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In situ tensile strength degradation of glass fiber in polymer composite
Scripta mater. 44 (2001) 785–789 www.elsevier.com/locate/scriptamat
IN SITU TENSILE STRENGTH DEGRADATION OF GLASS FIBER IN POLYMER COMPOSITE Kin Liao and Elaine Y.M. Tan School of Materials Engineering, Nanyang Technological University, Singapore 639798 (Received May 11, 2000) (Accepted in revised form October 10, 2000) Keywords: Aging; Fiber; Composites; Tensile strength Introduction With the current rapid growth of using fiber-reinforced polymeric composite materials for infrastructure applications, there is an increasing demand for long term durability data [1]. Aging of fiber reinforced composites in corrosive environments, especially for glass fiber composites, results in irreversible changes of its constituents (fiber, matrix, and the fiber/matrix interphase region), which lead to strength degradation, even under no externally applied load. Although changes in the constituents of a composite during environmental aging may all contribute to strength and property degradation, contribution from fiber degradation alone could constitute a significant part for glass fiber composites because of the susceptibility of glass fibers to stress corrosion. Despite the fact that the mechanism of stress corrosion of glass and glass fibers has been established [2– 4], and numerous studies on property retention/degradation of glass fiber reinforced composites under environmental aging have been carried out during the past several decades [1], direct evidence of in situ glass fiber strength degradation in the composite as a result of environmental loading is scare [5,6]. In a previous study [6], it was shown that the in situ tensile strength of glass fibers in a pultruded composite was reduced as a result of environmental aging in water at room and elevated temperatures. To further confirm the finding, a study on the in situ strength degradation of E-glass fibers in an epoxy matrix model composite undergo environmental aging was carried out. Experimental evidence of in situ tensile strength degradation of glass fibers in the composite is presented in this paper. It is well known that the tensile fracture surfaces of many ceramic and polymeric materials exhibit a distinct mirror-mist-hackle pattern, shown in Fig. 1. For most fracture surfaces with such a pattern, the mirror region containing the crack-initiating surface flaw is usually located around the circumference of the fiber while fracture mirror located inside the fiber can only be found occasionally, indicating that the strength of these glass fibers is essentially surface-flaw controlled. It has been well established that the tensile strength of glass and ceramic materials is related to the fracture mirror size through the following semi-empirical relation r 1/ 2 u ⫽ A where r is the radius of the fracture mirror, u the tensile strength (fracture stress), and A a material constant. Since u is proportional to r⫺1/2, the tensile strength of a fiber can be estimated by measuring fracture mirror size, if the material constant A is known. The constant A can be easily obtained from a set of calibration data. An estimation of the constant A for E-glass fiber is 1.47 MN/m3/2 [5]. It should 1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00661-8
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Figure 1. SEM image of fiber fracture surface showing typical surface features: mirror, mist, and hackle pattern.
be mentioned that not all the fractured fiber surfaces exhibit a distinctive mirror-mist-hackle pattern. Some fibers displayed a rough fracture surface without a recognizable pattern while others displayed a smooth fracture surface, thought to be locations with low strength [7]. Low-stress fractures are not taken into consideration in this study. Although by excluding low stress fractures the fiber strength distribution may not represent the actual distribution, it nevertheless represents an upper limit and therefore a comparison at different aging times is still viable in revealing the effects of environmental aging on fiber strength. Experimental Model composite panels of unidirectional E-glass fiber reinforced epoxy were fabricated using a hand laying method. Small fiber bundles each containing approximately 50 – 60 individual fiber were obtained by carefully splitting a larger bundle of E-glass fiber roving. These smaller glass fiber bundles were wrapped in parallel around a 15 cm ⫻ 15 cm Perplex glass plate with double sided tapes placed at the ends which fixes the fiber bundles in place. 2-mm thick metallic spacers were placed along the sides of the Perplex glass plate to ensure that the end product is a plate with uniform thickness. Some tension was applied to the fiber bundles while they were being wrapped. De-gassed epoxy resin was poured on to one side of the Perplex glass plate with banks placed along its 4 sides. The epoxy resin and the fiber were kept under a light pressure via a top plate while the epoxy was being cured. The process was repeated for the other side of the Perplex glass plate after the first side was being cured. A room temperature curing epoxy is chosen so that thermal residual stress resulting from curing is kept at minimal. Small specimens with nominal dimensions of 35mm ⫻ 5mm ⫻ 2mm were then obtained from the panel. Each of these specimens contains 3 or 4 fiber bundles. Small notch of about 1mm deep was put in the middle of each specimen to ensure that the model composite breaks in the middle for proper examination. These notched specimens were aged in water at 25° C for 500 h and 4320 h, and subsequently tested to failure under tension. The masses of three samples were measured at regular intervals to monitor moisture absorption. Three samples were tested after a same aging time. The tensile strength of glass fibers is estimated by measuring the size of the fracture mirror of the failed fibers using a scanning
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Figure 2. Sorption curve of model composite in water at 25° C.
electronic microscope (SEM). At least 50 measurements were made randomly from samples aged for same period of time. A total of 160 individual fibers were examined. Results and Discussion Sorption behavior of the model composite is shown in Fig. 2, where % mass change is plotted against square root of immersion time. The sorption curve exhibits a Fickian type behavior, with moisture saturation level at about 2.0% after 500 h. At 4320 h the mass change has dropped to about 1.9%, possibly because of dissolution of the matrix material. Results of in situ fiber strength measurements for as-fabricated samples, and water-aged samples are shown in Fig. 3, where the failure probability, Pf, determined using the median rank method, is plotted against r⫺1/2. Since we are only interested in the trend of environmental strength reduction, r⫺1/2 is used instead of u. Noticeable difference between aged samples and as-fabricated samples is seen from Fig. 3. As expected, the effect of environmental aging is to shift the accumulative failure probability curves to larger mirror sizes, indicating a reduction in tensile strength. The three accumulative failure probability curves are separated from each other for the most part, with some overlapping of the 500 h data and the 4320 h data at the high strength end. A time effect is also obvious from Fig. 3, that is, the longer the aging time, the lower the fiber tensile strength. Results of zero stress aging of glass fibers in water from previous studies suggest that strength degradation is negligible at room temperature in the neighborhood of 107 s [8,9]. Hasløv et al showed
Figure 3. Failure probability curves of residual tensile strength of glass fibers in the model composite.
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that no strength degradation has occurred in acrylate coated optical fibers before 300 days of stress-free aging in water at 65° C [8]. Comparing with previous studies, fiber strength degradation is obvious after 500 h (1.8 ⫻ 106 s) of immersion, which has occurred earlier than zero-stress aging of fibers reported in previous studies [8,9]. This discrepancy suggests that the accelerated rate of in situ fiber strength degradation could have arisen from moisture-induced tensile stress because of moisture absorption and subsequent stress corrosion. It has been shown, using a concentric cylinder model of fiber surrounded by a polymeric matrix that significant amount of tensile stress can be introduced to the fiber at a relatively low moisture level if the coefficient of moisture expansion of the matrix is high [10]. For epoxy, the coefficient of moisture expansion is about 8.0 ⫻ 10⫺3, a value that will result in significant tensile stress in the fibers even at low moisture level. Thus, it is both the moisture-induced stress and the corrosive action of the water that are responsible for the reduction of tensile strength of glass fibers in the model composite under stress free aging. It is seen from Fig. 3 that the failure probability curve of the 500 h data are closer to that of the 4320 h data than to that of the as-fabricated data, implying that more tensile strength reduction has occurred in the first 500 h of aging, and the reduction rate seems to be slowed down at longer times. It has been shown that radial (tensile) stress at the fiber/matrix interface can also be significant at high moisture levels, thus promoting fiber/matrix debonding [10]. It is believed that the fiber/matrix interphase was damaged or destroyed after moisture ingress, causing moisture-induced tensile stress in the fibers to be relaxed, and thus resulted in a reduction of degradation rate. This is further supported by the fact that pullout length of glass fibers from the epoxy matrix after aging for 4320 h is much longer than those from the 500 h samples. The data trend shown in Fig. 3 is very similar to what was obtained from a previous study on pultruded composite [6], in which tensile strength of the composite was found to be degraded after stress-free aging in water. Thus it can be deduced that strength degradation of the composite could be attributed, at least in part, to in situ fiber strength degradation and the moisture-induced internal stress is playing a significant role. Summary Using fracture mirror measurements, it is shown that in situ tensile strength degradation of E-glass fiber in an epoxy matrix model composite has taken place when aged in water at 25° C after 500 h. Compared to data of stress-free aging of E-glass fibers, faster in situ strength degradation of glass fiber is seen in the model composite, suggesting that both the moisture-induced tensile stress and the corrosive action of water play an important role in strength degradation of glass fiber composites during environmental aging. Acknowledgments The author wishes to thank E. Chow, R. Reyes, and J. Loke for their help in the laboratory through the TER program. References 1. 2. 3. 4.
K. Liao, C. R. Schultheisz, D. L. Hunston, and L. C. Brinson, SAMPE J. Adv. Mater. 30(4), 3 (1998). R. J. Charles, J. Appl. Phys. 29(11), 1549 (1958). A. G. Metcalfe and G. K. Schmitz, Glass Technol. 13(1), 5 (1972). S. M. Wiederhorn and L. H. Bolz, J. Am. Ceram. Soc. 53(10), 543 (1970).
Vol. 44, No. 5
5. 6. 7. 8. 9. 10.
IN SITU DEGRADATION OF GLASS FIBER
A. C. Jaras, B. J. Norman, and S. C. Simmens, J. Mater. Sci. 18, 2459 (1983). K. Liao, C. R. Schultheisz, and D. L. Hunston, Composites Part B. 30, 485 (1999). A. J. Eckel and R. C. Bradt, J. Am. Ceram. Soc. 72(3), 455 (1989). P. Hasløv, K. B. Jensen, and N. H. Skovgaard, J. Am. Ceram. Soc. 77(6), 1531 (1994). J-W. Leclercq and A. H. E. Breuls, SPIE 2290, 64 (94). K. Liao, Composites Part B, submitted.
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IN SITU TENSILE STRENGTH DEGRADATION OF GLASS FIBER IN POLYMER COMPOSITE Kin Liao and Elaine Y.M. Tan School of Materials Engineering, Nanyang Technological University, Singapore 639798 (Received May 11, 2000) (Accepted in revised form October 10, 2000) Keywords: Aging; Fiber; Composites; Tensile strength Introduction With the current rapid growth of using fiber-reinforced polymeric composite materials for infrastructure applications, there is an increasing demand for long term durability data [1]. Aging of fiber reinforced composites in corrosive environments, especially for glass fiber composites, results in irreversible changes of its constituents (fiber, matrix, and the fiber/matrix interphase region), which lead to strength degradation, even under no externally applied load. Although changes in the constituents of a composite during environmental aging may all contribute to strength and property degradation, contribution from fiber degradation alone could constitute a significant part for glass fiber composites because of the susceptibility of glass fibers to stress corrosion. Despite the fact that the mechanism of stress corrosion of glass and glass fibers has been established [2– 4], and numerous studies on property retention/degradation of glass fiber reinforced composites under environmental aging have been carried out during the past several decades [1], direct evidence of in situ glass fiber strength degradation in the composite as a result of environmental loading is scare [5,6]. In a previous study [6], it was shown that the in situ tensile strength of glass fibers in a pultruded composite was reduced as a result of environmental aging in water at room and elevated temperatures. To further confirm the finding, a study on the in situ strength degradation of E-glass fibers in an epoxy matrix model composite undergo environmental aging was carried out. Experimental evidence of in situ tensile strength degradation of glass fibers in the composite is presented in this paper. It is well known that the tensile fracture surfaces of many ceramic and polymeric materials exhibit a distinct mirror-mist-hackle pattern, shown in Fig. 1. For most fracture surfaces with such a pattern, the mirror region containing the crack-initiating surface flaw is usually located around the circumference of the fiber while fracture mirror located inside the fiber can only be found occasionally, indicating that the strength of these glass fibers is essentially surface-flaw controlled. It has been well established that the tensile strength of glass and ceramic materials is related to the fracture mirror size through the following semi-empirical relation r 1/ 2 u ⫽ A where r is the radius of the fracture mirror, u the tensile strength (fracture stress), and A a material constant. Since u is proportional to r⫺1/2, the tensile strength of a fiber can be estimated by measuring fracture mirror size, if the material constant A is known. The constant A can be easily obtained from a set of calibration data. An estimation of the constant A for E-glass fiber is 1.47 MN/m3/2 [5]. It should 1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00661-8
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Figure 1. SEM image of fiber fracture surface showing typical surface features: mirror, mist, and hackle pattern.
be mentioned that not all the fractured fiber surfaces exhibit a distinctive mirror-mist-hackle pattern. Some fibers displayed a rough fracture surface without a recognizable pattern while others displayed a smooth fracture surface, thought to be locations with low strength [7]. Low-stress fractures are not taken into consideration in this study. Although by excluding low stress fractures the fiber strength distribution may not represent the actual distribution, it nevertheless represents an upper limit and therefore a comparison at different aging times is still viable in revealing the effects of environmental aging on fiber strength. Experimental Model composite panels of unidirectional E-glass fiber reinforced epoxy were fabricated using a hand laying method. Small fiber bundles each containing approximately 50 – 60 individual fiber were obtained by carefully splitting a larger bundle of E-glass fiber roving. These smaller glass fiber bundles were wrapped in parallel around a 15 cm ⫻ 15 cm Perplex glass plate with double sided tapes placed at the ends which fixes the fiber bundles in place. 2-mm thick metallic spacers were placed along the sides of the Perplex glass plate to ensure that the end product is a plate with uniform thickness. Some tension was applied to the fiber bundles while they were being wrapped. De-gassed epoxy resin was poured on to one side of the Perplex glass plate with banks placed along its 4 sides. The epoxy resin and the fiber were kept under a light pressure via a top plate while the epoxy was being cured. The process was repeated for the other side of the Perplex glass plate after the first side was being cured. A room temperature curing epoxy is chosen so that thermal residual stress resulting from curing is kept at minimal. Small specimens with nominal dimensions of 35mm ⫻ 5mm ⫻ 2mm were then obtained from the panel. Each of these specimens contains 3 or 4 fiber bundles. Small notch of about 1mm deep was put in the middle of each specimen to ensure that the model composite breaks in the middle for proper examination. These notched specimens were aged in water at 25° C for 500 h and 4320 h, and subsequently tested to failure under tension. The masses of three samples were measured at regular intervals to monitor moisture absorption. Three samples were tested after a same aging time. The tensile strength of glass fibers is estimated by measuring the size of the fracture mirror of the failed fibers using a scanning
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Figure 2. Sorption curve of model composite in water at 25° C.
electronic microscope (SEM). At least 50 measurements were made randomly from samples aged for same period of time. A total of 160 individual fibers were examined. Results and Discussion Sorption behavior of the model composite is shown in Fig. 2, where % mass change is plotted against square root of immersion time. The sorption curve exhibits a Fickian type behavior, with moisture saturation level at about 2.0% after 500 h. At 4320 h the mass change has dropped to about 1.9%, possibly because of dissolution of the matrix material. Results of in situ fiber strength measurements for as-fabricated samples, and water-aged samples are shown in Fig. 3, where the failure probability, Pf, determined using the median rank method, is plotted against r⫺1/2. Since we are only interested in the trend of environmental strength reduction, r⫺1/2 is used instead of u. Noticeable difference between aged samples and as-fabricated samples is seen from Fig. 3. As expected, the effect of environmental aging is to shift the accumulative failure probability curves to larger mirror sizes, indicating a reduction in tensile strength. The three accumulative failure probability curves are separated from each other for the most part, with some overlapping of the 500 h data and the 4320 h data at the high strength end. A time effect is also obvious from Fig. 3, that is, the longer the aging time, the lower the fiber tensile strength. Results of zero stress aging of glass fibers in water from previous studies suggest that strength degradation is negligible at room temperature in the neighborhood of 107 s [8,9]. Hasløv et al showed
Figure 3. Failure probability curves of residual tensile strength of glass fibers in the model composite.
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that no strength degradation has occurred in acrylate coated optical fibers before 300 days of stress-free aging in water at 65° C [8]. Comparing with previous studies, fiber strength degradation is obvious after 500 h (1.8 ⫻ 106 s) of immersion, which has occurred earlier than zero-stress aging of fibers reported in previous studies [8,9]. This discrepancy suggests that the accelerated rate of in situ fiber strength degradation could have arisen from moisture-induced tensile stress because of moisture absorption and subsequent stress corrosion. It has been shown, using a concentric cylinder model of fiber surrounded by a polymeric matrix that significant amount of tensile stress can be introduced to the fiber at a relatively low moisture level if the coefficient of moisture expansion of the matrix is high [10]. For epoxy, the coefficient of moisture expansion is about 8.0 ⫻ 10⫺3, a value that will result in significant tensile stress in the fibers even at low moisture level. Thus, it is both the moisture-induced stress and the corrosive action of the water that are responsible for the reduction of tensile strength of glass fibers in the model composite under stress free aging. It is seen from Fig. 3 that the failure probability curve of the 500 h data are closer to that of the 4320 h data than to that of the as-fabricated data, implying that more tensile strength reduction has occurred in the first 500 h of aging, and the reduction rate seems to be slowed down at longer times. It has been shown that radial (tensile) stress at the fiber/matrix interface can also be significant at high moisture levels, thus promoting fiber/matrix debonding [10]. It is believed that the fiber/matrix interphase was damaged or destroyed after moisture ingress, causing moisture-induced tensile stress in the fibers to be relaxed, and thus resulted in a reduction of degradation rate. This is further supported by the fact that pullout length of glass fibers from the epoxy matrix after aging for 4320 h is much longer than those from the 500 h samples. The data trend shown in Fig. 3 is very similar to what was obtained from a previous study on pultruded composite [6], in which tensile strength of the composite was found to be degraded after stress-free aging in water. Thus it can be deduced that strength degradation of the composite could be attributed, at least in part, to in situ fiber strength degradation and the moisture-induced internal stress is playing a significant role. Summary Using fracture mirror measurements, it is shown that in situ tensile strength degradation of E-glass fiber in an epoxy matrix model composite has taken place when aged in water at 25° C after 500 h. Compared to data of stress-free aging of E-glass fibers, faster in situ strength degradation of glass fiber is seen in the model composite, suggesting that both the moisture-induced tensile stress and the corrosive action of water play an important role in strength degradation of glass fiber composites during environmental aging. Acknowledgments The author wishes to thank E. Chow, R. Reyes, and J. Loke for their help in the laboratory through the TER program. References 1. 2. 3. 4.
K. Liao, C. R. Schultheisz, D. L. Hunston, and L. C. Brinson, SAMPE J. Adv. Mater. 30(4), 3 (1998). R. J. Charles, J. Appl. Phys. 29(11), 1549 (1958). A. G. Metcalfe and G. K. Schmitz, Glass Technol. 13(1), 5 (1972). S. M. Wiederhorn and L. H. Bolz, J. Am. Ceram. Soc. 53(10), 543 (1970).
Vol. 44, No. 5
5. 6. 7. 8. 9. 10.
IN SITU DEGRADATION OF GLASS FIBER
A. C. Jaras, B. J. Norman, and S. C. Simmens, J. Mater. Sci. 18, 2459 (1983). K. Liao, C. R. Schultheisz, and D. L. Hunston, Composites Part B. 30, 485 (1999). A. J. Eckel and R. C. Bradt, J. Am. Ceram. Soc. 72(3), 455 (1989). P. Hasløv, K. B. Jensen, and N. H. Skovgaard, J. Am. Ceram. Soc. 77(6), 1531 (1994). J-W. Leclercq and A. H. E. Breuls, SPIE 2290, 64 (94). K. Liao, Composites Part B, submitted.
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