Abstract
In the present study, temperature and frequency effects are studied involving carbon fibre reinforced polymeric materials with unidirectional fibers. Before testing, laminates were preserved in a deep freezer at −80, −20, 0, and 25°C for 60 days. Compressive, tensile, and stiffness behaviors of the laminates were assessed. The results confirmed that the compressive strength, tensile strength, and tensile modulus of laminates severely deteriorate at high temperatures. This might happen because of the weakening of the fibre/matrix interface, resulting in the load-carrying capacity of the carbon fibre being severely reduced. Lower temperatures did not significantly affect the mechanical performance of the laminates. This is due to minor deformation of the frozen laminates and closely compacted epoxy chain segments. The effects of temperature and vibration on the storage modulus, loss modulus, and damping behaviour of laminates are discussed. The results confirm that a reduction in mechanical performance is a strongly temperature-dependent phenomenon. Laminate damping properties are also evaluated. According to the results of the experiments, −80°C has the greatest permanence. Finally, the accuracy of the results on storage modulus was compared with empirical models. The model suggested by Gibson et al. provided the most accurate estimates for the storage modulus of the laminates. Other models were less accurate and gave non-conservative estimates.
Introduction
Fibre-reinforced polymeric (FRP) composites have received considerable attention for components used in aerospace, automobiles, and wind turbine blades.1–2 Currently, carbon fibre reinforced polymeric (CFRP) composites have been the choice to develop large wind turbine blade structures.3–5 A blade can be exposed to moisture, temperature, and mechanical loads during its life span. Moisture often induces swelling and forms residual stresses, leading to degradation.6–8 Moisture absorption in a polymeric matrix can influence the thermophysical, mechanical, and chemical properties.9–11 Stress can develop under loading conditions on tension, compression, and fatigue leading to interlaminar cracks, and consequently delamination.12–13 At low temperatures, the relaxation of residual stress is low, and this may lead to larger debonded, causing brittle failure. Zafar et al.
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assessed the effects of long-term moisture on the mechanical behavior of CFRP laminates. Results indicated that the glass transition temperature (
Composite structures that use thermosetting polymer matrix as a binder deteriorate when exposed to elevated temperatures.
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The polymer matrix will soften and decompose when the temperature exceeds
Understanding the viscoelastic properties of CFRP composites at different states is essential. Melo and Radford 24 used the DMA tool to assess the viscoelastic properties of CFRP laminates. They observed a decrease in storage modulus and an increase in damping properties as the temperature changed. Many researchers have been developing empirical models to validate the experimental results. 25 Hawileh et al. 26 reported an analytical model to predict the tensile strength and elastic modulus of FRP composites at elevated temperatures. The test data and the proposed analytical model agreed well.
Carbon fibre reinforced polymeric composite has been used to produce the spar cap sections of the blades to increase their stiffness. The spar cap sections are exposed to fluctuating wind loads and different environments. It is important to assess the mechanical response of CFRP materials under different environmental conditions before using them for designing purposes.
This paper presents further research about the mechanical response of CFRP under different environmental conditions. Tensile and compressive responses under low and high-temperature tests were assessed. Additionally, storage modulus, loss modulus, damping ratio and
Experimental program
Material properties
Properties of carbon fibre and epoxy resin. 27
Laminate preparation and test methods
Laminates were prepared under ASTM standards.28–30 Three-ply carbon fibre with epoxy matrix was used to prepare laminates. Fourteen-ply carbon fibre was used to prepare the samples for the dynamical mechanical analysis. Laminates were produced using the resin transfer moulding (RTM) method and cured for 24 h at 25°C, then post-cured at 65°C for 16 h. Laminates were cooled at room temperature and tabs were produced by the hand lay-up method using plain weave glass fibres. Laminates were cut using a CNC machine with a tolerance of 0.02 mm, then cleaned, and flash was removed with sandpaper.
Matrix digestion using burn-off method was used to determine the volume fractions according to ASTM 3171 standard.
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The volume fraction of fibres was found to be 55%. Laminates preparation process, fibre orientation and testing methods are shown in Figure 1. A flow chart for specimen preparation.
Compressive and tensile strength tests were carried out using a Lloyd LR testing machine. The testing machine was equipped with a 30 kN load cell, and measurements were taken at a rate of 2 mm/min. Laminates were preserved in a deep freezer at −80, −20, and 0°C for 60 days. Compressive tests were performed at −80, −20, and 0°C, and tensile tests were performed at −20 and 0°C. Additional tests were carried out at 25, 50, 75, and 100°C. A heat-con thermocouple was used to measure the temperatures. An epsilon digital extensometer of 25 mm gauge length was used.
DMA tool was used to assess the change in storage modulus, loss modulus, and damping factors of laminates as a function of temperature and frequency. These tests were carried out under ASTM D5023 standard, using DMA Q 800 TA Instrument. Three-point bending modes were used. The heating rate was increased at 2°C/min, and frequencies were set at 1, 10 and 100 Hz. Liquid nitrogen was used as a cooling agent and test temperatures ranged from −80 to 140°C, −20 to 140°C, 0 to 140°C, and 20 to 160°C. Dimensions were set at a height of 4.57 ± 0.03 mm, a width of 13 ± 0.02 mm, and a length of 64 ± 0.02 mm.
Weibull distribution
Tensile and compressive results were analysed using the Weibull distribution.
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The basic form of the cumulative probability density function is given by
Experimental results and discussions
Compressive tests
A compressive test was performed to assess the response of CFRP laminates as a function of temperature. As shown in Figure 2, at testing temperatures of −20 and 100°C, the highest and lowest compressive strength were obtained. When the temperatures were raised from 0 to 100°C, the compressive strength decreased by 1.22%, 2.97%, 32.28%, 67.84%, and 93.11%, respectively, compared to −20°C laminate. The compressive strength of the laminates was affected less and was almost the same when the temperature dropped from 25 to −80°C. The reason is that the polymer chain segment motion is frozen and more closely compacted due to absorbed moisture. The compressive strength of the laminates was degraded little when the temperature was below −20°C. This could occur due to rapid fibre/matrix interface debonding and delamination caused by more moisture absorption and swelling.
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The compressive strength properties of the laminates under different temperatures.
The compressive strength response between 50 and 75°C was reduced due to resin softening as approached the
Compressive strength properties with the cumulative failure probability distribution of laminates are shown in Figure 3. A higher reduction in compressive strength was observed as the testing temperature increased from ambient to higher temperatures. Additionally, the accumulated compressive failure probability of each laminate was observed using the curves. The distribution of test results was characterized by the shape parameter (m) and the scale parameter The cumulative failure probability of the laminates under different temperatures.
Tensile tests
A tensile test was performed to assess the tensile performance of CFRP laminates at various temperatures. As shown in Figure 4(a) and (b), the tensile and stiffness properties of laminates decreased as the temperature increased from 0 to 100°C. The tensile strength decreased by 0.76%, 13.19%, 19.82%, and 67.58% when compared to the tensile strength at 0°C. The reduction in tensile modulus was about 3.24%, 12.66%, 19.28%, and 67.09%. A high level of moisture swelling may increase plasticization to lower elastic deformation. Lower tensile properties were observed on −20°C laminates. This might occur due to the presence of moisture, which degrades the interface strength rapidly and induces debonding and delamination.
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The tensile strength and stiffness of the laminates were evaluated between 50 and 75°C. Results show that the tensile and stiffness properties were reduced as the temperature increased. In this case, the force transfer capacity of the resin was reduced due to the resin’s softening effects.
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At 100°C, the lowest tensile strength and stiffness properties were obtained. The cumulative failure probability of laminates at different temperatures is shown in Figure 5. Average tensile failure stress (a) and tensile stiffness (b) of CFRP laminates at different temperatures. Cumulative failure probability of laminates at different temperatures.

Dynamic mechanical analysis
Storage modulus
Storage modulus is a measure of how stiff or flimsy a sample is. It provides useful information about the degree of cross-linking and fibre/matrix interfacial strength.36–38 Figure 6 shows the Storage modulus versus temperature of CE control (a), 0°C (b), −20°C (c), −80°C (d) groups of laminates.
Loss modulus
Figure 7 plots Loss modulus with the temperature of CE (a), 0°C (b), −20°C (c), −80°C (d) groups of laminates.
The Tg values of the laminates were assessed. It is observed that Tg of CE laminate shifted to a higher temperature as the frequency increased. This shows that the epoxy molecules need a higher temperature to increase their mobility. In the case of 0, −20, and −80
Damping factor
Figure 8 shows the damping behavior of the laminates. Results show that the damping properties of the laminates increased slightly up to
Comparison of experimental results and prediction models
The storage modulus results of laminates are compared with existing empirical models. Bai and Keller
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summarized the available models used to assess the mechanical behavior of FRP composites at elevated temperatures. According to Gibson et al.,
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the modulus of FRP composites at a temperature
An empirical model proposed by Gu and Asaro
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computes the mechanical properties of the laminates as a function of temperature using degradation parameters. The equation is given by
Mahieux and Reifsnider
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predicted the degradation of stiffness of FRP as a function of temperature which has the effect of breaking, relaxing and increasing intermolecular bonds in the polymeric matrix. Their empirical model is given by Damping factor with temperature of CE (a), 0°C (b), −20°C (c), −80°C (d) groups of laminates.

Figure 9 shows the comparison between Comparison of measured 
Figure 10 shows the comparison between Comparison of measured 
Figure 11 shows a comparison of measured Comparison of measured 
Figure 12 illustrates the comparisons of Comparison of measured 
Proposed empirical model
Coefficients based on equation (2) for
Conclusions
The mechanical properties of CFRP laminates at different testing temperatures and frequencies were assessed using the DMA tool and a tensile testing machine. The following observations and conclusions could be drawn: 1. The compressive strength, tensile strength, and tensile modulus of laminates are not significantly affected below 25°C. This indicates that the epoxy chain segment motion was frozen and tightly compacted, and small elastic deformations occur at low temperatures. 2. At a temperature of 75°C, the compressive strength, tensile strength, and tensile modulus of laminates decrease by 66.85%, 19.20%, and 16.58%, respectively, compared to room temperature. When the test temperature increases to 100°C, the compressive strength, tensile strength, and tensile modulus of laminates decrease by 92.89%, 67.33%, and 65.99%, respectively, compared to room temperature. In this case, the load-carrying capacity of the carbon fibre is severely compromised. 3. The stiffness parameters of laminates decreased as the testing temperature increased. This happens due to the presence of higher mobilization of the epoxy matrix in the rubbery region. The highest damping property was observed on −80°C laminates. Based on this, CFRP material is noted to have good vibration capacity in moist conditions and is recommended for use in the spar cap sections. 4. Finally, the storage modulus results were compared with empirical models to reduce the material, test costs, and design time. The empirical model developed by Gibson et al.
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was found to be accurate in predicting the storage modulus of laminates in the temperature and frequency ranges implemented in the present study.
Supplemental Material
Supplemental Material - Mechanical behaviour of carbon fibre reinforced polymer composite material at different temperatures: Experimental and model assessment
Supplemental Material for Mechanical behaviour of carbon fibre reinforced polymer composite material at different temperatures: Experimental and model assessment by Getahun Tefera, Sarp Adali and Glen Bright in Polymers and Polymer Composites
Footnotes
Acknowledgements
The authors gratefully acknowledge the support provided by UKZN, NRF and DDU.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
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References
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