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Effect of hygrothermal aging on static behavior of quasi-isotropic CFRP composite laminate
Composites Communications 17 (2020) 51–55
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Short Communication
Effect of hygrothermal aging on static behavior of quasi-isotropic CFRP composite laminate Alok Behera *, Ashok Vishwakarma, M.M. Thawre, Atul Ballal Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology (VNIT), Nagpur, 440010, India
A R T I C L E I N F O
A B S T R A C T
Keywords: CFRP Quasi-isotropic Hygrothermal Static strength FESEM TGA
The Carbon fiber reinforced polymer (CFRP) composites used in offshore and marine industries tend to deal with high temperatures and humid conditions throughout its life. This work investigates the effect of long term hygrothermal aging on the physical and mechanical properties of CFRP composites. The IMA/M21 carbon fiber epoxy quasi-isotropic Multidirectional laminate with stacking sequence [�45, 0, 90]2S was fabricated using vacuum assisted resin transfer moulding. The static specimens were immersed in room temperature (RT) and comparatively high temperature (HT) tap water at 70 � C up to saturation. The kinetics of moisture absorption was monitored up to saturation as a function of time which followed a two-stage model. The static strength of virgin, RT aged and HT aged specimens were determined with a constant cross-head speed of 1 mm/min. The post-saturation static strength showed a major drop however reduction in compressive strength was higher than tensile strength in a matrix dominated failure. Similarly, the hygrothermal aged specimen showed higher moisture absorption and strength reduction as compared to RT aging. The SEM and FESEM micrographs clearly revealed matrix erosion and fiber-matrix interfacial debonding after saturation. The TGA analysis of the moisture absorbed specimen showed a degraded physical property.
1. Introduction In the 21st century, the Fiber Reinforced Polymer (FRP) composites have grown strength to strength commercially by replacing traditional material in many applications. Recently, the use of FRP has spread across industries like aerospace, civil, offshore, sports, automobile, and marine, etc. The Carbon (CFRP), Glass (GFRP) and Kevlar (KFRP) Fiber Reinforced Polymer composites have seen the highest increase in the application. However, apart from the aerospace industry, FRP is still to prove its applicability as the primary structural applications for diverse uses. The application of CFRP has seen the highest growth in the aero space industry due to its superior fatigue strength and higher specific strength. The durability of CFRP composites exposed to moisture, high temperature, rain or humid conditions in a long run needs to be addressed further to make it applicable for outdoor applications with limited monitoring or inspection. Numerous researchers have studied the physical and mechanical durability of different polymer matrix composites under hygrothermal aging [1]. Hygrothermal aging is referred to as accelerate aging which involves long term water immersion at high temperature up to
saturation. The moisture diffusivity in composite primarily depends on time, temperature, medium, matrix property, curing technique, and fiber orientation, etc. The CFRP composites exposed to external envi ronments deteriorate physically and mechanically with respect to time. The consequences of hygrothermal aging on mechanical properties like tensile [2], compressive [3], short shear [4], flexural [5], 3-point bending [6], low-velocity impact [7] had been studied extensively. Kafodya et al. reported a significant decrease in a matrix dominated short beam shear strength whereas the fiber dominated tensile strength showed negligible degradation [8]. Similarly, Botelho et al. reported a 21% and 18% reduction in interlaminar shear strength of [0/0]s and [0/90]s CFRP laminate respectively. The higher moisture absorption in UD was one of the primary reasons for the difference [9]. Similarly, Huo et al. investigated the three-dimensional moisture diffusivity of Car bon/bismaleimide UD and cross-ply laminate [6]. They found that 90 � C hygrothermal aging was more prone to strength degradation as compared to 50 � C. This dependency post hygrothermal aging me chanical property on orientation, temperature, moisture content, matrix type, and aging time needs to be addressed further. The hygrothermal aging temperature leads to degradation
* Corresponding author. E-mail address: [email protected] (A. Behera). https://doi.org/10.1016/j.coco.2019.11.009 Received 12 June 2019; Received in revised form 5 November 2019; Accepted 6 November 2019 Available online 13 November 2019 2452-2139/© 2019 Elsevier Ltd. All rights reserved.
A. Behera et al.
Composites Communications 17 (2020) 51–55
also notable variation in mechanical performance [5,6,20–25]. Hence, for the current study, a middling 70 � C hygrothermal aging temperature was selected. This work investigates the effect of long term hygrothermal and room temperature aging on the physical and mechanical properties of CFRP quasi-isotropic (QI) laminate composite. The static strength of room temperature (RT) aged and virgin specimens was compared with high temperature (HT) 70 � C water aged specimens to enlight the effect of hygrothermal aging on fiber, matrix, and fiber-matrix interface. The obtained results provide new insight into the effect of long term water absorption on the static response of QI CFRP laminate. Fig. 1. (a) Environmental chamber (B) Static test setup.
2. Material and methodology
mechanisms like plasticization, variation in glass transition tempera ture, chemical transformation, and reduction in residual strength of matrix [10–15]. The hygrothermal aging temperature must be decided carefully as the mechanical Performance of fiber, matrix and fiber-matrix interface degrades with higher temperature [16]. Bunsell et al. had reported degrading mechanical properties of epoxy matrix laminates above 70� C and hence the temperature must be chosen below 60 � C. However, this transition temperature where the matrix gets physically damaged depends on glass transition temperature of that particular matrix [17,18]. Gentry et al. in a very important finding had reported that the aging temperature can be decided as close as 20 � C less than the glass transition temperature of epoxy [19]. However, many researchers had employed an aging temperature in the range of 50–90 � C and had not only reported desired acceleration in moisture diffusion but
2.1. Composite laminate The CFRP quasi-isotropic multidirectional laminate with stacking sequence [�45, 0, 90]2S was fabricated using vacuum-assisted resin transfer moulding. The IMA carbon fiber/M21 epoxy prepreg had a thickness and width of 0.18 mm and 300 mm respectively. The prepreg was cut into 300 mm � 450 mm layers and the desired stacking sequence was crafted. The laminates were cured using the autoclaving process at 180 � C with a heating rate of 2 � C/minute. A 59% fiber volume was maintained during fabrication and the laminate was 3.1 mm thick. 2.2. Hygrothermal aging The QI laminate specimens and coupons were fully immersed in two
Fig. 2. Specimen geometry for static testing.
Fig. 3. Water absorption vs. Time graph for QI CFRP (a) HT (b) RT. 52
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Composites Communications 17 (2020) 51–55
periods. The water absorption rate (WA) was measured using Equation (1). WA ¼
Wt Wi � 100 % Wi
(1)
where Wt is the weight of coupon after particular aging time “t” and Wi is the initial weight of coupon before water immersion. The mechanical testing was performed immediately after the water absorbed specimens reached their saturation level. 2.3. Mechanical characterization The static test for virgin, RT aged and HT aged specimens were carried out using Instron 8802 Universal Testing Machine (Capacity: 250KN) as shown in Fig. 1 (b). At least three replicate specimens were tested at a constant crosshead speed of 1 mm/min. The detailed tensile and compression specimen geometry is shown in Fig. 2 as per ASTM D3039 and ASTM D3410 standards, respectively. Glass fiber tabs were joined in both the ends to avoid slippage and stress intensity factor. The RT and HT water absorbed specimens were tested after it reached a saturation point. The failed specimen were analyzed using Scanning Electron Microscopy (SEM) [Make: Joel, Model: 6380] to find the mechanism behind the failure. An Auto platinum sputter [Make: Ins tron] was used to make the failure surface conducting.
Fig. 4. Static response of QI CFRP Virgin, HT and RT specimens.
beakers containing tap water. Among which, One was kept in the lab oratory room temperature (RT) condition and the other was held inside an Instron environment chamber at a constant high temperature (HT) of 70 � C up to saturation as shown in Fig. 1 (a). The glass transition tem perature (Tg) of employed M21 epoxy is 195 � C as per the manufacturer datasheet which is well above the experimental aging temperature [26]. The edge of the specimens and coupons were not sealed or coated to reduce water ingress. Three coupons were used for measuring the water absorption rate and the average result of the same are reported further. The effect of the aforementioned hygrothermal aging was compared with RT water aging for an identical duration. The year-round RT aging includes the atmospheric temperature in the range of 15–45 � C which can be realistic for any practical application. The coupons were withdrawn from water at a regular interval to measure the mass change. They were wiped using tissue paper before weighing it with the accuracy of 0.1 mg. Initially, the change in weight was measured with a few hours gap. The interval of measurement was increased with the passage of time and it was almost 15 days during final
3. Result and discussions 3.1. Water absorption effects The increase in water absorption percentage as a function of square root time for QI CFRP laminate is shown in Fig. 3. The average trend of three coupons was calculated using Equation (1) which showed an almost similar increase in weight. The HT aged coupons took more than a year to get saturated and the average saturated water absorption was 1.61% as shown in Fig. 3 (a). The water absorption was rapid up to initial 1.3% which comprises 1/4th of the total time. Subsequently, the moisture diffusion became almost stable and a mere 0.3% increase in WA took 3/4th of total time. This change in the water absorption trend is
Fig. 5. Scanning electron micrographs of untested (a) virgin (b) RT aged specimen, Fiber breakage in (c) virgin (d) RT aged specimen. 53
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Composites Communications 17 (2020) 51–55
Fig. 6. Scanning electron micrographs of HT moisture aged CFRP showing (a)Scaling (b) Delamination (c)Fiber breakage (d)Matrix cracking (e) Fibermatrix debonding.
denoted as a two-stage model [8]. This is different from Fick’s law which very frequently governs the moisture diffusion trend in pure polymers. However, the moisture diffusivity mechanism in QI CFRP is different due to the presence of fibers and fiber-matrix interface along with the polymer. The RT aged specimen moisture-absorbing capacity was 25% lower (1.2%) as compared to the HT specimen as shown in Fig. 3 (b) which proves the fact that higher temperature leads to an increase in moisture diffusivity.
increase in post aging tensile strength [23]. However, similar to the percentage of water absorption, RT specimens showed lower strength reduction as compared to HT specimens. This confirms a declining tendency of strength and the increasing possibility of matrix plastici zation with respect to raising in temperature [20,21]. The RT specimens with a year-round aging temperature of 15–45 � C showed very minor strength degradation as compared to HT specimens under tensile or compression load. A matrix dominated failure in compression tested HT specimens resulted in a sudden brittle failure due to increased plasticity. This reduced the post-aging compression strength in the HT specimens. The post HT aged FTIR analysis showed remarkable changes like omitted CH2 bending vibration and replacement of -CH2 group with hydroxyl (-OH) as a result of water. This reflected matrix degradation and increased plasticity due to long-run high-temperature aging [23,24]. Very similar FTIR spectra were also observed for RT specimens. Simi larly, the TGA analysis of HT and RT specimens reflected a 3% and 2% decline in glass transition temperature respectively as compared to virgin composite. The glass transition temperature plays a vital role in composite property and any decrease in it reflects property change of matrix and fiber-matrix interface.
3.2. Static response of virgin and water absorbed specimens The tensile and compressive test results of the virgin, RT and HT aged samples are presented in Fig. 4 which revealed a reduction in post aging strength. The high-temperature aging increased the plasticity of the matrix which affects the fiber-matrix interfacial bonding of composites. The tensile strength always depends on fiber properties whereas compressive strength depends on matrix properties. The degradation in fiber-matrix bonding and plasticization of matrix resulted in inappro priate load transfer. This results in a decrease in strength post-HT aging [8]. Surprisingly, few researchers had reported that the presence of fi bers aligned in loading direction, plasticization of matrix and removal of residual stresses generated during the curing process resulted in an 54
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Composites Communications 17 (2020) 51–55
3.3. Fracture analysis [4]
The FESEM micrographs of untested virgin CFRP and RT aged sur face revealed precise bonding between fiber/matrix and the interface between two layers as shown in Fig. 5 (a) and (b), respectively. There were hardly any pre-existing matrix cracking or voids on the virgin surface. Similarly, apart from scaling and very minor matrix cracks, there was hardly any notable defect observed in RT aged specimen as shown in Fig. 5 (b). Fiber failure was dominant in the failure of virgin and RT aged specimens as shown in Fig. 5 (c) and (d) respectively. The post-failure SEM micrographs of HT aged QI CFRP is shown in Fig. 6 (a–d). A white colored scaling was spotted on the post-aging surface as shown in Fig. 6 (a). This scaling was one of the reasons for the plastic behaviour of the matrix. The delamination and fiber breakage as shown in Fig. 6 (b–c)was the dominated mode of failure under tensile loading in both virgin and aged specimens. Similarly, the matrix cracking and fiber-matrix interfacial debonding were dominated mode of failure in compressive specimens as shown in Fig. 6 (d–e) compres sive. TheThe fiber-matrix interfacial debonding was clearly visible in the FESEM micrograph at 4500x magnification. These debonding not only reduced the compressive strength but also act as a void for moisture storage.
[5] [6]
[7]
[8]
[9] [10] [11]
4. Conclusion
[12]
In this work, a detailed analysis of the consequences of long term hygrothermal aging on mechanical properties of CFRP quasi-isotropic laminate composites was carried out. The laminates kept at room tem perature and 70 � C water were saturated at 1.20% and 1.61% respec tively and both followed a typical two-stage model. The hygrothermal aging decreased the tensile strength due to degradation in fiber-matrix bonding, a decrease in glass transition temperature and plasticization of the matrix. The matrix dominated compressive strength reduced due to the plasticization of matrix and fiber-matrix debonding. However, the magnitude of these factors was negligible for room temperature aged specimens reflecting the high-temperature effect. Much more detailed studied on high-temperature long-run aging with different lay-ups, temperature and loading conditions can increase the applicability of CFRP composite in various industries that are prone to environmental damages.
[13] [14] [15] [16]
[17] [18] [19]
Declaration of competing interest
[20]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[21]
Acknowledgement
[22]
Authous will like to thank Director, VNIT, Nagpur and HOD, Department of Metallurgical and Materials Engineering, VNIT, Nagpur for the constant support during this research.
[23] [24]
References
[25]
[1] A. Katunin, A. Gnatowski, W. Kajzer, Evolution of static and dynamic properties of GFRP laminates during ageing in deionized and seawater, Adv. Compos. Lett. 24 (2015) 47–52. [2] LV da Silva, FW da Silva, J.R. Tarpani, MM. de C. Forte, S.C. Amico, Ageing effect on the tensile behavior of pultruded CFRP rods, Mater. Des. 110 (2016) 245–254, https://doi.org/10.1016/j.matdes.2016.07.139. [3] J.P. Johnston, K.C. Liu, M. Yekani Fard, A. Chattopadhyay, Mechanical properties and damage characterization of triaxial braided composites in environmental
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Yungang, Thermal ageing effects on mechanical properties and barely visible impact damage behavior of a carbon fiber reinforced bismaleimide composite, Mater. Des. 115 (2017) 213–223, https://doi.org/ 10.1016/j.matdes.2016.11.062. I. Kafodya, G. Xian, H. Li, Durability study of pultruded CFRP plates immersed in water and seawater under sustained bending: water uptake and effects on the mechanical properties, Compos Part B 70 (2015) 138–148, https://doi.org/ 10.1016/j.compositesb.2014.10.034. E.C. Botelho, L.C. Pardini, M.C. Rezende, Hygrothermal effects on the shear properties of carbon fiber/epoxy composites, J. Mater. Sci. 41 (2006) 7111–7118, https://doi.org/10.1007/s10853-006-0933-7. S.W. Wen, J.Y. Xiao, Y.R. Wang, Accelerated ageing behaviors of aluminum plate with composite patches under salt fog effect, Compos Part B 44 (2013) 266–273. Z.Y. Lu, G.J. Xian, H. Li, Effects of exposure to elevated temperatures and subsequent immersion in water or alkaline solution on the mechanical properties of pultruded BFRP plates, Compos Part B 77 (2015) 421–430. A. Abbasi, P.J. Hogg, Temperature and environmental effects on glass fibre rebar: modulus, strength and interfacial bond strength with concrete, Compos Part B 36 (2005) 394–404. K. Liao, C.R. Schultheisz, D.L. Hunston, Effects of environmental aging on the properties of pultruded GFRP, Compos Part B 30 (1999) 485–493. M. Beringhier, M. Gigliotti, A novel methodology for the rapid identification of the water diffusion coefficients of composite materials, Compos Part A 68 (2015) 212–218. D. Perreux, C. Suri, A study of the coupling between the phenomena of water absorption and damage in glass/epoxy composite pipes, Compos. Sci. Technol. 57 (1997) 1403–1413. B. Yang, J. Zhang, L. Zhou, M. Lu, W. Liang, Z. Wang, Effect of fiber surface modification on water absorption and hydrothermal aging behaviors of GF/pCBT composites, Compos. B Eng. 82 (2015) 84–91, https://doi.org/10.1016/j. compositesb.2015.08.056. A.R. Bunsell, Hygrothermal ageing of composite materials, in: chez Proc. Composite Materials in Petroeum Industry, Les Rencontres scientifiques de l’IFP, Recueil-Malmaison, 1994. O.K. Joshi, The effect of moisture on the shear properties of carbon fiber composites, Composites 14 (1983) 196–200, https://doi.org/10.1016/0010-4361 (83)90002-2. T. Gentry, L. Bank, A. Barkatt, L. Prian, Accelerated test methods to determine the long-term behavior of composite highway structures subject to environmental loading, J. Compos. Technol. Res. 20 (1998) 38–50, https://doi.org/10.1520/ CTR10499J. B. Yang, J. Zhang, L. Zhou, Z. Wang, W. Liang, Effect of fiber surface modification on the lifetime of glass fiber reinforced polymerized cyclic butylene terephthalate composites in hygrothermal conditions, Mater. Des. 85 (2015) 14–23, https://doi. org/10.1016/j.matdes.2015.07.010. B. Yang, J. Zhang, L. Zhou, Z. Wang, W. Liang, Effect of fiber surface modification on the lifetime of glass fiber reinforced polymerized cyclic butylene terephthalate composites in hygrothermal conditions, Mater. Des. 85 (2015) 14–23, https://doi. org/10.1016/j.matdes.2015.07.010. S. Alessi, G. Pitarresi, G. Spadaro, Effect of hydrothermal ageing on the thermal and delamination fracture behaviour of CFRP composites, Compos. B Eng. 67 (2014) 145–153, https://doi.org/10.1016/j.compositesb.2014.06.006. B. Hong, G. Xian, Ageing of a thermosetting polyurethane and its pultruded carbon fiber plates subjected to seawater immersion, Constr. Build. Mater. 165 (2018) 514–522, https://doi.org/10.1016/j.conbuildmat.2018.01.042. Y M, E.L. Mahi, A. 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Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Short Communication
Effect of hygrothermal aging on static behavior of quasi-isotropic CFRP composite laminate Alok Behera *, Ashok Vishwakarma, M.M. Thawre, Atul Ballal Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology (VNIT), Nagpur, 440010, India
A R T I C L E I N F O
A B S T R A C T
Keywords: CFRP Quasi-isotropic Hygrothermal Static strength FESEM TGA
The Carbon fiber reinforced polymer (CFRP) composites used in offshore and marine industries tend to deal with high temperatures and humid conditions throughout its life. This work investigates the effect of long term hygrothermal aging on the physical and mechanical properties of CFRP composites. The IMA/M21 carbon fiber epoxy quasi-isotropic Multidirectional laminate with stacking sequence [�45, 0, 90]2S was fabricated using vacuum assisted resin transfer moulding. The static specimens were immersed in room temperature (RT) and comparatively high temperature (HT) tap water at 70 � C up to saturation. The kinetics of moisture absorption was monitored up to saturation as a function of time which followed a two-stage model. The static strength of virgin, RT aged and HT aged specimens were determined with a constant cross-head speed of 1 mm/min. The post-saturation static strength showed a major drop however reduction in compressive strength was higher than tensile strength in a matrix dominated failure. Similarly, the hygrothermal aged specimen showed higher moisture absorption and strength reduction as compared to RT aging. The SEM and FESEM micrographs clearly revealed matrix erosion and fiber-matrix interfacial debonding after saturation. The TGA analysis of the moisture absorbed specimen showed a degraded physical property.
1. Introduction In the 21st century, the Fiber Reinforced Polymer (FRP) composites have grown strength to strength commercially by replacing traditional material in many applications. Recently, the use of FRP has spread across industries like aerospace, civil, offshore, sports, automobile, and marine, etc. The Carbon (CFRP), Glass (GFRP) and Kevlar (KFRP) Fiber Reinforced Polymer composites have seen the highest increase in the application. However, apart from the aerospace industry, FRP is still to prove its applicability as the primary structural applications for diverse uses. The application of CFRP has seen the highest growth in the aero space industry due to its superior fatigue strength and higher specific strength. The durability of CFRP composites exposed to moisture, high temperature, rain or humid conditions in a long run needs to be addressed further to make it applicable for outdoor applications with limited monitoring or inspection. Numerous researchers have studied the physical and mechanical durability of different polymer matrix composites under hygrothermal aging [1]. Hygrothermal aging is referred to as accelerate aging which involves long term water immersion at high temperature up to
saturation. The moisture diffusivity in composite primarily depends on time, temperature, medium, matrix property, curing technique, and fiber orientation, etc. The CFRP composites exposed to external envi ronments deteriorate physically and mechanically with respect to time. The consequences of hygrothermal aging on mechanical properties like tensile [2], compressive [3], short shear [4], flexural [5], 3-point bending [6], low-velocity impact [7] had been studied extensively. Kafodya et al. reported a significant decrease in a matrix dominated short beam shear strength whereas the fiber dominated tensile strength showed negligible degradation [8]. Similarly, Botelho et al. reported a 21% and 18% reduction in interlaminar shear strength of [0/0]s and [0/90]s CFRP laminate respectively. The higher moisture absorption in UD was one of the primary reasons for the difference [9]. Similarly, Huo et al. investigated the three-dimensional moisture diffusivity of Car bon/bismaleimide UD and cross-ply laminate [6]. They found that 90 � C hygrothermal aging was more prone to strength degradation as compared to 50 � C. This dependency post hygrothermal aging me chanical property on orientation, temperature, moisture content, matrix type, and aging time needs to be addressed further. The hygrothermal aging temperature leads to degradation
* Corresponding author. E-mail address: [email protected] (A. Behera). https://doi.org/10.1016/j.coco.2019.11.009 Received 12 June 2019; Received in revised form 5 November 2019; Accepted 6 November 2019 Available online 13 November 2019 2452-2139/© 2019 Elsevier Ltd. All rights reserved.
A. Behera et al.
Composites Communications 17 (2020) 51–55
also notable variation in mechanical performance [5,6,20–25]. Hence, for the current study, a middling 70 � C hygrothermal aging temperature was selected. This work investigates the effect of long term hygrothermal and room temperature aging on the physical and mechanical properties of CFRP quasi-isotropic (QI) laminate composite. The static strength of room temperature (RT) aged and virgin specimens was compared with high temperature (HT) 70 � C water aged specimens to enlight the effect of hygrothermal aging on fiber, matrix, and fiber-matrix interface. The obtained results provide new insight into the effect of long term water absorption on the static response of QI CFRP laminate. Fig. 1. (a) Environmental chamber (B) Static test setup.
2. Material and methodology
mechanisms like plasticization, variation in glass transition tempera ture, chemical transformation, and reduction in residual strength of matrix [10–15]. The hygrothermal aging temperature must be decided carefully as the mechanical Performance of fiber, matrix and fiber-matrix interface degrades with higher temperature [16]. Bunsell et al. had reported degrading mechanical properties of epoxy matrix laminates above 70� C and hence the temperature must be chosen below 60 � C. However, this transition temperature where the matrix gets physically damaged depends on glass transition temperature of that particular matrix [17,18]. Gentry et al. in a very important finding had reported that the aging temperature can be decided as close as 20 � C less than the glass transition temperature of epoxy [19]. However, many researchers had employed an aging temperature in the range of 50–90 � C and had not only reported desired acceleration in moisture diffusion but
2.1. Composite laminate The CFRP quasi-isotropic multidirectional laminate with stacking sequence [�45, 0, 90]2S was fabricated using vacuum-assisted resin transfer moulding. The IMA carbon fiber/M21 epoxy prepreg had a thickness and width of 0.18 mm and 300 mm respectively. The prepreg was cut into 300 mm � 450 mm layers and the desired stacking sequence was crafted. The laminates were cured using the autoclaving process at 180 � C with a heating rate of 2 � C/minute. A 59% fiber volume was maintained during fabrication and the laminate was 3.1 mm thick. 2.2. Hygrothermal aging The QI laminate specimens and coupons were fully immersed in two
Fig. 2. Specimen geometry for static testing.
Fig. 3. Water absorption vs. Time graph for QI CFRP (a) HT (b) RT. 52
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Composites Communications 17 (2020) 51–55
periods. The water absorption rate (WA) was measured using Equation (1). WA ¼
Wt Wi � 100 % Wi
(1)
where Wt is the weight of coupon after particular aging time “t” and Wi is the initial weight of coupon before water immersion. The mechanical testing was performed immediately after the water absorbed specimens reached their saturation level. 2.3. Mechanical characterization The static test for virgin, RT aged and HT aged specimens were carried out using Instron 8802 Universal Testing Machine (Capacity: 250KN) as shown in Fig. 1 (b). At least three replicate specimens were tested at a constant crosshead speed of 1 mm/min. The detailed tensile and compression specimen geometry is shown in Fig. 2 as per ASTM D3039 and ASTM D3410 standards, respectively. Glass fiber tabs were joined in both the ends to avoid slippage and stress intensity factor. The RT and HT water absorbed specimens were tested after it reached a saturation point. The failed specimen were analyzed using Scanning Electron Microscopy (SEM) [Make: Joel, Model: 6380] to find the mechanism behind the failure. An Auto platinum sputter [Make: Ins tron] was used to make the failure surface conducting.
Fig. 4. Static response of QI CFRP Virgin, HT and RT specimens.
beakers containing tap water. Among which, One was kept in the lab oratory room temperature (RT) condition and the other was held inside an Instron environment chamber at a constant high temperature (HT) of 70 � C up to saturation as shown in Fig. 1 (a). The glass transition tem perature (Tg) of employed M21 epoxy is 195 � C as per the manufacturer datasheet which is well above the experimental aging temperature [26]. The edge of the specimens and coupons were not sealed or coated to reduce water ingress. Three coupons were used for measuring the water absorption rate and the average result of the same are reported further. The effect of the aforementioned hygrothermal aging was compared with RT water aging for an identical duration. The year-round RT aging includes the atmospheric temperature in the range of 15–45 � C which can be realistic for any practical application. The coupons were withdrawn from water at a regular interval to measure the mass change. They were wiped using tissue paper before weighing it with the accuracy of 0.1 mg. Initially, the change in weight was measured with a few hours gap. The interval of measurement was increased with the passage of time and it was almost 15 days during final
3. Result and discussions 3.1. Water absorption effects The increase in water absorption percentage as a function of square root time for QI CFRP laminate is shown in Fig. 3. The average trend of three coupons was calculated using Equation (1) which showed an almost similar increase in weight. The HT aged coupons took more than a year to get saturated and the average saturated water absorption was 1.61% as shown in Fig. 3 (a). The water absorption was rapid up to initial 1.3% which comprises 1/4th of the total time. Subsequently, the moisture diffusion became almost stable and a mere 0.3% increase in WA took 3/4th of total time. This change in the water absorption trend is
Fig. 5. Scanning electron micrographs of untested (a) virgin (b) RT aged specimen, Fiber breakage in (c) virgin (d) RT aged specimen. 53
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Composites Communications 17 (2020) 51–55
Fig. 6. Scanning electron micrographs of HT moisture aged CFRP showing (a)Scaling (b) Delamination (c)Fiber breakage (d)Matrix cracking (e) Fibermatrix debonding.
denoted as a two-stage model [8]. This is different from Fick’s law which very frequently governs the moisture diffusion trend in pure polymers. However, the moisture diffusivity mechanism in QI CFRP is different due to the presence of fibers and fiber-matrix interface along with the polymer. The RT aged specimen moisture-absorbing capacity was 25% lower (1.2%) as compared to the HT specimen as shown in Fig. 3 (b) which proves the fact that higher temperature leads to an increase in moisture diffusivity.
increase in post aging tensile strength [23]. However, similar to the percentage of water absorption, RT specimens showed lower strength reduction as compared to HT specimens. This confirms a declining tendency of strength and the increasing possibility of matrix plastici zation with respect to raising in temperature [20,21]. The RT specimens with a year-round aging temperature of 15–45 � C showed very minor strength degradation as compared to HT specimens under tensile or compression load. A matrix dominated failure in compression tested HT specimens resulted in a sudden brittle failure due to increased plasticity. This reduced the post-aging compression strength in the HT specimens. The post HT aged FTIR analysis showed remarkable changes like omitted CH2 bending vibration and replacement of -CH2 group with hydroxyl (-OH) as a result of water. This reflected matrix degradation and increased plasticity due to long-run high-temperature aging [23,24]. Very similar FTIR spectra were also observed for RT specimens. Simi larly, the TGA analysis of HT and RT specimens reflected a 3% and 2% decline in glass transition temperature respectively as compared to virgin composite. The glass transition temperature plays a vital role in composite property and any decrease in it reflects property change of matrix and fiber-matrix interface.
3.2. Static response of virgin and water absorbed specimens The tensile and compressive test results of the virgin, RT and HT aged samples are presented in Fig. 4 which revealed a reduction in post aging strength. The high-temperature aging increased the plasticity of the matrix which affects the fiber-matrix interfacial bonding of composites. The tensile strength always depends on fiber properties whereas compressive strength depends on matrix properties. The degradation in fiber-matrix bonding and plasticization of matrix resulted in inappro priate load transfer. This results in a decrease in strength post-HT aging [8]. Surprisingly, few researchers had reported that the presence of fi bers aligned in loading direction, plasticization of matrix and removal of residual stresses generated during the curing process resulted in an 54
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Composites Communications 17 (2020) 51–55
3.3. Fracture analysis [4]
The FESEM micrographs of untested virgin CFRP and RT aged sur face revealed precise bonding between fiber/matrix and the interface between two layers as shown in Fig. 5 (a) and (b), respectively. There were hardly any pre-existing matrix cracking or voids on the virgin surface. Similarly, apart from scaling and very minor matrix cracks, there was hardly any notable defect observed in RT aged specimen as shown in Fig. 5 (b). Fiber failure was dominant in the failure of virgin and RT aged specimens as shown in Fig. 5 (c) and (d) respectively. The post-failure SEM micrographs of HT aged QI CFRP is shown in Fig. 6 (a–d). A white colored scaling was spotted on the post-aging surface as shown in Fig. 6 (a). This scaling was one of the reasons for the plastic behaviour of the matrix. The delamination and fiber breakage as shown in Fig. 6 (b–c)was the dominated mode of failure under tensile loading in both virgin and aged specimens. Similarly, the matrix cracking and fiber-matrix interfacial debonding were dominated mode of failure in compressive specimens as shown in Fig. 6 (d–e) compres sive. TheThe fiber-matrix interfacial debonding was clearly visible in the FESEM micrograph at 4500x magnification. These debonding not only reduced the compressive strength but also act as a void for moisture storage.
[5] [6]
[7]
[8]
[9] [10] [11]
4. Conclusion
[12]
In this work, a detailed analysis of the consequences of long term hygrothermal aging on mechanical properties of CFRP quasi-isotropic laminate composites was carried out. The laminates kept at room tem perature and 70 � C water were saturated at 1.20% and 1.61% respec tively and both followed a typical two-stage model. The hygrothermal aging decreased the tensile strength due to degradation in fiber-matrix bonding, a decrease in glass transition temperature and plasticization of the matrix. The matrix dominated compressive strength reduced due to the plasticization of matrix and fiber-matrix debonding. However, the magnitude of these factors was negligible for room temperature aged specimens reflecting the high-temperature effect. Much more detailed studied on high-temperature long-run aging with different lay-ups, temperature and loading conditions can increase the applicability of CFRP composite in various industries that are prone to environmental damages.
[13] [14] [15] [16]
[17] [18] [19]
Declaration of competing interest
[20]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[21]
Acknowledgement
[22]
Authous will like to thank Director, VNIT, Nagpur and HOD, Department of Metallurgical and Materials Engineering, VNIT, Nagpur for the constant support during this research.
[23] [24]
References
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