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Environmental effects on the hybrid glass fiber/carbon fiber composites
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Content
ENVIRONMENTAL EFFECTS ON THE HYBRID GLASS FIBER/CARBON FIBER
COMPOSITES
by
Yun-I Tsai
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)
December 2009
Copyright 2009 Yun-I Tsai
ii
Acknowledgements
First of all, I would like to thank my advisor, Dr. Steven R. Nutt, for five years of
great assistance and the opportunity to work under his guidance. His excellent insight in
science and engineering broadened my view and led me through the challenges to the end
of my Ph.D. work. Without him, it would have been impossible to carry out this work.
Next, I would like to thanks Dr. Eric Bosze who taught me the knowledge of this
work, especially, in the toughest time of the beginning of this project. He is the greatest
senior leader that I ever have and I deeply appreciate his help.
I also want to thank my thesis committee: Dr. Edward Goo and Dr. Charles Sammis
for their agreement to serve on my Ph.D. guidance committee and taking the time and
effort to evaluate my work.
I also would like to thank Mr. Warren Haby, a laboratory manager at University of
Southern California, for his great advice and technical support facilitating experiments.
Also thanks to the folks at the USC Composites Center for interesting discussions and
being fun to be with.
I am also grateful to all my friends. They help me in both academic research and
private life in USC.
Finally, I would have done nothing if not for the love, patience, support, and
encouragement from my parents, Chi-Wen Tsai and Shiow-Mei Hu, as well as my sister
Yun-Chyi Tsai. I also would like to thank my grandparents, Sheng-Chieh Tsai and Mei-
Fen Lin Tsai. Last but not least, I would like to thanks my girlfriend I-Chieh Chen for her
support and encouragement. Thank you, my family, and I love you forever.
iii
Tables of Contents
Acknowledgements............................................................................................................. ii
List of Tables ...................................................................................................................... v
List of Figures.................................................................................................................... vi
Abstract.................. …………………………………………….………………………….ix
Chapter 1 Introduction................................................................................................... 1
1.1 Composite Materials ................................................................................................. 1
1.2 Overhead Conductors................................................................................................ 2
1.3 Hybrid Glass Fiber/Carbon Fiber Composites.......................................................... 6
1.4 Research Objectives................................................................................................ 10
Chapter 1 References ................................................................................................... 12
Chapter 2 Water Immersion Effects on the Hybrid GF/CF Composites..................... 13
2.1 Motivation............................................................................................................... 13
2.2 Experimental Procedure.......................................................................................... 14
2.2.1 Materials .......................................................................................................... 14
2.2.2 Conditioning .................................................................................................... 17
2.2.3 Mechanical, thermal properties and visual determination ............................... 20
2.3 Results and Discussions.......................................................................................... 20
2.3.1 Weight change mechanism .............................................................................. 20
2.3.2 Mechanical properties and physical characteristics......................................... 32
2.3.3 Recovery ability............................................................................................... 38
2.4 Conclusion .............................................................................................................. 40
Chapter 2 References ................................................................................................... 41
Chapter 3 Hygrothermal Effects on the Hybrid GF/CF Composites .......................... 43
3.1 Motivation............................................................................................................... 43
3.2 Experimental Procedure.......................................................................................... 44
3.2.1 Materials .......................................................................................................... 44
3.2.2 Conditioning .................................................................................................... 44
3.2.3 Mechanical, thermal properties and visual determination ............................... 45
3.3 Results and Discussion ........................................................................................... 46
3.3.1 Weight change mechanism .............................................................................. 46
3.3.2 Mechanical properties and physical characteristics......................................... 52
3.3.3 Recovery ability............................................................................................... 59
3.4 Conclusions............................................................................................................. 61
Chapter 3 References ................................................................................................... 62
iv
Chapter 4 Thermal Effects on the Hybrid GF/CF Composites ................................... 64
4.1 Motivation............................................................................................................... 64
4.2 Experimental Procedure.......................................................................................... 65
4.2.1 Materials .......................................................................................................... 65
4.2.2 Conditioning .................................................................................................... 65
4.2.3 Mechanical properties, thermal stability and visual determination ................. 66
4.3 Results and Discussion ........................................................................................... 67
4.3.1 Tensile strength-temperature Dependence....................................................... 67
4.3.2 High temperature isothermal aging.................................................................. 74
4.4 Conclusions............................................................................................................. 85
Chapter 4 References ................................................................................................... 86
Chapter 5 Thermal Cycling Effects on the Hybrid GF/CF Composites...................... 88
5.1 Motivation............................................................................................................... 88
5.2 Experimental Procedure.......................................................................................... 89
5.2.1 Materials .......................................................................................................... 89
5.2.2 Conditioning .................................................................................................... 89
5.2.3 Mechanical properties, thermal stability and visual determination ................. 92
5.3 Results and Discussion ........................................................................................... 93
5.3.1 Thermal cycling between 45 and 215 °C.......................................................... 93
5.3.2 Thermal cycling between -40 and 180 °C ........................................................ 98
5.4 Conclusions........................................................................................................... 106
Chapter 5 References ................................................................................................. 107
Chapter 6 Conclusions and Future Works................................................................. 108
Bibliography ....................................................................................................................111
List of Tables
Table 2.1 Diffusivity coefficients in 40, 60, and 90 °C water
immersion.
31
Table 2.2 Reduction (Red.) of the SBS and T
g
values in 40, 60, and
90°C water immersion.
37
Table 2.3 Retention (Ret.) of the SBS and T
g
values after removing
absorbed moisture in 40, 60, and 90 °C water immersion.
39
Table 3.1 Summarizes the retained SBS strength and T
g
in wet state
in 60 °C/85% R.H. air exposure.
58
Table 3.2 Summarizes the retained SBS strength and T
g
in dry state
in 60 °C/85% R.H. air exposure.
60
Table 4.1 Summarizes the retained T
g
of the hybrid GF/CF
composites at 180, 200, and 220 °C in 4, 12, and 20-week
isothermal aging.
84
Table 5.1 Glass transition temperature ( ) of the hybrid GF/CF
composites in unaged, 4, and 150 thermal cycles.
g
T
105
v
vi
List of Figures
Figure 1.1 The scheme of the aluminum conductor steel reinforced
conductor (ACSR).
4
Figure 1.2 The sag of the overhead conductor.
5
Figure 1.3 The scheme of aluminum composite core conductor
(ACCC).
8
Figure 1.4 The sag of various overhead conductors as a function of
temperature.
9
Figure 2.1 The scheme of cross-section of the pultruded composite
core, showing the carbon fiber composite core in the
center and the glass fiber composite shell around it.
16
Figure 2.2 Weight change of the hybrid GF/CF composites as a
function of time in 100 °C oven.
19
Figure 2.3 Weight change versus the square root of time (s
1/2
) for the
hybrid GF/CF composites in 40, 60, and 90 °C water
immersion.
26
Figure 2.4 Diffusion coefficients under 40, 60, and 90 °C water
immersion.
27
Figure 2.5 Moisture content distribution-radial position profiles in (a)
40°C (b) 60 °C (c) 90 °C water immersion.
28
Figure 2.6 The percent weight change at 60 °C as a function of time
for all-GF and all-CF 6.3 mm diameter composite rods.
29
Figure 2.7 The schemes of the GF/CF interface region and moisture
diffusion paths.
30
Figure 2.8 (a) T
g
change as a function of the water temperature and
time. (b) T
g
change as a function of water absorption
amount in the initial period with water temperature at less
than 1% water absorption. The inset shows the complete
data.
34
vii
Figure 2.9 (a) SBS strength change as a function of time and water
temperature. (b) SBS strength change as a function of the
water absorption amount in the initial period with water
temperature at less than 1% water absorption. The inset
shows the complete data.
35
Figure 2.10 The surface discoloration and roughness of the hybrid
GF/CF composites at various immersion temperatures.
36
Figure 3.1 Weight change versus the square root of time (s
1/2
) for the
for capped and uncapped hybrid GF/CF composites
exposed to 60 °C/85% humidity air.
50
Figure 3.2 Weight loss as a function of time for uncapped hybrid
GF/CF composites in 60 °C vacuum oven.
51
Figure 3.3 (a) T
g
change as a function of exposure time. (b) T
g
change
as a function of percent weight change.
55
Figure 3.4 (a) SBS strength change as a function of time. (b) SBS
strength change as a function of percent weight change.
56
Figure 3.5 Tensile strength change as a function of time. (b) Tensile
strength change as a function of percent weight change.
57
Figure 4.1 Tensile strength change as a function of temperature.
71
Figure 4.2 Normalized storage modulus and tensile strength as a
function of temperature.
72
Figure 4.3 SEM image of the microcracks in the matrix of -20 °C
specimen.
73
Figure 4.4 Normalized tensile strength of the hybrid GF/CF
composites versus time at 180, 200, and 220 °C isothermal
aging.
77
Figure 4.5 Normalized weight change of the hybrid GF/CF
composites at (a) 180 °C (b) 200 °C (c) 220 °C 1 and 6
weeks isothermal aging.
78
Figure 4.6 Normalized storage modulus of the hybrid GF/CF
composites at (a) 180 °C (b) 200 °C (c) 220 °C isothermal
aging.
79
viii
Figure 4.7 Normalized storage modulus versus normalized tensile
strength at (a) 180 °C (b) 200 °C (c) 220 °C isothermal
aging.
80
Figure 4.8 SEM images of (a) the fiber/matrix interface for the
unaged specimens. (b) very little matrix adhered to the
fibers at 220 °C 4-week isothermal aging.
81
Figure 4.9 (a) Example of failure mode for unaged specimens. (b) Example
of failure mode for 220 °C 4-week isothermal aging.
82
Figure 4.10 The surface discoloration and roughness of the hybrid GF/CF
composites at various temperature aging.
83
Figure 5.1 (a) Experimental record of a temperature cycle (between
45 and 215 °C). (b) Experimental record of a temperature
cycle (between -40 and 180 °C).
91
Figure 5.2 Normalized tensile strength as a function of thermal cycle.
95
Figure 5.3 (a) Equivalent thermal cycles for estimating 215 °C
isothermal aging. (b) The comparison of the normalized
tensile strength between thermal cycling and the
corresponding isothermal aging.
96
Figure 5.4 (a) Example of failure mode for thermal cycling
specimens. (b) Little matrix adhered to the fibers for the
hybrid GF/CF composites in thermal cycling.
97
Figure 5.5 (a) Normalized tensile strength versus thermal cycles. (b)
Normalized shear strength versus thermal cycles.
101
Figure 5.6 Normalized storage modulus as a function of temperature
for unaged, 4 and 150 thermal cycles.
102
Figure 5.7 Normalized storage modulus and tensile strength as a
function of thermal cycle.
103
Figure 5.8 (a) Equivalent thermal cycles for estimating 180 °C
isothermal aging. (b) The comparison of the normalized
tensile strength between thermal cycling and the
corresponding isothermal aging.
104
ix
Abstract
Fiber reinforced polymer composites (FRPCs) have been widely used to replace
conventional metals due to the high specific strength, fatigue resistance, and light weight.
In the power distribution industry, an advanced composites rod has been developed to
replace conventional steel cable as the load-bearing core of overhead conductors. Such
conductors, called aluminum conductor composite core (ACCC) significantly increases
the transmitting efficiency of existing power grid system without extensive rebuilding
expenses, while meeting future demand for electricity. In general, the service life of such
overhead conductors is required to be at least 30 years. Therefore, the long-term
endurance of the composite core in various environments must be well-understood.
Accelerated aging by hygrothermal exposure was conducted to determine the effect
of moisture on the glass fiber (GF)/carbon fiber (CF) hybrid composites. The influence of
water immersion and humid air exposure on mechanical properties is investigated.
Results indicated that immersion in water is the most severe environment for such hybrid
GF/CF composites, and results in greater saturation and degradation of properties. When
immersed directly in water, the hybrid GF/CF composites exhibit a moisture uptake
behavior that is more complex than composite materials reinforced with only one type of
fiber. The unusual diffusion behavior is attributed to a higher packing density of fibers at
the annular GF/CF interface, which acts as a temporary moisture barrier. Moisture uptake
leads to the mechanical and thermal degradation of such hybrid GF/CF composites.
Findings presented here indicate that the degradation is a function of exposure
temperature, time, and moisture uptake level. Results also indicate that such hybrid
x
GF/CF composites recover short beam shear (SBS) strength and glass transition
temperature (T
g
) values comparable to pre-aged samples after removal of the absorbed
moisture.
In the hygrothermal environment (60°C/85% R.H. air), the hybrid GF/CF
composites exhibit theoretical Fickian behavior before reaching the pseudo-saturation
level. However, with continued exposure, the composites then exhibit weight loss, a
phenomenon attributed to a combination of a dehydration reaction and a hydrolysis
reaction. These reactions resulted in loss of low molecular weight molecules present in
the epoxy matrix. Results indicate that a small amount of moisture-induced damage
occurred in the hybrid composites, although the mechanical and thermal properties were
only slightly diminished. In fact, the retained tensile strength was equivalent to the rated
tensile strength.
The influence of thermal exposure on the strength of hybrid GF/CF composites was
thoroughly investigated. Results indicate that tensile strength and modulus degrade as a
function of temperature and time. When the aging temperature is close to T
g
, degradation
was increased along with increasing temperature. Results also indicate that storage
modulus can be used as an index to predict the temperature dependence of tensile
strength and the long-term isothermal aging behavior, at least within specific domains.
The effect of thermal cycling on the hybrid composites was also investigated.
Thermal degradation during thermal cycling is attributed to two distinctive mechanisms.
The primary degradation results from isothermal aging, while additional thermal fatigue
damage occurs at sub-zero temperatures due to increasing residual stress. An empirical
xi
normalized tensile strength-storage modulus correlation provides an index of thermal
fatigue damage.
Various forms of environmental aging and their effects on mechanical and thermal
properties of hybrid GF/CF composites were assessed. The results and analysis provides
an insight into fundamental mechanisms of degradation associated with overhead
conductors with the hybrid GF/CF composite cores.
1
Chapter 1 Introduction
1.1 Composite Materials
Advances in technology require materials with unique property combinations that
cannot be achieved with conventional metal alloys, ceramics, or polymeric materials.
This is particularly true for applications that require durability under extreme conditions
such as aerospace, automobile, and infrastructure industry.
A composite material is formed by the combination of two or more distinct materials
to create a new material with enhanced properties. In short, superior mechanical, physical
and chemical properties are achieved by combination of two or more distinct materials.
The main advantages that drive the use of composites are weight reduction, corrosion
resistance, wear resistance, electromagnetic transparency, part-count reduction, thermal-
acoustical insulation, great specific strength, enhanced fatigue life and low thermal
expansion. The most common composites are those made with strong fibers, especially
with continuous long fibers. Particles and flakes are also used as reinforcements, but they
are not as effective as fibers.
In recent decades, the advent of commercial-grade carbon fiber and low-cost
manufacturing processes has led to new applications for composite materials in the
industry. Fiber reinforced polymer composites (FRPCs) are widespread applied in
aerospace and sporting goods, but new applications are arising in other industrial sectors,
such as marine, civil infrastructure, and automotive. One new example of an emerging
non-aerospace application is the composite reinforcement of high-voltage overhead
conductors, which may eventually replace conventional steel-reinforced conductors and
significantly improve the transmitting efficiency of the power grid system.
1.2 Overhead Conductors
Aluminum conductor (AC) is used widespread as it has the advantage of better
resistivity/weight than copper, as well as being cheaper. The conventional aluminum
conductor steel reinforced (ACSR) was introduced in the early 20th century, as shown in
Figure 1.1. ACSR offers optimal strength for line design and are primarily used for
medium and high voltage lines. Variable steel core stranding enables desired strength to
be achieved without sacrificing ampacity.
Because the conductor is a flexible object with uniform weight per unit length, the
geometric shape of a conductor strung on electric towers approximates that of a catenary.
The sag of the conductor (vertical distance between the highest and lowest point of the
curve) varies depending on the temperature, as shown in Figure 1.1. There are two ways
to increase the capacity without extensive rebuilding power grid system. One method is
to carry more aluminum wires on the overhead conductors, but it would result in the
greater loading on the supporting core. Another way to increase the power handling
capacity is increasing temperature of the conductor, but a minimum overhead clearance
must be maintained for safety. Because of the large coefficient of thermal expansion
(CTE) value (Steel: /°C), ACSR was limited below 100°C, with emergency
excursion to as high as 120°C, to prevent the excessive sag. It restricts the transmitting
efficiency of power grid system. In order to achieve a higher allowable operating
temperature with low sag in existing design clearance and loads, several overhead
6
10 5 . 11
−
×
2
3
conductors, such as aluminum conductor alloy reinforced (ACAR), all aluminum alloy
conductor (AAAC), and Aluminum conductor composite reinforced (ACCR) were
introduced. Although such aluminum-based core conductors have smaller CTE value and
afford low sag characteristics, the improvement is insufficient to satisfy the future
demand.
Figure 1.1 The scheme of the aluminum conductor steel reinforced conductor (ACSR).
4
Figure 1.2 The sag of the overhead conductor.
5
6
1.3 Hybrid Glass Fiber (GF)/Carbon Fiber (CF) Composites
In the past decades, the use of the electric power has become more important and the
efficiency of the power grid should be immediately improved to satisfy the increasing
demand. According to the official investigation, the demand for electricity is expected to
increase 20% over the next ten year. However, the transmission capacity is expected to
only increase 5% in the existing power grid system [1]. Nowadays, the existing power
grid system faces a serious challenge and to resolve the deficiency of electric power
requires immediate attention. To avoid extensive rebuilding and reduce outage time, a
more powerful power grid system which uses the existing tower infrastructure will be a
prior concern. Based on such principle, several aluminum conductors with aluminum-
based core have been introduced to increase the transmission efficiency, however the
improvement is limited. Therefore, the technology of fiber reinforced polymer
composites was applied in the manufacture of overhead conductors for the first time.
An advanced aluminum composite core conductor (ACCC), as shown in Figure 1.3,
has been introduced and shows the significant thermal sag-resistance [2]. ACCC could
allow much higher operating temperature (180°C) and it can increase the power capacity
significantly to improve the transmitting efficiency. The core technology of the ACCC is
the development of a hybrid glass fiber (GF)/carbon fiber (CF) composites rod which is
manufactured by pultrusion method and developed to replace the conventional or high-
strength steel core of ACSR to support the whole conductor’s weight. The internal core is
reinforced with carbon fiber which provides an exceptionally high level of tensile
strength and slightly negative CTE. The outer shell of the composite rod is reinforced
with fiberglass which improves the core’s flexural strength, reinforces impact resistance
7
and prevents a galvanic corrosion [4,5]. Previous investigation indicated that ACCC
shows at least 50% less thermal sag than ACSR at the same temperature as shown in
Figure 1.4 [2,3]. Due to the excellent specific strength and low sag of the hybrid GF/CF
composites, the long-span crossing of ACCC can be achieved with shorter electric towers
or the numbers of tower building can be reduced by a longer-span crossing.
Last but not least, ACCC allows the incorporation of 25% to 30% more aluminum
(conductive material) without any weight penalty due to the lighter hybrid GF/CF
composite core. ACCC not only endures higher operating temperature but also carries
more conductive aluminum. This last invention can significantly increase the transmitting
efficiency of the existing power grid system and satisfy the electric demand in the next
decades.
Figure 1.3 The scheme of aluminum composite core conductor (ACCC).
8
Figure 1.4 The sag of various overhead conductors as a function of temperature.
9
10
1.4 Research Objectives
Academic and applied interests provided motivation for these studies. A hybrid
GF/CF composites rod is used to replace the conventional steel as the core of the
overhead conductors for supporting whole weight. For such an application, the service
life should span several decades and little or no maintenance is expected. The influence
of environmental exposures and the long-term retention of properties should be much
taken into account. In general, the environmental factors primarily consist of moisture,
temperature, radiation, aggressive chemicals, and combinations of these factors with
mechanical loads. These environmental attacks could affect the composite materials and
result in the degradation of mechanical, thermal, and physical properties in diverse ways.
Among these environmental factors, the influence of moisture exposure and thermal
effect are the most important concern of the application of overhead conductors.
In general, moisture in any form is unfriendly to polymer composites [6]. However,
moisture attacks (such as rain and humid air) are inevitable for the overhead conductor.
However, the influence of moisture on the hybrid GF/CF composites pultrusion rod has
never been reported. Besides, to understand the extent of degradation on such hybrid
GF/CF composites at various hygrothermal environments is helpful to establish an
accurate prediction on service life.
On another hand, the transmitting efficiency of the existing power grid system
strongly depends on the operating temperature. The higher operating temperature can be
allowed, the more efficient electric transmission can be achieved. Similarly, the thermal
effect on the hybrid GF/CF pultrusion composites is still absent. In this dissertation,
various thermal effects (such as temperature, isothermal aging, and thermal fatigue) on
11
the kinetic behavior of properties are evaluated to build a safety limit for the overhead
conductor.
Mechanical and physical properties, including short beam shear (SBS) strength,
tensile strength, and glass transition temperature (T
g
), were measured to assess the
influence of environmental exposures. In addition, the recovery ability of such hybrid
GF/CF composites was investigated by removing the absorbed moisture. The failure
mode and crack propagation behavior were investigated via scanning electron
microscopy (SEM) and optical microscopy. The dissertation advanced our understanding
of environmental attack-property relationship for the hybrid GF/CF composites, tried to
establish a reliable service limit prediction, and provided a scientific basis for future
applications.
12
Chapter 1 References
1. Statement of David N. Cook general counsel north American electric reliability
counsel, National energy policy with respect to federal, State and local
impediments to the sitting of energy infrastructure, Senate committee on energy
and natural resources, Washington, D.C. (2001).
2. Alawar A, Bosze EJ, Nutt SR. A composite core conductor for low sag at high
temperatures. IEEE Trans Power Delivery 2005; 20 (3): 2193-2199.
3. Alawar A, Bosze EJ, Nutt SR. A hybrid numerical method for calculation of sag
of composite cinductors. Electric power Systems Delivery 76 (2006): 389-394.
4. Boyd J, Chang G, Webb W, Speak S. Galvanic corrosion effects on carbon fiber
composites, 36
th
International SAMPE Symposium and Exhibition: How
Concept Becomes Reality, April 15-18 (1991): 1217-1231.
5. Boyd J and Speak S. Galvanic corrosion effects on carbon fiber composites:
results from accelerated tests, 37
th
International SAMPE Symposium and
Exhibition: Materials Working for you in the 21
st
Century, March 9-12 (1992):
1184-1198.
6. Shen CH, Springer GS. Moisture absorption of graphite-epoxy composites
immersed in liquids and in humid air. J Comp Mater 10 (1976): 2-20.
13
Chapter 2 Water Immersion Effects on the Hybrid GF/CF Composites [1]
2.1 Motivation
Fiber reinforced polymer composites (FRPCs) have been used widespread due to the
excellent specific strength, great fatigue resistance, and light weight. A hybrid glass fiber
(GF)/carbon fiber (CF) composites reinforcement of high-voltage overhead conductor
eventually replaces the conventional steel-reinforced conductor. It could significantly
improve the transmitting efficiency of power grid system to satisfy the future electric
demand [2,3]. For such an application, where the service life can span several decades
and little or no maintenance is expected, the long-term retention of properties on
environmental exposures is important concerns. To design for such service life requires
the ability to forecast changes in material properties as a function of environmental
exposure, including bulk properties and the integrity of fiber-matrix interfaces.
The primary environmental exposures consist of moisture, temperature, radiation,
and aggressive chemicals. Among these factors, moisture exists everywhere in air and its
influence is inevitable. In generally, moisture in any form is unfriendly for polymer
composites. Moisture could degrade the mechanical and physical properties of composite
materials in adverse ways, as described in multiple studies [4-19]. Matrix and/or interface
degradation resulting from moisture absorption is a concern in most composite
applications subject to normal atmospheric moisture, which can range from precipitation
to mild humidity. Hygrothermal degradation have been reported for GF-epoxy and CF-
epoxy composites exposed to humid air or submerged in water [10,11]. Epoxies are
sensitive to moisture absorption because of the ubiquitous polar groups in the
networks which results in attractive interactions with polar water molecules [18,19]. In
addition, several studies also indicated that complete immersion in water represents the
most severe environment for the polymer composites, while humid air generally results in
lower maximum moisture content [10-13]. To understand the overall moisture effects on
the hybrid GF/CF composites, accelerated water immersion experiment were performed
in this dissertation.
−
OH
The primary objective is to investigate the influence of water immersion on such
hybrid GF/CF composites. Three immersing conditions (40 °C corresponded to mild
condition, 60 °C to moderate condition, and 90 °C to severe condition) were investigated
in this dissertation. The weight change and associated kinetic behaviors on properties of
such hybrid GF/CF composites are measured to establish the correlations. The second
objective is to determine the extent to which these same properties could be recovered by
removing the absorbed moisture. These issues are particularly relevant to the long-term
durability in the intended application, and to similar composites intended for
infrastructure applications.
2.2 Experimental Procedure
2.2.1 Materials
The unidirectional hybrid CF/GF reinforced composite rod, 9.53 mm (2.6 in.) in
diameter, as shown in Figure 2.1., was manufactured by pultrusion method with using a
propriety epoxy formulation and an anhydride curing agent. The outer shell of the
14
15
composite rod is reinforced with glass fiber (GF) while the inner core is reinforced with
carbon fiber (CF). The diameter of CF core is ~7 mm, and the total fiber volume fraction
is 67%. In addition, all-CF and all-GF reinforced composite rod, 6.3 mm in diameter,
with slightly varied fiber volume fraction was produced as the control group to determine
the diffusion coefficients of all-CF and all-GF composites, respectively.
Figure 2.1 The scheme of cross-section of the pultruded composite core, showing the
carbon fiber composite core in the center and the glass fiber composite shell around it.
16
2.2.2 Conditioning
Specimens were cut to a length of 66.5 mm and silicone sealant was applied to the
ends to prevent moisture penetration from the cut ends. Prior to immersion, all specimens
were air-dried in a 100 °C oven for 2 days to remove the retained moisture from storage.
To assess dryness, specimens were dried in an oven at 100°C for up to 10 days, profiling
the weight change. Figure 2.2 shows the measured weight loss as a function of time and it
reveals an apparent slope change after 2-day drying. The weight loss (0.07%) in the first
2 days is attributed to moisture removal. An additional 0.01% weight loss was observed
in the following 8 days of drying. The weight loss in this period is regarded as a result of
a dehydration reaction of the matrix. In order to prevent further thermal reaction, 2-day
drying was adopted in this dissertation. All specimens were weighed using an analytical
balance (ACCULAB LA-60) with 0.01mg accuracy. Specimens were then placed in large
Pyrex dishes containing deionized (DI) water at 40, 60 and 90 °C, respectively.
Specimens were removed from the baths at predetermined times up to 32-week
immersion. Before measuring the weight, all immersed specimens were wiped off the
residual surface water carefully. All specimens were subsequently weighed to determine
the weight change mechanism. The weight change was calculated according to:
*100%
wo
o
WW
W
−
(1)
where W
w
is the wet weight, and W
o
is the dry weight.
After weighing, specimens were divided into two groups of three specimens each.
The first group, in the “wet” state, immediately measures the short beam shear (SBS)
strength and glass transition temperature (T
g
). The second group was dried in the 100 °C
17
18
oven for 2 days and then the same measurements were run in the “dry” state. Drying
process was performed in this dissertation to determine if any decrease in property values
resulting from moisture absorption is reversible by removing the absorbed moisture.
0 200 400 600 800 1000
99.86
99.88
99.90
99.92
99.94
99.96
99.98
100.00
Weight Change, (%)
Time, (Sec
1/2
)
100
o
C Oven
Figure 2.2 Weight change of the hybrid GF/CF composites as a function of time in 100 °C
oven.
19
20
2.2.3 Mechanical, thermal properties and visual determination
Short beam shear (SBS) strength was measured in accordance with ASTM D4475-
02 in a commercial instrument (INSTRON 5567), using a span length 6 times the
diameter and a crosshead displacement rate of 1.3 mm/min.
Dynamic mechanical analysis (DMA) was performed to determine the change in
glass transition temperature (T
g
). A dual cantilever beam clamp was employed using a
commercial DMA instrument (TA Instruments DMA2980). Sample beams 60 × 9.5 × 1.6
mm were cut from the carbon core of the rod. A load frequency of 1 Hz was used over a
temperature range of 25 to 250 °C, and T
g
was determined from the peak of loss modulus
curve.
Transverse sections were cut and polished using conventional polishing techniques
and then examined microscopically (Olympus AH3-UMA) to detect evidence of physical
damages and cracks. In addition, a dye penetrate method was used to inspect microcracks.
2.3 Results and Discussion
2.3.1 Weight change mechanism
The weight change mechanism is shown in Figure 2.3. The percent weight change is
plotted as a function of the square root of time (s
1/2
) for different immersed temperatures.
Each point represents the average of measurements on six specimens, and the error bar is
the standard deviation value. The maximum weight change values for 40, 60, and 90°C
specimens were 0.53 ± 0.03%, 0.90 ± 0.04% and 11.74 ± 1.22% respectively after 5300
hours (32 weeks). The solid lines are the theoretical Fickian diffusion curves obtained by
fitting the moisture absorption, M
t
, equation [21]:
∞
∞
=
∑
− − = M t D
a
M
n
n
n
t
] ) exp(
4
1 [
1
2
2 2
α
α
(2)
where is the saturation level of water absorption, D is the diffusion coefficient, a is
the radius, and
∞
M
n
α is the root of the zero order Bessel function. The Bessel function
appears in this equation because of the cylindrical geometry of the samples. The
saturation level is assumed to be a constant for the case of complete immersion in
water [9]. By fitting the initial slope to equation (2) and choosing 1% as the saturation
point, then D can be determined. Table 2.1 summarizes the D values for 40, 60, and 90 °C
water immersions, respectively. Figure 2.4 shows a plot of ln(D) versus inverse
temperature, yielding the expected linear dependence [8].
th
n
∞
M
Plots of moisture content distribution as a function of radial position within the
hybrid GF/CF composites are shown in Figure 2.5(a)-(c) for the 40, 60, and 90 °C,
respectively. The numbers on the curves represent the critical immersed times given in
sec
1/2
which were determined from Figure 2.3. The curves at specific times are
determined by the equation [21]:
∑
∞
=
−
− =
1 1
0
2
) (
) ( ) exp( 2
1 ) , (
n n n
n n
a J
r J t D
a
t r C
α α
α α
(3)
where r is the radial position of the specimen, t is the time, J
o
is the Bessel function of
zero order, J
1
is the Bessel function of first order, and D is the diffusion coefficient at
each temperature, listed in Table 2.1. The GF/CF interface is located at 3.5 mm in these
figures and it is an important region for the moisture uptake mechanism.
21
22
In this dissertation, 40 and 60 °C specimens did not reach saturation level in 32-week
immersion, but 90 °C specimens apparently exceeded the saturation level (1%). Figure 2.3
shows that the weight change for the 40, 60, and 90 °C specimens are coincident with the
theoretical Fickian curves in its initial 1344 hours (~2200 s
1/2
), 1000 hours (~1900 s
1/2
),
and 625 hours (~1500 s
1/2
) respectively. Nevertheless, 40 and 60 °C specimens behave
non-Fickian diffusion early before reaching the saturation. To determine if the deviation
from the predicted behavior is attributable to phenomena peculiar to the CF/GF interface,
all-CF and all-GF reinforced composite rods are also measured the weight change in
60°C water. Figure 2.6 shows that there is no evidence of a change in the diffusion rate
before reaching saturation point and both rods behave Fickian diffusion until post
saturation. Therefore, the distinctly non-Fickian diffusion behavior on such hybrid GF/CF
composites is attributed primarily to the presence of the radial GF/CF interface.
The hybrid GF/CF composite features a thick GF outer shell and a CF core, each
comprising ~50% of the cross-section. This core-shell structure of the hybrid GF/CF
composite is largely responsible for the complex weight change behavior observed in
Figure 2.3. Note that the weight change mechanism of the hybrid GF/CF composites
shown in Figure 2.3 is noticeably different from the all-GF and all-CF composites shown
in Figure 2.6. For the hybrid GF/CF composites, moisture first penetrates the outer GF
shell, reaches the GF/CF interface, and then diffuses into the inner CF core. While the
moisture concentration of the GF shell approached the critical level, the GF/CF interface
would dominate the moisture uptake mechanism by acting as the moisture barrier.
23
Figure 2.3 shows two-stage weight gain for 40 and 60 °C specimens before reaching
saturation. The two-stage weight gain (non-Fickian diffusion) is primary interest in this
dissertation. In addition, a third stage of weight change was observed for 90 °C specimens.
It associated with supersaturation and irreversible cracking damage.
Stage I. The weight gain is accurately described by fitting a theoretical Fickian curve.
The diffusion kinetics during Stage I are primarily determined by the GF shell. The stage
ends while the moisture content in the GF shell reached a critical level. At this point, the
diffusion rate apparently changed. For 40 and 60 °C specimens, the slope of weight gain
changed at 2200 sec
1/2
and 1900 sec
1/2
, respectively, as shown in Figure 2.3, signaling the
end of Stage I. Figure 2.5 shows that the moisture content reached 70-75% of saturation
in the GF shell and was between 35-50% at the GF/CF interface in the end of stage I,
depending on the immersed temperature. Note that there is no clear indication between
Stage I and II for 90 °C specimens.
Stage II. The change in diffusion rate between stage I and II observed in Figure 2.3
is clearly associated with the GF/CF interface, and multiple explanations are considered
to explain this phenomenon. First, the difference in the diameter of CF and GF could
cause an increase in path length, introducing a tortuosity factor which is inversely related
to the diffusion rate [22, 23]. This phenomenon was examined by simply calculating the
increase in the path length around the fibers and comparing the new length to the length
of a straight line connecting the same start and end points. However, calculations
revealed no significant difference (<5%) in the diffusion path length for the GF shell and
the CF core.
24
Closer examination of the GF/CF interface revealed a slight inter-mingling of CF
with the larger GF, resulting in closer packing, as shown in Figure 2.7. The smaller CF
(~7μm) inserted into the space between the larger GF (~20 μm). Image analysis using the
linear intercept method reveals that the matrix fractions are 45-47% for both the GF shell
and CF core. However, the matrix fraction area is only 32% in the region of GF/CF
interface. Consequently, the diffusion path is changed from the relatively wide inter-fiber
spacing of GF shell to the narrow inter-fiber spacing at GF/CF interface region. The
closer packing at the annular interface region retards the diffusion of moisture, and acts as
an effective (albeit accidental) moisture barrier.
The inhomogeneous GF/CF interface works as moisture barrier in the Stage II,
altering the moisture diffusion mechanism. Nevertheless, immersed temperature also
affected the diffusion kinetics. Because the diffusion coefficient (D) of the 90 °C
specimens is nearly 2 and 1.5 times the rate of the 40 and 60 °C specimens, the moisture
diffusion kinetics is not apparently slowed by the GF/CF interface. In this dissertation, 40
and 60 °C specimens ended in Stage II, but the final data point of 60 °C samples showed a
sudden increase in diffusion rate, suggesting the possible onset of Stage III, although no
additional data were recorded.
Stage III. The slope of the weight gain sharply increased and the value of weight
gain quickly exceeded the theoretical 1% saturation point. The onset of the Stage III for
the 90 °C specimens occurred at a weight gain of ~0.8%. At this point, the water content
at the GF/CF interface reaches 80 - 90% of saturation and the GF shell was completely
saturated as shown in Figure 2.5(c). The increasing slope of the weight gain in this period
is attributed to the development of microcracking [6]. However, the only evidence of
25
cracking in 90 °C samples appeared at >2.1% weight gain, when multiple macrocracks
appeared along the CF/GF interface and branched into the GF core and CF shell. Few
cracks developed from the surface were also observed and it is attributed to the surface
glass fiber peel-off.
0 1000 2000 3000 4000 5000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2000 2400 2800 3200 3600 4000 4400
2
4
6
8
10
12
14
Weight change, (%)
Time, (Sec
1/2
)
40
o
C
60
o
C
90
o
C
Stage I
Stage II
Stage III
Stage I
Stage II
Stage III
Stage I
Stage II
Figure 2.3 Weight change versus the square root of time (s
1/2
) for the hybrid GF/CF
composites in 40, 60, and 90 °C water immersion.
26
2.7 2.8 2.9 3.0 3.1 3.2
-30.0
-29.5
-29.0
-28.5
-28.0
-27.5
-27.0
-26.5
ln(D)
1000/T, (K
-1
)
Figure 2.4 Diffusion coefficients under 40, 60, and 90 °C water immersion.
27
(a)
(b)
(c)
Figure 2.5 Moisture content distribution-radial position profiles in (a) 40 °C (b) 60 °C
(c) 90 °C water immersion.
28
0 500 1000 1500 2000 2500 3000 3500
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Weight change, (%)
Time, (Sec
1/2
)
CF
GF
Figure 2.6 The percent weight change at 60 °C as a function of time for all-GF and all-
CF 6.3 mm diameter composite rods.
29
Figure 2.7 The schemes of the GF/CF interface region and moisture diffusion paths.
30
Table 2.1 Diffusivity coefficients in 40, 60, and 90 °C water immersion.
Water 40°C 60°C 90°C
D ) (
1 2 −
s m
13
10 42 . 1
−
×
13
10 84 . 3
−
×
12
10 7 . 2
−
×
31
32
2.3.2 Mechanical properties and physical characteristics
T
g
and SBS strength of the hybrid GF/CF composites were measured to determine
the influence of the moisture uptake. Figure 2.8(a) shows the T
g
drop as a function of
time at carious immersed temperatures and Figure 2.8 (b) shows the correlation between
T
g
and percent moisture uptake. In the equivalent moisture uptake level, immersed at
higher temperature yields higher retained T
g
(or smaller degradation). For example, in
0.5% weight gain, the retained T
g
were 165, 175, and 190°C for 40, 60, and 90°C water
immersion, respectively. The inset in Figure 2.8(b) shows how the T
g
changed over the
entire immersion time. Table 2.2 summarizes T
g
for unaged and 32 week immersion
specimens (for the 90 °C specimens, the reported T
g
corresponds to 1% moisture
saturation). Figure 2.7(a) shows the dependence of the SBS on time at various
temperatures, while Figure 2.7(b) shows the SBS dependence on the amount of moisture
uptake. Table 2.2 also summarizes the corresponding SBS strength. The reductions in T
g
and SBS strength of such hybrid GF/CF composites in water immersion show similar
trends that are apparent by comparing Figure 2.8 and 2.9.
Surface discoloration of such hybrid GF/CF composites is observed after extended
immersion as shown in Figure 2.10. The surface color changed from brown to a tan/white,
which is attributed to changes in the matrix chemistry, as report elsewhere [7]. The
surface condition also showed different degrees of degradation. The surface of 40 °C
specimens remained smooth, much like the unaged composites. However, 60 and 90 °C
specimens appeared rougher surfaces due to the partial debonding of glass fibers at the
surface.
33
Polished cross-sections of the 40 and 60 °C specimens shows no evidence of micro-
or macro-cracking after 32-week immersion, and attempts to detect microcracks using
fluorescent dye penetrant were also negative. In addition, polished cross-sections of 90 °C
specimens near the saturation point (at the end of the Stage I/II) show no evidence of
microcracking. The multiple macrocracks in 90 °C specimens were observed while weight
gain is higher than 2.1% which exceeds the theoretical saturation level.
0 1000 2000 3000 4000 5000
40
60
80
100
120
140
160
180
200
220
Tg, (
o
C)
Time, (Hr)
40
o
C
60
o
C
90
o
C
(a)
0.0 0.2 0.4 0.6 0.8 1.0 1.2
150
160
170
180
190
200
210
01 2 5 6 7 8
40
60
80
100
120
140
160
180
200
Tg, (
o
C)
40
o
C
60
o
C
90
o
C
Water absorption, (%)
(b)
Figure 2.8 (a) T
g
change as a function of the water temperature and time. (b) T
g
change as
a function of water absorption amount in the initial period with water temperature at less
than 1% water absorption. The inset shows the complete data.
34
0 1000 2000 3000 4000 5000 6000
0
10
20
30
40
50
Shear strength, (MPa)
Time, (Hr)
40
o
C
60
o
C
90
o
C
(a)
0.00.2 0.40.6 0.81.0 1.2
33
36
39
42
45
48
51
54
0 1 2 567 8 9
0
10
20
30
40
50
40
o
C
60
o
C
90
o
C
Shear strength, (MPa)
Water absorption, (%)
(b)
Figure 2.9 (a) SBS strength change as a function of time and water temperature. (b) SBS
strength change as a function of the water absorption amount in the initial period with
water temperature at less than 1% water absorption. The inset shows the complete data.
35
Figure 2.10 The surface discoloration and roughness of the hybrid GF/CF composites at
various immersion temperatures.
36
37
Table 2.2 Reduction (Red.) of the SBS and T
g
values in 40, 60, and 90 °C water
immersion.
Shear strength (MPa) Red. (%) T
g
( °C) Red. (%)
Unaged 48 209
40°C (32Ws) 41.8 87 160.3 77
60°C (32Ws) 38.2 80 146 70
90°C (4Ws) 37 77 153.5 73
38
2.3.3 Recovery ability
The reversibility of mechanical and physical properties after water immersion is
assessed in the present study. Table 2.3 summarizes the recovery ability and percent
retention of thermal and mechanical properties. After 32-week immersion, 40 and 60 °C
specimens show comparable recovery of 85-98% in SBS strength and 77-91% in T
g
after
drying. Specimens immersed in 90°C water for 4 weeks, which are close to the saturation
point, also show the comparable recovery. In summary, removal of absorbed moisture
results in near full recovery of both T
g
and SBS strength for such hybrid GF/CF
composites, while the moisture level is below the saturation. However, once the moisture
saturation level (1%) is exceeded, both mechanical and thermal properties do not fully
recover due to the permanent matrix damage on the matrix. In particular, when moisture
levels exceeded 2%, the damage is appeared in the cracking form.
Degradation resulting from the aqueous environment is often caused by damage to
the fiber/matrix interface, and generally results in a decrease in strength and modulus [16].
Moisture could soften the matrix, degrade the stress transfer function, and result in a
substantial loss of T
g
and SBS strength. In this dissertation, removal of absorbed moisture
led to significant recovery (77-98% of original value) for thermal and mechanical
properties. However, full recovery was not achieved, showing that at least some
permanent degradation occurred.
39
Table 2.3 Retention (Ret.) of the SBS and T
g
values after removing absorbed moisture in
40, 60, and 90 °C water immersion.
Shear strength (MPa) Ret. (%) T
g
( °C) Ret. (%)
Unaged 48 209
40°C (32Ws) 47.2 98 189.5 91
60°C (32Ws) 40.7 85 160.2 77
90°C (4Ws) 41.8 87 168.4 81
40
2.4 Conclusions
Accelerated water immersion experiments are conducted to determine the influence
of moisture on the hybrid GF/CF composites. The hybrid GF/CF composites exhibit a
more complex moisture uptake mechanism than the single fiber type reinforced
composite materials. Non-theoretical Fickian diffusion is attributed to a higher density
packing of fibers located at the annular GF/CF interface region, which acted as a
temporary moisture barrier. In aerospace applications, it is common practice to insert a
thin GF layer between metallic structures and attached CF composite parts to prevent
galvanic corrosion. However, in this dissertation, the GF/CF interface is embedded deep
beneath the outer surface, and each constituent comprises ~50% of the cross-section.
Thus, the moisture absorption mechanism of such hybrid GF/CF composites is quite
different from typical aerospace parts, with the GF layer dominating the diffusion rate for
a long time before the GF layer even begins to affect the diffusion behavior. Although
moisture results in the mechanical and thermal degradation, the hybrid GF/CF composites
retain comparable SBS strength and T
g
after removing the absorbed moisture. In
conclusion, the water immersion only causes a slight amount of permanent damage to the
critical fiber/matrix interfaces that influence the parts ultimate strength before reaching
the saturation.
41
Chapter 2 References
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on thermal and mechanical properties of carbon fiber/fiberglass hybrid composites.
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2. Alawar A, Bosze EJ, Nutt SR. A composite core conductor for low sag at high
temperatures. IEEE Trans Power Delivery 2005; 20 (3): 2193-2199.
3. Bosze EJ, Alawar A, Bertschger O, Tsai Y-I, Nutt SR. High-temperature strength
and storage modulus in unidirectional hybrid composites. Comp Sci Tech 66
(2006): 1963-1969.
4. Selzer R, Friedrich K. Mechanical properties and failure behaviour of carbon
fibre-reinforced polymer composites under the influence of moisture. Comp A 28
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5. Biro DA, Pleizer G, Deslandes Y. Application of the microbond technique: effects
of hydrothermal exposure on carbon-fiber/epoxy interfaces. Comp Sci Tech 46
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6. Zhou J, Lucas JP. The effects of a water environment on anomalous absorption
behavior in graphite/epoxy composites. Comp Sci Tech 53 (1995): 57-64.
7. Ellyin F, Master R. Environmental effects on the mechanical properties of glass-
fiber epoxy composite tubular specimens. Comp Sci Tech 64 (2004): 1863-1874.
8. Imaz JJ, Rodrigurz JL, Rubio A, Mondragon I. Hydrothermal environment
influence on water diffusion and mechanical behaviour of carbon fiber/epoxy
laminates. J Mater Sci Let 20 (1991): 662-665.
9. Wan YZ, Wang YL, Huang Y , He BM, Hah KY . Hydrothermal aging behaviour of
VARTMed three-dimensional braided carbon-epoxy composites under external
stresses. Comp A 36 (2005): 1102-1109.
10. Shen CH, Springer GS. Moisture absorption of graphite-epoxy composites
immersed in liquids and in humid air. J Comp Mater 10 (1976): 2-20.
11. Loos AC, Springer GS. Moisture absorption of graphite-epoxy composites
immersed in liquids and in humid air. J Comp Mater 13 (1979): 131-147.
12. Collings TA, Copley SM. On the accelerated aging of CFRP. Comp 14 (1983):
180-188.
13. Bullions TA, Loos AC, McGrath JE. Moisture sorption effects and properties of a
carbon fiber-reinforced phenylethynyl-terminated poly(etherimide). J Comp
Mater 37 (2003): 791-809.
42
14. Luo HL, Lian JJ, Wan YZ, Huang Y, Wang YL, Jiang HJ. Moisture absorption in
VARTMed three-dimensional braided carbon-epoxy composites with different
interface conditions. Mater Sci & Eng A 425 (2006): 70-77.
15. Thwe MM, Liao K. Effects of environmental aging on the mechanical properties
of bamboo-glass fiber reinforced polymer matrix hybrid composites. Comp A 33
(2002): 43-52.
16. Wan YZ, Wang YL, Huang Y, Luo HL, He F, Chen GC. Moisture absorption in
three-dimensional braided carbon/Kevlar/epoxy hybrid composite for orthopedic
usage and its influence on mechanical performance. Comp A 37 (2006): 1480-
1484.
17. Lee MC, Peppas NA. Water transport in graphite/epoxy composites. J Appl Poly
Sci 47 (1993): 1349-1359.
18. Olmos D, Lopez-Moron R, Gonzalez-Benito J, The nature of the glass fibre
surface and its effect in the water absorption of glass fibre/epoxy composites. The
use of fluorescence to obtain information at the interface. Comp Sci Tech 66
(2006): 2758-2768.
19. Karad SK, Attwood D, Jones FR, Moisture absorption by cyanate ester modified
epoxy resin matrices. Part V: effect of resin structure. Comp A 36 (2005): 764-771.
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degradation of unidirectional polymer composite. Comp B 2001; 32: 365-370
21. Crank J. The mathematics of diffusion 2
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22. Wang H-T, Smith JM. Tortuosity factor of diffusion in catalyst pellets. ALChE
Journal 29 (1983): 132-136.
23. Bourg IC, Sposito G, Bourg ACM. The tortuosity of diffusion paths in saturated
compacted sodium bentonite. Clays in natural & engineered barriers for
radioactive waste confinement (2005); 149-150.
43
Chapter 3 Hygrothermal Effect on the Hybrid GF/CF Composites [1]
3.1 Motivation
Because of the suspension between two electric towers, it is inevitable for the
overhead conductors to contact humid air. To apply the composites technology into the
electric industry, understanding the moisture effects on the hybrid GF/CF composites is
essential. Chapter 2 has investigated the influence of water immersion on such hybrid
GF/CF composites as well as assessed associated kinetic behavior on their properties.
However, water immersion represents the most sever environment for the polymer
composites and it would result in the great damage or degradation. In order to establish
safe limits on service conditions, necessary inspection/replacement frequency, and for
design of appropriate protective measures, understanding the kinetics and mechanisms of
degradation of such hybrid GF/CF composites exposing to hygrothermal environment
(humid air) is necessary. The moisture uptake behavior in CF-polymer and GF-polymer
composites exhibited Fickian diffusion in some cases; while in others, non-Fickian
diffusion was also reported [2-13]. However, most of these reports documented the single
type of fiber reinforced laminated composites. A complete hygrothermal investigation on
the hybrid GF/CF unidirectional composites is absent [6].
60°C/85% R.H. is a common industrial standard to run an accelerated hygrothermal
experiment and is also adopted in this dissertation. The primary objective is to investigate
the hygrothermal effects and associated kinetic behavior on properties of such hybrid
GF/CF composites. The second objective is to determine the recovery ability of such
44
hybrid GF/CF composites after removing the absorbed moisture. This dissertation
provides a complete reference on the long-term hygrothermal durability of such hybrid
GF/CF composites.
3.2 Experimental procedures
3.2.1 Materials
The unidirectional hybrid CF/GF reinforced composite rod, 9.53 mm in diameter, as
shown in Figure 2.1, was manufactured by pultrusion method with using a propriety
epoxy formulation and an anhydride curing agent. The outer shell of the composite rod is
reinforced with glass fiber (GF) while the inner core is reinforced with carbon fiber (CF).
The diameter of CF core is ~7 mm, and the total fiber volume fraction is 67%.
3.2.2 Conditioning
Specimens were cut to lengths of 66.5 mm (2.6 in.) for weight change and short
beam (SBS) strength measurements. Such specimens were then divided into two groups.
The first group was capped on both cut ends with a silicone sealant to prevent moisture
diffusion through the cut ends. (Note that in service, composite’s ends will be encased in
metallic grips when conductors are strung between lattice towers). The second group was
not end-sealed and it allows the longitudinal diffusion. A third set of such hybrid GF/CF
composites were cut to lengths of 106.7 mm (42 in.) with ends-sealed for the tensile
strength measurements.
Prior to hygrothermal exposure, all specimens were air-dried in a 100 °C oven for 2
days to remove the retained moisture resulting from storage. 2-day drying was adopted in
this dissertation to prevent further thermal reaction. All specimens were weighed using an
analytical balance (ACCULAB LA-60) with 0.01mg accuracy. All specimens were then
placed in an environmental chamber (TPS T30RC-2) at 60 °C and 85% relative humidity
(R.H.) to investigate the influence of hygrothermal exposure. Specimens were then
removed from the environmental chamber periodically to measure the weight change and
to conduct thermal and mechanical measurements. Before measuring the weight, all
specimens were wiped off residual surface water carefully. The weight change is
calculated according to:
*100%
wo
o
WW
W
−
(1)
where W
w
is the wet weight, and W
o
is the dry weight.
After weighing, both capped and uncapped groups were divided into two subsets of
five specimens each. Specimens from the first subset were measured mechanical
properties and conducted thermal analysis in the “wet” state. Specimens from the second
subset were dried at 100 °C for 2 days to remove the absorbed moisture [6]. Then the
same measurements and analysis were performed in the “dry” state. Drying process was
performed to determine the extent to which decreases in property values resulting from
moisture absorption were reversible.
3.2.3 Mechanical, thermal properties and visual determination
Short beam shear (SBS) strength was measured in accordance with ASTM D4475-
02 in a commercial instrument (INSTRON 5567), using a span length 6 times the
diameter and a crosshead displacement rate of 1.3 mm/min.
45
46
Tensile strength was measured at room temperature (RT) in accordance to ASTM
D3916-02. A universal testing instrument (INSTRON 5585) with custom-made adhesive
gripping fixtures was used and the pulling rate is 0.2 in/min.
Dynamic mechanical analysis (DMA) was performed to determine the change in
glass transition temperature (T
g
) with exposure time. A dual cantilever beam clamp was
employed using a commercial DMA instrument (TA Instruments DMA2980). Sample
beams 60 × 9.5 × 1.6 mm were cut from the carbon core of the rod. A load frequency of 1
Hz was used over a temperature range of 25 to 250 °C, and T
g
was determined from the
peak of loss modulus curve.
Transverse sections were cut and polished using conventional polishing techniques
and then examined microscopically (Olympus AH3-UMA) to detect evidence of cracking.
In addition, dye penetrate method was used to detect the evidence of microcracks.
3.3 Results and Discussion
3.3.1 Weight change mechanism
Figure 3.1 shows weight change as a function of time for the hybrid GF/CF
composites exposed to 60 °C/85% R.H. air. The percent weight gain is plotted as a
function of the square root of the exposure time (s
1/2
). Each point represents the average
of six measurements on individual specimens, and the error bars show standard deviation
values. The solid lines superimposed over the data points are the best fit for theoretical
Fickian curves. In the initial 70-hour (~500 s
1/2
) exposure ( ~0.10% weight gain), capped
and uncapped specimens exhibited similar behavior. With exposure time increase, the
two groups showed the diverged behavior. Capped specimens reached the pseudo-
saturation level ( ~0.227%) in 7 weeks (2000 s
1/2
) and uncapped specimens reached a
higher pseudo-saturation level (0.423%) in 21 weeks. In addition, the weight loss was
observed for both groups after reaching the pseudo-saturation level.
The different weight gain rates of capped and uncapped specimens are attributed to
the different moisture diffusion pathways. For capped specimens, moisture only diffuses
in the radial direction. The corresponding diffusivity coefficient, D
r
, can be determined
by fitting the moisture absorption equation for M
r
[14]:
2
22
1
4
[1 exp( )]
rr
n
n
n
M DtM
a
α
α
∞
∞
=
=− −
∑
(2)
where is the saturation level of water absorption, is the radial diffusion
coefficient, a is the radius, and
∞
M
r
D
n
α is the root of the zero-order Bessel function. The
Bessel function appears in this equation due to the cylindrical geometry.
th
n
For uncapped specimens, moisture not only diffuses in radial direction but also
along longitudinal direction (from the unsealed ends). The longitudinal diffusivity can be
derived from Springer’s model [3].
r
f
f
L
D
v
v
D
2 / 1
) ( 2 1
1
π
−
−
= (3)
where is the longitudinal diffusion coefficient, is the radial diffusion coefficient,
and is the fiber volume fraction.
L
D
r
D
f
v
Using the measured fiber volume fraction (67%), the ratio of the longitudinal and
radial diffusion coefficients is 4.32, indicating that the rate of longitudinal diffusion along
the fiber/matrix interface is much greater than diffusion in the radial direction. The faster
47
48
longitudinal diffusion accounts for the great weight gain rate for uncapped specimens and
it results in the greater pseudo-saturation level.
Different maximum weight change and transitions (curve knees) were observed for
capped and uncapped specimens, as shown in Figure 3.1. These differences are attributed
to weight loss behavior resulting from hygrothermal exposure and the additional
diffusion pathway available in samples with uncapped ends. The dehydration reaction
resulting from thermal aging and hydrolysis resulting from moisture exposure contribute
to the weight loss behavior of polymer composites in hygrothermal environments in
several studies [15,16].
To investigate the dehydration reaction on the hybrid GF/CF composites, several
specimens were dried in a vacuum oven at 60 °C, the same temperature at which the rods
were exposed in the environmental chamber. Figure 3.2 shows the measured weight loss
as a function of time. Dried specimens lost 0.062% weight in 33 days ( ~1700 s
1/2
). The
observed weight loss is attributed to a thermal dehydration reaction in the epoxy, as
reported previously [15]. Both capped and uncapped samples showed similar weight loss
behavior and temperature dependence, indicating similar dehydration reactions.
The observed weight loss results from the combination of matrix dehydration, which
results primarily from thermal exposure, and matrix dissolution resulting from humidity
exposure. Hydrolysis irreversibly affected the chemical structure of epoxy and caused
weight loss during hygrothermal exposure [17]. Low molecular weight species from the
polymer matrix of composite materials dissolved in water and/or in humid air, causing
similar weight loss behavior [18,19]. The weight loss resulting from hydrolysis reaction
49
is related to the amount of absorbed moisture. Thus, uncapped specimens could react
with greater hydrolysis process than capped specimens during hygrothermal exposure.
The full weight change profile consisted of weight gain associated with moisture
absorption by the matrix, as well as weight loss associated with the loss of low molecular
weight species dissolving in the water. When the weight loss rate exceeds the moisture
absorption rate, a deflection appeared in the weight change curve, as shown in Figure 3.1.
In this dissertation, the knee of the weight change curve occurred at 7-week exposure for
capped specimens and at 21-week exposure for uncapped specimens. The uncapped
specimens showed more rapid moisture absorption and the greater maximum weight
change level than the capped specimens, as shown in Figure 3.1.
0 500 1000 1500 2000 2500 3000 3500
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
Weight change, (%)
Time, (Sec
1/2
)
Cap
Uncap
Figure 3.1 Weight change versus the square root of time (s
1/2
) for the for capped and
uncapped hybrid GF/CF composites exposed to 60 °C/85% humidity air.
50
0 400 800 1200 1600
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
Weight Change, (%)
Time, (Sec
1/2
)
60
o
C Vacuum Oven
Figure 3.2 Weight loss as a function of time for uncapped hybrid GF/CF composites in
60°C vacuum oven.
51
52
3.3.2 Mechanical properties and physical characteristics
Long-term hygrothermal exposure would cause degradation in the thermal and
mechanical properties of the polymer composites, as evidenced by changes in T
g
and SBS
strength [15]. Figure 3.3(a) shows the T
g
change as a function of exposure time, while
Figure 3.3(b) shows the T
g
dependence on percent weight change. T
g
reduced with
exposure time in a roughly linear fashion, while capped and uncapped specimens showed
nearly identical behavior. The slight decrease in T
g
is not unusual [15], and this was
accompanied by a slight decrease in shear strength, as well. Figure 3.4(a) shows that the
SBS strength decreased linearly with exposure time, while Figure 3.4(b) shows the SBS
strength dependence on percent of weight change. (Note that when the weight change
reached its pseudo-saturation level and then started to decrease, the T
g
and SBS strength
continuously decreased during weight loss). Table 3.1 summarizes the retained SBS
strength and T
g
of the unaged and 16-week 60 °C/85% R.H. exposure specimens.
During prolonged hygrothermal exposure, the physical aging (matrix swelling and
plasticization) and chemical aging that accompanies moisture absorption inevitably
causes degradation to composite properties. The reduction in T
g
is associated with the
diffusion of water molecules into the composites, disrupting Van der Waals and hydrogen
bonds in the epoxy resulting in increased chain mobility [20]. Figure 3.3 and 3.4 show
that capped and uncapped specimens exhibited similar trends in T
g
and SBS strength,
although the uncapped specimens absorbed much more moisture. Earlier work indicated
that water uptake in longitudinal directions could be accelerated by capillarity and
diffusion along fiber-matrix interfaces under hygrothermal condition [21]. For the
uncapped specimens, the excess moisture arising from longitudinal diffusion is primary
53
trapped along the matrix/fiber interface [21]. Thus, the matrix’s moisture uptake for
capped and uncapped specimens during hygrothermal exposure was roughly the same.
Likewise, because T
g
and SBS strength are essentially matrix properties, capped and
uncapped specimens experienced similar matrix degradation, leading to similar decreases
in T
g
and SBS strength.
In general, the change in measured properties paralleled the measured weight change,
and both were approximately linear with time. However, the experimental results also
show that the T
g
and SBS dependence on percent of weight change was not strictly linear
for capped specimens, as shown in Figure 3.3(b) and 3.4(b). Degradation resulting from
hygrothermal exposure is typically caused by moisture-induced damage, and the extent of
damage is related to the amount of moisture absorbed. In the case of hygrothermal
exposure, measured weight change values reflected both moisture absorption as well as
weight loss from thermal aging. Consequently, the weight change model, which accounts
only for moisture uptake, cannot be expected to accurately predict reductions in T
g
and
SBS strength.
Figure 3.5(a) shows the tensile strength as a function of exposure time and Figure
3.5(b) shows the correlation between tensile strength and percent weight change. The
hybrid GF/CF composites retained comparable tensile strength (94%) after 14-week
exposure. (Note that even after composite samples reached pseudo-saturation (at 7
weeks), the tensile strength matched or exceeded the rated tensile strength (RTS) of the
composite (153.8 kN).) The moisture uptake resulting from hygrothermal exposure could
result in matrix softening, diminishing load transfer, and reducing the fiber-matrix bond
strength, all of which cause some loss of mechanical and thermal properties. Nevertheless,
54
the fibers are nearly inert to hygrothermal exposure, and most of the adverse effects are
concentrated in the matrix and interfaces. The longitudinal tensile strength, a fiber-
dominated property, shows great moisture resistance than the SBS strength and/or T
g
,
both of which are strongly matrix-sensitive.
0 500 1000 1500 2000 2500 3000 3500
150
160
170
180
190
200
210
Tg, (
o
C)
Time, (Sec
1/2
)
Cap
Uncap
(a)
0.0 0.1 0.2 0.3 0.4
150
160
170
180
190
200
210
Tg, (
o
C)
Weight change, (%)
Cap
Uncap
(b)
Figure 3.3 (a) T
g
change as a function of exposure time. (b) T
g
change as a function of
percent weight change.
55
0 500 1000 1500 2000 2500 3000 3500
25
30
35
40
45
50
Shear strength, (MPa)
Time, (Sec
1/2
)
Cap
Uncap
(a)
0.0 0.1 0.2 0.3 0.4
25
30
35
40
45
50
Shear strength, (MPa)
Weight change, (%)
Cap
Uncap
(b)
Figure 3.4 (a) SBS strength change as a function of time. (b) SBS strength change as a
function of percent weight change.
56
0 500 1000 1500 2000 2500 3000
130
135
140
145
150
155
160
165
170
Tensile strength, (kN)
Time, (Sec
1/2
)
60
o
C 85% R.H.
RTS
(a)
0.00 0.05 0.10 0.15 0.20 0.25
130
135
140
145
150
155
160
165
170
Tensile strength, (kN)
Water absorption, (%)
60
o
C/85% R.H.
RTS
(b)
Figure 3.5 Tensile strength change as a function of time. (b) Tensile strength change as a
function of percent weight change.
57
58
Table 3.1 Summarizes the retained SBS strength and T
g
in wet state in 60 °C/85% R.H. air
exposure.
Shear strength (MPa) Wet (%) T
g
( °C) Wet (%)
unaged 46 205
Cap (16Ws) 39.4 86 181.1 88
Uncap (16Ws) 40.1 87 180.4 88
59
3.3.3 Recovery ability
The mechanical and physical reversibility of the hybrid GF/CF composites suffered
hygrothermal exposure was explored. Exposed specimens were dried in an oven at 100 °C
for 2 days to remove the absorbed moisture, after which SBS strength and T
g
measurements were performed. The recovered value and percent retention of these
properties are summarized in Table 3.2. While capped and uncapped specimens were
exposed to 60°C/85% R.H. air for 16 weeks, the recovery of SBS strength after drying
was 91-92%, while the recovery in T
g
was 88-89%.
The partial recovery of SBS strength and T
g
is attributed to the partial reversal of the
physical changes (such as matrix swelling and plasticization) induced by moisture
exposure. Matrix plasticization resulting from hygrothermal exposure is associated with
an increase in molecular mobility and a decrease in intermolecular cohesive forces, while
swelling is associated with softening and reduction in load transfer [15]. These partly
reversed processes are strongly dependent on the amount of absorbed moisture by
removing absorbed moisture (drying). Compared to water immersion, the extent of
recovery of the hybrid GF/CF composites in hygrothermal exposure shows less recovery
ability [6]. The difference in recovery ability can be attributed to the extent of moisture
absorption, which was far less in this hygrothermal exposure (60 °C/85% R.H. air).
60
Table 3.2 Summarizes the retained SBS strength and T
g
in dry state in 60 °C/85% R.H. air
exposure.
Shear strength (MPa) Dry (%) T
g
( °C) Dry (%)
unaged 46 205
Cap (16Ws) 41.9 91 181.1/182.5 89
Uncap (16Ws) 42.1 92 180.4/180.7 88
61
3.4 Conclusion
An accelerated hygrothermal exposure on the hybrid GF/CF composites was
conducted to determine the change in mechanical and thermal properties. DMA
measurement demonstrated that a moderate loss in T
g
, the extent of which depended on
exposure time. Mechanical measurements also revealed that the SBS strength and tensile
strength reduce with exposure time, albeit to a moderate extent. Weight change is
generally regarded as an index of the extent of degradation from aging. However, the
present study indicates that the weight change is only weakly correlated with the
decrements in T
g
and mechanical properties. Weight loss behavior during hygrothermal
exposure was observed after reaching the pseudo-saturation. This phenomenon is
attributed to a combination of dehydration reaction and hydrolysis reaction, which results
in the loss of low molecular weight molecules present in the epoxy matrix.
Slight moisture-induced damage occurred in the hybrid GF/CF composites during
hygrothermal exposure and it leads to slight mechanical and thermal degradation without
any cracking. Importantly, the retained tensile strength is equivalent to the rated tensile
strength (RTH) after 14-week 60 °C/85% R.H. exposure. However, few permanent
moisture-induced damages are inevitable. Thus, waterproof research (protective coatings
and nano-particle reinforcement) will be the future subject to reinforce the moisture-
resistant ability of such hybrid GF/CF composites.
62
Chapter 3 References
1. Tsai YI, Bosze EJ, Barjasteh E, Nutt SR, Hygrothermal Effects on Hybrid Glass
Fiber/Carbon Fiber Composites. Comp Sci Tech (In submission).
2. Olmos D, Lopez-Moron R, Gonzalez-Benito J, The nature of the glass fibre
surface and its effect in the water absorption of glass fibre/epoxy composites. The
use of fluorescence to obtain information at the interface. Comp Sci Tech 66
(2006): 2758-2768.
3. Karad SK, Attwood D, Jones FR, Moisture absorption by cyanate ester modified
epoxy resin matrices. Part V: effect of resin structure. Comp A 36 (2005): 764-771.
4. Loos AC, Springer GS, Moisture absorption of graphite-epoxy composites
immersed in liquids and in humid air. J Comp Mater 13 (1979): 131-146.
5. Shen CH, Springer GS, Moisture Absorption and Desorption of Composite
Materials. J Comp Mater 10 (1976): 2-20.
6. Tsai YI, Bosze EJ, Barjasteh E, Nutt SR, Influence of hygrothermal environment
on thermal and mechanical properties of carbon fiber/fiberglass hybrid composites.
Comp Sci Tech 69 (2009) 423-437.
7. Zhou JM, Lucas JP, The effects of a water environment on anomalous absorption
behavior in graphite/epoxy composites. Comp Sci Tech 53 (1995):57-64.
8. Wan YZ, Wang YL, Huang Y , He BM, Hah KY , Hygrothermal aging behaviour of
VARTMed three-dimensional braided carbon-epoxy composites under external
stresses. Comp A 36 (2005): 1102-1109.
9. Bullions TA, Loos AC, McGrath JE, Moisture sorption effects and properties of a
carbon fiber-reinforced phenylethynyl-terminated poly(etherimide). J Comp
Mater 37 (2003): 791-809.
10. Luo HL, Lian JJ, Wan YZ, Huang Y, Wang YL, Jiang HJ, Moisture absorption in
VARTMed three-dimensional braided carbon-epoxy composites with different
interface conditions. Mater Sci & Eng A 425 (2006): 70-77.
11. Thwe MM, Liao K, Effects of environmental aging on the mechanical properties
of bamboo-glass fiber reinforced polymer matrix hybrid composites. Comp A 33
(2002): 43-52.
12. Wan YZ, Wang YL, Huang Y, Luo HL, He F, Chen GC, Moisture absorption in
three-dimensional braided carbon/Kevlar/epoxy hybrid composite for orthopedic
usage and its influence on mechanical performance. Comp A 37 (2006): 1480-
1484.
63
13. Lee MC, Peppas NA, Water transport in graphite/epoxy composites. J Appl Poly
Sci 47 (1993): 1349-1359.
14. Crank J, The mathematics of diffusion 2
nd
edition. 71-74.
15. Karbhari VM, Durability of composites for civil structure applications. Woodhead
Publishing in Materials.
16. Rose N, Bras ML, Delobel R, Thermal oxidative degradation of an epoxy resin.
Polymer degradation and Stability. 42 (1993): 307-316.
17. Kootsookos A, Mouritz AP, Seawater durability of glass- and carbon-polymer
composites. Comp Sci Tech 64 (2004): 1503-1511.
18. Apicella A, Migliaresi C, Nicolais L, Iaccarino L, Roccotelli S, The water aging
of unsaturated polyester-based composites: influence of resin chemical structure.
Comp 14 (1983): 387-392.
19. Liao, K, Schultheisz CR, Hunston DL, Brinson LC, Long term durability of fiber
reinforced polymer matrix composite materials for infrastructure applications: a
review. J Adv Mater 54 (1998): 3-40.
20. Zhou JM, Lucas JP. Hygrothermal effects of epoxy resin. Part I: The nature of
water in epoxy. Polymer 40 (1999): 5505-5512.
21. Bao LR, Yee AF, Moisture diffusion and hygrothermal adding in bismaleimide
matrix carbon fiber composites – Part I: uni-weave composites. Comp Sci Tech 62
(2002): 2099-2110.
64
Chapter 4 Thermal effects on the Hybrid GF/CF Composites
4.1 Motivation
The hybrid GF/CF composite rod is used to sustain the whole weight of the
overhead conductor and its long-term retention of properties under various environmental
exposures is a significant concern. Chapter 2 and 3 has investigated the influence of
hygrothermal exposures (water immersion and humid air) on such hybrid GF/CF
composites. Nevertheless, the influence of temperature, which is strongly related to the
operation of overhead conductors, is another important environmental concern. To
establish a prediction of service life, a complete thermal investigation is necessary. This
chapter worked on the tensile strength-temperature dependence and the isothermal aging
effect on the hybrid GF/CF composites.
In general, the overhead conductors are controlled between 100 and 170 °C to
maintain the regular demand. However, it is possible to reach 200 °C or higher to increase
the transmitting efficiency as needed. The influence of the cold environment is also taken
into account for using the overhead conductor in the frigid region such as Alaska and
Siberia. Previous studies had reported that the temperature dependence and damage
mechanisms of polymer composites in the low and high temperatures [1-10]. Several
studies also assessed the influence of high-temperature isothermal aging on the polymer
composites and indicated that fiber reinforced polymer composites (FRPCs) show good
resistance to long-term thermal aging due to the inherent low thermal conductivity and
high dimensional stability [11-13]. However, most of these works focused on single-
fiber-type unidirectional or angle-ply laminates. The thermal investigation on the hybrid
GF/CF unidirectional composites is still absent.
The first objective of the dissertation is to investigate the tensile strength-
temperature dependence of such hybrid GF/CF composites from the very cold
temperature (-60 °C) up to very high temperature (240 °C). The second objective is to
understand the associated kinetic behavior of properties in the high temperatures aging,
which are below or above the glass transition temperature ( ). These issues are
particularly relevant to the long-term thermal durability of the intended application, and
to similar composites intended for infrastructure applications.
g
T
4.2 Experimental Procedures
4.2.1 Materials
The unidirectional hybrid CF/GF reinforced composite rod, 9.53 mm in diameter, as
shown in Figure 2.1, was manufactured by pultrusion method with using a propriety
epoxy formulation and an anhydride curing agent. The outer shell of the composite rod is
reinforced with glass fiber (GF) while the internal core is reinforced with carbon fiber
(CF). The diameter of CF core is ~7 mm, and the total fiber volume fraction is 67%.
4.2.2 Conditioning
Specimens were cut to lengths of 122 mm (48 in.) in preparation for tensile strength
measurements. In the tensile strength-temperature dependence experiment, specimens
were soaked in an environmental chamber (Instron 3119-403) at specific temperatures
(between -60 and 240 °C) for 15 minutes prior to pulling to failure at the pre-determined
65
temperature. In the isothermal aging experiment, specimens were isothermally aged in an
environmental chamber (Instron 3119-403) at 180, 200, and 220 °C for 20 weeks,
respectively. At predetermined intervals, specimens were removed from the
environmental chamber to measure the retained tensile strength at room temperature (RT).
4.2.3 Mechanical, thermal properties and visual determination
Tensile strength is measured at RT in accordance to ASTM D3916-02. A universal
testing instrument (INSTRON 5585) with custom-made adhesive gripping fixtures was
used and the pulling rate is 0.2 in/min.
Dynamic mechanical analysis (DMA) was performed to measure changes in glass
transition temperature ( ) and storage modulus of the hybrid composites. Samples were
sectioned from the carbon core of the hybrid composites and polished to 60 × 9.5 × 1.6
mm and a dual cantilever beam clamp was employed using a commercial instrument (TA
Instruments DMA2980). Specimens were cyclically loaded at 1 Hz and heated at a ramp
rate of 5 °C/min from 25 to 250 °C and T
g
T
g
was determined from the peak of loss modulus
curve.
Thermogravimetric analysis (TGA) was performed to evaluate the thermal stability
of the hybrid composites. Samples were cut from the carbon fiber core of the hybrid
GF/CF composites and the weights were controlled between 20-30 mg for accuracy using
an analytical balance (ACCULAB LA-60) with 0.01mg accuracy. Samples were heated
at 3 °C/min from 25 to 550 °C in a commercial instrument (TA Instruments TGA 2050).
Transverse sections of the thermal cycling specimens were cut and polished by using
conventional polishing techniques. Then, the polished sections were examined
66
microscopically by optical microscope (Olympus AH3-UMA) and scanning electronic
microscope (JEOL 6610) to detect the evidence of physical damages.
4.3 Results and Discussion
4.3.1 Tensile strength-temperature dependence
The retained tensile strength of the hybrid GF/CF composites at specific
temperatures (between -60 and 240 °C) are shown in Figure 4.1. Between -20 and 25 °C,
the tensile strength seem to maintain constant (less than 1% variance) without any
reduction or increment. In the cold region (below -20 °C), the tensile strength decreased
slightly with temperature decrease at a rate of 156.54 N/°C. At -60°C, the hybrid GF/CF
composites lose 3.4% tensile strength (compared with 25 °C). In the high temperature
region, a bilinear tensile strength loss behavior was observed as shown in Figure 4.1.
Between 25 and 185 °C, the tensile strength dropped with temperature increase at a rate of
138 N/ °C. At 185 °C, the hybrid GF/CF composites retained comparable tensile strength
(86.66%). When the temperature increases, a greater tensile strength loss rate of 567
N/ °C was observed. At 240°C, the hybrid GF/CF composites only retained 67.79%
tensile strength. In this dissertation, the abrupt tensile strength degradation takes place
over 185 °C due to being close to the glass transition temperature ( =205 °C).
g
T
The storage modulus in viscoelastic solids measures the stored energy, representing
the elastic portion. Previous study indicated that the molecular segmental mobility
becomes important only when the matrix is aged in the glassy state at 20 − ≤
g aged
T T °C
[14]. Figure 4.2 shows the normalized storage modulus of such hybrid GF/CF composites
67
as a function of temperature. Because the properties of glass fiber (GF) and carbon fiber
(CF) are stable in this temperature region, changes in the storage modulus reflect changes
in the epoxy matrix and/or the fiber/matrix interface. DMA measurement shows that the
storage modulus of such hybrid GF/CF composites drops abruptly at 185 °C, as expected.
The temperature dependence of tensile strength and that of the storage modulus are
correlated and can be understood in terms of shear stresses at the fiber /matrix interface
[2]. Figure 4.2 also shows the comparison of the normalized tensile strength versus
temperature superimposed on the normalized storage modulus. Between 25 and 185 °C,
the normalized storage modulus shows identical behavior with the normalized tensile
strength, as shown in equation (1). However, in the high temperature region (over 185 °C)
and cold temperature region (below 25 °C), the temperature dependence differed.
n
To
T
To
T
E
E
] [
) (
) (
) (
) (
=
σ
σ
n = 1 (between 25 and 185 °C) (1)
where
) (T
σ is the strength at temperature T ,
) (To
σ is the strength at the reference
temperature , is the storage modulus at temperature
o
T
) (T
E T , is the storage
modulus at the reference temperature , and is an empirical value.
) (To
E
o
T n
The tensile strength-storage modulus dependence can be understood in terms of
micro-structural changes in the matrix. The matrix gradually softens as the temperature
increases. While temperature is close to , the matrix becomes rubbery and loses much
of the ability to transfer stress between fibers. In addition, the difference in the coefficient
of thermal expansion (CTE) for fibers and matrix gives rise to thermal stresses, which
increase interfacial stresses. The tensile property is related to the fiber-matrix adhesion
g
T
68
69
and the ability of the matrix to transfer stress from highly stressed fibers to other fibers
via shear, which could be represented in the storage modulus format. Nevertheless, this
relationship between tensile property and matrix’s load-transfer ability becomes
divergent in the high temperature region (over 185 °C). Figure 4.2 shows that the
normalized tensile strength retained a much greater level than the corresponding
normalized storage modulus over 185 °C. For example, at 240 °C, the hybrid GF/CF
composites retained 67.79% tensile strength, but only kept 20% storage modulus. This
great storage modulus loss represents that the matrix has barely load transfer ability.
However, the carbon fibers which are largely responsible for the tensile strength are not
vulnerable at this temperature region and carry most of tensile load by themselves. At
high temperature, the storage modulus shows great temperature sensitivity than the
tensile property.
In the low temperature region (below 25 °C), the opposite tensile-storage behavior
was observed as shown in Figure 4.2. As temperature decreases, the tensile strength
decreases but the storage modulus increases. Previous study indicated that increases in
storage modulus are indicative of matrix hardening and increased matrix stiffness [6]. In
the low temperature region, brittle matrix also affects the failure mechanism of the
composite materials. The matrix shrinkage is associated with cooling and also gives rise
to increase clamping force on the fibers. Figure 4.1 shows that a slight tensile strength
increase (less than 1%) is observed between -20 °C and 25 °C. However, an apparent
reduction in tensile strength was observed below -20 °C. The tensile strength degradation
is attributed to the increment of residual stress, which results in the development of
microcracking in the matrix and at the fiber/matrix interface [15]. The appearance of
residual stresses inside the materials due to the mismatch between the coefficient of
thermal expansion (CTE) of fibers and matrix strongly dominates the failure mechanism
of the composites. Assuming uniform strain in the longitudinal direction and uniform
stress in the transverse direction, the matrix longitudinal stresses are derived [15]:
) /( ) )( )( (
m m f f o m f m f f mL
E V E V T T E E V + − − = α α σ (2)
where E is the elastic modulus, V is the volume fraction, α is the coefficient of thermal
expansion, T is the temperature,
L
σ is the longitudinal stress, and subscripts m and f
refer to matrix and fiber, respectively. is the stress-free temperature, usually taken as
the cure temperature of the composites.
o
T
For the hybrid GF/CF composites used in this dissertation, =3100 MPa,
m
E
m
α = mm/mm/ °C, =72300 MPa,
6
10 * 50
g
E
g
α = mm/mm/ °C, =230000 MPa,
6
10 * 4 . 5
c
E
c
α = mm/mm/ °C, =200 °C, and the volume fraction is 67% for both GF shell
and CF core. At -60 °C, the axial residual (tensile) stress in the matrix of the GF shell and
the CF core are 35.2 and 40.5 MPa, respectively. These values correspond to 70.4% and
81 % of the epoxy matrix tensile strength (50 MPa). This calculation shows that the
residual stresses induced at lower temperatures could have significant effects on the
hybrid GF/CF composites. In the low temperature region, microcracking is formed in the
matrix or at the interface and then extended to the macrocracks and the interface
debonding [15], as shown in Figure 4.3.
6
10 * 6 . 0 −
o
T
70
-60 -30 0 30 60 90 120 150 180 210 240
0
10
20
30
40
50
60
70
80
90
100
156.54 N/
o
C
567.48 N/
o
C
138.37 N/
o
C
Normalized tensile Strength, (%)
Temperature, (
o
C)
Figure 4.1 Tensile strength change as a function of temperature.
71
-50 0 50 100 150 200 250
10
20
30
40
50
60
70
80
90
100
110
Normalized storage modulus →
↑
Normalized tensile strength
Normalized storage/tensile, (%)
Temperature, (
o
C)
Figure 4.2 Normalized storage modulus and tensile strength as a function of temperature.
72
Figure 4.3 SEM image of the microcracks in the matrix of -20 °C specimen.
73
4.3.2 High temperature isothermal aging
Long-term thermal aging may result in further damage to the matrix and reduce the
tensile strength. Figure 4.4 shows the normalized tensile strength as a function of time at
180, 200, and 220 °C isothermal aging, respectively. The tensile strength shows linear
degradation behavior. After 20-week isothermal aging, the hybrid GF/CF composites
retained 82%, 81%, and 73% tensile strength at 180, 200, and 220 °C isothermal aging,
respectively.
The thermal stability of the hybrid GF/CF composites is also affected during the
long-term isothermal aging as well as tensile strength. Table 4.1 shows the change at
180, 200, and 220°C in 4, 12, and 20-week isothermal aging, respectively. For 180 and
200°C specimens, increased with time increase in the initial isothermal aging period
and reached its maximum value of 231 and 223 °C in 12 and 4 weeks, respectively.
increase is attributed to an increase in cross-link density in the polymer matrix (post-
curing). After reach the maximum, T
g
T
g
T
g
T
g
dropped with time increase and it is attributed to
chain rupture in the matrix [17]. For 220 °C specimens, no increment in T
g
was observed.
This dissertation indicated that there is no post-curing occurred while aging temperature
is higher than the curing temperature.
Figure 4.5 shows the weight change as a function of temperature from TGA
measurements. It shows that the thermal decomposition of the epoxy starts at ~300 °C.
Therefore, the isothermal aging temperatures selected in this dissertation are not expected
to cause drastic changes in the matrix. The overlapped curves for specimens which aged
in different times reflect the stability of the matrix at the selected aging temperature.
74
Figure 4.5 also shows that the final weight levels varied with 1- and 6-week aging at 180,
200, and 220 °C, respectively. As the aging temperature increased, the difference of final
weight between 1- and 6-week aging is greater. It indicated that the matrix loss is a
function of aging time and temperature. The matrix loss leads to the increment of the
fiber weight fraction of such GF/CF composites during isothermal aging, as shown in
Figure 4.5. For example, in 220 °C isothermal aging, the final weight level for 6-week
aging is greater than the one for 1-week aging and it represents that the fiber weight
fraction of 6-week aging specimens is greater. This matrix loss is attributed to a series of
thermal degradations, including the slow-rate degradation which consists of chain
scission (breakage of weak bonds), vaporization of low molecular weight product, and
dehydration reaction [16-17].
Figure 4.6 shows the normalized storage modulus as a function of time for 180, 200,
and 220 °C isothermal aging, respectively. Figure 4.7 shows the tensile strength-storage
modulus correlation for 180, 200, and 220 °C isothermal aging, respectively. For 180 °C
isothermal aging, (which is below storage modulus drop temperature of 185 °C), the
normalized storage modulus shows an identical behavior with the normalized tensile
strength, as shown in Figure 4.7(a). An empirical correlation is proposed to describe the
observed correlation.
n
t T
t T
t T
t T
E
E
] [
) , (
) , (
) , (
) , (
2
1
2
1
=
σ
σ
n = 1 (3)
where
) , (
1
t T
σ is the normalized strength at temperature T with aging time ,
1
t
) , (
2
t T
σ is the
normalized strength at temperature T with aging time , is the normalized
2
t
) , (
1
t T
E
75
storage modulus at temperature T with aging time , is the normalized storage
modulus at temperature T with aging time , and is an empirical exponent.
1
t
) , (
2
t T
E
2
t n
Normalized storage modulus, which is successfully used to describe the tensile
strength-temperature dependence behavior, also perfectly profiles the tensile strength
change in isothermal aging. However, the empirical correlation is not matched in high
temperature aging, as shown in Figure 4.7(b) and (c). Isothermal aging in the higher
temperatures (above 185 °C) would result in the much degradation/damage on the matrix
and makes the tensile strength and storage modulus behave deviated.
76
When the aged hybrid GF/CF composites were cooled down to the room
temperature (RT), the soft matrix would re-harden, but the permanent degradation is
inevitable [11]. Chemical bonds are formed at the fiber/matrix interface and residual
stresses can influence the interfacial shear strength [18]. Isothermal aging at high
temperature would cause debonding in the fiber/matrix interface and weakens the ability
of load transfer. Figure 4.8 compares the fiber/matrix interfaces of unaged specimens and
those aged at 220 °C for 4 weeks. After tensile fracture, unaged specimens show strong
matrix adhesion, while aged samples show little matrix adhesion, indicating irreversible
changes in fiber/matrix bonding induced by high temperature isothermal aging. The
apparent fiber/matrix detachment resulting from high temperature isothermal aging is
also evident in the macroscopic failure mode of the composites, as shown in Figure 4.9.
The fracture end of the aged hybrid GF/CF composites is brush-like in appearance, while
the unaged specimens show much greater matrix adhesion (to fibers). Surface
discoloration and roughness are also observed in this dissertation as shown in Figure 4.10
and this phenomenon is attributed to the surface thermal oxidation [19].
0 5 10 15 20 25
0
10
20
30
40
50
60
70
80
90
100
Normalized tensile strength, (%)
Time, (Weeks)
180
o
C
200
o
C
220
o
C
Figure 4.4 Normalized tensile strength of the hybrid GF/CF composites versus time at
180, 200, and 220 °C isothermal aging.
77
50 100 150 200 250 300 350 400 450 500
75
80
85
90
95
100
Weight change, (%)
Temperature, (
o
C)
1 Week
6 Weeks
↑
↓
(a)
50 100 150 200 250 300 350 400 450 500
75
80
85
90
95
100
↑
↓
Weight change, (%)
Temperature, (
o
C)
1 Week
6 Weeks
(b)
50 100 150 200 250 300 350 400 450 500
75
80
85
90
95
100
↓
↑
Weight change, (%)
Temperatyre, (
o
C)
1 Week
6 Weeks
(c)
Figure 4.5 Normalized weight change of the hybrid GF/CF composites at (a) 180 °C (b)
200°C (c) 220 °C 1 and 6 weeks isothermal aging.
78
0 50 100 150 200 250 300
20
30
40
50
60
70
80
90
100
Normalized storage Modulus, (%)
Temperature, (
o
C)
Unaged
4 weeks
12 weeks
20 weeks
(a)
0 50 100 150 200 250 300
20
30
40
50
60
70
80
90
100
Normalized storage modul;us, (%)
Temperature, (
o
C)
Unaged
4 weeks
12 weeks
20 weeks
(b)
0 50 100 150 200 250 300
10
20
30
40
50
60
70
80
90
100
Normalized storage modulus, (%)
Temperature, (
o
C)
Unaged
4 week
12 weeks
20 weeks
(c)
Figure 4.6 Normalized storage modulus of the hybrid GF/CF composites at (a) 180 °C (b)
200°C (c) 220 °C isothermal aging.
79
04 8 12 16 20
0
10
20
30
40
50
60
70
80
90
100
Normalized tensile/storage
Time, (Week)
Tensile strength
Storage modulus
(a)
0 5 10 15 20
0
10
20
30
40
50
60
70
80
90
100
Normalized tensile/storage
Time, (Week)
Tensile strength
Storage modulus
(b)
0 5 10 15 20
0
10
20
30
40
50
60
70
80
90
100
Normalized tensile/storage
Time, (Week)
Tensile strength
Storage modulus
(c)
Figure 4.7 Normalized storage modulus versus normalized tensile strength at (a) 180 °C
(b) 200 °C (c) 220 °C isothermal aging.
80
(a)
(b)
Figure 4.8 SEM images of (a) the fiber/matrix interface for the unaged specimens. (b)
very little matrix adhered to the fibers at 220 °C 4-week isothermal aging.
81
(a)
(b)
Figure 4.9 (a) Example of failure mode for unaged specimens. (b) Example of failure
mode for 220 °C 4-week isothermal aging.
82
Figure 4.10 The surface discoloration and roughness of the hybrid GF/CF composites at
various temperature aging.
83
84
Table 4.1 Summarizes the retained T
g
of the hybrid GF/CF composites at 180, 200, and
220°C in 4, 12, and 20-week isothermal aging.
180°C aging 200 °C aging 220 °C aging
4 Weeks 217.66°C 222.72 °C 195.58 °C
12 Weeks 231.11°C 222.48 °C 186.25 °C
20 Weeks 230.27°C 217.10 °C 174.66 °C
85
4.4 Conclusions
The tensile strength-temperature dependence and the influence of high temperature
isothermal aging on the hybrid GF/CF composites were investigated in this dissertation.
It indicated that the retained tensile strength degrades as a function of aging temperature
and time. Dynamic mechanical analysis (DMA) also shows that the storage modulus
which represents the fiber-matrix adhesion and the ability of the matrix to transfer stress
changes a function of temperature. The normalized storage modulus and tensile strength
appeared identical behavior between 25 and 185 °C, in accordance with the previous work
[2]. The empirical correlation offers a simple method to predict thermal durability of such
hybrid GF/CF composites at certain temperature domains. However, in the cold (below -
25°C) and high (above 185 °C) temperature region, the empirical correlation diverges.
Thermogravimetric analysis (TGA) also shows that no structural damage (matrix’s
decomposition) took place in this temperature range, while only slow-rate degradation
(chain scission, vaporization of low molecular weight product, and dehydration reaction)
occurred. It indicated that the thermal degradation on such GF/CF composites is only
attributed to the weak fiber/matrix interface.
This dissertation provides a reference on thermal investigation of the hybrid GF/CF
composites and intended application. Nevertheless, the temperature of the overhead
conductor is fluctuating during the practical operation. In this case, the influence of
thermal fatigue behavior resulting from the thermal cycling should be concerned to
establish a reliable prediction of the service life.
86
Chapter 4 References
1. Alawar A, Bosze EJ, Nutt SR, A composite core conductor for low sag at high
temperatures. IEEE Transactions on Power Delivery 20 (2005): 2193-2199.
2. Bosze EJ, Alawar A, Bertschger O, Tsai Y-I, Nutt SR, High-temperature strength
and storage modulus in unidirectional hybrid composites. Comp Sci Tech 66
(2006) 1963-1969.
3. Dimitrienko Yui. Unidirectional composites under high temperatures. In:
Thermomechanics of composites under high temperatures. London: Kluwer
Academic Publishers (1999).
4. Nakada M, Miyno Y, Kinoshita M, Koga R, Okuya T. Time temperature
dependence of tensile strength of unidirectional CFPR. J of Comp Mater 36
(2002): 2567-2581.
5. Sanchez-Saez S, Gomez-del T, Barbero E, Zaera R, Navarro C. Static behavior
of CFRPs at low temperatures. Comp B 33 (2002): 383-390.
6. Rivera J, Karbhari VM. Cold-temperature and simultaneous aqueous
environment related degradation of carbon/vinylester composites. Comp B 33
(2002): 17-24.
7. Xu D, Liu R, Xia J, Zhao J, Shen W. Fracture behavior of glass –cloth/polyester
composite laminate at low temperature. Journal of Reinforced Plastics and
Composites 4 (1985):205-211.
8. Dutta PK, Hui D. low-temperature and freeze-thaw durability of thick
composites. Comp B 27 (1996): 371-379.
9. Kreibich VT, Lohse F, Schmid R. Polymers in low temperature technology. In:
Clark AF, Reed RP, Hartwing G, editors. Nonmetallic materials and composites
at low temperatures, NY: Plenum Press (1979): 1-32.
10. Hartwig G. Mechanical and electrical low temperature properties of high
polymers. In: Clark AF, Reed RP, Hartwing G, editors. Nonmetallic materials
and composites at low temperatures, NY: Plenum Press (1979): 33-50.
11. Leveque D, Schieffer A, Mavel A, Maire JF. Analysis of how thermal aging
affects the long-term mechanical behavior and strength of polymer-matrix
composites. Comp Sci Tech 65 (2005) 395-401.
12. Akay M, Spratt GR. Evaluation of thermal aging of a carbon fiber reinforced
bismalemide. Comp Sci Tech 68 (2008) 3081-3086.
87
13. Wolfrum J, Eibl S, Lietch L. Rapis evaluation of long-term thermal degradation
of carbon fiber epoxy composites. Comp Sci Tech 69 (2009) 523-530.
14. Yuan J. Isothermal cure and degradation of epoxy FR-4 laminates: The time-
temperature relationship. International SAMPE Technical Conference 23 (1991)
403-414.
15. Hahn HT. Residual stresses in polymer matrix composite laminated. J Comp
Mater 10 (1976): 266-278.
16. Yuan J. The thermal degradation and decomposition of brominated epoxy FR-4
laminates. IEEE (1993) 330-335.
17. Rose N, Bras ML, Delobel R, Thermal oxidative degradation of an epoxy resin.
Polymer degradation and Stability. 42 (1993): 307-316.
18. Parlevliet PP, Bersee HEN, Beukers A. Residual stresses in thermoplastic
composites – a study of the literature. Part III: Effects of thermal residual
stresses. Comp A 38 (2007): 1581-1596.
19. Barjasteh E, Bosze EJ, Tsai YI, Nutt SR, “Thermal aging of fiberglass/carbon-
fiber hybrid composites”. Journal of composites A. (In submission)
88
Chapter 5 Thermal Cycling Effects on the Hybrid GF/CF Composites
5.1 Motivation
Among environmental attacks, thermal effect is an important concern for the
overhead conductors. The operation temperature is strongly related to the transmitting
efficiency of the power grid system. However, the high temperature also leads to the
greater degradation of the hybrid GF/CF composites. Chapter 4 has discussed the tensile
strength-temperature dependence and isothermal aging behavior of the hybrid GF/CF
composites. Therefore, this chapter assessed another thermal effect “thermal fatigue
behavior”.
Although the operation temperature of the overhead conductors is generally
controlled between 100 and 170 °C, the temperature is fluctuating rather than constant in
the actual operation depending on the demand. Besides, the difference between peak and
non-peak temperature could be significant. Chapter 4 indicated that the hybrid GF/CF
composites retain comparable tensile properties even in the high-temperature isothermal
aging, but the thermal fatigue damage during the fluctuant operation temperatures should
be taken into account. Several studies indicated that thermal cycling accelerates the
thermal degradation in the fatigue format (matrix cracking) [1-8]. However, most of these
works focused on the thin film or thin unidirectional or angle-ply laminates [2-6]. The
thermal cycling investigation on the bulk hybrid GF/CF composites is absent.
The primary objective of this dissertation is to investigate the influence of thermal
cycling and associated kinetic behavior on mechanical and thermal properties. The hybrid
89
GF/CF composites were tested in two different thermal cycling conditions to investigate
the thermal fatigue mechanism. In addition, a simplified oxidation model was used to
transfer the thermal cycling to the equivalent isothermal aging. Such transformation not
only shorten a long time thermal cycling measurement to a relatively short isothermal
aging, but also help us to figure the thermal fatigue mechanism during thermal cycling.
5.2 Experimental Procedures
5.2.1 Materials
The unidirectional hybrid CF/GF reinforced composite rod, 9.53 mm in diameter, as
shown in Figure 2.1, was manufactured by pultrusion method with using a propriety
epoxy formulation and an anhydride curing agent. The outer shell of the composite rod is
reinforced with glass fiber (GF) while the internal core was reinforced with carbon fiber
(CF). The diameter of CF core is ~7 mm, and the total fiber volume fraction is 67%.
5.2.2 Conditioning
Two thermal cycling experiments are carried out to investigate the thermal fatigue
damage. The first experiment assessed the thermal cycling effect between normal
temperature and high temperature (between 45 and 215 °C) with prolonged 215 °C
isothermal aging. Figure 5.1(a) profiles the real time of each cycle. Specimens were
placed in an environmental chamber (Instron 3119-403) with mechanical loading of 8000
lbf. Such specimens ran the tensile strength measurement in 20, 30, and 40 thermal cycles,
respectively. The second experiment assessed the thermal cycling effect between low
temperature and high temperature (between -40 and 180 °C) with moderate -40 and 180 °C
isothermal aging. Figure 5.1(b) describes the real time of each cycle. The specimens were
90
placed in an environmental chamber (TPS T30RC-2) with free mechanical loading. Such
specimens were removed from the environmental chamber in predetermined intervals (4,
18, 30, 74, 100, and 150 cycles). The specimens then ran the tensile strength and short
beam shear (SBS) strength measurements to determine the influence of thermal cycling.
To get the average value, five specimens were tested in the SBS strength measurements
and three specimens were tested in the tensile strength measurements.
0 100 200 300 400 500
0
50
100
150
200
250
Temperature, (
o
C)
Time, (Mins)
(a)
0 50 100 150 200 250 300
-50
0
50
100
150
200
Temperature, (
o
C)
Time, (Min)
(b)
Figure 5.1 (a) Experimental record of a temperature cycle (between 45 and 215 °C). (b)
Experimental record of a temperature cycle (between -40 and 180 °C).
91
92
5.2.3 Mechanical, thermal properties and visual determination
Short beam shear (SBS) strength was measured at room temperature (RT) in
accordance with ASTM D4475-02 using a load frame (INSTRON 5567). Such specimens
were cut to a length of 66.5 mm (2.6 in.).The span length is six times the diameter and the
crosshead displacement rate is 1.3 mm/min.
Tensile strength was measured at room temperature (RT) in accordance to ASTM
D3916-02. A universal testing instrument (INSTRON 5585) with custom-made adhesive
gripping fixtures was used. Such specimens were cut to a length of 121.92 mm (48 in.)
and the pulling rate is 0.2 in/min.
Dynamic mechanical analysis (DMA) was performed to determine the change of the
glass transition temperature (T
g
) and the storage modulus. A dual cantilever beam clamp
was employed using a commercial instrument (TA Instruments DMA2980). Specimens
were cut from the carbon core of the rod to a standard dimension of 60 × 9.5 × 1.6 mm.
Such specimens were cyclically loaded at 1 Hz and heated at a ramp rate of 5 °C/min from
25 to 250 °C and T
g
was determined from the peak of loss modulus curve.
Transverse sections of the thermal cycling specimens were cut and polished by using
conventional polishing techniques. Then, the polished sections were examined
microscopically by optical microscope (Olympus AH3-UMA) and scanning electronic
microscope (JEOL 6610) to detect the evidence of physical damages.
93
5.3 Results and Discussion
5.3.1 Thermal cycling between 45 and 215 °C
The operation temperature of the overhead conductors is fluctuating and 215 °C is
regarded as the maximum temperature. To investigate the influence of high temperature
endurance and thermal fatigue resistance is required. The hybrid GF/CF composites ran
the thermal cycling test between normal temperature and high temperature (between 45
and 215 °C) with prolonged 215 °C isothermal aging as shown in Figure 5.1(a). Figure 5.2
shows that the normalized tensile strength degrades as a function of thermal cycle. The
hybrid GF/CF composites retained 88.7 % tensile strength in the 40 thermal cycles.
Chapter 3 indicated that the higher temperature would result in the greater mechanical
loss. However, the thermal fatigue mechanism from thermal cycling behavior on such
hybrid GF/CF composites is necessary to be investigated. An oxidation model was used
to transfer each thermal cycle to a set of isothermal stages [6]. It indicated that the more
isothermal stages can be made, the more ideal thermal cycling-isothermal aging
transform can be achieved. Based on the purpose of separating the isothermal aging effect
and the additional thermal fatigue damage, a two-stage model is performed in this
dissertation. When the retained tensile strength of thermal cycling is less than the
equivalent isothermal aging, it represents that thermal fatigue damage was occurred
during thermal cycling.
Figure 5.3(a) shows one thermal cycle and its equivalent 215 °C isothermal aging.
Because the thermal degradation is the function of temperature and time, the two-stage
model is possible to accumulate the greater thermal degradation on the hybrid GF/CF
composites due to the extended 215 °C isothermal aging duration. Because the heating
94
and cooling duration (1 hr total) for each cycle are much shorter than the 215 °C
isothermal aging duration (8 hrs), the influence of the extended 215 °C isothermal aging
from two-step transfer on the hybrid GF/CF composites was not significant. Thus, the
thermal cycling-isothermal aging transformation by two-step model is successful in this
case. Figure 5.3(b) shows the comparison of normalized tensile strength between thermal
cycling and the equivalent 215 °C isothermal aging. The identical tensile degradation
behavior of such hybrid GF/CF composites was observed in this dissertation. It indicated
that the thermal degradation in this thermal cycling (between 45 and 215 °C) could be
completely attributed to 215 °C isothermal aging without extra thermal fatigue damage. In
addition, the failure mechanism under thermal cycling is investigated in this dissertation.
Figure 5.4(a) shows the failure mode of the thermal cycling specimens after tensile test.
The brush-like appearance of the fracture surface looks similar as the isothermal aging.
Figure 5.4(b) shows the image of fiber/matrix interface from SEM observation. The little
fibers adhesion and thermal-induced fiber/matrix debonding are observed as well as the
isothermal aging.
95
0 010 20 30 40 5
75
80
85
90
95
100
105
Normalized tensile strength, (%)
Cycles, (#)
Thermal cycling
Figure 5.2 Normalized tensile strength as a function of thermal cycle.
0 100 200 300 400 500
0
50
100
150
200
250
Temperature, (
o
C)
Time, (Mins)
Thermal Cycling
Isothermal aging
(a)
0 10 2030 405
75
80
85
90
95
100
105
0
Normalized tensile strength, (%)
Cycles, (#)
Thermal cycling
Isothermal aging
(b)
Figure 5.3 (a) Equivalent thermal cycles for estimating 215 °C isothermal aging. (b) The
comparison of the normalized tensile strength between thermal cycling and the
corresponding isothermal aging.
96
(a)
(b)
Figure 5.4 (a) Example of failure mode for thermal cycling specimens. (b) Little matrix
adhered to the fibers for the hybrid GF/CF composites in thermal cycling.
97
5.3.2 Thermal cycling between -40 and 180 °C
During thermal cycling, the primary thermal degradation takes place in the high
temperature region and the thermal fatigue occurs in the low temperature region [1,5].
Previous study also indicated that the freeze-thaw exposure results in fiber/matrix
debonding and matrix microcracking [9]. The first thermal cycling experiment showed
that no additional thermal fatigue damage account for thermal degradation during high
temperature thermal cycling. Thus, the second thermal cycling experiment (between -40
and 180 °C) was performed to determine if any thermal fatigue damage occurs in the low
temperature. Figure 5.5(a) shows the normalized tensile strength as a function of thermal
cycle. The hybrid GF/CF composites show a linear degradation at a rate of 52 N/cycle to
94.1% tensile strength in 150 thermal cycles. Figure 5.5(b) also shows the normalized
SBS strength degrades as the function of thermal cycle. The hybrid GF/CF composites
retained 98.4% SBS strength in 150 cycles. In this thermal cycling (between -40 and
180°C), the hybrid GF/CF composites showed great thermal resistance in the limited
period (150 cycles).
The thermal stability would be regarded as another index of the thermal degradation
on such hybrid GF/CF composites. Table 5.1 summarizes the retained for unaged, 4,
and 150 cycles, respectively. It shows drop as the function of thermal cycle and the
hybrid GF/CF composites retained comparable (96.6%) in 150 thermal cycles. The
reduction in is attributed to the chain rupture of the epoxy matrix due to thermal
attack [10].
g
T
g
T
g
T
g
T
98
99
Storage modulus can be understood in terms of shear stresses at the fiber/matrix
interface and represent the change in the polymer matrix and/or the fiber/matrix interface
[11]. Figure 5.6 shows the normalized storage modulus of the hybrid GF/CF composites
for unaged, 4 and 150 cycles, respectively. In the initial 4 cycles, the significant down
shift of storage modulus was observed. Then the degradation rate slowed down with
thermal cycles increase. Chapter 3 has indicated that the hybrid GF/CF composites show
the identical behavior between the normalized storage modulus and the normalized
tensile strength in 180 °C isothermal aging. But the empirical correlation is not equal to 1
in this thermal cycling, as shown in Figure 5.7. This deviation could be regarded as the
evidence of the extra thermal fatigue damage from thermal cycling.
Figure 5.8(a) shows one thermal cycle and its equivalent 180 °C isothermal aging. In
this thermal cycling, the heating and cooling duration (100 minutes total) for each cycle
are much longer than 180 °C isothermal aging duration (30 minutes). The excessive
180°C isothermal aging duration from two-step model would result in the greater
mechanical degradation in the equivalent isothermal ageing than the original thermal
cycling. Figure 5.8(b) shows the normalized tensile strength for the thermal cycling and
the equivalent 180 °C isothermal aging, respectively. Because of the two-step
transformation, the hybrid GF/CF composites for thermal cycling should retain greater
tensile strength than the equivalent isothermal aging one which suffered overdone
thermal attack. However, this dissertation showed that the retained tensile strength for
thermal cycling was much less than its equivalent 180 °C isothermal aging. The excessive
tensile strength loss could be attributed to the extra thermal fatigue damage during
100
thermal cycling and the difference could be regarded as the extent of thermal fatigue
damage. In this thermal cycling experiment (between -40 and 180 °C), the hybrid GF/CF
composites suffered the greatest thermal attack in the 180 °C isothermal aging duration
and also undergo the severest thermal fatigue damage in the -40 °C cooling duration, as
expected [5,9].
0 20 40 60 80 100 120 140 160
75
80
85
90
95
100
105
Normalized tensile strength, (%)
Cycles, (#)
Thermal cycling
(a)
0 20 40 60 80 100 120 140 160
90
92
94
96
98
100
102
Normalized shear strength, (%)
Cycles, (#)
Thermal cycling
(b)
Figure 5.5 (a) Normalized tensile strength versus thermal cycles (b) Normalized shear
strength versus thermal cycles.
101
25 50 75 100 125 150 175 200 225
20
30
40
50
60
70
80
90
100
Normalized storage modulus, (%)
Temperatue, (
o
C)
Unaged
4 Cycles
150 Cycles
Figure 5.6 Normalized storage modulus as a function of temperature for unaged, 4 and
150 thermal cycles.
102
0 20 40 60 80 100 120 140 160
0.5
0.6
0.7
0.8
0.9
1.0
Normalized Modulus/Strength
Cycles, (#)
Normalized tensile strength
Normalized storage modulus
Figure 5.7 Normalized storage modulus and tensile strength as a function of thermal
cycle.
103
0 50 100 150 200
0
20
40
60
80
100
120
140
160
180
200
Temperature, (
o
C)
Time, (Mins)
Thermal Cycling
Isothermal aging
(a)
0 20 40 60 80 100 120 140 160
80
85
90
95
100
105
Normalized tensile strength, (%)
Cycles, (#)
Thermal cycling
Isothermal aging
(b)
Figure 5.8 (a) Equivalent thermal cycles for estimating 180 °C isothermal aging. (b) The
comparison of the normalized tensile strength between thermal cycling and the
corresponding isothermal aging.
104
Table 5.1 Glass transition temperature ( ) of the hybrid GF/CF composites in unaged, 4,
and 150 thermal cycles.
g
T
Unaged 4 cycles 150 cycles
g
T 205°C 203 °C 198 °C
105
106
5.4 Conclusions
Two thermal cycling experiments were run in this dissertation to investigate the
influence of thermal cycling on the hybrid GF/CF composites. The first experiment
assessed thermal cycling between 45 and 215 °C with long-term isothermal aging at
215°C. It indicates that the thermal degradation is completely attributed to 215 °C
isothermal aging and no thermal fatigue damage occurred during thermal cycling. The
second experiment assessed thermal cycling between -40 and 180 °C with moderate
isothermal aging at-40 and 180 °C. It indicates that the major strength loss is attributed to
180°C isothermal aging. Nevertheless, thermal fatigue damage which occurs in the sub-
zero temperature also aggravates the thermal degradation in both mechanical and thermal
properties.
An empirical normalized tensile strength-storage modulus correlation is calculated
in this dissertation. The unequal value could be used as an index of thermal fatigue
damage. This dissertation the thermal fatigue damage only occurs in the freeze region due
to the increasing residual stress and is unrelated with the temperature difference. For the
application of overhead conduction, the thermal fatigue damage on such hybrid GF/CF
composites could be ignored in the regular operation. However, the thermal fatigue effect
should be concerned as applying in the sub-zero environment. Last but not least, the
hybrid GF/CF composites can replace the conventional metal to be used in severe
temperature changing environment due to the excellent thermal fatigue resistance.
107
Chapter 5 References
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polymeric composites: prediction of thermal cycling effect from isothermal data.
Comp A 31 (2000): 945-957.
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5. Lafarie-Frenot MC, Rouquie S. Influence of oxidative environments on damage
in c/epoxy laminates subjected to thermal fatigue. Comp Sci Tech 64 (2004)
1725-1735.
6. Lafarie-Frenot MC, Rouquie S, Ho NQ, Bellenger V. Comparison of damage
development in C/epoxy laminates during isothermal aging or thermal cycling.
Comp A 37 (2006):662-671.
7. Biernacki K, Szyszkowski W, Yannacopoulos S. An experimental study of large
scale model composite materials under thermal fatigue. Comp A 30 (1999):1027-
1034.
8. Joffe R, Varna J. Effect of thermal fatigue on intralaminar cracking in laminates
loaded in tension. The 17
th
ICCM International Conference (2009): F13:4
9. Rivera J, Karbhari. Cold-temperature and simultaneous aqueous environment
related degradation of carbon/vinylester composites. Comp B 33 (2002):17-24.
10. Leveque D, Schieffer A, Mavel A, Maire JF. Analysis of how thermal aging
affects the long-term mechanical behavior and strength of polymer-matrix
composites. Comp Sci Tech 65 (2005) 395-401.
11. Bosze EJ, Alawar A, Bertschger O, Tsai Y-I, Nutt SR, High-temperature strength
and storage modulus in unidirectional hybrid composites. Comp Sci Tech 66
(2006) 1963-1969.
108
Chapter 6 Conclusion and Future Works
Because conventional steel-reinforced aluminum conductor (ACSR) can not satisfy
future transmission demands, an aluminum composite core conductor (ACCC) has been
developed. The new conductor is expected to significantly improve transmission
efficiency and overcome the deficiencies of the electric power grid in the next decades. In
this new conductor, a hybrid glass fiber (GF)/carbon fiber (CF) reinforced polymer
composite replaces the conventional steel cable core of overhead conductors, resulting in
markedly less sag at high temperatures. For such infrastructure applications, various
forms of environmental attack must be investigated and understood to establish service
life. In this project, we have comprehensively investigated the different environmental
effects on such advanced hybrid GF/CF composites.
In Chapters 2 and 3, we assessed the effect of moisture, including water and humid
air. Immersing the polymer composites in water represents the most severe hygrothermal
environment. The maximum saturation level for such hybrid GF/CF composites in water
immersion is several times the saturation level in humid air. Non-Fickian diffusion
behavior was observed for low-temperature water immersion due to the GF/CF interface,
which worked as a temporary moisture barrier. However, the excessive moisture which
was concentrated at the GF/CF interface led to the development of cracking. The cracks
initially developed along the GF/CF interface and branched to the GF shell or the CF core.
In the hygrothermal environment, the hybrid GF/CF composites showed different weight-
change behavior, following theoretical Fickian diffusion behavior before reaching
pseudo-saturation. Weight-loss was observed during 60 °C/85% R.H. air exposure, and
109
this was attributed to thermal reactions in the matrix, particularly dehydration reaction
and hydrolysis process. Due to the absorbed moisture, the mechanical and thermal
properties of the hybrid GF/CF composites degraded as a function of temperature and
time. In addition, the ability to recover property levels by removing absorbed moisture
was also investigated. The hybrid GF/CF composites showed recovery the extent of
which depended on the amount of moisture absorption. However, the moisture-induced
damage in the matrix and fiber/matrix interface caused only moderate permanent loss of
properties.
In Chapters 4 and 5, the distinctive thermal effects on the hybrid GF/CF composites,
such as tensile strength-temperature dependence, isothermal aging, and thermal cycling,
were investigated. The retained tensile strength of the hybrid GF/CF composites
decreased as a function of temperature and time. However, an abrupt drop occurred at
185°C, which was near the glass transition temperature (T
g
) of the GF/CF composites.
The results indicated that the storage modulus can be used to predict the tensile strength-
temperature dependence. The empirical storage modulus-tensile strength correlation also
shows identical behavior for isothermally aged specimens. The failure mechanism of the
GF/CF composites changed after thermal attack and was attributed to weakening of the
fiber/matrix interface.
Although the thermal degradation is a function of temperature and the most severe
thermal degradation occurred at the highest temperatures, thermal fatigue during thermal
cycling occurred primarily due to low-temperature exposure. Thermal fatigue damage
was associated with the development of cracking in the matrix. In the low-temperature
region, the increasing thermally-induced internal stress damaged the matrix and caused
110
microcracking. In contrast with the effects of moisture exposure, the hybrid GF/CF
composites showed excellent thermal resistance and retained mechanical and thermal
properties after moderate isothermal aging and thermal cycling.
This dissertation has shown that environmental attacks can lead to various extents of
thermal and mechanical degradation in such hybrid GF/CF composites. Thus, using a
protective coating to prevent environmental attack of the hybrid composites will be an
important subject for future work. Except for the effects of moisture and temperature,
other environmental factors, such as ultraviolet (UV) light exposure, and aggressive
chemical corrosion (acid rain), also should be considered in the future. Finally, the
combined effects of environmental attack and mechanical load must be understood to
predict service life.
Lastly, because the overhead conductors typically are suspended between two lattice
towers, wind forces would result in Aeolian vibration, a flexural fatigue-like behavior on
such overhead conductors. In general, fiber-reinforced composites exhibit greater fatigue
resistance than conventional metallic alloys. However, the hybrid GF/CF composites
under moderate flexural fatigue loading should be investigated to meet the target of 30-
year service life (or longer).
111
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Abstract (if available)
Abstract
Fiber reinforced polymer composites (FRPCs) have been widely used to replace conventional metals due to the high specific strength, fatigue resistance, and light weight. In the power distribution industry, an advanced composites rod has been developed to replace conventional steel cable as the load-bearing core of overhead conductors. Such conductors, called aluminum conductor composite core (ACCC) significantly increases the transmitting efficiency of existing power grid system without extensive rebuilding expenses, while meeting future demand for electricity. In general, the service life of such overhead conductors is required to be at least 30 years. Therefore, the long-term endurance of the composite core in various environments must be well-understood.
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Asset Metadata
Creator
Tsai, Yun-I
(author)
Core Title
Environmental effects on the hybrid glass fiber/carbon fiber composites
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Materials Science
Publication Date
10/20/2009
Defense Date
10/14/2009
Publisher
University of Southern California
(original),
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Tag
carbon fiber,composites,glass fiber,hygrothermal,OAI-PMH Harvest,thermal
Language
English
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Nutt, Steven R. (
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), Goo, Edward K. (
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), Sammis, Charles G. (
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boldlike@hotmail.com,yunitsai@usc.edu
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