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vinyl ester composite for infrastructure applications
International Journal of Fatigue 22 (2000) 53–64 www.elsevier.com/locate/ijfatigue
Characterization of fatigue and combined environment on durability performance of glass/vinyl ester composite for infrastructure applications F. McBagonluri, K. Garcia, M. Hayes, K.N.E. Verghese, J.J. Lesko
*
Materials Response Group, Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA Received 8 February 1999; received in revised form 20 June 1999; accepted 1 August 1999
Abstract As composite materials find increased use in infrastructure applications, where design lives are typically much longer than those in aerospace, the issue of durability becomes more critical. The tolerance of composites to damage induced by cyclic loading and moisture ingress is of utmost importance. This study highlights the effects of short-term cyclic moisture aging on the strength and fatigue performance of a glass/vinyl ester pultruded composite system. In particular, it addresses the change in quasi-static properties and tension–tension (R=0.1) fatigue behavior of a commercial glass/vinyl ester system in fresh and salt water. The quasi-static tensile strength was seen to reduce by 24% at a moisture concentration of 1% by weight. This reduction in strength was not recoverable even when the material was dried, suggesting that the exposure to moisture caused permanent damage in the material system. Even though the fatigue damage process of the unaged or ‘as-delivered’, fresh-water- and salt-water-saturated material was similar, the cyclic moisture absorption–desorption experiments altered the fatigue performance of the composite system tested. Results were consistent with Mandell’s postulate that fatigue failure in glass-fiber-reinforced polymeric composites is a fiber-dominated mechanism with a characteristic slope of 10% UTS/decade. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Glass fiber; Vinyl ester; Fatigue; Salt water; Accelerated aging
1. Introduction Despite the tremendous progress that has been made in the sector of infrastructure composite materials and engineering within the past few years [1–7], there still remain technological challenges. These challenges include the development of reliable test methods, less expensive yet sophisticated and specialized materials, viable and robust life prediction tools, as well as consideration of the environmental and mechanical response of these materials. Other significant hurdles include the absence of comprehensive data characterizing the longterm durability of glass-fiber-reinforced polymeric composites coupled with the absence of adequate established standards for repair, design and maintenance.
* Corresponding author. Tel.: +1-540-231-5259; fax: +1-540-2319187. E-mail address: [email protected] (J.J. Lesko)
Significant attempts have been made to address some of these pressing issues. Some of the work that serves as a precedent to the results presented in this paper originates from Mandell and Meier [8]. Mandell and Meier note that fatigue failure in general is characterized by the progressive accumulation of cracks in the matrix and at the fiber/matrix interface, resulting in a loss of remaining strength and stiffness. This progressive reduction in remaining strength reaches a limiting point where it equals the applied stress and consequently results in failure. This general trend in failure does not apply explicitly to tensile failure in glass-reinforced composites, where failure appears to be fiber-dominated, and it is independent of matrix or interface type. Mandell noted that this damage mechanism for fiber-reinforced composite was true for cyclic as well as static fatigue (creep), and that the stress level versus cycles-to-failure curve properties are translated into withstanding crack initiation and propagation. However, Mandell cautions that exceptions to the fiber-dominated mechanism may
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exist especially when a severe fatigue mechanism is at play; for instance, in woven fabric reinforcement and in very ductile matrices. Mandell [9] conducted a series of experiments using polymeric matrix, glass-reinforced composites. These tests indicated an inverse stress versus life slope of 10% of the ultimate tensile strength (UTS) per decade for different matrix types, volume fractions, glass fiber types and architectures. Mandell concluded, based on this evidence, that the 10% UTS/decade slope was characteristic of the fatigue behavior of glass-fiberreinforced polymeric-matrix composites. Jones et al. [10] also reported similar observations. Transverse tensile tests conducted by Bradely and Grant [11,12] indicated that the strength after saturation depends heavily on both the properties of the matrix and the shear strength of the interface. An environmental scanning electron microscope (ESEM) was used to facilitate in situ damage evolution and propagation observation. Environments such as immersion at room temperature in fresh and simulated sea-water at atmospheric pressure and 20.7 MPa to simulate deep-water applications were investigated. Microindentation tests were also performed to evaluate the interfacial shear strength and to compare it to the trends observed in the strength data. It was noted that composites made of very brittle matrices promoted damage via matrix cracking, leading to low strengths. Further, these composites showed an increase in strength after saturation because of the plasticization effect of the matrix. On the other hand, the strength of composites made of matrices that are more ductile but develop poor fiber/matrix interfaces remained unchanged throughout the study since the damage was always restricted to the fiber/matrix interface region. Composites that did show significant changes in strength after saturation also indicated a change in damage mode, namely from being matrix-dominated before aging to interfacial after saturation. The moisture uptake data also showed that saturation contents for salt-water-aged samples were less than for their corresponding fresh-water counterparts. This was attributed to the osmotic effect in the salt-water solutions. Little difference in strength was therefore seen to exist between the three exposure environments. These attempts seek to facilitate an understanding of the response of fiber-reinforced composite systems to the applied environment, and also to serve as a precursor to the development of reliable analytical and life prediction tools. Fiber-reinforced composite data currently available are industry-specific, with most of the current data belonging to the aerospace and petrochemical industries where years of experience with composites have resulted in a database, while little data are available for the marine and infrastructure sectors. The absence of data on glass-fiber-reinforced composites for marine and infrastructure applications, where longevity is of interest, has been in part responsible for their slow acceptance. This paper therefore documents a novel approach to
the evaluation of polymeric-matrix composites for their hygrothermal–mechanical properties, and contributes to the current discourse on their applications as infrastructure materials. This discussion looks at the effect of moisture adsorption, cycling and comparative fatigue in water and in a simulated sea environment on the shortterm durability of pultruded glass-reinforced polymeric composites by performing in situ fatigue cycling in both fresh- and salt-water environments.
2. Experimental procedure The material system used in this study was EXTREN, which was pultruded by Strongwell, Inc. (Bristol, VA). The glass/vinyl ester composite consists of five main layers: three layers of unidirectional glass sandwiched between two substantially thicker layers of continuous strand mat for a total thickness of 0.125 in. (3.175 mm). Strongwell reported the average fiber volume fraction to be between 0.28 and 0.30, which was obtained through matrix burn-off tests. The pultruded material was delivered as boards, 1.5 ft (45.7 cm) by 4 ft (122 cm), which were cut into coupons using a water-cooled diamond saw. Coupon sizes of 9 in. (22.9 cm) by 1 in. (2.5 cm) were used for specimens loaded in a fluid cell (described below). All other tests were performed on 6 in. (15.2 cm) by 1 in. (2.5 cm) coupons. The coupons were cut with the long direction parallel to the unidirectional glass fibers. The edges of all the coupons were sanded smooth with 180 grit sandpaper to ensure uniformity of the specimens and to prevent premature failure due to edge effects. The edges of the specimens were then coated with a Buehler two-part Epoxide resin and cured for 2 h at 65°C to minimize edge absorption during exposure of the material to humid environments. Pre-conditioned specimens were placed in Plexiglass racks designed to ensure equal and maximum exposure of all surfaces of the specimen to both water and 3.5% by weight salt solution. The aging apparatus consisted of 20 gallon glass tanks which facilitated specimen monitoring without disturbing the set-up. Heaters were mounted in each corner of the rectangular tank. An Omega controller was used to maintain temperatures in the tank by regulating power to the heaters and monitoring tank temperature. Two pumps were placed in the tank to ensure circulation and uniform heat distribution. The tank was covered with bubble wrap and a Plexiglass lid, and the entire tank was insulated with Styrofoam. For tests employing salt water, in order to ensure consistency in the concentration of salt in solution, a fresh solution was made for each batch. The salinity of the solution was monitored by a salimeter and adjusted periodically by adding fresh solution. Specimens subjected to cyclic moisture absorption–
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desorption were placed in tanks similar to the one described above. To facilitate desorption, specimens were removed from the tanks and placed in a convection oven at the same temperature as the water bath from which they were removed. The specimens were kept in the oven for a fixed length of time. The average absorption–desorption cycle length was approximately 1 month, with 28 days in a water bath and 3 days in an oven. In all, five cycles were applied to the specimens. In an attempt to try and separate temperature effects from that of moisture, a batch of samples was subjected to pure thermal aging in a convection oven at the same temperature as that of the water bath. The duration of exposure was equal to one cycle as explained above. The complete test matrix is shown in Table 1. Quasistatic tests were performed at a loading rate of 0.05 in. (0.127 cm) per minute. Fatigue tests were conducted at a maximum-to-minimum load ratio of R=0.1 at frequencies of 2 Hz and 10 Hz. No detectable heating of the specimens was observed during the fatigue tests. A fluid cell (Fig. 1) was used for the fatigue tests conducted in salt water. The fluid cell [2,3] consists of two 4 in. (10.2 cm) by 3 in. (7.62 cm) Plexiglass plates, each with a recess to accommodate the specimen once closed. The specimen was sandwiched between the plates and sealed with a silicone adhesive. Inlet and outlet hoses were attached to the cell. which were then attached to a temperature-controlled water circulator. The specimen/fluid cell assembly could be inserted into a test frame and fatigue tests could be conducted on the specimen. The fluid cell had no effect on the failure of the samples. This was checked through a series of tests with and without the cell.
Table 1 Test matrix detailing specimen pre-conditioning and test conditions Pre-conditioning
Mechanical test Quasi-static
Fatigue
Test condition Air As-delivered Tap water (45°C) Salt water (65°C) Thermal (45°C) Two cycles/dry Two cycles/wet Five cycles/dry Five cycles/wet a b
Fluid cell
× × × × × × × ×
Tested at 10 Hz and 20 Hz. Tested at 2 Hz and 10 Hz.
Air ×a ×
×
Fluid cell
×b
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3. Results 3.1. Quasi-static properties following moisture aging and cycling The quasi-static properties were evaluated for the asreceived, salt-aged and water-aged EXTREN material system using Weibull statistical methods. The plot of the residual quasi-static properties normalized to the asreceived quasi-static properties is shown in Fig. 2 with the corresponding standard deviation bars. The distribution of static strength properties does not seem to vary significantly in shape. The dimensionless shape parameter a is constant for the three conditions, namely: asreceived (dry), wet and salt water. Table 2 summarizes the mechanical properties of the composite after exposure to the different aging environments. Also listed in the table are the A and B allowables for the strength of the composite. Fig. 3 shows the moisture weight gain during the cyclic absorption–desorption experiment. The numbers on the plot serve as locators and are points at which samples were removed for testing. These numbers will be used to present data in this paper. The stiffness, strength and strain-to-failure for the cyclically aged material are illustrated in Fig. 4. As seen in the chart, there is a significant difference between the as-delivered and the cyclically aged materials. There is however a larger difference in strength than in stiffness, but, in both cases, there appears to be a significant change. The properties of the thermally aged material do not appear to differ from those of the as-delivered material. 3.2. Enviro-mechanical fatigue performance The fatigue curves for the as-delivered, fresh-wateraged and salt-water-aged materials are shown in Fig. 5. The curves for the fresh-water- and salt-water-aged materials are nearly identical. The slopes of all S–N curves appear to be similar, suggesting the existence of a common damage mechanism. Fig. 6 shows the comparative S–N plot for samples tested dry at two different test frequencies. There seems to be some difference in the degradation of the glass-fiber-reinforced composite under these conditions as evinced by the slope of the fits. Fig. 7 represents the S–N curves for the as-delivered, wet and five-cycles moisture-aged specimens. On Fig. 3, these would denote points 1, 2 and 7. The curves appear to be slightly different. The material tested was, however, in different states (i.e., dry after five cycles and wet after one cycle). It is seen that the effect of moisture on the slope of the S–N curve is negligible even though the specimens were subjected to both isothermal as well as cyclic aging conditions. Fig. 8(a), (b) and (c) show the residual strength changes for the as-delivered, wet and salt-water-aged specimens.
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Fig. 1.
Fig. 2.
Experimental set-up showing the hygrothermal chamber and a schematic illustration of the fluid cell.
Properties of as-delivered, fresh-water-aged and salt-water-aged materials along with the A and B allowables.
4. Discussion 4.1. Quasi-static performance following moisture aging and cycling The results indicate that the quasi-static modulus in the dry specimens underwent a change of 11% and the strength a reduction of 32% following aging in salt water. The Poisson’s ratio increased by 6%. Data from
the fresh-water-aged specimens indicate a decrease in modulus of about 11% and 32% in ultimate tensile strength, from dry to water-aged. There was no change in Poisson’s ratio for this case. The results indicate little difference between the material properties of the waterand salt-solution-aged materials. The results of the cyclic moisture ingression indicate that the quasi-static material properties are reduced significantly after the first conditioning cycle, but damage does not continue to
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Table 2 Mechanical properties along with A and B allowables for strength on as-delivered, tap-water-aged and salt-water-aged specimens (SD=standard deviation)
As-delivered Tap water Salt water
Fig. 3.
Modulus (GPa)
Poisson’s ratio
Strength (MPa)
A allowable B allowable
Mean
SD
Mean
SD
Mean
SD
Mean
Mean
15.5 13.3 13.85
0.67 1.2 2
0.31 0.31 0.33
0.03 0.03 0.03
212 145.5 145.05
17.95 13.4 14.47
149.9 102.6 100.2
179.6 122.6 121.7
Moisture uptake plot during the cyclic aging process. Tests were performed by immersion in a water tank at 45°C.
Fig. 4. Normalized stiffness, strength and strain-to-failure of cyclically aged material. The legend follows the format [x, description, y], where ‘x’ stands for the number of cycles and ‘y’ stands for the corresponding location shown in Fig. 3.
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Fig. 5. S–N curves of the as-delivered, water-aged and 3.5% salt-solution-aged materials.
Fig. 6. Effect of test frequency: S–N curves of the material tested at 2 Hz and 10 Hz.
accumulate with additional cycles and exposure time. Performing a test of means with a significance level of 1% on the cyclically aged materials indicates that we must accept the null hypothesis that the strength of the as-delivered material is different from that of the other materials tested except for the material subjected to thermal relaxation. The implications from this statement are that all other material states are different from the asdelivered material, while the thermally relaxed material is similar. Aging in water has significant effects on the material, and most of these effects are irreversible upon desorption. The test of means analysis also indicated that there is a significant reduction in strength from the first
moisture cycle, similar to what was observed by Schultheisz et al. [13] for glass/epoxy composites, and further cycling did not cause significant changes in material properties. The properties at the second and fifth cycles are similar to those at the wet and dry stages. This confirmed the idea that the initial damage is not recoverable when the material is allowed to return to its initial moisture content, suggesting that there is damage to either the matrix, interphase, fiber or a combination. Nevertheless, it is also true that samples that were dried after the first cycle showed some signs of partial recovery in strength. This indicates that a part of the total drop in properties can be attributed to matrix plasticization. The remaining,
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Fig. 7. S–N curves for as-delivered, one-cycle/wet and five-cycles/dry materials.
irrecoverable part is probably due to degradation of the interface and fiber. It is also conceivable that residual stresses which develop in the composite during its production could also play a part in the mechanical property changes observed. However, Fig. 4 shows that that the ‘thermal’ data are statistically similar to the initial properties; thus, the degradation cannot be attributed to thermal relaxation of residual stresses. Preliminary investigation using scanning electron microscopy (SEM) [Fig. 9(a) and (b)] indicates the existence of damage to both the fiber itself and cracks about the interface region in the fresh-water-aged sample, compared with the as-delivered. Weitsman [14] reports similar findings for graphite/epoxy composites. The ongoing study is focused on quantifying the damage in the aged composites and relating the damage to aging history. 4.2. Enviro-mechanical fatigue performance The relationship between the as-delivered and aged materials is the shift in the intercept of the S–N curve (Fig. 5), or the ultimate tensile strength (UTS). When the data for each material state are normalized with respect to their respective UTS, the curves collapse on top of each other. Conducting a statistical test of coincidence [15] indicates that the slopes of the lines are similar. The lines are normalized and plotted in Fig. 10. This similarity in slope is independent of prior moisture exposure, moisture content or type of moist environment. In other words, the aged and the unaged EXTREN underwent a similar fatigue-induced failure mechanism. The slope B of the S–N curves for all test conditions ranges from 9.9 to 12.7%.
The fatigue frequency effect depicted on Fig. 6 shows a slight increase in damage at 2 Hz compared with that at 10 Hz. This could be due to the statistical nature of fatigue data or due to damage accumulation with increasing time spent at a given applied stress level, as detailed by McBagonluri [1]. The S–N curves for the as-delivered, one-cycle/wet and five-cycles/dry materials, normalized with respect to their respective Sult, are depicted in Fig. 11. Statistical analysis performed on the as-delivered and onecycle/wet data indicated that the two normalized S–N curves are the same. The five-cycles/dry material is within the 95% confidence bands of the dry material, but the curve is not similar to the as-delivered and onecycle/wet curves. The slope of the S–N curve decreases, as does its intercept. Performing tests of means on the individual data sets with a significance level of 1% indicates that the null hypothesis must be accepted that the cyclically aged wet material and cycled dry material are different. Test of means also indicates that the wet cycled materials are similar and the dry cycled materials are similar. This indicates the importance of the final state the material was in when it was tested. It does, however, also suggest that the number of cycles does not prove to be very critical, a conclusion which is consistent with the data as seen in Fig. 4. The residual strength curves for the as-delivered, tapwater-aged and salt-water-aged specimens are shown in Fig. 8(a), (b) and (c). The curves were generated by fitting the data with Eq. (1), a form of the residual strength equation suggested by Broutman and Sahu [16]. This equation is derived by considering remaining strength to be a measure of damage and postulating that normalized remaining strength (referenced to the initial strength) is
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Fig. 8.
Remaining strength curves for: (a) as-delivered material; (b) one-cycle/wet material; (c) salt-water-aged material.
an internal state variable. Using thermodynamic arguments for the identified ‘critical element’ [17] and assuming a power-law form for the kinetics equation, the following equation is derived for a ‘critical element’ in the laminate:
冉 冊冉 冊
Sa n a Sres ⫽1⫺ 1⫺ . Sult Sult N
(1)
In this equation, Sres is the residual strength, Sult is the ultimate tensile strength of the material, Sa is the applied
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Fig. 9. SEM micrographs of: (a) as-delivered material; (b) fresh-water-aged (magnification is ×3000 in both micrographs). The micrograph in (b) clearly indicates the existence of damage both in the form of fiber breakage and fiber/matrix interface cracking.
stress on the material, n is the number of loading cycles applied to a specimen, and N is the life of the specimen. The parameter a is a characteristic of the material included to account for the non-linearity of the strengthreduction curve and may be determined by curve-fitting the data to Eq. (1). Curves of the form of Eq. (1) were fit to the data for each load level and average values for a range from 1.0 to 1.2. The damage mechanisms for the moisture-cycled material appear to be changing. One possible reason could be toughening of the matrix due to plasticization. Glass/vinyl ester fatigue data discussed here for various conditions — namely, temperature, moisture type and frequency effects — are shown with Mandell’s data in Fig. 12. It is interesting to note that the slopes B on these
plots are between 10% and 11% UTS/decade, which is consistent with the results obtained by Mandell [9]. Mandell and co-workers investigated the fatigue effect due to fiber orientation, fiber volume fraction, resin type and glass fiber type in polymeric composites. Mandell et al. observed a similar trend in the fatigue slope for the aforementioned variables, concluding that the failure mechanism could be attributed to a fiber-dominated process. In other words, glass-fiber composite fatigue failure seems to occur as a result of gradual deterioration of the load-bearing fibers, and is independent of fiber volume fraction, resin type, glass fiber type and fiber orientation. The results presented in this paper suggest that this process is also not influenced by short-term exposure to fresh and salt water, frequency of testing and exposure temperature.
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Fig. 10.
Normalized S–N curves for as-delivered, water-aged and 3.5% salt-solution-aged materials.
Fig. 11.
Normalized S–N curves for as-delivered, one-cycle/wet and five-cycles/dry materials.
Subsequent work by Sims and Gladman [18] on the effect of pre-conditioning (boiled and unboiled), preinduced mechanical damage and orientation of reinforced material on glass fiber fatigue collaborated Mandell’s findings. Sims and Gladman concluded that, “Independent of pre-conditioning treatment, orientation of reinforcement or preloading-induced mechanical damage, Mandell’s stipulation lays credence to the concept of a monotonic fatigue failure mechanism for glass fiberreinforced composite”. These results seem to validate Mandell’s postulate that a fiber-dominated mechanism is responsible for final composite failure, and further
extended Mandell’s variables to include pre-conditioning, preloading and fiber architecture (woven). One could also infer that the effect of temperature is implicit in the pre-conditioning aspect of Sims and Gladman’s affirmation.
5. Conclusions A fiber-dominated mechanism has been established as the dominant mechanism responsible for the failure of EXTREN under dry, wet and simulated sea-water
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Fig. 12. Mandell’s relationship between tensile strength and slope of the S–log N curve, ‘B’, for different materials. Values are included for EXTREN for various frequencies and testing conditions. The image on the right is an enhanced view of the data indicated on the left.
environments. The aged material, in either fresh or salt water, experiences a one-time drop in strength with moisture ingress. Results obtained for the inverse slopes of the stress levels versus life diagrams are consistent with results reported by Mandell and co-workers for glass reinforcement. The temperature effect on fatigue response did not depend on the presence or absence of the ambient fluid. Furthermore, fatigue damage evolution and subsequent failure of fiber-reinforced composites have been found to be independent of moisture content or moisture regime in the short term, although long-term aging and moisture ingress appear to affect the fatigue performance of the material.
Acknowledgements The authors would like to thank the Building and Fire Research Laboratory at the National Institute of Standards and Technology (NIST), the National Science Foundation (NSF) Career Award, Strongwell and 3M for their support.
References [1] McBagonluri-Nuuri DF. Simulation of fatigue performance and creep rupture of glass-reinforced polymeric composites for infrastructure applications. MS thesis, Virginia Polytechnic Institute and State University, 1998. http://scholar.lib.vt.edu/ theses/available/etd-72198-155929/ [2] Hayes MD. Characterization and modeling of a fiber-reinforced polymeric composite structural beam and bridge structure for use in the Tom’s Creek Bridge Rehabilitation Project. MS thesis, Virginia Polytechnic Institute and State University, 1998. [3] Hayes MD, Garcia K, Verghese N, Lesko JJ. The effects of moisture on the fatigue behavior of glass/vinyl ester composite. In: Saadatmanesh H, Ehsani MR, editors. Fiber composites in infrastructure, Proceedings ICCI ’98, Tucson, AZ, 5–7 January, University of Arizona, 1998.
[4] McBagonluri F, Hayes M, Garcia K, Lesko JJ. Durability of Eglass vinyl ester in an elevated temperature simulated sea water environment. In: Proceedings of the Seventh International Conference on Marine Applications of Composite Materials (MACM ’98), Melbourne, FL, 17–19 March, Composites Education Association, Inc, 1988. [5] McBagonluri F, Hayes M, Garcia K, Lesko JJ. Durability of glass/vinyl ester composites in a simulated sea environment. In: Proceedings of the American Society of Mechanical Engineers (ASME) Conference, Dallas, TX, 15–18 November. Massachusetts: ASME Technical Publishing Department, 1997. [6] Lesko JJ, Hayes M, McBagonluri F, Verghese N, Garcia K. Environmental–mechanical durability of glass/vinyl ester composites. In: Proceedings of the Third International Conference on Progress in Durability Analysis of Composites Systems (DURACOSYS), Blacksburg, VA, 14–17 September. Rotterdam: A.A. Balkena, 1997. [7] Demers C. E-glass fiber-reinforcement polymeric composites, tension–tension axial fatigue life diagram. Presented at The National Seminar on Advanced Composite Material Bridges, 5– 7 May 1997. Federal Highway Administration (FHWA). [8] Mandell JF, Meier U. Effects of stress ratio, frequency, loading time on tensile fatigue of glass-reinforced epoxy. In: O’Brien TK, editor. Long-term behavior of composites. Philadelphia (PA): American Society for Testing and Materials, 1983:55–77. [9] Mandell JF. Fatigue behavior of fiber–resin composites. In: Pritchard G, editor. Developments in reinforced plastics 2. London: Applied Sciences Publisher, 1978. [10] Jones CJ, Dickson RF, Adams T, Reither H, Harris B. The environmental fatigue behavior of reinforced plastics. Proc R Soc Lond A 1984;396:313–38. [11] Bradely WL, Grant TS. The effects of the moisture absorption in interfacial strength of polymeric composites. J Mater Sci 1995;30:5537–42. [12] Grant TS, Bradely WL. In situ observations in SEM of degradation of graphite/epoxy composite materials due to seawater immersion. J Compos Mater 1995;29(7):852–67. [13] Schultheisz CR, McDonough WG, Kondagunta S, Schutte CL, Macturk KS, McAuliffe M, Hunston DL. Effect of moisture on E-glass/epoxy interfacial and fiber strengths. Presented at: 13th Symposium on Composite Materials: Testing and Design. Philadelphia (PA): American Society for Testing and Materials, 1996. [14] Weitsman Y. Coupled damage and moisture-transport in fiberreinforced, polymeric composites. Int J Solid Struct 1987;23(7):1003–25.
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[15] Standard practice for statistical analysis of linear or linearized stress–life and strain–life fatigue data. ASTM E 739. Philadelphia (PA): American Society for Testing and Materials. [16] Broutman LJ, Sahu S. A new damage theory to predict cumulative fatigue damage in fiberglass reinforced plastics. In: Composite materials: testing and design (second conference). ASTM STP 497, Philadelphia (PA): American Society for Testing and Materials, 1972:170–88.
[17] Reifsnider KL, Stinchcomb WW. A critical element model of the residual strength and life of fatigue-loaded composite coupons. In: Hahn HT, editor. Composite materials: fatigue and fracture. ASTM STP 407, Philadelphia (PA): American Society for Testing and Materials, 1986:298–303. [18] Sims GD, Gladman DG. Effect of test conditions on the fatigue strength of a glass-fabric laminate. Part B — Specimen condition. Plast Rubb Mater Appl 1980;1:122–8.
Characterization of fatigue and combined environment on durability performance of glass/vinyl ester composite for infrastructure applications F. McBagonluri, K. Garcia, M. Hayes, K.N.E. Verghese, J.J. Lesko
*
Materials Response Group, Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA Received 8 February 1999; received in revised form 20 June 1999; accepted 1 August 1999
Abstract As composite materials find increased use in infrastructure applications, where design lives are typically much longer than those in aerospace, the issue of durability becomes more critical. The tolerance of composites to damage induced by cyclic loading and moisture ingress is of utmost importance. This study highlights the effects of short-term cyclic moisture aging on the strength and fatigue performance of a glass/vinyl ester pultruded composite system. In particular, it addresses the change in quasi-static properties and tension–tension (R=0.1) fatigue behavior of a commercial glass/vinyl ester system in fresh and salt water. The quasi-static tensile strength was seen to reduce by 24% at a moisture concentration of 1% by weight. This reduction in strength was not recoverable even when the material was dried, suggesting that the exposure to moisture caused permanent damage in the material system. Even though the fatigue damage process of the unaged or ‘as-delivered’, fresh-water- and salt-water-saturated material was similar, the cyclic moisture absorption–desorption experiments altered the fatigue performance of the composite system tested. Results were consistent with Mandell’s postulate that fatigue failure in glass-fiber-reinforced polymeric composites is a fiber-dominated mechanism with a characteristic slope of 10% UTS/decade. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Glass fiber; Vinyl ester; Fatigue; Salt water; Accelerated aging
1. Introduction Despite the tremendous progress that has been made in the sector of infrastructure composite materials and engineering within the past few years [1–7], there still remain technological challenges. These challenges include the development of reliable test methods, less expensive yet sophisticated and specialized materials, viable and robust life prediction tools, as well as consideration of the environmental and mechanical response of these materials. Other significant hurdles include the absence of comprehensive data characterizing the longterm durability of glass-fiber-reinforced polymeric composites coupled with the absence of adequate established standards for repair, design and maintenance.
* Corresponding author. Tel.: +1-540-231-5259; fax: +1-540-2319187. E-mail address: [email protected] (J.J. Lesko)
Significant attempts have been made to address some of these pressing issues. Some of the work that serves as a precedent to the results presented in this paper originates from Mandell and Meier [8]. Mandell and Meier note that fatigue failure in general is characterized by the progressive accumulation of cracks in the matrix and at the fiber/matrix interface, resulting in a loss of remaining strength and stiffness. This progressive reduction in remaining strength reaches a limiting point where it equals the applied stress and consequently results in failure. This general trend in failure does not apply explicitly to tensile failure in glass-reinforced composites, where failure appears to be fiber-dominated, and it is independent of matrix or interface type. Mandell noted that this damage mechanism for fiber-reinforced composite was true for cyclic as well as static fatigue (creep), and that the stress level versus cycles-to-failure curve properties are translated into withstanding crack initiation and propagation. However, Mandell cautions that exceptions to the fiber-dominated mechanism may
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exist especially when a severe fatigue mechanism is at play; for instance, in woven fabric reinforcement and in very ductile matrices. Mandell [9] conducted a series of experiments using polymeric matrix, glass-reinforced composites. These tests indicated an inverse stress versus life slope of 10% of the ultimate tensile strength (UTS) per decade for different matrix types, volume fractions, glass fiber types and architectures. Mandell concluded, based on this evidence, that the 10% UTS/decade slope was characteristic of the fatigue behavior of glass-fiberreinforced polymeric-matrix composites. Jones et al. [10] also reported similar observations. Transverse tensile tests conducted by Bradely and Grant [11,12] indicated that the strength after saturation depends heavily on both the properties of the matrix and the shear strength of the interface. An environmental scanning electron microscope (ESEM) was used to facilitate in situ damage evolution and propagation observation. Environments such as immersion at room temperature in fresh and simulated sea-water at atmospheric pressure and 20.7 MPa to simulate deep-water applications were investigated. Microindentation tests were also performed to evaluate the interfacial shear strength and to compare it to the trends observed in the strength data. It was noted that composites made of very brittle matrices promoted damage via matrix cracking, leading to low strengths. Further, these composites showed an increase in strength after saturation because of the plasticization effect of the matrix. On the other hand, the strength of composites made of matrices that are more ductile but develop poor fiber/matrix interfaces remained unchanged throughout the study since the damage was always restricted to the fiber/matrix interface region. Composites that did show significant changes in strength after saturation also indicated a change in damage mode, namely from being matrix-dominated before aging to interfacial after saturation. The moisture uptake data also showed that saturation contents for salt-water-aged samples were less than for their corresponding fresh-water counterparts. This was attributed to the osmotic effect in the salt-water solutions. Little difference in strength was therefore seen to exist between the three exposure environments. These attempts seek to facilitate an understanding of the response of fiber-reinforced composite systems to the applied environment, and also to serve as a precursor to the development of reliable analytical and life prediction tools. Fiber-reinforced composite data currently available are industry-specific, with most of the current data belonging to the aerospace and petrochemical industries where years of experience with composites have resulted in a database, while little data are available for the marine and infrastructure sectors. The absence of data on glass-fiber-reinforced composites for marine and infrastructure applications, where longevity is of interest, has been in part responsible for their slow acceptance. This paper therefore documents a novel approach to
the evaluation of polymeric-matrix composites for their hygrothermal–mechanical properties, and contributes to the current discourse on their applications as infrastructure materials. This discussion looks at the effect of moisture adsorption, cycling and comparative fatigue in water and in a simulated sea environment on the shortterm durability of pultruded glass-reinforced polymeric composites by performing in situ fatigue cycling in both fresh- and salt-water environments.
2. Experimental procedure The material system used in this study was EXTREN, which was pultruded by Strongwell, Inc. (Bristol, VA). The glass/vinyl ester composite consists of five main layers: three layers of unidirectional glass sandwiched between two substantially thicker layers of continuous strand mat for a total thickness of 0.125 in. (3.175 mm). Strongwell reported the average fiber volume fraction to be between 0.28 and 0.30, which was obtained through matrix burn-off tests. The pultruded material was delivered as boards, 1.5 ft (45.7 cm) by 4 ft (122 cm), which were cut into coupons using a water-cooled diamond saw. Coupon sizes of 9 in. (22.9 cm) by 1 in. (2.5 cm) were used for specimens loaded in a fluid cell (described below). All other tests were performed on 6 in. (15.2 cm) by 1 in. (2.5 cm) coupons. The coupons were cut with the long direction parallel to the unidirectional glass fibers. The edges of all the coupons were sanded smooth with 180 grit sandpaper to ensure uniformity of the specimens and to prevent premature failure due to edge effects. The edges of the specimens were then coated with a Buehler two-part Epoxide resin and cured for 2 h at 65°C to minimize edge absorption during exposure of the material to humid environments. Pre-conditioned specimens were placed in Plexiglass racks designed to ensure equal and maximum exposure of all surfaces of the specimen to both water and 3.5% by weight salt solution. The aging apparatus consisted of 20 gallon glass tanks which facilitated specimen monitoring without disturbing the set-up. Heaters were mounted in each corner of the rectangular tank. An Omega controller was used to maintain temperatures in the tank by regulating power to the heaters and monitoring tank temperature. Two pumps were placed in the tank to ensure circulation and uniform heat distribution. The tank was covered with bubble wrap and a Plexiglass lid, and the entire tank was insulated with Styrofoam. For tests employing salt water, in order to ensure consistency in the concentration of salt in solution, a fresh solution was made for each batch. The salinity of the solution was monitored by a salimeter and adjusted periodically by adding fresh solution. Specimens subjected to cyclic moisture absorption–
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desorption were placed in tanks similar to the one described above. To facilitate desorption, specimens were removed from the tanks and placed in a convection oven at the same temperature as the water bath from which they were removed. The specimens were kept in the oven for a fixed length of time. The average absorption–desorption cycle length was approximately 1 month, with 28 days in a water bath and 3 days in an oven. In all, five cycles were applied to the specimens. In an attempt to try and separate temperature effects from that of moisture, a batch of samples was subjected to pure thermal aging in a convection oven at the same temperature as that of the water bath. The duration of exposure was equal to one cycle as explained above. The complete test matrix is shown in Table 1. Quasistatic tests were performed at a loading rate of 0.05 in. (0.127 cm) per minute. Fatigue tests were conducted at a maximum-to-minimum load ratio of R=0.1 at frequencies of 2 Hz and 10 Hz. No detectable heating of the specimens was observed during the fatigue tests. A fluid cell (Fig. 1) was used for the fatigue tests conducted in salt water. The fluid cell [2,3] consists of two 4 in. (10.2 cm) by 3 in. (7.62 cm) Plexiglass plates, each with a recess to accommodate the specimen once closed. The specimen was sandwiched between the plates and sealed with a silicone adhesive. Inlet and outlet hoses were attached to the cell. which were then attached to a temperature-controlled water circulator. The specimen/fluid cell assembly could be inserted into a test frame and fatigue tests could be conducted on the specimen. The fluid cell had no effect on the failure of the samples. This was checked through a series of tests with and without the cell.
Table 1 Test matrix detailing specimen pre-conditioning and test conditions Pre-conditioning
Mechanical test Quasi-static
Fatigue
Test condition Air As-delivered Tap water (45°C) Salt water (65°C) Thermal (45°C) Two cycles/dry Two cycles/wet Five cycles/dry Five cycles/wet a b
Fluid cell
× × × × × × × ×
Tested at 10 Hz and 20 Hz. Tested at 2 Hz and 10 Hz.
Air ×a ×
×
Fluid cell
×b
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3. Results 3.1. Quasi-static properties following moisture aging and cycling The quasi-static properties were evaluated for the asreceived, salt-aged and water-aged EXTREN material system using Weibull statistical methods. The plot of the residual quasi-static properties normalized to the asreceived quasi-static properties is shown in Fig. 2 with the corresponding standard deviation bars. The distribution of static strength properties does not seem to vary significantly in shape. The dimensionless shape parameter a is constant for the three conditions, namely: asreceived (dry), wet and salt water. Table 2 summarizes the mechanical properties of the composite after exposure to the different aging environments. Also listed in the table are the A and B allowables for the strength of the composite. Fig. 3 shows the moisture weight gain during the cyclic absorption–desorption experiment. The numbers on the plot serve as locators and are points at which samples were removed for testing. These numbers will be used to present data in this paper. The stiffness, strength and strain-to-failure for the cyclically aged material are illustrated in Fig. 4. As seen in the chart, there is a significant difference between the as-delivered and the cyclically aged materials. There is however a larger difference in strength than in stiffness, but, in both cases, there appears to be a significant change. The properties of the thermally aged material do not appear to differ from those of the as-delivered material. 3.2. Enviro-mechanical fatigue performance The fatigue curves for the as-delivered, fresh-wateraged and salt-water-aged materials are shown in Fig. 5. The curves for the fresh-water- and salt-water-aged materials are nearly identical. The slopes of all S–N curves appear to be similar, suggesting the existence of a common damage mechanism. Fig. 6 shows the comparative S–N plot for samples tested dry at two different test frequencies. There seems to be some difference in the degradation of the glass-fiber-reinforced composite under these conditions as evinced by the slope of the fits. Fig. 7 represents the S–N curves for the as-delivered, wet and five-cycles moisture-aged specimens. On Fig. 3, these would denote points 1, 2 and 7. The curves appear to be slightly different. The material tested was, however, in different states (i.e., dry after five cycles and wet after one cycle). It is seen that the effect of moisture on the slope of the S–N curve is negligible even though the specimens were subjected to both isothermal as well as cyclic aging conditions. Fig. 8(a), (b) and (c) show the residual strength changes for the as-delivered, wet and salt-water-aged specimens.
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Fig. 1.
Fig. 2.
Experimental set-up showing the hygrothermal chamber and a schematic illustration of the fluid cell.
Properties of as-delivered, fresh-water-aged and salt-water-aged materials along with the A and B allowables.
4. Discussion 4.1. Quasi-static performance following moisture aging and cycling The results indicate that the quasi-static modulus in the dry specimens underwent a change of 11% and the strength a reduction of 32% following aging in salt water. The Poisson’s ratio increased by 6%. Data from
the fresh-water-aged specimens indicate a decrease in modulus of about 11% and 32% in ultimate tensile strength, from dry to water-aged. There was no change in Poisson’s ratio for this case. The results indicate little difference between the material properties of the waterand salt-solution-aged materials. The results of the cyclic moisture ingression indicate that the quasi-static material properties are reduced significantly after the first conditioning cycle, but damage does not continue to
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Table 2 Mechanical properties along with A and B allowables for strength on as-delivered, tap-water-aged and salt-water-aged specimens (SD=standard deviation)
As-delivered Tap water Salt water
Fig. 3.
Modulus (GPa)
Poisson’s ratio
Strength (MPa)
A allowable B allowable
Mean
SD
Mean
SD
Mean
SD
Mean
Mean
15.5 13.3 13.85
0.67 1.2 2
0.31 0.31 0.33
0.03 0.03 0.03
212 145.5 145.05
17.95 13.4 14.47
149.9 102.6 100.2
179.6 122.6 121.7
Moisture uptake plot during the cyclic aging process. Tests were performed by immersion in a water tank at 45°C.
Fig. 4. Normalized stiffness, strength and strain-to-failure of cyclically aged material. The legend follows the format [x, description, y], where ‘x’ stands for the number of cycles and ‘y’ stands for the corresponding location shown in Fig. 3.
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Fig. 5. S–N curves of the as-delivered, water-aged and 3.5% salt-solution-aged materials.
Fig. 6. Effect of test frequency: S–N curves of the material tested at 2 Hz and 10 Hz.
accumulate with additional cycles and exposure time. Performing a test of means with a significance level of 1% on the cyclically aged materials indicates that we must accept the null hypothesis that the strength of the as-delivered material is different from that of the other materials tested except for the material subjected to thermal relaxation. The implications from this statement are that all other material states are different from the asdelivered material, while the thermally relaxed material is similar. Aging in water has significant effects on the material, and most of these effects are irreversible upon desorption. The test of means analysis also indicated that there is a significant reduction in strength from the first
moisture cycle, similar to what was observed by Schultheisz et al. [13] for glass/epoxy composites, and further cycling did not cause significant changes in material properties. The properties at the second and fifth cycles are similar to those at the wet and dry stages. This confirmed the idea that the initial damage is not recoverable when the material is allowed to return to its initial moisture content, suggesting that there is damage to either the matrix, interphase, fiber or a combination. Nevertheless, it is also true that samples that were dried after the first cycle showed some signs of partial recovery in strength. This indicates that a part of the total drop in properties can be attributed to matrix plasticization. The remaining,
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Fig. 7. S–N curves for as-delivered, one-cycle/wet and five-cycles/dry materials.
irrecoverable part is probably due to degradation of the interface and fiber. It is also conceivable that residual stresses which develop in the composite during its production could also play a part in the mechanical property changes observed. However, Fig. 4 shows that that the ‘thermal’ data are statistically similar to the initial properties; thus, the degradation cannot be attributed to thermal relaxation of residual stresses. Preliminary investigation using scanning electron microscopy (SEM) [Fig. 9(a) and (b)] indicates the existence of damage to both the fiber itself and cracks about the interface region in the fresh-water-aged sample, compared with the as-delivered. Weitsman [14] reports similar findings for graphite/epoxy composites. The ongoing study is focused on quantifying the damage in the aged composites and relating the damage to aging history. 4.2. Enviro-mechanical fatigue performance The relationship between the as-delivered and aged materials is the shift in the intercept of the S–N curve (Fig. 5), or the ultimate tensile strength (UTS). When the data for each material state are normalized with respect to their respective UTS, the curves collapse on top of each other. Conducting a statistical test of coincidence [15] indicates that the slopes of the lines are similar. The lines are normalized and plotted in Fig. 10. This similarity in slope is independent of prior moisture exposure, moisture content or type of moist environment. In other words, the aged and the unaged EXTREN underwent a similar fatigue-induced failure mechanism. The slope B of the S–N curves for all test conditions ranges from 9.9 to 12.7%.
The fatigue frequency effect depicted on Fig. 6 shows a slight increase in damage at 2 Hz compared with that at 10 Hz. This could be due to the statistical nature of fatigue data or due to damage accumulation with increasing time spent at a given applied stress level, as detailed by McBagonluri [1]. The S–N curves for the as-delivered, one-cycle/wet and five-cycles/dry materials, normalized with respect to their respective Sult, are depicted in Fig. 11. Statistical analysis performed on the as-delivered and onecycle/wet data indicated that the two normalized S–N curves are the same. The five-cycles/dry material is within the 95% confidence bands of the dry material, but the curve is not similar to the as-delivered and onecycle/wet curves. The slope of the S–N curve decreases, as does its intercept. Performing tests of means on the individual data sets with a significance level of 1% indicates that the null hypothesis must be accepted that the cyclically aged wet material and cycled dry material are different. Test of means also indicates that the wet cycled materials are similar and the dry cycled materials are similar. This indicates the importance of the final state the material was in when it was tested. It does, however, also suggest that the number of cycles does not prove to be very critical, a conclusion which is consistent with the data as seen in Fig. 4. The residual strength curves for the as-delivered, tapwater-aged and salt-water-aged specimens are shown in Fig. 8(a), (b) and (c). The curves were generated by fitting the data with Eq. (1), a form of the residual strength equation suggested by Broutman and Sahu [16]. This equation is derived by considering remaining strength to be a measure of damage and postulating that normalized remaining strength (referenced to the initial strength) is
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Fig. 8.
Remaining strength curves for: (a) as-delivered material; (b) one-cycle/wet material; (c) salt-water-aged material.
an internal state variable. Using thermodynamic arguments for the identified ‘critical element’ [17] and assuming a power-law form for the kinetics equation, the following equation is derived for a ‘critical element’ in the laminate:
冉 冊冉 冊
Sa n a Sres ⫽1⫺ 1⫺ . Sult Sult N
(1)
In this equation, Sres is the residual strength, Sult is the ultimate tensile strength of the material, Sa is the applied
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Fig. 9. SEM micrographs of: (a) as-delivered material; (b) fresh-water-aged (magnification is ×3000 in both micrographs). The micrograph in (b) clearly indicates the existence of damage both in the form of fiber breakage and fiber/matrix interface cracking.
stress on the material, n is the number of loading cycles applied to a specimen, and N is the life of the specimen. The parameter a is a characteristic of the material included to account for the non-linearity of the strengthreduction curve and may be determined by curve-fitting the data to Eq. (1). Curves of the form of Eq. (1) were fit to the data for each load level and average values for a range from 1.0 to 1.2. The damage mechanisms for the moisture-cycled material appear to be changing. One possible reason could be toughening of the matrix due to plasticization. Glass/vinyl ester fatigue data discussed here for various conditions — namely, temperature, moisture type and frequency effects — are shown with Mandell’s data in Fig. 12. It is interesting to note that the slopes B on these
plots are between 10% and 11% UTS/decade, which is consistent with the results obtained by Mandell [9]. Mandell and co-workers investigated the fatigue effect due to fiber orientation, fiber volume fraction, resin type and glass fiber type in polymeric composites. Mandell et al. observed a similar trend in the fatigue slope for the aforementioned variables, concluding that the failure mechanism could be attributed to a fiber-dominated process. In other words, glass-fiber composite fatigue failure seems to occur as a result of gradual deterioration of the load-bearing fibers, and is independent of fiber volume fraction, resin type, glass fiber type and fiber orientation. The results presented in this paper suggest that this process is also not influenced by short-term exposure to fresh and salt water, frequency of testing and exposure temperature.
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Fig. 10.
Normalized S–N curves for as-delivered, water-aged and 3.5% salt-solution-aged materials.
Fig. 11.
Normalized S–N curves for as-delivered, one-cycle/wet and five-cycles/dry materials.
Subsequent work by Sims and Gladman [18] on the effect of pre-conditioning (boiled and unboiled), preinduced mechanical damage and orientation of reinforced material on glass fiber fatigue collaborated Mandell’s findings. Sims and Gladman concluded that, “Independent of pre-conditioning treatment, orientation of reinforcement or preloading-induced mechanical damage, Mandell’s stipulation lays credence to the concept of a monotonic fatigue failure mechanism for glass fiberreinforced composite”. These results seem to validate Mandell’s postulate that a fiber-dominated mechanism is responsible for final composite failure, and further
extended Mandell’s variables to include pre-conditioning, preloading and fiber architecture (woven). One could also infer that the effect of temperature is implicit in the pre-conditioning aspect of Sims and Gladman’s affirmation.
5. Conclusions A fiber-dominated mechanism has been established as the dominant mechanism responsible for the failure of EXTREN under dry, wet and simulated sea-water
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Fig. 12. Mandell’s relationship between tensile strength and slope of the S–log N curve, ‘B’, for different materials. Values are included for EXTREN for various frequencies and testing conditions. The image on the right is an enhanced view of the data indicated on the left.
environments. The aged material, in either fresh or salt water, experiences a one-time drop in strength with moisture ingress. Results obtained for the inverse slopes of the stress levels versus life diagrams are consistent with results reported by Mandell and co-workers for glass reinforcement. The temperature effect on fatigue response did not depend on the presence or absence of the ambient fluid. Furthermore, fatigue damage evolution and subsequent failure of fiber-reinforced composites have been found to be independent of moisture content or moisture regime in the short term, although long-term aging and moisture ingress appear to affect the fatigue performance of the material.
Acknowledgements The authors would like to thank the Building and Fire Research Laboratory at the National Institute of Standards and Technology (NIST), the National Science Foundation (NSF) Career Award, Strongwell and 3M for their support.
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[4] McBagonluri F, Hayes M, Garcia K, Lesko JJ. Durability of Eglass vinyl ester in an elevated temperature simulated sea water environment. In: Proceedings of the Seventh International Conference on Marine Applications of Composite Materials (MACM ’98), Melbourne, FL, 17–19 March, Composites Education Association, Inc, 1988. [5] McBagonluri F, Hayes M, Garcia K, Lesko JJ. Durability of glass/vinyl ester composites in a simulated sea environment. In: Proceedings of the American Society of Mechanical Engineers (ASME) Conference, Dallas, TX, 15–18 November. Massachusetts: ASME Technical Publishing Department, 1997. [6] Lesko JJ, Hayes M, McBagonluri F, Verghese N, Garcia K. Environmental–mechanical durability of glass/vinyl ester composites. In: Proceedings of the Third International Conference on Progress in Durability Analysis of Composites Systems (DURACOSYS), Blacksburg, VA, 14–17 September. Rotterdam: A.A. Balkena, 1997. [7] Demers C. E-glass fiber-reinforcement polymeric composites, tension–tension axial fatigue life diagram. Presented at The National Seminar on Advanced Composite Material Bridges, 5– 7 May 1997. Federal Highway Administration (FHWA). [8] Mandell JF, Meier U. Effects of stress ratio, frequency, loading time on tensile fatigue of glass-reinforced epoxy. In: O’Brien TK, editor. Long-term behavior of composites. Philadelphia (PA): American Society for Testing and Materials, 1983:55–77. [9] Mandell JF. Fatigue behavior of fiber–resin composites. In: Pritchard G, editor. Developments in reinforced plastics 2. London: Applied Sciences Publisher, 1978. [10] Jones CJ, Dickson RF, Adams T, Reither H, Harris B. The environmental fatigue behavior of reinforced plastics. Proc R Soc Lond A 1984;396:313–38. [11] Bradely WL, Grant TS. The effects of the moisture absorption in interfacial strength of polymeric composites. J Mater Sci 1995;30:5537–42. [12] Grant TS, Bradely WL. In situ observations in SEM of degradation of graphite/epoxy composite materials due to seawater immersion. J Compos Mater 1995;29(7):852–67. [13] Schultheisz CR, McDonough WG, Kondagunta S, Schutte CL, Macturk KS, McAuliffe M, Hunston DL. Effect of moisture on E-glass/epoxy interfacial and fiber strengths. Presented at: 13th Symposium on Composite Materials: Testing and Design. Philadelphia (PA): American Society for Testing and Materials, 1996. [14] Weitsman Y. Coupled damage and moisture-transport in fiberreinforced, polymeric composites. Int J Solid Struct 1987;23(7):1003–25.
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[15] Standard practice for statistical analysis of linear or linearized stress–life and strain–life fatigue data. ASTM E 739. Philadelphia (PA): American Society for Testing and Materials. [16] Broutman LJ, Sahu S. A new damage theory to predict cumulative fatigue damage in fiberglass reinforced plastics. In: Composite materials: testing and design (second conference). ASTM STP 497, Philadelphia (PA): American Society for Testing and Materials, 1972:170–88.
[17] Reifsnider KL, Stinchcomb WW. A critical element model of the residual strength and life of fatigue-loaded composite coupons. In: Hahn HT, editor. Composite materials: fatigue and fracture. ASTM STP 407, Philadelphia (PA): American Society for Testing and Materials, 1986:298–303. [18] Sims GD, Gladman DG. Effect of test conditions on the fatigue strength of a glass-fabric laminate. Part B — Specimen condition. Plast Rubb Mater Appl 1980;1:122–8.