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Fiber-reinforced syntactic foams
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Content
FIBER-REINFORCED SYNTACTIC FOAMS
by
Yi-Jen Huang
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)
May 2009
Copyright 2009 Yi-Jen Huang
ii
Acknowledgements
First, I would like to acknowledge the support and guidance provided by my
advisor, Professor Steven R. Nutt, during my study for Ph.D. Professor Nutt was always
there to listen and to give advice. He is responsible for involving me in the projects in the
first place. He taught me how to ask questions and express my ideals. He showed me
different ways to approach a research problem and the need to be persistent to accomplish
any goal.
Besides my advisor, I would like to thank the rest of my thesis committee: Dr.
Edward Goo and Dr. Charles Sammis for their agreement to serve on my PhD guidance
committee and taking the time and effort to evaluate my work.
I am sincerely grateful to Mr. Warren Haby, a laboratory manager at University of
Southern California, for his great advice and technical support facilitating experiments.
He always helps me to overcome the challenging research that lies behind it. He has been
a friend and mentor. He had confidence in me when I doubted myself, and brought out
the good ideas in me. Also thanks to the folks at the USC Composites Center for
interesting discussions and being fun to be with.
Last, but not least, I thank my family in Taiwan and my parents, Chung-Tien
Huang, and Chin-Yu Wang, for educating me with aspects from both music and sciences,
for unconditional support and encourage pursuing my interests.
iii
Tables of Contents
Acknowledgements ............................................................................................................. ii
Tables of Contents ............................................................................................................. iii
List of Tables ...................................................................................................................... v
List of Figures ................................................................................................................... vii
Abstract.................. …………………………………………….………………………….ix
Chapter 1 Introduction ................................................................................................... 1
1.1 Composite Materials ........................................................................................... 1
1.2 Syntactic Foams .................................................................................................. 2
1.3 Fiber-reinforced Syntactic Foams ....................................................................... 3
1.4 Research Objectives ............................................................................................ 4
References ....................................................................................................................... 6
Chapter 2 Fiber-Reinforced Syntactic Foam Based on Amino Microspheres .............. 8
2.1 Motivation ................................................................................................................. 8
2.2 Experimental ............................................................................................................. 9
2.2.1 Materials ............................................................................................................ 9
2.2.2 Synthesis of fiber reinforced syntactic foams .................................................. 10
2.2.3 Mechanical Properties ...................................................................................... 11
2.3 Results ..................................................................................................................... 15
2.3.1 Compression response ..................................................................................... 15
2.3.2 Tensile response ............................................................................................... 18
2.3.3 Shear response ................................................................................................. 25
2.4 Discussion ............................................................................................................... 29
2.5 Conclusions ............................................................................................................. 35
References ..................................................................................................................... 37
Chapter 3 Fiber-Reinforced Syntactic Foam Based on Phenolic Microspheres ......... 38
3.1 Motivation ............................................................................................................... 38
3.2 Experimental ........................................................................................................... 39
3.2.1 Materials .......................................................................................................... 39
3.2.2 Synthesis of fiber reinforced syntactic foams .................................................. 40
3.2.3 Mechanical Properties ...................................................................................... 41
3.3 Results and Discussion ........................................................................................... 45
3.3.1 Compression .................................................................................................... 45
3.3.2 Tension ............................................................................................................. 53
3.3.3 Shear ................................................................................................................ 55
3.3.4 Specific strength and specific stiffness ............................................................ 59
iv
3.4 Conclusions ............................................................................................................. 63
References ..................................................................................................................... 65
Chapter 4 Fiber-Reinforcement of Hybrid Syntactic Foams ....................................... 67
4.1 Motivation ............................................................................................................... 67
4.2 Experimental ........................................................................................................... 69
4.2.1 Materials .......................................................................................................... 69
4.2.2 Synthesis of hybrid foams ................................................................................ 70
4.2.3 Synthesis of fiber-reinforced hybrid foams (FRHFs) ...................................... 71
4.2.4 Mechanical properties ...................................................................................... 74
4.3 Prediction of heat expandable foam density ........................................................... 74
4.4 Results and Discussion ........................................................................................... 80
4.4.1 Compression of hybrid foams .......................................................................... 80
4.4.2 Mechanical response of fiber-reinforced hybrid foams (FRHFs) .................... 84
4.5 Conclusions ............................................................................................................. 90
References ..................................................................................................................... 92
Chapter 5 Composite Truss Core for Sandwich Structures ......................................... 95
5.1 Motivation ............................................................................................................... 95
5.2 Material ................................................................................................................... 97
5.3 Results and Discussion ......................................................................................... 101
5.3.1 Unit cell architecture and relative density ..................................................... 101
5.3.2 Analytical prediction of the pyramidal truss core response ........................... 103
5.3.3 Compressive Response .................................................................................. 106
5.3.4 Shear Response .............................................................................................. 109
5.3.5 Comparison to Conventional Honeycombs ................................................... 113
5.4 Conclusions ........................................................................................................... 116
References ................................................................................................................... 118
Chapter 6 Suggestions for Future Works .................................................................. 121
Bibliography ....................................................................................................................122
v
List of Tables
Table 2.1 Properties of aramid (Kevlar-49) and carbon (C-30) fibers
13
Table 2.2 Compressive strengths and moduli of neat and fiber-
reinforced syntactic foams
18
Table 2.3 Tensile strengths and moduli of neat and fiber-reinforced
syntactic foam
25
Table 2.4 Shear strength and modulus of neat and fiber-reinforced
syntactic foams
29
Table 2.5 Calculated fiber efficiency factors for compressive
strengths and moduli
32
Table 2.6 Calculated fiber efficiency factors for shear strengths and
moduli
33
Table 2.7 Calculated fiber efficiency factors for tensile strengths and
moduli
34
Table 3.1 Properties of carbon (C-30) fibers
43
Table 3.2 Compression test
47
Table 3.3 Tensile test
55
Table 3.4 Shear test
59
Table 4.1 The composition and compressive responses of 24 hybrid
foam samples
73
Table 4.2 The calculated density of the HEM in hybrid foams
77
Table 4.3 The mean diameter and shell thickness of the HEMs
relative to the correspond density
80
Table 4.4 Comparisons of commercial PVC foam, hybrid foams and
FRHF
86
Table 5.1 Compressive responses
109
vi
Table 5.2 Shear responses
112
Table 5.3 Specific properties of Sample 1 and conventional
honeycombs
116
vii
List of Figures
Figure 2.1 Sample of carbon fiber-reinforced syntactic foam based on
amino resin microspheres
14
Figure 2.2 Definitions of N-type and P-type foams
15
Figure 2.3 Compression stress-strain curves for syntactic foam
samples with density = 300 kg/m
3
17
Figure 2.4
The neat foam and the N-type fiber-reinforced foam after
compression test
21
Figure 2.5 The P-type fiber-reinforced foam after compression test.
22
Figure 2.6 Tensile stress-strain curves for syntactic foam samples
with density = 300 kg/m
3
23
Figure 2.7 The N-type fiber-reinforced foams fractured after tensile
test
24
Figure 2.8 Shear stress-strain curves for syntactic foam samples with
density = 300 kg/m
3
27
Figure 2.9 The N-type fiber-reinforced foam fractured after shear test
28
Figure 3.1 Schematic morphology of typical syntactic foam
44
Figure 3.2 Definition of N-type and P-type foams
45
Figure 3.3 Compressive stress-strain curves for syntactic foams
(density = 300 kg/m
3
)
50
Figure 3.4 Neat (left) and N-type (right) foams after compression test
51
Figure 3.5 SEM image of the neat foam (density = 300 kg/m
3
)
52
Figure 3.6 SEM image of the N-type foam (density = 300 kg/m
3
)
53
Figure 3.7 Tensile stress-strain curves for syntactic foams (density =
250 kg/m
3
)
58
Figure 3.8 Shear stress-strain curves for syntactic foams (density =
250 kg/m
3
)
62
viii
Figure 3.9 Specific strength and specific stiffness for the P-type
foams and the neat foams
63
Figure 4.1 The synthesis process for FRHFs
74
Figure 4.2 Compressive stress (4.2a) and modulus (4.2b) of hybrid
foams vary with resin weight percentage
79
Figure 4.3 SEM image of (85/15) and (0/100) hybrid foams
84
Figure 4.4 Compressive stress (4.4a) and modulus (4.4b) of hybrid
foams and FRHFs
89
Figure 4.5 SEM image of (40/60) FRHF, showing interlacing of
fibers in webbing
90
Figure 5.1 The mold used to prepare rigid carbon fiber core
100
Figure 5.2 The rigid carbon fiber core
101
Figure 5.3 Unit cell of pyramidal truss
103
Figure 5.4 An edge fixed solid cylindrical strut under compressive
force
105
Figure 5.5 Compressive stress and strain responses
111
Figure 5.6 Shear stress and strain responses 115
ix
Abstract
Long fibers are generally preferred for reinforcing foams for performance reasons.
However, uniform dispersion is difficult to achieve because they must be mixed with
liquid resin prior to foam expansion. New approaches aiming to overcome such problem
have been developed at USC’s Composites Center. Fiber-reinforced syntactic foams with
long fibers (over 6 mm in length) manufactured at USC’s Composites Center have
achieved promising mechanical properties and demonstrated lower density relative to
conventional composite foams.
Fiber-reinforced syntactic foams were synthesized from thermosetting polymeric
microspheres (amino and phenolic microspheres), as well as thermoplastic PVC heat
expandable microspheres (HEMs). Carbon and/or aramid fibers were used to reinforce
the syntactic foams. Basic mechanical properties, including shear, tensile, and
compression, were measured in syntactic foams and fiber-reinforced syntactic foams.
Microstructure and crack propagation behavior were investigated by scanning electron
microscope and light microscopy. Failure mechanisms and reinforcing mechanisms of
fiber-reinforced syntactic foams were also analyzed.
As expected, additions of fiber reinforcements to foams enhanced both tensile and
shear properties. However, only limited enhancement in compression properties was
observed, and fiber reinforcement was of limited benefit in this regard. Therefore, a
hybrid foam design was explored and evaluated in an attempt to enhance compression
properties. HEMs were blended with glass microspheres to produce hybrid foams, and
hybrid foams were subsequently reinforced with continuous aramid fibers to produce
x
fiber-reinforced hybrid foams. Mechanical properties of these foams were evaluated.
Findings indicated that the production of hybrid foams was an effective way to enhance
the compressive properties of syntactic foams, while the addition of fiber reinforcements
enhanced the shear and tensile performance of syntactic foams.
Another approach to produce ultralight sandwich core materials was explored in
which towpreg (fiber bundles impregnated with resin) were configured to produce 3D
pyramidal truss structures. The composite truss structures were subsequently filled with
foam to improve resistance to buckling. Mechanical properties of the foam-filled truss
structures were measured and contrasted with analytical predictions based on simple truss
theory. Results indicated that combination of foams and carbon fiber truss structures had
synergistic effects that enhanced the capacity to carry compressive and shear loads.
1
Chapter 1 Introduction
1.1 Composite Materials
Advances in technology require materials with unique property combinations that
cannot be achieved in conventional metal alloys, ceramics, or polymeric materials. This
is particularly true for applications that require durability under extreme conditions, such
as aerospace [1], underwater vessels [2], and ground vehicles [3]. Aircraft engineers, for
example, continually seek lower-density materials that are stiff and durable enough to
withstand severe abrasion and impacts while resisting corrosion [4, 5]. Traditionally,
strong and stiff materials are also relatively dense.
The development of composite materials has broadened the range of material
properties by expanding possible combinations. A composite can be generally described
as a multiphase material that exhibits a superior combination of properties with a
significant proportion of both constituent phases. In other words, superior properties, are
fashioned by judicious combination of two or more distinct materials. Many composites
offer property trade-offs as well.
Scientists and engineers have been producing new generations of extraordinary
materials by combining various elements such as metal, ceramics, and polymers.
Classification of composite materials can be organized into three main categories -
particle-reinforced, fiber-reinforced, and structural composites. Most composites have
been created to improve combinations of mechanical characteristics such as stiffness,
toughness, and ambient and high-temperature strength.
2
1.2 Syntactic Foams
Syntactic foams, a type of hollow particle filled polymer composite, represent an
important class of composites used in applications requiring ultra-low density [6-8].
Syntactic foams consist of hollow microspheres and a resinous binder. The microspheres
can be phenolic, glass, carbon, ceramic, or polymer-based materials. Epoxy, phenolic,
and polyester are common choices for the thermosetting binder [9, 10]. Syntactic foams
are considered closed-cell foams because of the continuous cell walls and the existence of
complete enclosed-cells in the final product. This type of foam offers some advantages
over other foam structures. For example, compared to conventional foams, syntactic
foams generally absorb less moisture, are more impact resistant, and are stronger in
compression.
The resistance of microspheres to compressive loads results in high specific
compressive strength for syntactic foams. However, the tensile and shear strengths of
syntactic foams are generally inferior to conventional foams. Results from observation of
tensile and shear fracture surfaces revealed that failures are primarily interfacial or
matrix-dominated, causing much lower strengths [11]. For example, compressive strength
of a typical glass microspheres syntactic foam (density = 600 kg /m
3
) showed 66 MPa,
much higher than tensile strength and shear strengths of 23 MPa and 26 MPa,
respectively [12]. In foam-core sandwich structures, shear and compression properties are
often critical to structural performance. Traditional syntactic foams are typically stiff and
brittle due to the use of glass microspheres. In contrast, modern syntactic foams based on
polymeric microspheres are naturally softer and less brittle [13]. However, such
3
advantage/disadvantage relations can be altered by fiber reinforcement and thus become
competitive to other cellular core materials used in sandwich panels.
1.3 Fiber-reinforced Syntactic Foams
Research on the fiber reinforcement of foams has been conducted for the past two
decades. Reviews by Methven and Dawson [14], Titow [15], and Ashida [16] provide
detailed descriptions of reinforcement methods, models and properties on different types
of composite foams. Performance improvements have been achieved several research
teams using fiber reinforcement. For example, Karthikeyan et al. [9, 17] improved
flexural strength by 30% compared to glass microspheres syntactic foam by adding 3
wt% of short (6mm) glass fibers, and increased compressive strength by 15-20% by
adding 5 wt% fibers. However, no improvements were observed when 10-14 wt% glass
fiber were added to glass microsphere-based syntactic foams by Gupta et al. [18,19].
Instead, Karthikeyan et al. [20] found a greater number of voids in foams with higher
fiber loadings that resulted in decreasing compressive strength with higher fiber content
using ultrasonic imaging techniques. Relation between mechanical characteristics of
fiber-reinforced syntactic foams and fiber reinforcements has become clearer as a result
of these works.
Mechanical characteristics of fiber-reinforced syntactic foam depend not only on
fiber properties, but also on degrees to which applied load is transferred to fibers by the
matrix. The strength of the interfacial bond between the fiber and matrix phases is the key
to the extent of load transfer. Also, fiber arrangement or orientation is significantly
4
influential to fiber-reinforced syntactic foam strength and other properties. With respect
to orientation, two extreme scenarios are considered - (1) parallel alignment of the fibers
in a single direction, and (2) totally random alignment. Continuous fibers are usually
aligned, whereas discontinuous fibers may be aligned randomly or partially oriented.
Aside from the arrangement or orientation of fibers, fiber length also plays an
important role in mechanical performance of fiber-reinforced syntactic foams. Longer
fibers are generally preferred for performance reasons. Fibers that exceed the critical
length are less easily pulled from the surrounding foam matrix, thus exploiting higher
strength and stiffness of fibers with improved crack resistance and damage tolerance.
However, long fibers pose processing challenges, particularly in dispersion and mixing
phases during foam synthesis [21]. Longer fibers greatly increase melt viscosity,
interfering with mixing and foam expansion. To overcome such problems, new
approaches have been developed at USC’s Composites Center. Fiber-reinforced syntactic
foams with long fibers (over 6 mm in length) manufactured at USC’s Composites Center
not only achieved promising mechanical properties, but also demonstrated a relatively
lower density when compared with conventional ones. These new approaches will be
presented in the following chapters.
1.4 Research Objectives
Academic and applied interests provided motivation for these studies. Efforts were
spent initially on synthesis of fiber-reinforced syntactic foams from thermosetting
polymeric microspheres (amino and phenolic microspheres), as well as thermoplastic
5
PVC heat expandable microspheres (HEMs). Carbon and/or aramid fibers were used to
reinforce syntactic foams. Basic mechanical properties, including shear, tensile, and
compression, were measured in syntactic foams and fiber-reinforced syntactic foams. The
microstructures and crack propagation behavior were investigated via scanning electron
microscopy and light microscopy. Failure mechanisms and reinforcing mechanisms of
fiber-reinforced syntactic foams were analyzed. These studies of syntactic foam systems
have advanced our understanding of basic structure-property relationship for composite
foams, and have provided a scientific basis for future applications of syntactic foam
systems.
Another approach to produce long fiber-reinforced foams was also investigated. Pre-
impregnated carbon fiber tows (towpreg) were configured to produce 3D pyramidal truss
structures for use as cores for prototype sandwich panels. Mechanical properties of
prototype panels were measured and contrasted with analytical predictions based on
simple truss theory. Despite curvature of the trusses, the specific properties were
generally comparable or superior to those of traditional honeycombs. The failure
mechanism of the sandwich structures subjected to compression and shear loading was
investigated. Results of the study indicated that composite truss cores showed load-
carrying abilities after the peak shear strength. The load-carrying ability was attributed to
the ability of surviving struts to carry tensile load. Moreover, the injection of foams into
composite truss cores provided mechanical support to the trusses and had synergistic
effects that enhanced the capacity to carry compressive and shear loads.
6
Chapter 1 References
1. Carter JT, Emmerson GT, Lo Faro C, McGrail PT, and Moore DR. The
development of a low temperature cure modified epoxy resin system for
aerospace composites. Composites Part A 2003; 34(1): 83-91.
2. Messager T, Pyrz M, Gineste B, and Chauchot P. Optimal laminations of thin
underwater composite cylindrical vessels. Composite Structures 2002; 58(4): 529-
537.
3. Torre L, and Kenny JM. Impact testing and simulation of composite sandwich
structures for civil transportation. Composite Structures 2000; 50 (3): 257-267.
4. Krebs J, Bhattacharyya D, and Friedrich K. Production and evaluation of
secondary composite aircraft components - a comprehensive case study.
Composites Part A 1997; 28(5): 481-489.
5. Modi OP, Saxena M, Prasad BK, Yegneswaran AH, and Vaidya ML. Corrosion
behaviour of squeeze-cast aluminum alloy-silicon carbide composites. Journal of
Materials Science 1992; 27(14): 3897-3902.
6. Gupta N, Kishore, Sankaran S, and Raman Nagar CV. On the characterization of
syntactic foam core sandwich composites for compressive properties, J
Reinforced Plast Compos 1999; 18(14): 1347-1357.
7. Jize NN, Hiel C, and Ishai O. Mechanical performance of composite sandwich
beams with syntactic foam cores, Compos Mater 1996; 12: 125-138.
8. Hiel C, Dittman D, and Ishai O. Composite sandwich construction with syntactic
foam core, Composites 1993; 24(5): 447-450.
9. Karthikeyan CS, Sankaran S, and Kishore, Influence of chopped strand fibers on
the flexural behavior of a syntactic foam core system, Polym Int 2000; 49: 158-
162.
10. Narkis M, Kenig S, and Puterman M, Three phase syntactic foams. Polym
Compos 1984; 5(2): 159-164.
11. Ashida K. Foamed composites. In: A.H. Landrock, Editor, Handbook of plastic
foams, Noyes Publications, Park Ridge, New Jersey 1995, p. 455.
12. Gupta N, Woldesenbet E, Kishore and Sankaran S. Response of syntactic foam
core sandwich structured composites to three-point bending. J Sandwich Struct
Mater 2002; 4: 249–272.
7
13. Lawrence E, and Pyrz R. Viscoelastic properties of polyethylene syntactic foam
with polymer microballoons. Polym Polym Compos 2001; 9(4): 227-238.
14. Methven JM, and Dawson JR. Reinforced foams. In: N.C. Hilyard, Editor,
Mechanics of cellular plastics, Scientific and Technical Books, New York 1982, p.
323-358.
15. Titow WV. Reinforced foams in reinforced thermoplastics, Halsted Press, New
York 1975, p. 264-278.
16. Ashida K. Foamed composites. In: A.H. Landrock, Editor, Handbook of plastic
foams, Noyes Publications, Park Ridge, New Jersey 1995, p. 163-183.
17. Karthikeyan CS, Sankaran S, Kumar MNJ, and Kishore. Processing and
compressive strengths of syntactic foams with and without fibrous reinforcement,
J Appl Polym Sci 2001, 81: 405-411.
18. Gupta N, Karthikeyan CS, Sankaran S, and Kishore, Correlation of processing
methodology to the physical and mechanical properties of syntactic foams with
and without fibers, Mater Charact 1999; 43: 271-277.
19. Gupta N, Woldesenbet E, and Kishore, Compressive fracture features of syntactic
foams - microscopic examination, J Mater Sci 2002; 37: 3199-3209.
20. Karthikeyan CS, Murthy CRL, Sankaran S, and Kishore, Characterization of
reinforced syntactic foams using ultrasonic imaging technique, Bull Mater Sci
1999; 22(4): 811-815.
21. Cotgreave TC, and Shortall JB. The mechanism of reinforcement of polyurethane
foam by high-modulus chopped fibers. J Mater Sci 1977; 12: 708-717.
8
Chapter 2 Fiber-Reinforced Syntactic Foam Based on Amino Microspheres
2.1 Motivation
This chapter discusses the effect of fiber reinforcement in thermosetting polymer
syntactic forms. Amino resins, as thermosetting materials, are produced during reactions
of amino group bearing compounds and formaldehyde. Urea-formaldehyde and
melamine-formaldehyde resins, the two most popular and viable aminos, are the aminos
of interest in this chapter. Cost-effectiveness of these two aminos allows them to be
widely used in commercial products such as soundproof devices or unbreakable plates.
Work with urea and urea-formaldehyde began as early as in 1880s by French and
German scientists named Einhorn, Holzer, and Goldschmidt et al. These amino resins
were commercially introduced to the U.S. in 1928 by H. John and F. Pollack. In 1935, a
new light-colored, enhanced water and heat resistance amino called melamine-
formaldehyde resin were introduced. Despite its higher cost, according to the literature by
Einhorn and Hamburger [1], it brought the application of amino resins to a broader
spectrum of usage in forms of powder, particle board, varnish resin, or fine divided solids
for construction industry or as depot fertilizers [2]. One of the most common usages of
amino resins is its application as wall material in microspheres production. Although
there are a multitude of possible wall materials, amino resins still play an important role
due to its availability and low-cost as being selected in microspheres preparations. The
greater importance of Amino microspheres in pharmaceutical industries and boarder
applications in areas such as marine transportations are the chief advances in its
9
developments [3, 4]. Other application for amino microspheres is the pressure-sensitive
copy papers [5].
Formation process of amino microspheres is done in an interfacial reaction where
encapsulation took place during two separate phases [6]. Water immiscible substances,
urea or melamine resins in most cases, are emulsified or dispersed in water and later be
brought into contact with water soluble amino resin. Note that MINOSET is a
commercial product and the exact synthesis method of MINOSET is not available.
This chapter introduces a new type of microshere based on fire-resistant amino
resin. The goal of this study was to synthesize fiber-reinforced amino syntactic foam and
to evaluate composite foams in its use as sandwich core material. In this chapter,
mechanical behavior of composite syntactic foams was measured with variations in terms
of fiber lengths, type and arrangements on the tensile, shear and compressive properties.
Prototypes of sandwich panels were used for comparison with properties of that of balsa
wood as reference.
2.2 Experimental
2.2.1 Materials
Hollow amino resin microspheres (MINOSET Asia Pacific Microspheres Corp.,)
which consist of a mixture of urea-formaldehyde and melamine-formaldehyde resins,
were used in this experiment of syntactic foams fabrication. Amino resin is a
thermosetting material and therefore crosslinks to form rigid microspheres. Typical
applications of these microspheres are functional fillers in plasitc composites and in
10
adhesives due to its reduced density yet enhanced flame retardancy and compressive
strength. Mean diameter of these microspheres was 55 μm, with true density of 0.22 –
0.29 g/cc, and hydrostatic compressive strength of 4.5 MPa [7]. Phenolic resol resin
(Schenectady International, Inc.) was chosen as the polymer binder. Two types of fibers
were selected as reinforcements - aramid (Kevlar - 49) (Dupont Corp.) and carbon (C –
30) fibers (SGL Carbon AG), and average lengths were ~12 mm and ~ 24 mm. Properties
of Kevlar – 49 and C - 30 fibers are presented in Table 2.1 [8, 9].
2.2.2 Synthesis of fiber reinforced syntactic foams
All fiber-reinforced foams were fabricated to a standard composition of 4 wt %
fiber, 19 wt % phenolic resin and 77 wt % amino microspheres. Density of all forms was
measured to be 300 kg/m
3
. Pre-forms were made by first treating fibers and microspheres
with a 5 wt % solution of phenolic resin in ethyl alcohol and mixing. Next, pre-forms
were dried for 24 hours at room temperature, followed by hot compaction for 40 minutes
at 150 °C. The resulting fiber-reinforced syntactic foams had a layered structure
perpendicular to the direction of compaction, as shown in Figure (2.1b). The fibers were
randomly oriented within the plane normal to the compaction direction, designated the
XY plane, and tended to lie in this plane, as shown in figure (2.1a). Unreinforced
syntactic foams were fabricated for use as a control material, and these samples consisted
of 23 wt % phenolic resin and 77 wt % microspheres.
11
2.2.3 Mechanical Properties
Compressive, shear, and tensile tests were performed on composite syntactic
foams in accordance with ASTM standards D 1621 – 73, C – 273, D 1623 – 78,
respectively. The specimen dimensions for both compressive and tensile testing was 25.4
x 25.4 x 25.4 mm
3
, while shear specimens were 25.4 x 25.4 x 6 mm
3
. Two types of fiber
arrangement were adopted as shown in Figure (2.2). In the first type, designated N, the
XY fiber plane was normal to the load axis, while in the second type, designated P, the
XY plane was parallel to the load axis. More than five samples were tested for each
material condition. Samples were examined before and after testing with an optical
microscope.
12
Table 2.1 Properties of aramid (Kevlar-49) and carbon (C-30) fibers
Sample
Density
(g/cm
3
)
Fiber diameter
(μm)
Tensile modulus
(GPa)
Tensile strength
(GPa)
Kevlar-49 1.44 12 131 3.8
C-30 1.81 7 225 3.5
13
Figure 2.1 Sample of carbon fiber-reinforced syntactic foam based on amino resin
microspheres (2.1a) XY plane of the sample, (2.1b) thickness of the sample.
10 mm
(2.1a)
(2.1b)
14
Figure 2.2 Definitions of N-type and P-type foams
15
2.3 Results
2.3.1 Compression response
Compression tests on neat foams and fiber-reinforced foams were conducted and
influences of fiber arrangement, fiber length, and fiber type were analyzed. Figure (2.3a)
showed the stress-strain curves of neat foam samples and N-type composite foam
samples. Despite the little effect on modulus or failure strain, N-type composite foams
showed slightly higher strengths. However, the curves illustrate the different failure
behavior of composite foams compared with neat foams, and the influence of fiber
orientation. While both neat foams and N-type composite foams showed monotonically
increasing stress after yielding, neat foams showed a lower slope. In Figure (2.3b), P-type
composite foams showed much higher yield points comparing to neat foams, with a
decreased stress after the peak.
The compressive strengths and moduli of foam samples are summarized in Table
2.2. The P-type fiber-reinforced foam samples have compressive strengths of 7.5 - 9.3
MPa. Neat foam and N-type composite foams have compressive strengths of 4.2 MPa
and 4.3 – 4.8 MPa, respectively. The compressive moduli of the composite foams display
only minor sensitivity to fiber arrangement, although the P-type foams reinforced with
carbon fibers showed a modulus increase of 40 % over the neat foam.
16
Figure 2.3 Compression stress-strain curves for syntactic foam samples with density
= 300 kg/m
3
. 1) Neat foam, 2) N-type short aramid fiber-reinforced foam, 3) N-type short
C fiber-reinforced foam, 4) N-type long aramid fiber-reinforced foam, 5) N-type long C
fiber-reinforced foam, 6) P-type short aramid fiber-reinforced foam, 7) P-type short C
fiber-reinforced foam, 8) P-type long C fiber-reinforced foam, 9) P-type long aramid
fiber-reinforced foam.
(2.3b)
(2.3a)
17
Table 2.2 Compressive strengths and moduli of neat and fiber-reinforced syntactic
foams
Sample
Modulus
(MPa)
Modulus
(%)
Strength
(MPa)
Strength
(%)
Neat foam 308 100 4.20 100
N-type short (12 mm) carbon
fiber-reinforced foam
287 93 4.50 107
N-type long (24mm) carbon
fiber-reinforced foam
293 95 4.82 115
P-type short (12 mm) carbon
fiber-reinforced foam
427 139 7.50 179
P-type long (24mm) carbon
fiber-reinforced foam
432 140 8.68 206
N-type short (12 mm) aramid
fiber-reinforced foam
300 97 4.31 103
N-type long (24mm) aramid
fiber-reinforced foam
284 92 4.43 105
P-type short (12 mm) aramid
fiber-reinforced foam
304 99 7.60 181
P-type long (24mm) aramid
fiber-reinforced foam
310 100.6 9.33 222
18
Figure (2.4) showed compression testing results of neat foams and N-type fiber-
reinforced foams. Fractures on corner and edge of neat foam are direct evidence of its
brittle behavior. However, the retained corner and edge of N-type composite foams
suggested increased fracture resistance and overall structural integrity. A different failure
mode on P-type foams was observed (Figure 2.5) with vertical cracks evident extending
parallel to preferred orientation of fibers.
Lengths of fiber had shown only a minor effect on compressive strength of
composite foams. Strength enhancement on N-type foams was negligible. Longer fibers
(24 mm) produced only 15-20% greater strength than short ones (12 mm). Effects of fiber
type were primarily manifest in P-type foams, where moduli of Carbon fiber foams were
~40% higher than aramid fiber foams.
2.3.2 Tensile response
Tensile tests were conducted on neat foams and composite foams with different
fiber type, fiber arrangement, and fiber length. Representative stress–strain curves are
shown in Figure (2.6). The stress-strain response of the neat and fiber-reinforced foams
differed substantially, particularly after peak stress, as shown in Figure (2.6a). The neat
foam exhibited brittle stress–strain behavior, and after the peak stress, abruptly lost all
load-carrying capacity. In contrast, N-type composite foams continued to carry
significant loads well beyond the peak stress, although the peak stress was reached at
slightly smaller strains. Even P-type foams continued to carry some loads beyond peak
stress, although the loads were substantially reduced, as shown in Figure (2.6b). As
19
expected, the stress drops were larger in P-type foams than in N-type foams. Figure (2.7)
shows fracture surfaces of N-type composites foams reinforced with carbon and aramid
fibers. The carbon fibers tend to be straight, while the aramid fibers are more curved. The
average length of the exposed fiber ends was ~ 7 mm.
Table 2.3 summarized strength and modulus among different forms. As expected,
P-type foams demonstrated the greatest increase in strength, with a surprising seven-fold,
accompanied by an 8-fold increase in modulus. These enhancements were achieved with
4wt% fiber reinforcement. N-type composite foams showed increases in strength and
modulus of ~1.3. The research discovered that there is only a minor effect on elastic
moduli among fiber types, as the moduli of carbon-fiber-reinforced foams and aramid-
fiber-reinforced foams differed by merely 10%. On the other hand, fiber type showed a
greater effect in foam strength. In P-type foams, aramid fibers-reinforced foams resulted
in 30-40% higher strengths than carbon fibers-reinforced foams. In N-type foams, fiber
type caused relatively smaller differences in strength on the order of 15% or less. Fiber
length had the least effect. Longer fibers (24 mm) showed only slightly greater tensile
moduli and strength (about 15% greater) than shorter ones (12 mm).
20
Figure 2.4 The neat foam and the N-type fiber-reinforced foam after compression test:
(2.4a) Neat foam; (2.4b) N-type short carbon fiber-reinforced foam; (2.4c) N-type short
aramid fiber-reinforced foam. Arrows show the direction of compression
(2.4a) (2.4b) (2.4c)
21
Figure 2.5 The P-type fiber-reinforced foam after compression test:
(2.5a) P-type
short carbon fiber-reinforced foam; (2.5b) P-type short aramid fiber-reinforced foam.
Arrows show the direction of compression
(2.5a) (2.5b)
10 mm
22
Figure 2.6 Tensile stress-strain curves for syntactic foam samples with density = 300
kg/m
3
. 1) Neat foam, 2) N-type short carbon fiber-reinforced foam, 3) N-type long
carbon fiber-reinforced foam, 4) N-type short aramid fiber-reinforced foam, 5) N-type
long aramid fiber-reinforced foam, 6) P-type short carbon fiber-reinforced foam, 7) P-
type long carbon fiber-reinforced foam, 8) P-type short aramid fiber-reinforced foam, 9)
P-type long aramid fiber-reinforced foam.
(2.6a)
(2.6b)
23
(2.7b)
Figure 2.7 The N-type fiber-reinforced foams fractured after tensile test: (2.7a) N-
type long aramid fiber-reinforced foam; (2.7b) N-type long carbon fiber-reinforced foam.
(2.7a)
1 mm
1 mm
24
Table 2.3 Tensile strengths and moduli of neat and fiber-reinforced syntactic foam
Sample
Modulus
(MPa)
Modulus
(%)
Strength
(MPa)
Strength
(%)
Neat foam 70 100 1.19 100
N-type short (12 mm) carbon
fiber-reinforced foam
77 110 1.37 115
N-type long (24mm) carbon
fiber-reinforced foam
86 123 1.43 120
P-type short (12 mm) carbon
fiber-reinforced foam
492 703 5.14 432
P-type long (24mm) carbon
fiber-reinforced foam
561 801 6.00 504
N-type short (12 mm) aramid
fiber-reinforced foam
87 124 1.5 126
N-type long (24mm) aramid
fiber-reinforced foam
95 136 1.64 138
P-type short (12 mm) aramid
fiber-reinforced foam
438 626 7.02 590
P-type long (24mm) aramid
fiber-reinforced foam
494 706 8.44 709
25
2.3.3 Shear response
The stress–strain curves from shear tests are presented in Figure (2.8). Moduli of
N-type composite foams tend to be similar to that of neat foams. P-type foams with long
fibers showed enhanced moduli. Composite foams generally showed higher peak stress
than neat foams. After peak stress, neat foams and P-type carbon fiber foams showed
brittle behavior, while the other composite foams showed gradually decreasing stress
after peak stress. Representative fracture surfaces from N-type composite foams are
shown in Figure (2.9). The extremely fibrous and brush-like surfaces indicates extensive
fiber pullout. Fibers appeared to undergo pullout rather than fracture and resulted in
gradual stress drop after peak stress.
Data from the shear tests are summarized in Table 2.4. Fiber arrangement had a
significant effect on shear moduli and shear strengths. As shown in Table 2.4, P-type and
N-type composite foams, when compared with neat foam, showed strength increments of
3.3 times and 1.3 times, respectively. Similarly, the moduli increment factors for P- and
N-type composite foams were up to 2.3 and 1.2, respectively. Both carbon and aramid
fiber-reinforced N-type foams showed strength and modulus values similar to neat foam.
In contrast, P-type aramid fiber-reinforced foams showed increases in shear strength of
up to 3.3 times, while P-type carbon fiber-reinforced foams showed increases of 2.4 times.
Moduli increments for foams with carbon and aramid fiber reinforcement were similar -
2.3 and 2.0 times, respectively.
26
Figure 2.8 Shear stress-strain curves for syntactic foam samples with density = 300
kg/m
3
. 1) Neat foam, 2) N-type short aramid fiber-reinforced foam, 3) N-type long
aramid fiber-reinforced foam, 4) N-type short C fiber-reinforced foam, 5) N-type long C
fiber-reinforced foam, 6) P-type short C fiber-reinforced foam, 7) P-type short aramid
fiber-reinforced foam, 8) P-type long aramid fiber-reinforced foam, 9) P-type long C
fiber-reinforced foam
(2.8b)
(2.8a)
27
(2.9a)
(2.9b)
Figure 2.9 The N-type fiber-reinforced foam fractured after shear test: (2.9a) N-type
long carbon fiber-reinforced foam; (2.9b) N-type long aramid fiber-reinforced foam.
10 mm
28
Table 2.4 Shear strength and modulus of neat and fiber-reinforced syntactic foams
Sample
Modulus
(MPa)
Modulus
(%)
Strength
(MPa)
Strength
(%)
Neat foam 72 100 2.12 100
N-type short (12 mm) carbon
fiber-reinforced foam
72 100 2.43 115
N-type long (24mm) carbon
fiber-reinforced foam
83 115 2.80 132
P-type short (12 mm) carbon
fiber-reinforced foam
125 174 4.10 193
P-type long (24mm) carbon
fiber-reinforced foam
167 231 5.17 244
N-type short (12 mm) aramid
fiber-reinforced foam
71 99 2.14 101
N-type long (24mm) aramid
fiber-reinforced foam
75 104 2.23 105
P-type short (12 mm) aramid
fiber-reinforced foam
99 138 3.81 180
P-type long (24mm) aramid
fiber-reinforced foam
145 201 7.14 336
29
Fiber length and orientations had major effects on shear properties in some foams.
For example, long fiber based foams (24 mm) consistently resulted in higher moduli and
strengths than shorter fiber based foams (12 mm). For N-type composite foams, the
longer fibers produced only modest increments (17%) compared to shorter 12 mm fibers.
However, for P-type foams, longer fibers resulted in 46% higher shear moduli, and 156%
higher shear strengths compared with shorter fiber P-type foams.
2.4 Discussion
Perhaps the most striking effect of the fiber reinforcement of foams is the fact that
enhancements in certain mechanical properties of several hundred percent were achieved
with the addition of only 4 wt% of fiber. In order to estimate the fiber contribution to the
performance of the composite foams, fiber efficiency factors for strengths (σ) and moduli
(E) were calculated from a simple rule of mixtures, as shown below.
E E V K E V
V K V
r u u Ef f f
r u u f f f
= +
= + σ σ σ
σ
Where E
r
, E
u
and E
f
are the moduli of the reinforced foams, unreinforced foam and fibers,
respectively, and V
u
and V
f
are the volume fractions of unreinforced foam and fibers,
respectively.
30
In expressions, K
Ef
and f K
σ
are the fiber efficiency factors, ranging from slightly
negative to 0.36. Tables 2.5-2.7 show the calculated fiber efficiency factors for the
different tests. The reason for the negative fiber efficiency factor is that the adhesion
between the fiber and microspheres is not perfect. The imperfect bonding creates defects
in the reinforced foam and reduces certain mechanical properties. As intuitively expected,
fiber efficiency factors were consistently higher for P-type foams than for N-type foams.
The fiber distribution is roughly laminar; and consequently the fibers contribute more to
the mechanical properties of the P-type foams than to those of the N-type foams.
Likewise, fiber efficiency factors for long fiber-reinforced foams were always higher than
for short fiber-reinforced foams. Further discussion of these phenomena is presented in
the following paragraphs.
The critical fiber length is an important factor that describes the threshold for
efficient stress transfer in composites [10]. This is especially true in cellular composites,
where the matrix is porous and stress transfer is less efficient. Fibers that exceed the
critical length cannot be easily pulled out of the surrounding matrix unless they break,
thus realizing the full strengthening potential of fiber reinforcements [10, 11]. The critical
lengths for carbon and aramid fibers can be estimated using the expression [11]:
σ
t
π R
2
= 2σ
s
π RL
c
Where σ
t
and σ
s
are the fiber tensile strength and interface shear strength, and R and L
c
represent the fiber radius and fiber critical length, respectively.
31
Table 2.5 Calculated fiber efficiency factors for compressive strengths and moduli
Sample
Ef
K
f
K
σ
N-type short (12 mm) carbon
fiber-reinforced foam
-0.012 0.013
N-type long (24mm) carbon fiber-
reinforced foam
-0.008 0.027
P-type short (12 mm) carbon
fiber-reinforced foam
0.077 0.14
P-type long (24mm) carbon fiber-
reinforced foam
0.080 0.18
N-type short (12 mm) aramid
fiber-reinforced foam
-0.004 0.004
N-type long (24mm) aramid fiber-
reinforced foam
-0.018 0.008
P-type short (12 mm) aramid
fiber-reinforced foam
-0.001 0.10
P-type long (24mm) aramid fiber-
reinforced foam
0.004 0.15
32
Table 2.6 Calculated fiber efficiency factors for shear strengths and moduli
Sample
Ef
K
f
K
σ
N-type short (12 mm) carbon
fiber-reinforced foam
3E-4 0.013
N-type long (24mm) carbon fiber-
reinforced foam
0.007 0.028
P-type short (12 mm) carbon
fiber-reinforced foam
0.034 0.081
P-type long (24mm) carbon fiber-
reinforced foam
0.061 0.13
N-type short (12 mm) aramid
fiber-reinforced foam
-3E-4 0.001
N-type long (24mm) aramid fiber-
reinforced foam
0.003 0.004
P-type short (12 mm) aramid
fiber-reinforced foam
0.023 0.050
P-type long (24mm) aramid fiber-
reinforced foam
0.062 0.15
33
Table 2.7 Calculated fiber efficiency factors for tensile strengths and moduli
Sample
Ef
K
f
K
σ
N-type short (12 mm) carbon
fiber-reinforced foam
0.005 0.008
N-type long (24mm) carbon fiber-
reinforced foam
0.010 0.010
P-type short (12 mm) carbon fiber-
reinforced foam
0.27 0.16
P-type long (24mm) carbon fiber-
reinforced foam
0.31 0.20
N-type short (12 mm) aramid
fiber-reinforced foam
0.015 0.009
N-type long (24mm) aramid fiber-
reinforced foam
0.022 0.013
P-type short (12 mm) aramid fiber-
reinforced foam
0.31 0.17
P-type long (24mm) aramid fiber-
reinforced foam
0.36 0.21
34
The interface shear strength for the foam matrix can be approximated by the shear
strength of the foam matrix [11]. Based on these assumptions, the calculated critical
lengths for carbon and aramid fibers in the syntactic foam matrix were 2.9 mm and 5.4
mm, respectively. Thus, even the shortest C fibers (used 12 mm) were substantially
longer than the critical length (2.9 mm), while the short aramid fibers used were only
slightly longer than the critical length (5.4 mm).
The greatest performance enhancements were observed in P-type composite
foams. Fiber efficiency factors for P-type foams were always significantly higher than for
N-type foams. For example, a maximum K
Ef
of 0.36 was measured for the tensile
modulus of P-type aramid fiber foams with long fibers. This value is much higher than
the efficiency factor for N-type aramid fiber foams in the same condition, which was only
0.02. Fibers in syntactic foams were not strongly oriented in a single direction, as shown
in Figure (2.1), and hence efficiency factors did not approach 1 for any of the foams.
Shear and tensile results exhibit trends consistent with previous work on
composite foams synthesized from expandable PVC microspheres [12]. In that work, the
unidirectional fiber performs had a layered structure in which fibers extended
predominantly in one direction but tended to interlock within the layers. Composite
foams produced from unidirectional webbing exhibited tensile strengths in the X, Y, and
Z directions in the ratio of 3: 1.4: 1. The same composite foams showed shear strengths in
the X, Y, and Z directions in the ratio of 1.75: 1.28: 1. In the present work, as in the work
cited above, the maximum efficiencies of shear and tensile tests were achieved when
fibers were oriented predominantly along the loading direction.
35
The strong influence of fiber orientation on compression response was manifested
in amino resin syntactic foams with medium density (300 kg/m
3
). In particular,
significant enhancement was observed when fibers were primarily oriented parallel to the
compressive load. This kind of fiber arrangement is analogous to a unidirectional fiber
composite. Failure modes for unidirectional fiber composites subject to a longitudinal
compressive load include (a) transverse tensile failure; (b) fiber microbuckling (either
with the matrix still elastic, preceded by matrix yielding, or preceded by constituent
debonding), and (c) shear failure [13]. Compressive failure of P-type foams may be
initiated by transverse tensile strain originating from the Poisson effect. This would result
in cracks at the interface of the fiber and the porous matrix.
2.5 Conclusions
Composite syntactic foams comprised of amino resin microspheres were
fabricated with small additions of chopped carbon and aramid fibers. Shear, compression,
and tensile properties of composite foams were evaluated with arrangements of fibers
with different axis orientations, and properties were compared to those of similar foam
without fibers. In one foam orientation, properties were enhanced 30 – 40%, while in the
other orientation, compression strength increased by a factor of up to 2; tensile strength
increased by a factor of up to 7, and the elastic modulus increased by a factor of up to 8.
Shear strength and modulus increased by a factor of up to 3.3 and 2.3, respectively.
The performance of the composite syntactic foams can be compared with
conventional core materials used in sandwich structure. The properties of the composite
36
syntactic foams fabricated in the present study generally surpassed those of commercial
PVC foams of comparable density (300 kg/m
3
). The shear strength of the composite
syntactic foams would be comparable to balsa wood of comparable density and to some
types of honeycomb [14]. In general, however, compression properties of the composite
foams showed only modest enhancement, and fiber reinforcement was of limited benefit
in this regard. Before foams can compete with balsa and honeycomb cores, new strategies
will be required to enhance compression properties. Nevertheless, because of the
enhanced mechanical performance and moderate cost, the fiber-reinforced syntactic foam
may be eligible for certain structural applications in ships, transportation land vehicles,
and aircraft. Syntactic composite foams with densities of 100 – 200 kg/m
3
would expand
the range of potential applications.
37
Chapter 2 References
1. Leinhorn, A, and Hamburger A. Ber. Deutschen Chemischen Gesellschaft 1908;
41: 24.
2. Updeqraff IH. Amino Resins. In: Encyclopedia of Polymer Science and
Engineering 1985; 1: 752.
3. Little SR, Lynn DM, Puram SV, and Langer R. Formulation and characterization
of poly (β amino ester) microparticles for genetic vaccine delivery. Journal of
Controlled Release 2005; 107(3): 449-462.
4. Huang YJ, Vaikhanski L, and Nutt SR. 3D long fiber-reinforced syntactic foam
based on hollow polymeric microspheres. Composites Part A 2006; 37(3): 488-
496.
5. Kajtna J, Likozar B, Golob J, and Krajnc M. The influence of the polymerization
on properties of an ethylacrylate/2-ethyl hexylacrylate pressure-sensitive adhesive
suspension. International Journal of Adhesion and Adhesives 2008; 28(7): 382-
390.
6. Dietrich K, Bonatz E, Geistlinger H, Herma H, Nastke R, Purz HJ, Schlawne M,
and Teige W. Amino resin microcapsules. Acta Polymerica 1989; 40(5): 325-331.
7. Technical information. Asia Pacific Corp. 2003.
8. Technical information. DuPont Co. 2003.
9. Technical information. SGL Carbon AG. 2003.
10. Methven J, and Dawson JR. Reinforced foams in mechanics of cellular plastics.
In: H.C. Hilyard, Editor, Scientific and technical books 1982, p. 323-358.
11. Cotgreave TC, and Shortall JB. The mechanism of reinforcement of polyurethane
foam by high-modulus chopped fibers. J Mater Sci 1977; 12: 708-717.
12. Vaikhanski L, and Nutt SR. Fiber reinforced composite foam from expandable
PVC microspheres. Composites: Part A 2003; 34: 1245-1253.
13. Agarwal BD, and Broutman LJ. Analysis and performance of fiber composites,
Wiley, New York, 1990 p. 449.
14. Technical information, Baltek Corp. 2003.
38
Chapter 3 Fiber-Reinforced Syntactic Foam Based on Phenolic Microspheres
3.1 Motivation
Among the two most widely used thermosetting resins, amino resin was discussed
in previous chapter and phenolic resin will be the topic of this chapter. Phenolic resin was
the first synthesized thermosetting resin in 1907. There have been intense studies for
reaction mechanisms in Phenolic resin synthesis and its reactions with other substances
[1-4]. Formaldehyde and tri-phenol reactions are methods used to form phenolic resins
and are reasons of their dark, opaque color that results to limitation of their usage in light-
colored products. Phenolic resins are structurally weaker then amino resins. Cellulose-
filled amino resins typically have a tensile strength of 90 MPa while cellulose-filled
phenolic resins only have 62 MPa. Even though both of them are thermosetting resins,
they show different effectiveness in fiber reinforcements to syntactic forms.
Producing phenolic microspheres involves a few steps [5, 6]. First, feed phenolic
resin with surfactants into a mixer, and heat to required temperature. This creates
polymer beads. The beads need to be pumped into a disk sprayer using a displacement
pump. Condensation products will be dispersed in the top spray chamber and heated by
heat a carrier as a combustion product of burning propane in a furnace. As the solvent
evaporates, the microspheres will expand and solidify. The condensation reaction is
obtained by suspension polymerization. This technique is useful for preparation of
polymer beads in size ranging from 10 to 2000 micrometers. A typical oil and water
procedure involves suspension of an immiscible, oil-soluble monomer in water [7, 8].
39
The suspension is stirred to directly form polymerized spherical particles with initiators
soluble in monomer phase [9, 10].
Phenolic microspheres are very cost attractive, and having good FST properties.
Phenolic microspheres also form excellent chemical bonds with organic base resins such
as epoxy, polyester, polyurethane and others to produce a composite with very good
physical properties. No coupling agents are required to bond phenolic microspheres and
resin matrix. Inherent bonding of spheres with base resin prevents the filler from scarring
or popping from surface of the composite and therefore making phenolic microspheres
ideal for great outdoor applications given their outstanding smoothness.
This chapter undertakes an investigation in mechanical properties in synthesized
phenolic microsphere foams reinforced by a small addition of chopped carbon fibers.
Particularly, the aim of this study was to focus on enhancing the specific strength and
specific stiffness of the composite foam. Composite foams with different densities and
fiber orientations were produced, and the relations between the foam density and the
mechanical properties, including compression, tension, and shear, were determined. The
result illustrated an approach to produce composite syntactic foams with superior specific
strength.
3.2 Experimental
3.2.1 Materials
Hollow phenolic microspheres (PHENOSET® BJO-0930 Asia Pacific
Microspheres Corp.) were used in syntactic foams fabrication with different fiber
40
loadings. Phenolic microspheres are used as a filler of polymer composites and adhesive
to reduce density, yet enhance flame retardancy and compressive strength. The mean
diameter of microsphere was 70 μm, with true density of 210 – 250 kg/m
3
, and
hydrostatic compressive strength being 3.4 MPa [11]. Phenolic resol resin (Schenectady
International, Inc.) was chosen as polymer binder. Carbon fiber (C-30, SGL Carbon AG)
with an average length of ~12 mm was selected as reinforcement. Table 3.1 presents
properties of C-30 fibers [12].
3.2.2 Synthesis of fiber reinforced syntactic foams
Figure (3.1) shows the microstructure of typical syntactic foams consisting voids,
resins, and microspheres. Voids are included to obtain low density foams while retaining
high strength. In the fiber-reinforced samples, the carbon fibers span thousands of
microspheres. In this study, all fiber-reinforced foams were fabricated with a composition
of 4 wt % fiber, 19 wt % phenolic resin and 77 wt % phenolic microspheres. The
syntactic foams were synthesized with three settings of density: 250, 300, and 350 kg/m
3
.
Pre-forms were obtained from the process of immersing and mixing fibers and
microspheres in a 5 wt % solution of phenolic resin in ethyl alcohol. Next, pre-forms
were dried for 24 hours at 20°C, followed by hot compaction and curing for 40 minutes at
150oC. Six pre-forms were stacked with parallel fiber layers before compacting.
Compaction pressure was set to be just sufficient to achieve the desired sample volume
(and density). Fibers were randomly oriented within plane normal to the compaction
direction. Neat syntactic foams were fabricated as a control material using the same
41
processing conditions, and consisted of 23 wt % phenolic resin and 77 wt % phenolic
microspheres.
3.2.3 Mechanical Properties
Compressive, shear, and tensile tests on composite syntactic foam were performed
according to ASTM standards D 1621 – 73, C – 273, D 1623 – 78, respectively.
Specimen dimensions for compressive and tensile testing were both 25.4×25.4×25.4 mm
3
,
with shear specimens of 25.4×25.4×6.2 mm
3
. Two fiber plane orientations were used in
testing. In the first orientation, designated as N, the XY fiber plane was perpendicular
(normal) to the load axis, as shown in Figure (3.2), while in the second orientation,
designated as P, the sample was turned 90 degrees, with the fiber plane parallel to the
load axis, as shown in Figure (3.2). At least five samples were tested for each material
condition. Samples were examined before and after testing with a light microscope.
42
Table 3.1 Properties of carbon (C-30) fibers
Sample
Density
(kg/m
3
)
Fiber diameter
(μm)
Tensile modulus
(GPa)
Tensile strength
(GPa)
C-30 1810 7 225 3.5
43
Figure 3.1 Schematic morphology of typical syntactic foam
44
Figure 3.2 Definition of N-type and P-type foams
45
3.3 Results and Discussion
3.3.1 Compression
Compressive tests were conducted on the neat foams and fiber-reinforced foams,
and the influence of foam density and fiber preferred orientation was analyzed. Table 3.2
summarized compressive strength and modulus values for each foam samples. As
expected, both the compressive strength and modulus showed an increase in foam density.
Compared to neat foams, compressive property of P-type composite foams increased
significantly yet N-type foams showed only similar value to that of neat foams. The neat
foam was tested in both the compaction direction and 90 degree to the compaction
direction, and no significant change in the strength values was observed.
Often, foam properties comply with a simplified power law [13]:
A(ρ)=A
0
ρn [1]
where A being foam property, and ρ being foam density. A
0
represents a factor reflecting
properties of solid cell wall materials, and n represents the exponent. By fitting
compressive modulus data to the power law, exponent values were calculated 1.01, 1.83,
and 1.58 for P-type foam, neat foam, and N-type foam, respectively. Corresponding
correlation factors were computed 1, 0.99, and 0.89, respectively. As expected, even
though fiber orientation strongly affected exponent in the density study range,
compressive modulus of fiber-reinforced syntactic foams exhibited in accordance with
the power law.
46
Table 3.2 Compression test
Sample
D = 250 kg/m
3
D = 300 kg/m
3
D = 350 kg/m
3
Stress
(MPa)
Modulus
(MPa)
Stress
(MPa)
Modulus
(MPa)
Stress
(MPa)
Modulus
(MPa)
Neat
1.87 ±
0.12
65 ± 5
2.83 ±
0.25
93 ± 6
4.55 ±
0.31
120 ± 11
N-type
2.06 ±
0.18
70 ± 5
3.16 ±
0.26
89 ± 7
4.05 ±
0.27
120 ± 10
P-type
4.80 ±
0.34
216 ± 19
6.27 ±
0.57
259 ± 23
8.33 ±
0.65
303 ± 27
47
Figure (3.3) displays compressive stress-strain curves for syntactic foams (density
= 300 kg/m
3
). Different failure behaviors of fiber-reinforced foam and the influences of
fiber orientation were illustrated. A monotonically increasing stress is observed on both
neat foam and N-type composite foams and is believed as the result of compaction of
voids after yielding [14]. However, when the strain exceeded 20%, the compressive stress
of the neat foam dropped, a consequence of fractures at sample corners and edges, as
shown in Figure (3.4). In neat foams, fracture occurs along a plane inclined 45 degrees to
the load axis. Compressive strength was diminished by voids in the samples due to stress
concentration at void tips. Cracks initiate at these sites and propagate either along
microsphere surfaces or through the microspheres. To clarify the fracture mechanism of
the neat foam, fracture surfaces were examined by scanning electron microscope (SEM)
(Figure 3.5). Most microspheres fractured after the compressive test and provide
evidence that cracks propagated through the microspheres.
Figure (3.4) shows that the N-type foam retained shape and that cracks
propagated within the fiber layer plane, approximately normal to the stress axis. For N-
type foams, failure occurred parallel to the fiber layer planes, which are defined in Figure
(3.2). Introducing the fiber layers led to additional voids between the fiber layer planes
and the matrix, increasing the local void fraction at interfacial areas between the fiber
layer and the matrix. This can be explained by the interlacing of the fibers, as shown in
Figure (3.6). The SEM image shows interstitial spaces devoid of microspheres that are
created by fiber intersections. Such spaces account for the weakest part of N-type foams
as a severe limitation on compressive strength. Thus, the compressive strength of N-type
48
foams was limited by the interfacial strength between the fiber layer planes and the
matrix instead of the matrix strength.
Figure (3.3) showed a different failure mode that was observed from P-type form.
The sharp decrease of compressive stress after peak stress is believed the direct result of
separating fibers and supporting matrix. Cracks propagated in P-type foams initially
along 45-degree plane and defected along preferred fiber orientation. This transverse
tensile separation, or debonding, in lateral direction was evident to Poisson’s effect. Such
separation prevents the matrix from transferring loads to fibers and reduces the resistance
of compressive load. Substantial enhancement in compressive modulus was observed
from the steep slope of P-type loading curve, attributing to the stiffness of the carbon
fibers and the supporting foam cells [15].
49
Figure 3.3 Compressive stress-strain curves for syntactic foams (density = 300 kg/m
3
)
50
Figure 3.4 Neat (left) and N-type (right) foams after compression test. N type foams
represent the fiber reinforced foams with the XY fiber plane perpendicular (normal) to
the load axis.
51
Figure 3.5 SEM image of the neat foam (density = 300 kg/m
3
)
52
Figure 3.6 SEM image of the N-type foam (density = 300 kg/m
3
)
53
3.3.2 Tension
The tensile strength and modulus values for foam samples are summarized in
Table 3.3. For the neat and P-type foams, both tensile strength and modulus increased
with increasing foam density. However, for N-type foams, the tensile strength was
insignificantly dependent on foam density. For neat foams, increasing density from 250
kg/m
3
to 350 kg/m
3
resulted in increases in tensile strength and modulus of 36% and 31%,
respectively. For P-type foams, increasing density from 250 kg/m
3
to 350 kg/m
3
resulted
in an increase of 210% in both tensile stress and modulus. The tensile strength values for
all N-type foams were ~1 MPa, although the tensile modulus increased from 120 MPa to
160 MPa when the foam density increased from 250 kg/m
3
to 350 kg/m
3
.
Fracture of neat foams occurred in the middle of the gage section. SEM
examination of the fracture surface revealed that fracture propagated through the
microspheres. Volume fraction of voids played an important role in controlling tensile
strength and modulus of neat foams. According to Luxmoore and Owen [16], a crack will
initiate from an oversized void with a composite subject to tensile loading. Therefore, as
foam density increased, strength and modulus are expected to increase, too. Foam failure
occurred in N-type foams along preferred fiber plane, normal to load axis. Similar to
compression loading, fracture occurred at sites where displayed a greatest local volume
fraction of voids. Strength and stiffness of N-type foams were also found insensitive to
foam density.
54
Table 3.3 Tensile test
Sample
D = 250 kg/m
3
D = 300 kg/m
3
D = 350 kg/m
3
Stress
(MPa)
Modulus
(MPa)
Stress
(MPa)
Modulus
(MPa)
Stress
(MPa)
Modulus
(MPa)
Neat
1.33 ±
0.11
140 ± 12
1.64 ±
0.13
160 ± 14
1.95 ±
0.16
184 ± 11
N-type
1.01 ±
0.09
120 ± 11
1.06 ±
0.08
150 ± 13
1.18 ±
0.07
160 ± 14
P-type
1.76 ±
0.12
188 ± 13
2.71 ±
0.22
262 ± 22
3.72 ±
0.22
395 ± 31
55
Failures in P-type foams involved two components: failure of the matrix, and
pullout of the fibers. Carbon fibers bridged foam cracks and resisted crack opening after
matrix failure. Matrix failure accounted for the most controlling failure initiation. Fibers
of P-type foams carried much applied load that led to a tested tensile strength of 30~90%
greater than neat foams with comparable density. P-type foams tensile modulus values
also demonstrated a greater 34%, 63%, and 114% for three different densities in
reflection of fiber-dominated behavior. Denser matrix was proved to provide better fiber
support and better reinforcement effect.
Tensile stress-strain curves for composite syntactic foams are shown in Figure
(3.7). The curves illustrate different failure behavior of the P-type foam compared with
the neat and the N-type foams. Both the neat and the N-type composite foams exhibit
brittle fracture after yielding, while the P-type foams carry substantial loads long after
yielding. The post-yield behavior derives from crack bridging and fiber pullout, processes
which boost fracture toughness. In contrast, fiber orientations in N-type foams do little to
enhance the load-carrying capacity of the composite foam, and these foams behave much
like neat foams.
3.3.3 Shear
Shear strength and modulus on each foam sample can be found in Table 3.4. For
neat foams, the shear strength and modulus increased slightly with increasing foam
density. When density was increased from 250 kg/m
3
to 350 kg/m
3
, the shear stress and
modulus increased 3% and 44%, respectively. Similarly, for P-type fiber-reinforced
56
foams, shear strength and modulus increased 22% and 20% respectively with increasing
density. However, when compared to the neat foam, the P-type foam (350 kg/m
3
density)
showed 150% and 102% increases in shear strength and modulus, respectively. N-type
foams showed insignificant density dependence on both shear strength and modulus.
However, when compared to neat foam, the N-type foams showed 60% increase in shear
strength, although the shear modulus was similar to the neat foams at all foam densities.
The shear properties of the neat and the N-type foams do not show a strong
dependence on density, unlike the tensile and compressive properties. Cracks in neat
foams propagated primarily through the interfaces between adjoining microspheres. In
this case, shear strength as well as shear modulus were primarily determined by shear
properties of resin binder. On contrary, cracks in N-type foams propagated in planes
parallel to the preferred fiber plane. Therefore, local volume fraction of voids control
largely in shear strength and modulus rather than global density of the matrix. Fibrous
and brush-like surface of N-type foams was an indication of extensive fiber pullout in
shear tests. Process of fiber pullout absorbed energy [17] and increased shear strength of
N-type foam. In P-type foams, interlaced fibers resists shear loads and shear fracture,
enhancing shear strength. The interlacing of fibers leads to the enhanced shear
performance of the foam.
57
Figure 3.7 Tensile stress-strain curves for syntactic foams (density = 250 kg/m
3
)
58
Table 3.4 Shear test
Sample
D = 250 kg/m
3
D = 300 kg/m
3
D = 350 kg/m
3
Stress
(MPa)
Modulus
(MPa)
Stress
(MPa)
Modulus
(MPa)
Stress
(MPa)
Modulus
(MPa)
Neat
2.32 ±
0.19
25 ± 2
2.35 ±
0.21
29 ± 3
2.38 ±
0.21
36 ± 4
N-type
3.81 ±
0.28
31 ± 2
3.84 ±
0.34
31 ± 4
3.85 ±
0.33
31 ± 3
P-type
4.85 ±
0.39
61 ± 5
5.34 ±
0.48
63 ± 6
5.94 ±
0.47
73 ± 8
59
Figure (3.8) illustrated typical shear stress-strain curves for syntactic foams.
Brittle behavior after peak stress was commonly observed on all curves. Modulus of the
N-type foam tends to be similar to the neat foam, while P-type foams show enhanced
modulus. This phenomenon is attributed to the larger number of voids introduced by the
fiber layer planes for the N-type foams, which weaken the resistance to shear loads.
3.3.4 Specific strength and specific stiffness
The specific strength and specific stiffness for the P-type foams and the neat
foams are shown in Figure (3.9). The specific strength is known as the strength-to-weight
ratio and the specific stiffness is known as the stiffness-to-weight ratio. Compared with
the neat foams, P-type foams displayed a much higher specific strength and stiffness in
compression, tension, and shear. P-type foams (350 kg/m
3
) exhibited the greatest specific
strength and specific stiffness in compression and tension, while P-type foams (250
kg/m
3
) showed the greatest specific strength and stiffness in shear. Unlike the specific
tensile and compressive strengths, as density of P-type foams increased by 40%, the
specific shear strength and specific shear modulus actually decreased by 22% and 20%.
On contrary, density dependence of specific shear strength and shear modulus was
relatively weak for all foam types, due to shear strength and modulus being largely
controlled by local volume fraction of voids rather than global density of matrix. The
specific compressive strength and the specific Tensile strength increased with increasing
composite foam density. However, the specific shear strength decreased with increasing
composite foam density, as shown in Figure (3.9a). E. M. Wouterson et al. reinforced the
60
epoxy resin with phenolic microspheres, and concluded that the specific compressive
stiffness leveled off as the increase in stiffness was counter-balanced by the increased
density of the composite [18]. A similar phenomenon occurred in the present work,
where the specific stiffness for compressive and shear loading was essentially
independent of foam density, as shown in Figure (3.9b).
61
Figure 3.8 Shear stress-strain curves for syntactic foams (density = 250 kg/m
3
).
62
Figure 3.9 Specific strength and specific stiffness for the P-type foams and the neat
foams. The bold solid lines are the lines for the P-type foams and the dash lines are the
lines for the neat foams.
(3.9b)
(3.9a)
63
3.4 Conclusions
Composite syntactic foams composed of phenolic resin microspheres were
fabricated with a small addition of chopped carbon fiber. Compressive, tensile, and shear
properties of foams with different fiber orientations and densities were evaluated. The
experimental results lead to several conclusions. Substantially strengthened syntactic
foams can be obtained by adding a relatively small amount of short carbon fibers (i.e., 4
wt %). Fiber orientation and foam density accounts largely for property enhancements.
Significant improvements in specific properties in compression, tension and shear can be
achieved for composite foams with fiber layer planes parallel to the loading direction (P-
type). Fiber-reinforced syntactic foams and composite laminates are directly influenced
by fiber orientation. Relationship of foam properties and foam density, over the density
range experiment, exhibited power law behavior albeit with different exponents.
Syntactic foams reinforced with other fibers (e.g., glass fiber, aramid fiber, etc.)
are expected to exhibit similar enhancements in specific strength and specific modulus.
Different reinforcing fibers should allow one to tailor property enhancements, by
exploiting the specific fiber properties and their distribution in the matrix. For example,
more flexible fibers should permit more random orientations, and thus the property
enhancement could be less anisotropic. Reinforcing syntactic foams with fibers provides
effective strengthening and toughening of the foams. Furthermore, interlacing of the
fibers can enhance isotropic toughness through crack bridging and deflection. Efficient
load transfer may make it possible to reduce the amount of resinous binding material,
thereby reducing composite density while retaining high strength and high modulus.
64
Fiber reinforcement affords enhancements in strength, stiffness, and toughness that are
difficult or impossible through traditional methods such as changing the type,
concentration, size, and wall thickness of microspheres, or modifying the interface
between the microspheres and the binding material. In terms of practical relevance, fiber
reinforcement also enhances core-skin adhesion in sandwich structures, while elevating
core properties to ranges comparable to those of honeycomb cores.
65
Chapter 3 References
1. Nylen P, and Sunderland E. Modern Surface Coatings; Wiley, London, 1965.
2. Carswell TS. Phenoplasts their structure, properties, and chemical technology.
New York: Interscience Publishers, 1947.
3. Fry JS, Merriam CN, and Boyd WH. Chemistry and Technology of Phenolic
Resins and Coatings in ACS Symposium Series. Applied Polymer Science.
Washington, DC: American Chemical Society, 1985. p. 1147.
4. Kirk-Othmer, Ed. Encyclopedia of Chemical Technology, 3rd edition; Wiley:
New York, 1983; vol. 17: Phenolic Resins.
5. Gould DF, Phenolic Resins; Reinhold: New York, 1959.
6. Singh A, and Lal D. Effect of reaction parameters on the particle sizes of
crosslinked spherical phenolic beads by suspension polymerization of phenol and
formaldehyde. Journal of Applied Polymer Science Volume 100 Issue
3, Pages 2323 – 2330.
7. Svec F, and Frechet JMJ. Science 1996; 273:205.
8. Lewandowski K, Svec F, and Frechet JMJ. Journal of Applied Polymer Science
1998; 67: 597.
9. Jo YD, Park KS, Ahn JH, and Ihm SD. Eur Polem J 1996; 65: 1257.
10. Coutinho HMB, Neves MAFS, and Dias ML. J. Apply Polym Sci 1997; 65: 1257.
11. Technical information, Asia Pacific Corp. 2003.
12. Technical information, SGL Carbon AG 2003.
13. Gibson LJ, and Ashby MF. Cellular Solids, Structure and Properties, Cambridge
University Press, Cambridge, 1997.
14. Gupta N, Kishore, Woldesenbet E, and Sankaran S. J. Mater. Sci. 2001: 36: 4485.
15. Kelly A, Cahn R, and Bever M. Concise Encyclopedia of Composite Materials,
MIT Press Cambridge. Massachusetts 1994; 1.
16. Luxmoore R, and Owen DRJ. Mechanics of Cellular Plastics. Applied Science
Publishers, London 1982; 359.
17. Masud, and Zaman KB. J. of Mater. Processing Technology 2006; 172: 258.
66
18. Wouterson E, Boey F, Hu X, and Wong S. Composites Science and Technology
200; 65: 1840.
67
Chapter 4 Fiber-Reinforcement of Hybrid Syntactic Foams
4.1 Motivation
Rigid foams are widely used as core materials for sandwich panels in structural
applications, including marine vessels, surf boards, rail cars, air vehicles, and wind
turbine blades [1-3]. Rigid, structural foams such as polymethacrylimide (PMI) and
partly cross-linked polyvinyl chloride (PVC) (e.g. Rohacell
and Divinycell
), are
typically produced by expanding liquid polymers. PMI foam is widely used in demanding
applications requiring high strength and/or thermoformability [4-6]. Likewise, PVC
foams are widely used in transportation vehicles, wind turbine blades, and construction
applications [7, 8]. Both types of rigid foam exhibit low density, attractive process
characteristics (such as formability), and ranges of mechanical and physical properties.
Rigid foams also can be produced by combining hollow microspheres with a resin
binder to form syntactic foams [9]. The microspheres usually consist of ceramic materials
such as glass and silica, or of polymeric materials (e.g. epoxy, phenolics, polyester
chloride and unsaturated polyester) [10]. Typically, resin binders are thermosetting resins
such as epoxy, phenolic, and polyester [11, 12]. Compared with conventional foams,
syntactic forms generally exhibit superior compressive strength and reduced moisture
uptake. Because of these characteristics, syntactic foams are frequently used in marine
applications and lightweight panels [13, 14]. Properties of syntactic foams can be further
enhanced by the addition of fibers [15-19].
68
Hybrid composites that include multiple types of reinforcement have been widely
studied [20-22]. This approach is employed to bestow synergistic properties of chosen
fillers and matrix, the hope being that more than one type of reinforcement can be
combined to create multiple advantages, either in performance, weight, and/or cost.
Optimizing such hybrid composites requires extensive investigation to understand the
mechanical properties [23]. The multiple benefits of Nevertheless, hybrid composites
have seen increased commercial significance in recent years because of the multiple
benefits of hybrid reinforcement. First, hybrid materials can be designed with specific
properties to match an end use because of a wider spectrum of tailor-made physical and
mechanical properties. Secondly, economic advantages are realized by the reduced cost
of reinforcement and filler materials of hybrid composites over conventional composites.
Finally, and perhaps most importantly, synergistic effects in hybrid composites can lead
to mechanical and functional improvements beyond those observed in conventional
composites [24].
In previous work [25], aramid fiber-reinforced syntactic foams were produced
using HEMs. Mechanical performance and formability of fiber-reinforced foams
surpassed unreinforced foams in terms of tensile and shear properties as well as damage
tolerance (crack resistance). However, aramid fibers did little to increase compressive
strength and modulus of the composite foams, both of which are critical for sandwich
core applications.
In the present work, FRHFs were prepared to achieve an improved balance of
properties, which were then compared with the compressive, shear, and tensile properties
of partly cross-linked PVC foams. The work consisted of two sections: 1) a study of
69
compressive properties of hybrid foams with different formulations, and 2) an analysis of
the effects of fiber reinforcement on a single hybrid foam formulation selected from the
first section. The hybrid foams were comprised of blends of rigid glass microspheres and
heat expandable (thermoplastic) microspheres, and the resulting compressive properties
were compared with those of foams comprised solely of HEMs. The hybrid composites
literature has dealt primarily with efforts to blend different fibers [26]. However, there
have been few reports on efforts to blend microspheres in foams, and this study
elucidates the effects of blending different microspheres in hybrid foams.
4.2 Experimental
4.2.1 Materials
The HEM selected (supplied by Expancel, Inc) was based on a thermoplastic
copolymer of polyacrylonitrile (PAN) and polyvinyl chloride (PVC) [27]. Each
microsphere consisted of a thermoplastic shell encapsulating a hydrocarbon blowing
agent such as isobutene. The glass microsphere selected (K25, supplied by 3M) was
hollow spheres with a reported crush strength 5.17 MPa. The average diameter of the
glass microspheres was 55 microns [28]. Phenolic resol resin (Schenectady International,
Inc.) was chosen as the polymer binder. For the preparation of FRHFs, non-woven
aramid fiber webbing (supplied by Tex Tech Industries Inc.) was selected. The webbing
was comprised of chopped aramid fiber (Kevlar 49) with an average length of 100mm.
The fiber arrangement of webbing was unidirectional, and the webbing preform had a
layered structure [29]. Fiber extended primarily in one direction, although there is some
70
waviness within the layers. A degree of fiber crossover between layers held the preform
together.
4.2.2 Synthesis of hybrid foams
Glass microspheres were mixed with HEMs in different ratios, designated as (G/H)
ratio (glass/heat expandable). The process of foam synthesis relied on microsphere
expansion in a closed mold. During heating, the blowing agent expanded the softened
shells of the microspheres, expanding the spheres from 10-12 microns to 30-50 microns
in diameter. The glass microspheres occupied empty spaces and prevented excessive
displacements during foam expansion, while the HEMs generated hydrostatic pressure
needed for thermo-welding.
Critical process parameters for foam synthesis included heating time, heating
temperature, resin binder content, and (G/H) ratio. The critical characteristic governing
mechanical behavior of unreinforced foams is density [30]. For foam synthesis, the
process temperature was 150 °C, and other parameters were selected to yield hybrid
foams of comparable density, typically ~100 kg/m
3
. The heating time was 45-60 minutes,
depending on the amount of HEM used in the particular sample. Shorter heating times
generally resulted in inadequate expansion and fusion of the HEMs, resulting in weak
foams. For heating times longer than 60 min (at 150°C), the HEMs shrank due to the
increased amount of gas diffused out from the shell of microspheres, thereby causing
surface cracks in the foams.
71
The experimental design consisted of four resin binder contents - 15%, 20%, 25%,
and 30% by weight. The design also included six (G/H) ratios - (0/100), (20/80), (40/60),
(50/50), (60/40), and (85/15). There were two main components - resin and microspheres
- in each sample, and thus 15 wt% resin was equivalent to 85 wt% microspheres, and the
weight percentage of the microspheres included both HEMs and glass microspheres. A
total of 24 samples were prepared, as summarized in Table 4.1.
4.2.3 Synthesis of fiber-reinforced hybrid foams (FRHFs)
The preparation of the FRHFs involved vibratory infiltration of dry microspheres
into the fiber webbing, as described in a previous report [31]. The synthesis process was
shown in Figure (4.1). The fiber composition was 4 wt% in all FRHFs. The webbing was
surface-treated to improve adhesion to the microspheres. Treatment was carried out in a
dilute solution of 5 wt% phenolic resin in acetone, followed by drying for 24 hours at
room temperature and curing for 30 minutes at 200°C. The glass microspheres and HEMs
were blended and infiltrated into the fiber webbing, then heated to 150 °C in a closed
mold to expand the microspheres and fuse them together.
72
Table 4.1 The composition and compressive responses of 24 hybrid foam samples
(G/H)
Resin
(wt%)
HEM (wt%)
Glass
Microsphere
(wt%)
Strength
(MPa)
Modulus
(MPa)
(0/100) 15 85 0 1.31 ± 0.10 83 ± 7
(20/80) 15 68 17 1.43 ± 0.12 92 ± 8
(40/60) 15 51 34 1.58 ± 0.13 125 ± 11
(50/50) 15 42.5 42.5 1.52 ± 0.12 121 ± 11
(60/40) 15 34 51 1.45 ± 0.11 120 ± 10
(85/15) 15 12.75 72.25 1.01 ± 0.08 80 ± 7
(0/100) 20 80 0 1.32 ± 0.11 84 ± 7
(20/80) 20 64 16 1.44 ± 0.11 91 ± 8
(40/60) 20 48 32 1.65 ± 0.13 130 ± 12
(50/50) 20 40 40 1.53 ± 0.12 120 ± 10
(60/40) 20 32 48 1.46 ± 0.12 119 ± 11
(85/15) 20 12 68 1.05 ± 0.08 80 ± 8
(0/100) 25 75 0 1.32 ± 0.10 84 ± 8
(20/80) 25 60 15 1.43 ± 0.11 91 ± 7
(40/60) 25 45 30 1.61 ± 0.12 128 ± 12
(50/50) 25 37.5 37.5 1.49 ± 0.12 117 ± 11
(60/40) 25 30 45 1.45 ± 0.11 120 ± 11
(85/15) 25 11.25 63.75 1.11 ± 0.09 80 ± 8
(0/100) 30 70 0 1.34 ± 0.10 85 ± 8
(20/80) 30 56 14 1.43 ± 0.11 90 ± 8
(40/60) 30 42 28 1.55 ± 0.11 121 ± 11
(50/50) 30 35 35 1.48 ± 0.12 115 ± 10
(60/40) 30 28 42 1.47 ± 0.13 121 ± 11
(85/15) 30 10.5 59.5 1.15 ± 0.08 81 ± 7
73
Figure 4.1 The synthesis process for FRHFs
74
4.2.4 Mechanical properties
Compressive, shear, and tensile tests were performed on hybrid foams in
accordance with ASTM standards (D 1621-73, C-273, D 1623-78). The specimen
dimensions for both compressive and tensile testing were 25.4 x 25.4 x 25.4 mm
3
, while
shear specimens were 25.4 x 25.4 x 6 mm
3
. In the compressive and tensile tests, the
loading direction was parallel to the preferred fiber orientation of the webbing, while for
the shear tests, the shear direction was perpendicular to the preferred orientation. At least
five samples were tested for each material condition.
4.3 Prediction of heat expandable foam density
Foam density plays the most critical role in determining foam properties [30]. An
approximate prediction of the density of the HEMs in hybrid foams can help explain the
effects of decreasing the HEM content while increasing the glass microsphere content.
Approximate values for the volume fraction of HEMs (X
E
) and the density of the HEMs
(D
E
) in hybrid foams were calculated and are listed in Table 4.2. The volume fraction of
glass, X
G
, can be obtained from equation (1) below.
) %(
) %(
G wt
D
D
D V
G wt m
V D
m
X
G
C
G C
C
C G
G
G
=
×
= = ………………………….(1)
75
Where m
C
, V
C
, and D
C
are the mass, volume, and density of the hybrid foam, and m
G
, D
G
,
and wt%(G) are the mass, density, and weight percentage of glass microspheres in the
hybrid foam.
The volume fraction of phenolic resin was ignored, since it was negligible
compared to the microspheres (<1%). Therefore, the volume fraction of the HEMs, X
E
,
can be expressed as equation (2).
G E
X X − = 1 ……………………………………………………………(2)
The density of the HEMs in hybrid foams (D
E
) can be calculated from equation (3) below.
) %(
1
) %(
E wt
X
D
X V
E wt m
X V
m
D
G
C
E C
C
E C
E
E
−
=
×
= = ……………………….(3)
Here, m
E
and wt%(E) are the mass and weight percentage of the HEMs in the hybrid
foam.
76
Table 4.2 The calculated density of the HEM in hybrid foams
(G/H) Resin (wt%) X
E
D
E
(kg/m
3
)
(20/80) 15 0.93 73
(40/60) 15 0.86 59
(50/50) 15 0.83 51
(60/40) 15 0.80 43
(85/15) 15 0.71 18
(20/80) 20 0.94 68
(40/60) 20 0.87 55
(50/50) 20 0.84 48
(60/40) 20 0.81 40
(85/15) 20
0.73
16
(20/80) 25 0.94 64
(40/60) 25 0.88 51
(50/50) 25 0.85 44
(60/40) 25 0.82 37
(85/15) 25 0.75 15
(20/80) 30 0.94 59
(40/60) 30 0.89 47
(50/50) 30 0.86 41
(60/40) 30 0.83 34
(85/15) 30
0.76
14
77
In Table 4.2, the lowest volume fraction of the HEMs was 0.71 for (85/15) foams.
Thus, the HEM constituent together with the binder can be considered as the matrix
phase, even in the (85/15) hybrid foams. These (85/15) foams exhibited extremely low
density (14~18 kg/m
3
), and low strength values, because the “matrix” did not transfer
stress effectively to the glass microspheres. For the (85/15) foams, the phenolic binder
helped bind the glass microspheres, and consequently, compressive strength values
increased with increasing resin content, as shown in Figure (4.2a). In other hybrid foams,
(20/80), (40/60), (50/50), and (60/40), the resin binding was essentially redundant, as the
heat expandable foams were sufficiently dense and well-bonded to transfer stress
effectively.
The relationship of the diameter of HEMs and the shell thickness of HEMs with
the corresponding HEM density, D
E
, is listed in Table 4.3 [27]. When the size of HEMs
expanded from 12 μm to 50 μm, D
E
decreased from 1000 kg/m
3
to 20 kg/m
3
. The shell
thickness of HEMs changed along with D
E
. The shell thickness decreased from 2 μm to
0.1 μm when D
E
decreases from 1000 kg/m
3
to 20 kg/m
3
. The changes in the shell
thickness of HEMs resulted in the measured change in the compressive properties of the
HEMs. That is, when the HEMs expanded, the shell thickness decreased and as a result,
the compressive strength and modulus of HEMs decreased.
78
(4.2a)
(4.2b)
Figure 4.2 Compressive stress (4.2a) and modulus (4.2b) of hybrid foams vary with
resin weight percentage
79
Table 4.3 The mean diameter and shell thickness of the HEMs relative to the
correspond density
Density (kg/m
3
) 1000 500 200 100 50 20
Mean Diameter of
HEMs (μm)
12 17 23 30 37 50
Shell Thickness of
HEMs (μm)
2 1 0.6 0.4 0.2 0.1
80
4.4 Results and Discussion
4.4.1 Compression of hybrid foams
Compressive tests were conducted on neat foams and hybrid foams to determine
the influence of resin content and the (G/H) ratio. The compressive strength and modulus
values of hybrid foams are shown in Figure (4.2a) and Figure (4.2b). For all resin
compositions, the (40/60) hybrid foams showed the highest compressive strength and
modulus values.
The (0/100) neat foams were conventional syntactic foams consisting of HEMs
and resin. The HEMs for all (0/100) foams were mutually fused. Consequently, relative
amounts of HEMs and resin had little effect on the compressive strength and modulus.
The (0/100) foams exhibited a mean compressive strength of 1.32 MPa and a
compressive modulus of 91 MPa. In comparison, the (20/80) hybrid foams exhibited a
mean compressive strength of 1.43 MPa and a modulus of 91 MPa. Again, the amount of
the HEM was sufficient to effectively fuse the microspheres, and the resin content had
little effect. One reason for the similar compressive properties of (20/80) hybrid foams
with different resin contents was the amount of glass microspheres used differed by only
3 wt%, from 14wt% to17 wt% as shown in Table 4.1.
The (85/15) hybrid foams exhibited lower strength and modulus values than the
(0/100) neat foams. The lower values were attributed to the thinner shell thickness of
HEMs of the hybrid foams. As shown in Table 4.2, these (85/15) hybrid foams contained
10.5~12.75 wt% HEM and D
E
were 14~18 kg/m
3
. As shown in Table 4.3, HEMs
expanded to ~50 microns in diameter in (85/15) hybrid foams, compared to 30 microns in
81
the (0/100) foams, as shown in Figure (4.3). While the HEMs expanded more, the
thickness of the shell decreased from 0.4 μm in (0/100) foams to 0.1 μm in (85/15)
hybrid foams. This thinnest shell thickness resulted in the lowest compressive strength
and modulus.
The (60/40) and (50/50) hybrid foams showed similar compression properties,
and both foams showed little dependence on resin content, as shown in Figure 4.2. The
compressive strength and modulus values for the (60/40) hybrid foams were ~1.45 MPa,
and 120 MPa, respectively. Similarly, the mean compressive strength and modulus values
for the (50/50) hybrid foams were 1.50 MPa, and 118 MPa. As shown in Table 4.2, the
calculated densities of HEM were 41, 44, 48, 51 kg/m
3
for (50/50) hybrid foams with
resin contents of 30%, 25%, 20%, and 15%, respectively. The (50/50) hybrid foams all
had similar HEM density values (41-51 kg/m
3
) and similar weight fractions of glass
microspheres. Consequently, the compressive properties were consistent and showed
little dependence on these parameters within the range of values tested. The (60/40)
hybrid foams showed the same trends as the (50/50) hybrid foams, and thus the
compressive properties were consistent in these foams as well.
The (40/60) hybrid foams exhibited the highest compressive strength and modulus
values for any resin content. The (40/60) hybrid foam with 20 wt% resin showed the
highest values for compressive strength (1.65 MPa) and compressive modulus (130 MPa).
Thus, as the glass microsphere content was increased, the compressive strength and
modulus values increased. However, based on the experimental design, in order to
achieve similar densities of hybrid foams (100 kg/m
3
); increasing the weight percentage
of glass microspheres required decreasing both the weight percentage and the volume
82
fraction of HEMs. According to the calculated density values (D
E
) in Table 4.2, D
E
was
lowest in the hybrid foam with the highest glass microsphere content. The lower D
E
had
larger mean diameters and thinner shell thicknesses, as shown in Table 4.3. As expected,
the thinner walls of the HEMs resulted in reduced compressive strength and modulus
values. Therefore, combining the effect of increasing glass microsphere content with the
compensating effect of decreasing D
E
led to identification of an optimal hybrid foam
composition (40/60) with maximum compression properties.
83
Figure 4.3 SEM image of (85/15) and (0/100) hybrid foams
(0/100)
50 microns
30 microns
(85/15)
84
4.4.2 Mechanical response of fiber-reinforced hybrid foams (FRHFs)
Fiber reinforcement of hybrid foams produced substantial increases in both tensile
strength and modulus, as shown in Table 4.4. For example, the tensile strength for the
(0/100) foams increased from 0.7 to 3.4 MPa, while the modulus increased from 15 to 97
MPa. Similarly, the tensile strength of the (40/60) hybrid foam increased from 0.7 to 3.3
MPa, while the modulus increased from 15 to 111 MPa. Note that the tensile strength
and modulus values for the (40/60) hybrid foam (without fiber additions) were nearly
identical to those of the (0/100) neat foam. Thus, the glass microspheres had a negligible
effect on the tensile properties of the FRHFs, as one would reasonably expect.
85
Table 4.4 Comparisons of commercial PVC foam, hybrid foams and FRHF
Compression Tensile Shear
Strength
(MPa)
Modulus
(MPa)
Strength
(MPa)
Modulus
(MPa)
Strength
(MPa)
Modulus
(MPa)
(0/100) neat
foam
1.3 ± 0.1 84 ± 7 0.7 ± 0.1 15 ± 1 1.2 ± 0.1 22 ± 3
(40/60)
hybrid foam
1.6 ± 0.1 130 ± 12 0.7 ± 0.1 15 ± 1 1.2 ± 0.1 24 ± 4
(0/100)
fiber-
reinforced
foams
1.2 ± 0.1 75 ± 7 3.4 ± 0.3 97 ± 10 1.7 ± 0.2 46 ± 4
(40/60)
FRHF
1.8 ± 0.1 147 ± 14 3.3 ± 0.3 111 ± 10 1.8 ± 0.2 45 ± 4
Commercial
cross-linked
PVC foam
1.6 ± 0.1 120 ± 11 3.2 ± 0.3 104 ± 9 1.5 ± 0.2 38 ± 4
86
The shear properties for the FRHFs showed trends similar to the tensile properties.
In particular, the shear strength for the (0/100) FRHF and the (40/60) FRHF was 1.7 MPa
and 1.8 MPa, respectively, as shown in Table 4.4 (these values represent increases of 40-
50% over the neat hybrid foam). Likewise, the shear modulus for the (0/100) and (40/60)
FRHFs was 46 MPa and 45 MPa, respectively (increases of ~90% relative to the
unreinforced hybrid foam, which was 24 MPa). Note that the shear strength and modulus
values for the (40/60) hybrid foam were almost identical to those of the (0/100) neat foam.
The results indicate that the addition of glass microspheres did not significantly affect the
shear or tensile properties of either the hybrid foams or the FRHFs. Thus, enhancement of
tensile and shear properties of FRHFs was attributed solely to the addition of fibers.
Because the effects of fiber reinforcement of syntactic foams have been considered
previously [25], the following discussion focuses on the compressive behavior of the
FRHFs.
The (40/60) hybrid foams with 20 wt% resin exhibited the highest compressive
strength and modulus values among all samples. Consequently, FRHFs with 20 wt%
resin were selected for further analysis. Five FRHFs were prepared, and the compressive
properties of these foams were summarized in Figures (4.4a) and (4.4b). The plots
illustrate the effects of different (G/H) ratios. For the (0/100) and (20/80) FRHFs, the
addition of aramid fibers decreased the compressive strength and the compressive
modulus of the hybrid foams. In contrast, the (40/60), (50/50), and (60/40) FRHFs
yielded higher compressive strength and modulus values compared with the unreinforced
hybrid foams. The compressive strength increased by 11-12% for all three foams, while
87
the compressive modulus increased by 11-13% for the (40/60), (50/50) and (60/40)
FRHFs.
The (20/80) hybrid foam showed greater compressive strength than the (20/80)
FRHF (Figure 4.4a). The lower compressive strength of the FRHF was attributed to the
presence of voids between the fibers. Introducing fibers led to voids which resulted from
the interlacing of the fibers, as shown in Figure (4.5). The SEM image shows interstitial
spaces created by fiber intersections that are devoid of microspheres. The different
behavior of the (40/60), (50/50), and (60/40) FRHFs compared with (20/80) FRHF was
attributed to the increased contents of glass microspheres in these foams. Effective fiber
reinforcement can be realized only when stress can be efficiently transferred to the fibers.
The glass microsphere was effective load-transfer material. When the content of glass
microspheres was increased, the load-transfer became more effective and the fiber-
reinforcing effects influenced the compressive behavior more strongly. Note that the
volume fraction of glass microspheres in the (40/60), (50/50), and (60/40) FRHFs was
more than double that of the (20/80) FRHF, thus making the fiber reinforcement more
effective in these materials.
The (40/60) hybrid foam and the (40/60) FRHF were also compared with the
cross-linked PVC foam, as shown in Table 4. The cross-linked PVC foam (with density =
100 kg/m
3
) was used as a control sample for reference. The data indicate that the
compression properties of the (40/60) hybrid foam were comparable to those of the
control foam, while the FRHF showed a 15% increase in strength and a 22% increase in
modulus. Comparisons of shear and tensile properties have been presented elsewhere [25].
88
(4.4a)
(4.4b)
Figure 4.4 Compressive stress (4.4a) and modulus (4.4b) of hybrid foams and FRHFs
89
Figure 4.5 SEM image of (40/60) FRHF, showing interlacing of fibers in webbing.
40 microns
90
4.5 Conclusions
Hybrid syntactic foams composed of glass microspheres and HEMs were
fabricated and basic mechanical properties were measured. The compressive strength and
modulus of hybrid foams with different (G/H) ratio were evaluated. The presence of glass
microspheres in the hybrid foams enhanced the compressive strength and modulus of the
hybrid foams by 25% and 54%, respectively. The optimal (G/H) ratio that produced the
highest compressive strength was 40/60, and the associated reinforcing mechanisms were
identified. As the weight percentage of glass microspheres was increased in a hybrid
foam, the compressive strength and modulus values were expected to increase also.
However, the results showed that increasing the glass microsphere content decreased both
the weight percentage and the volume fraction of HEMs. Calculations indicated that the
density of HEMs, D
E
, was lower in hybrid foams containing higher percentages of glass
microspheres. Consequently, low density HEMs had a detrimental effect on the
compressive properties. The combined effects of increasing the volume fraction of glass
microspheres and decreasing the density of HEMs in hybrid foams resulted in an optimal
(G/H) ratio that maximized compressive strength.
The addition of fiber reinforcements to hybrid foams enhanced both the tensile
and shear properties, as expected. In addition, most of the FRHFs showed modest
increases in compressive strength and modulus when fibers were oriented in the loading
direction. Compared with the cross-linked PVC foam, FRHFs showed increases in
compressive strength and shear strength of 15% and 20%, respectively. These properties
are critical for sandwich core materials, and they derive from the superior strength and
91
stiffness of the reinforced glass microspheres and fibers. The synthesis process of the
FRHFs is well-suited to scale-up and can be readily adapted to other types of
microspheres and fiber preforms. For example, changing the type of glass microspheres
may lead to further improvements in foam properties. The enhanced performance should
expand their applications as a core material for sandwich structures. Likewise, additions
of different reinforcing fibers should allow one to tailor property enhancements by
exploiting the specific fiber properties and their distribution in the foams.
92
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structural foam. International Journal of Solids and Structures 2000; 37(43):
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5. Burman M, and Zenkert D. Fatigue of foam core sandwich beams - 1: undamaged
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7. Gdoutos EE, Daniel IM, and Wang KA. Compression facing wrinkling of
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9. Shutov FA. Syntactic polymer foams. Advances in Polymer Science 1986; 73: 63-
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the flexural behavior of a syntactic foam core system. Polymer International 2000;
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12. Narkis M, Kenig S, and Puterman M. Three phase syntactic foams. Polymer
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foam core. Composites 1993; 24(5): 447–450.
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15. Huang Y, Vaikhanski L, and Nutt SR. 3D long fiber-reinforced syntactic foam
based on hollow polymeric microspheres. Composites Part A 2006; 37(3): 488-
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16. Karthikeyan CS, Sankaran S, Kumar MNJ, and Kishore. Processing and
compressive strengths of syntactic foams with and without fibrous reinforcement.
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17. Gupta N, Karthikeyan CS, Sankaran S, and Kishore. Correlation of processing
methodology to the physical and mechanical properties of syntactic foams with
and without fibers. Materials Characterization 1999; 43: 271–277.
18. Gupta N, Woldesenbet E, and Kishore. Compressive fracture features of syntactic
foams—microscopic examination. Journal of Materials Science 2005; 37(15):
3199–3209.
19. Karthikeyan CS, Murthy CLR, Sankaran S, and Kishore. Characterization of
reinforced syntactic foams using ultrasonic imaging technique. Bulletin of
Materials Science 1999; 22(4): 811–815.
20. Shan Y, and Liao K. Environmental fatigue behavior and life prediction of
unidirectional glass–carbon epoxy hybrid composites. International Journal of
Fatigue 2002; 24: 847-859.
21. Idicula M, Neelakantan NR, Oommen Z, Joseph K, and Thomas S. A study of the
mechanical properties of randomly oriented short banana and sisal hybrid fiber
reinforced polyester composites. Journal of Applied Polymer Science 2005; 96(5):
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25. Vaikhanski L, and Nutt SR. Fiber-reinforced composite foam from expandable
PVC microspheres. Composites Part A 2003; 34(12): 1245-1253.
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95
Chapter 5 Composite Truss Core for Sandwich Structures
5.1 Motivation
In the design of engineering structures for aerospace and transportation
applications, the factor of weight is generally a critical design constraint. Consequently,
lightweight materials are key enablers for future transportation vehicles, where weight
reductions are necessary for increased fuel efficiency [1]. The sandwich concept
combines low weight with high stiffness and strength [2], and sandwich structures are
employed for a wide range of weight-critical applications. Since the early fifties,
sandwich structures, particularly composite sandwich structures, have been exploited to
deliver excellent performance in aerospace and marine applications [3, 4]. In recent years,
sandwich composites have been introduced for ground vehicles as well.
Composite sandwich structures are not without drawbacks, such as prohibitively
high manufacturing costs, and susceptibility to moisture uptake and impact damage [5-8].
These production and maintenance problems motivated a recent effort to develop core
structures for sandwich constructions that were both affordable and impact resistant, and
offered mechanical efficiency comparable to conventional core materials used in aircraft,
marine or transportation vehicles [9, 10]. Currently, the core materials commonly used in
sandwich structures are either polymer foams or cellular honeycombs of paper, composite,
or metal [11-13]. Polymer foams and cellular honeycombs are susceptible to in-plane
shear, core compression failure, and face sheet debonding, respectively. In addition to
these damage mechanisms, sandwich structures often sustain damage from impacts
caused by accidental dropping of objects, vehicular collisions, or foreign object collisions,
96
all of which can adversely affect performance. Such impacts frequently occur in service
and may result in internal damage that is difficult to detect, compromising the strength
and stability of the structure. Also, impact loads may cause local delamination of face
sheets, which is generally difficult to detect and repair.
In order to overcome the problems described above, one approach considered in
the composite industry has involved use of core materials reinforced with 3D fiber
architectures and z direction reinforcements [14-16]. 3D fabric reinforcement provides
resistance to delamination and damage from an impact load [17]. The damage tolerance
and the impact resistance are increased because of the through-thickness fiber
reinforcement of the core, and in some cases, the mechanical interlocking provided by
fibers woven into the face sheets and extending through the core. 3D composites can be
made using techniques as diverse as stitching and z-rods (z-pinning) [18]. However, most
attention has been given to 3D composites manufactured by textile techniques, including
weaving, braiding, stitching, and knitting. These approaches have demonstrated that
composite structures made with 3D textile fabrics are potentially less expensive to
manufacture and provide better through- thickness mechanical properties than composites
made with the traditional 2D fabrics. However, 3D woven sandwich structures require in
situ core formation, which does not always result in high quality core material. Z-pinning
affords an alternative approach, although it introduces the risk of damage to the face
sheets and/or core material.
97
In the present work, prepreg carbon fiber tows (towpreg) were used to build 3D
truss structures for use as sandwich panel cores. Unlike most foam-core stitching
processes [19], no foam is involved or required. A conventional truss structure is
comprised of beams arranged in periodic configurations, and is widely used in
construction for mechanical efficiency. Several kinds of truss structures have been
analyzed, including the octet truss, tetrahedral lattice truss, pyramidal lattice truss and 3D
kagome [20-22]. The pyramidal lattice truss was selected for the present investigation
because of the ease of processing and the relatively lighter weight compared to other truss
configurations. Analytical prediction of the compressive and in-plane shear properties of
the core material was performed and compared with the results of mechanical testing.
The mechanical properties of the composite truss cores were comparable to commercial
grade honeycombs when normalized for density.
5.2 Material
Carbon fiber towpreg (Panex 35 continuous tow) was used to build the structure.
The towpreg was comprised of carbon fibers (~55 vol%) impregnated with epoxy and
partly cured to a tacky state. The tool used to form the truss structure is shown in the
Figure (5.1). The tool consists of multiple orthogonal rods at two different heights, and
all of the rods can be removed after the curing process. The final configuration of the
truss was controlled by adjusting the elevation and spacing of the rods in the tool. After
setting the rods, the towpreg was “woven” or wrapped over and under the rods in
orthogonal directions. Once configured, the assembly was cured in an oven at 110°C for
98
4 hours. After curing, the rods were removed, and the truss structure was removed from
the tool.
Figure (5.2) shows a typical sample produced by this process. The curvature of
the truss elements arises from the rods used in the tool, and the limited flexibility of the
CF towpreg. Cores with two relative densities were prepared - 80 kg/m
3
and 40 kg/m
3
.
Prototype sandwich beams were fabricated using aluminum sheets 0.508 mm thick. A
foam-filled variant of the truss cores was also fabricated using heat expandable PVC
foam with a density of 48 kg/m
3
. The foam was made from heat expandable microspheres
(Expancel DU 461), and filled the interstitial spaces between the trusses. There were four
types of samples prepared in this study. Sample 1 was a sandwich beam with a high-
density truss core (a relative density of 80 kg/m
3
). Sample 2 was a sandwich beam with a
foam-filled high-density truss core. Sample 3 was a sandwich beam with a low-density
truss core (a relative density of 40 kg/m
3
). Sample 4 was a sandwich beam with a foam-
filled low-density truss core.
Sandwich samples were tested in compression and shear in accordance with
standard protocols (ASTM C-365 and ASTM C-273). The loading rate was 10
-3
s
-1
and
the compression samples were 40 mm x 40 mm x 15.6 mm, while the shear samples were
40 mm x 320 mm x 15.6 mm. Five replicates were tested in each case to gauge the
variability in the test measurements.
99
Figure 5.1 The mold used to prepare rigid carbon fiber core
100
Figure 5.2 The rigid carbon fiber core
1cm
101
5.3 Results and Discussion
5.3.1 Unit cell architecture and relative density
A regular pyramidal consists of a quadrilateral base with triangular side surfaces
joined at one point. However, unlike regular pyramid, the struts of composite truss cores
were curved at the nodes where they joined. This was a consequence of the cylindrical
rods in the tool, and the limited formability of the CF towpreg, as discussed previously.
Nevertheless, the relative density of the structures can be calculated by starting with a
normal pyramidal model and assuming straight struts with length 1 and radius a as shown
in Figure (5.3). The equation below shows the geometric computation dictating the
relative density of the core:
( )
ρ
π
ω ω
π
ω ω
= =
4
2
2
2
2 2
2
a l
l l
a
l
cos sin
cos sin
(1)
The effective truss angles and lengths were determined to be ω = 48° and strut
length l = 21.05 mm. For Samples 1 and 2, the strut radius r was 1 mm, resulting in a
relative (macro) density for the composite truss core of 0.043. For Samples 3 and 4, the
strut radius was 0.71mm, and the relative density of the composite truss core was 0.022.
The macroscopic relative density for the composite truss cores (obtained by weighing the
truss cores and measuring the bulk volume) was 0.044 (83 kg/m
3
) for Samples 1 and 2,
and 0.022 (41 kg/m
3
) for Samples 3 and 4.
102
Figure 5.3 Unit cell of pyramidal truss
103
5.3.2 Analytical prediction of the pyramidal truss core response
A simple analysis was used to predict the stiffness and strength of the truss cores.
Based on the coordinate system in Figure (5.4), analytical expressions for the out-of-
plane axial stiffness E
33
, strength σ
33,
the transverse shear stiffness G
13,
and the strength
σ
13
of the pyramidal core can be obtained in terms of the core geometry and the elastic
properties of the truss material. The properties of the core can be evaluated by focusing
on the elastic deformation of a single strut resulting from an applied force.
In Figure (5.4), the fixed edge cylindrical strut of length 1 and radius a represents
a single strut of the pyramidal core. Originally, the faces embedded in the core in a fixed
position drawn in the Figure (5.4). Application of force F displaces the face to a new
position such that the top end of the strut moves freely along the x
3
-direction, but is fixed
in the x
1
and x
2
-directions. The imposed displacement δ is generated by the applied force
F, which is comprised of the axial force F
A
and the shear force F
S.
The F
A
and F
S
are
given by elementary beam theory as [23]
F E a
l
A S
= π
δ ω
2
sin
(2)
F
E I
l
S
S
=
12
3
δ ω cos
(3)
where I ≡ πa
4
/4 is the second moment of area of the strut cross-section, and E
S
is the
Young’s modulus of a single carbon fiber strut .
104
Figure 5.4 An edge fixed solid cylindrical strut under compressive force
X
1
X
3
X
2
Force F
δ
ω
Original position
New position
105
The total applied force F in the x
3
-direction follows as
F F F
E a
l
a
l
A S
S
= + = +
sin cos sin cos ω ω
π δ
ω ω
2
2
2
2
3 (4)
In Figure (5.3), there are four struts in a unit cell. The out-of plane axial stress σ
33
and
strain ε applied to the unit cell are related to the force F and displacement δ via
( )
σ
ω
ω
33
2 2 2
8
2
2
= =
F
l
F
l
cos
cos
(5)
ε
δ
ω
≡
l sin
(6)
The effective Young’s modulus can be obtained from equations (5) and (6) as
E
E
a
l
l
a
S
33
2
2
2
2
2
4
2
3 4
2
3
3
2
=
+
= +
π ω
ω
ω
ω
ρ ω
ρ
π
ω ω
sin
cos
sin
cos
sin sin cos (7)
The first and second terms in equation (7) represent the contributions to the stiffness of
the pyramidal core due to the stretching and bending of the struts, respectively.
Wallach and Gibson’s [24] analyzed the stiffness and strength of a pyramidal
truss core and reported approximate analytical expressions for the shear modulus,
compressive strength, and shear strength. Assuming the pyramidal truss core is
sufficiently symmetric that the transverse shear modulus is isotropic,
G
E
a
l
S
13
2
2
8
2 =
= π ω
ρ
ω sin sin (8)
106
Ideally, all four bars yield simultaneously, and the normal collapse strength σ
33
under
compressive load is
σ
σ
π
ω
ω
ρ ω
33
2
2
2
2
Y
a
l
=
=
sin
cos
sin (9)
where σ
Y
is the yield strength of the carbon fiber strut. The transverse shear strength σ
13
depends on the loading direction ψ as defined in Figure (5.3). The yield surface consists
of several collapse planes, each plane corresponding to two struts undergoing tensile
yield and two undergoing compression yield. Thus, the shear strength τ(ψ) is given by
( )
( ) ( )
τ ψ
σ
π
ω ψ ψ
ρ ω
ψ ψ
Y
a
l
=
+
=
+
2 1
2
2
2
cos cos sin
sin
cos sin
(10)
for ψ ≤ π/4. For the composite truss cores fabricated here, the angle ψ was 45º. Based
on the compression test on carbon fiber rods, the yield strength and the Young’s modulus
of a single carbon fiber strut were 350 MPa and 10 GPa, respectively. Substituting these
two values (σ
Y
and E
S
) into equations (7) ~ (10), the calculated values were listed in Table
1 for compressive responses and Table 2 for shear responses.
5.3.3 Compressive Response
Sandwich specimens with aluminum face sheets and the pyramidal truss cores
(comprised of sixteen pyramidal unit cells) were used in compression tests. Two
specimens with unfilled pyramidal truss cores, with relative densities of 80 kg/m
3
and 40
kg/m
3
, were tested. In addition, similar specimens featuring truss cores filled with heat
expandable PVC foam (density of 48 kg/m
3
) were tested and compared with the
107
specimens with unfilled cores. The compressive strength and modulus of all four samples
and the analytical predictions are listed in Table 5.1. The compressive strengths for the
samples with high-density truss cores, Samples 1 and 2, were 4.65 MPa and 5.39 MPa,
respectively, while the compressive moduli were 72 MPa and 94 MPa. The compressive
strengths for the samples with low-density truss cores, Samples 3 and 4, were 2.46 MPa
and 2.72 MPa, respectively, while the compressive moduli were 42 MPa and 51 MPa. As
expected, Sample 2, with the high-density truss core and heat expandable foam, showed
the highest compressive strength (5.39MPa) and modulus (94MPa) of the four samples.
The ratios of measured compressive strength-to-predicted strength were 0.57 and
0.60 for Samples 1 and 3, respectively. Similarly, the ratios for the experimental-to-
predicted values for compressive modules were 0.55 and 0.65 for Samples 1 and 3. The
fact that measured values were approximately 0.6 of predicted values was attributed to
the curvature of the struts. Note that the predictions assume a simplified geometry
characterized by straight struts. The curved struts were effectively pre-bent, resulting in a
reduced plastic buckling strength. As expected, the addition of heat expandable foam
enhanced both compressive strength and modulus of the truss core, resisting strut bending
and buckling. Comparing Sample 1 and 2, the latter showed a 16% increase in strength
and a 31% increase in modulus.
108
Table 5.1 Compressive responses
Stress (MPa) Modulus (MPa)
Prediction 8.23 130
Sample 1 4.61 ± 0.37 72 ± 9
Sample 2 5.39 ± 0.43 94 ± 12
Prediction 4.12 65
Sample 3 2.46 ± 0.17 42 ± 5
Sample 4 2.72 ± 0.19 51 ± 5
109
The corresponding stress-strain response is plotted in Figure (5.5). In all four
cases, an initial linear response was observed, followed by nonlinear regime in which the
slope decreased continuously. After a broad peak, the stress decreased with increasing
strain. The peak stress was reached at a strain of ~7% in all four samples. The nonlinear
regime corresponded to plastic buckling of the struts in the pyramidal core specimens
[21]. The truss core failure mechanism (elastic or plastic buckling or plastic yield)
depended on the slenderness ratio of the struts (length-to-radius). The slenderness ratio
was 21.05 for Samples 1 and 2, and the slenderness ratio was 29.65 for Samples 3 and 4.
Because the slenderness ratio and relative density of the truss core are interdependent,
both factors affect the truss core strength, and the failure mechanism (as well as the truss
material) [25].
5.3.4 Shear Response
The shear strength and modulus values for the four composite truss samples are
listed in Table 5.2, along with the analytical predictions. Sample 2, with the high-density
foam-filled truss core, showed the highest shear strength and modulus values (2.78 and
50 MPa) of the four samples. The high-density truss core exhibited higher shear strength
and modulus compared to the low-density core (compare Samples 1 and 3). The ratios of
measured-to-predicted values for shear strength were 0.43 and 0.44 for Samples 1 and 3,
while the ratios for shear modulus values were 0.78 and 0.68. As before, the differences
between the predicted and measured values were attributed to the truss curvature.
110
Figure 5.5 Compressive stress and strain responses
111
Table 5.2 Shear responses
Stress (MPa) Modulus (MPa)
Prediction 5.25 52.7
Sample 1 2.28 ± 0.17 41 ± 4
Sample 2 2.78 ± 0.19 50 ± 6
Prediction 2.63 26.4
Sample 3 1.15 ± 0.06 18 ± 2
Sample 4 1.60 ± 0.09 25 ± 2
112
The addition of heat expandable foam caused increases in shear strength and
modulus of the truss-core sandwich structures (see Table 5.2). Relative to the unfilled
truss core (Sample 3), Sample 4 showed increases in shear strength and modulus of 39%
and 38%. The foam lent support to the trusses, resisting bending and buckling. The shear
strength of the foam was only 0.35 MPa, and the shear modulus of the foam was 6 MPa.
The shear strength values of the foam-filled truss cores were 6% greater than the mere
sum of the two, indicating a modest synergistic effect. (Note that the in-plane shear
modulus for these samples did not exhibit simple Rule of Mixtures behavior because the
shear modulus was dominated by the CF trusses.)
The measured shear stress-strain curves for the pyramidal cores are shown in
Figure (5.6). The samples exhibited characteristics of truss-based sandwich cores [26],
including elastic behavior during initial loading, which continued as the load increased
until a peak stress was reached. The peak stress was followed by a brief stress plateau,
after which the load decreased sharply. The shear strength of the truss cores depended on
the initial failure mode of the truss members. In shear, two struts in each unit cell are
loaded in compression and two in tension. Mechanics-based simulations predict that
failure of such truss structures are most likely to initiate by buckling of the struts loaded
in compression [27]. After the buckling of the compression-loaded struts, the tension
struts continued to carry load until the onset of rupture of the nodes. Continued loading
produced a stress plateau, the duration (or net strain) for which was markedly different
between samples with or without foams (compared Samples 1 and 2). The presence of
foams extended the stress plateau by providing added support to trusses in the core,
resisting buckling and the initiation of failure. Note that the load shed by the ruptured
113
trusses redistributed to neighboring, intact struts, and some of the load was transferred via
the foam. This robustness in the presence of failed struts is a key attribute of the pyramid
truss configuration.
5.3.5 Comparison to Conventional Honeycombs
In Table 5.3, the specific strength and modulus of the sandwich beam with high-
density composite truss cores (Sample 1) are compared to HC sandwich panels
commonly used in industry (Gillfab 4030 and 4014, widely used in aircraft interiors).
Both sandwich panels featured aluminum facings bonded to aluminum honeycomb cores.
The thickness of skins was the same for all samples. The specific compressive strength
and modulus values for Sample 1 were 29% and 17% greater than those of the
commercial products. However, the specific shear strength and modulus values for
Sample 1 were 21% and 15% less than those of the commercial reference material
(Gillfab 4030). The specific properties were generally comparable to those of
conventional honeycombs, despite little effort to optimize the truss cores for mechanical
properties.
114
Figure 5.6 Shear stress and strain responses
115
Table 5.3 Specific properties of Sample 1 and conventional honeycombs
Sample 1 Gillfab 4030 Gillfab 4014
Density (kg/m
3
) 80 91.3 68.9
Specific Compressive
Strength (KN-m/kg)
58 45 41
Specific Compressive
Modulus (KN-m/kg)
900 767 609
Specific Shear
Strength (KN-m/kg)
28 36 26
Specific Shear
Modulus (KN-m/kg)
512 602 406
116
5.4 Conclusions
Composite truss cores were fabricated from towpreg, and yielded strength and
modulus values that were roughly 60% of predicted values. The truss cores featured
curved struts (because of the stiffness of the carbon towpreg used), and while this
curvature might impart some spring-like capacity for elastic energy absorption, the
geometry was not optimal for stiffness and strength. Nevertheless, the specific
compressive strength and modulus values were greater than those of aluminum
honeycombs, while the specific shear strength and modulus values were comparable to
aluminum honeycomb panels. With further optimization, it is expected that the truss
cores could surpass the mechanical efficiency of honeycombs, while simultaneously
reducing manufacturing costs.
The truss geometry is amenable to foam additions, which provide thermal and
acoustic insulation. To illustrate this concept, heat expandable foam was injected into a
composite truss core. The foam provided mechanical support to the trusses, increasing
resistance to bending and buckling, and undoubtedly provided a measure of
thermal/acoustic insulation.
There is room for additional development and optimization of the manufacturing
process, and of the truss geometry. This includes implementation of superior textile
technology, selection of fibers with greater bending flexibility (to achieve straight
trusses), and local reinforcement of the core / skin contact points (truss nodes). For
example, the application of textile technology would allow more precise location and
placement of the fiber towpregs, increasing property levels. In addition, the geometry of
117
the pyramidal lattice (angles, configurations, slenderness ratio, and straightness) strongly
affects the mechanical characteristics of the assembly.
118
Chapter 5 References
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impact response of multi-functional sandwich composites. Composite
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composite and FML-reinforced sandwich structures. Composites Science and
Technology 2004; 64(1): 35-54.
12. McCormacka TM, Millerb R, Keslerc O, and Gibson LJ. Failure of sandwich
beams with metallic foam cores. International Journal of Solids and Structures
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119
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50: 305–317.
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applications for advanced three-dimensional fibre textile composites. Compos
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Conference, Blacksburg, VA, 2001.
20. Hyun S, Karlsson AM, Torquato S, and Evans AG. Simulated properties of
Kagome and tetragonal truss core panels. International Journal of Solids and
Structures 2003; 40(25): 6989-6998.
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truss lattice material. Journal of the Mechanics and Physics of Solids 2001;
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120
24. Wallach JC, and Gibson LJ. Mechanical behavior of a three-dimensional truss
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121
Chapter 6 Suggestions for Future Works
Fiber reinforcements were expected to enhance tensile and shear properties and
were evident to do it in early chapters. However, they did not achieve a sound effect in
strengthening compression properties for composite foams, as shown in chapter 2, 3, and
4. FRHFs will continue to be required to if composite foams were to use as replacement
of balsa and honeycomb cores. Complete properties database for key formulations of
FRHFs will be a prioritized task for future work. FRHFs comprised of phenolic
microspheres and glass microspheres as well as amino microspheres and glass
microspheres will need to be further studied and evaluated. Those evaluations should
include compression strength and modulus, shear strength and modulus, and impact
toughness.
As previously mentioned in chapter 5, manufacturing process and truss geometry
were subjects for future development and optimization for composite truss core foams.
Wider selections of fibers for greater bending flexibility, such as aramid fibers, should be
studied and documented. Local reinforcement of the core, or skin contact points (truss
nodes) are subjects for improvement as well. Other than that, geometry of pyramidal
lattice (angles, configurations, slenderness ratio, and straightness) is also a critical next
subject to begin with.
122
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Abstract (if available)
Abstract
Long fibers are generally preferred for reinforcing foams for performance reasons. However, uniform dispersion is difficult to achieve because they must be mixed with liquid resin prior to foam expansion. New approaches aiming to overcome such problem have been developed at USC's Composites Center. Fiber-reinforced syntactic foams with long fibers (over 6 mm in length) manufactured at USC's Composites Center have achieved promising mechanical properties and demonstrated lower density relative to conventional composite foams.
Linked assets
University of Southern California Dissertations and Theses
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Asset Metadata
Creator
Huang, Yi-Jen
(author)
Core Title
Fiber-reinforced syntactic foams
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Materials Science
Publication Date
03/14/2009
Defense Date
02/06/2009
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
fiber-reinforced,OAI-PMH Harvest,syntactic foams,truss core
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Nutt, Steven R. (
committee chair
), Goo, Edward K. (
committee member
), Sammis, Charles G. (
committee member
)
Creator Email
yijenhua@usc.edu,yijenhua@yahoo.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-m2031
Unique identifier
UC184247
Identifier
etd-Huang-2670 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-207675 (legacy record id),usctheses-m2031 (legacy record id)
Legacy Identifier
etd-Huang-2670.pdf
Dmrecord
207675
Document Type
Dissertation
Rights
Huang, Yi-Jen
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Repository Name
Libraries, University of Southern California
Repository Location
Los Angeles, California
Repository Email
cisadmin@lib.usc.edu
Tags
fiber-reinforced
syntactic foams
truss core