Characterization of fatigue and combined environment on durability performance of glass/vinyl ester...

International Journal of Fatigue 22 (2000) 53–64www.elsevier.com/locate/ijfatigue

Characterization of fatigue and combined environment ondurability 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 thosein aerospace, the issue of durability becomes more critical. The tolerance of composites to damage induced by cyclic loading andmoisture ingress is of utmost importance. This study highlights the effects of short-term cyclic moisture aging on the strength andfatigue performance of a glass/vinyl ester pultruded composite system. In particular, it addresses the change in quasi-static propertiesand tension–tension (R=0.1) fatigue behavior of a commercial glass/vinyl ester system in fresh and salt water. The quasi-statictensile strength was seen to reduce by 24% at a moisture concentration of 1% by weight. This reduction in strength was notrecoverable even when the material was dried, suggesting that the exposure to moisture caused permanent damage in the materialsystem. Even though the fatigue damage process of the unaged or ‘as-delivered’, fresh-water- and salt-water-saturated material wassimilar, 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-domi-nated 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 madein the sector of infrastructure composite materials andengineering within the past few years [1–7], there stillremain technological challenges. These challengesinclude the development of reliable test methods, lessexpensive yet sophisticated and specialized materials,viable and robust life prediction tools, as well as con-sideration of the environmental and mechanical responseof these materials. Other significant hurdles include theabsence of comprehensive data characterizing the long-term durability of glass-fiber-reinforced polymeric com-posites coupled with the absence of adequate establishedstandards for repair, design and maintenance.

* Corresponding author. Tel.:+1-540-231-5259; fax:+1-540-231-9187.

E-mail address:[email protected] (J.J. Lesko)

0142-1123/00/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved.PII: S0142-1123 (99)00100-0

Significant attempts have been made to address someof these pressing issues. Some of the work that servesas a precedent to the results presented in this paper orig-inates from Mandell and Meier [8]. Mandell and Meiernote that fatigue failure in general is characterized bythe progressive accumulation of cracks in the matrix andat the fiber/matrix interface, resulting in a loss of remain-ing strength and stiffness. This progressive reduction inremaining strength reaches a limiting point where itequals the applied stress and consequently results in fail-ure. This general trend in failure does not applyexplicitly to tensile failure in glass-reinforced com-posites, where failure appears to be fiber-dominated, andit is independent of matrix or interface type. Mandellnoted that this damage mechanism for fiber-reinforcedcomposite was true for cyclic as well as static fatigue(creep), and that the stress level versus cycles-to-failurecurve properties are translated into withstanding crackinitiation and propagation. However, Mandell cautionsthat exceptions to the fiber-dominated mechanism may

54 F. McBagonluri et al. / International Journal of Fatigue 22 (2000) 53–64

exist especially when a severe fatigue mechanism is atplay; for instance, in woven fabric reinforcement and invery ductile matrices. Mandell [9] conducted a series ofexperiments using polymeric matrix, glass-reinforcedcomposites. These tests indicated an inverse stress versuslife 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 slopewas characteristic of the fatigue behavior of glass-fiber-reinforced polymeric-matrix composites. Jones et al. [10]also reported similar observations. Transverse tensiletests conducted by Bradely and Grant [11,12] indicatedthat the strength after saturation depends heavily on boththe properties of the matrix and the shear strength of theinterface. An environmental scanning electron micro-scope (ESEM) was used to facilitate in situ damage evol-ution and propagation observation. Environments suchas immersion at room temperature in fresh and simulatedsea-water at atmospheric pressure and 20.7 MPa to simu-late deep-water applications were investigated. Micro-indentation tests were also performed to evaluate theinterfacial shear strength and to compare it to the trendsobserved in the strength data. It was noted that com-posites made of very brittle matrices promoted damagevia matrix cracking, leading to low strengths. Further,these composites showed an increase in strength aftersaturation because of the plasticization effect of thematrix. On the other hand, the strength of compositesmade of matrices that are more ductile but develop poorfiber/matrix interfaces remained unchanged throughoutthe study since the damage was always restricted to thefiber/matrix interface region. Composites that did showsignificant changes in strength after saturation also indi-cated a change in damage mode, namely from beingmatrix-dominated before aging to interfacial after satu-ration. The moisture uptake data also showed that satu-ration contents for salt-water-aged samples were lessthan for their corresponding fresh-water counterparts.This was attributed to the osmotic effect in the salt-watersolutions. Little difference in strength was therefore seento exist between the three exposure environments.

These attempts seek to facilitate an understanding ofthe response of fiber-reinforced composite systems to theapplied environment, and also to serve as a precursor tothe development of reliable analytical and life predictiontools. Fiber-reinforced composite data currently avail-able are industry-specific, with most of the current databelonging to the aerospace and petrochemical industrieswhere years of experience with composites have resultedin a database, while little data are available for the mar-ine and infrastructure sectors. The absence of data onglass-fiber-reinforced composites for marine and infra-structure applications, where longevity is of interest, hasbeen in part responsible for their slow acceptance.

This paper therefore documents a novel approach to

the evaluation of polymeric-matrix composites for theirhygrothermal–mechanical properties, and contributes tothe current discourse on their applications as infrastruc-ture materials. This discussion looks at the effect ofmoisture adsorption, cycling and comparative fatigue inwater and in a simulated sea environment on the short-term durability of pultruded glass-reinforced polymericcomposites by performing in situ fatigue cycling in bothfresh- and salt-water environments.

2. Experimental procedure

The material system used in this study wasEXTREN, which was pultruded by Strongwell, Inc.(Bristol, VA). The glass/vinyl ester composite consistsof five main layers: three layers of unidirectional glasssandwiched between two substantially thicker layers ofcontinuous strand mat for a total thickness of 0.125 in.(3.175 mm). Strongwell reported the average fiber vol-ume fraction to be between 0.28 and 0.30, which wasobtained 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 couponsusing a water-cooled diamond saw. Coupon sizes of 9 in.(22.9 cm) by 1 in. (2.5 cm) were used for specimensloaded in a fluid cell (described below). All other testswere performed on 6 in. (15.2 cm) by 1 in. (2.5 cm) cou-pons. The coupons were cut with the long direction par-allel to the unidirectional glass fibers. The edges of allthe coupons were sanded smooth with 180 grit sandpaperto ensure uniformity of the specimens and to preventpremature failure due to edge effects. The edges of thespecimens were then coated with a Buehler two-partEpoxide resin and cured for 2 h at 65°C to minimizeedge absorption during exposure of the material tohumid environments.

Pre-conditioned specimens were placed in Plexiglassracks designed to ensure equal and maximum exposureof all surfaces of the specimen to both water and 3.5%by weight salt solution. The aging apparatus consistedof 20 gallon glass tanks which facilitated specimenmonitoring without disturbing the set-up. Heaters weremounted in each corner of the rectangular tank. AnOmega controller was used to maintain temperatures inthe tank by regulating power to the heaters and monitor-ing tank temperature. Two pumps were placed in thetank to ensure circulation and uniform heat distribution.The tank was covered with bubble wrap and a Plexiglasslid, and the entire tank was insulated with Styrofoam.For tests employing salt water, in order to ensure consist-ency in the concentration of salt in solution, a fresh sol-ution was made for each batch. The salinity of the sol-ution was monitored by a salimeter and adjustedperiodically by adding fresh solution.

Specimens subjected to cyclic moisture absorption–

55F. McBagonluri et al. / International Journal of Fatigue 22 (2000) 53–64

desorption were placed in tanks similar to the onedescribed above. To facilitate desorption, specimenswere removed from the tanks and placed in a convectionoven at the same temperature as the water bath fromwhich they were removed. The specimens were kept inthe oven for a fixed length of time. The average absorp-tion–desorption cycle length was approximately1 month, with 28 days in a water bath and 3 days in anoven. In all, five cycles were applied to the specimens.In an attempt to try and separate temperature effectsfrom that of moisture, a batch of samples was subjectedto pure thermal aging in a convection oven at the sametemperature as that of the water bath. The duration ofexposure was equal to one cycle as explained above.

The complete test matrix is shown in Table 1. Quasi-static tests were performed at a loading rate of 0.05 in.(0.127 cm) per minute. Fatigue tests were conducted ata maximum-to-minimum load ratio ofR=0.1 at fre-quencies of 2 Hz and 10 Hz. No detectable heating ofthe specimens was observed during the fatigue tests.

A fluid cell (Fig. 1) was used for the fatigue tests con-ducted in salt water. The fluid cell [2,3] consists of two4 in. (10.2 cm) by 3 in. (7.62 cm) Plexiglass plates, eachwith a recess to accommodate the specimen once closed.The specimen was sandwiched between the plates andsealed with a silicone adhesive. Inlet and outlet hoseswere attached to the cell. which were then attached to atemperature-controlled water circulator. Thespecimen/fluid cell assembly could be inserted into a testframe and fatigue tests could be conducted on the speci-men. The fluid cell had no effect on the failure of thesamples. This was checked through a series of tests withand without the cell.

Table 1Test matrix detailing specimen pre-conditioning and test conditions

Pre-conditioning Mechanical test

Quasi-static Fatigue

Test condition

Air Fluid cell Air Fluid cell

As-delivered × ×a

Tap water (45°C) × ×Salt water (65°C) × ×b

Thermal (45°C) ×Two cycles/dry ×Two cycles/wet ×Five cycles/dry × ×Five cycles/wet ×

a Tested at 10 Hz and 20 Hz.b Tested at 2 Hz and 10 Hz.

3. Results

3.1. Quasi-static properties following moisture agingand cycling

The quasi-static properties were evaluated for the as-received, salt-aged and water-aged EXTREN materialsystem using Weibull statistical methods. The plot ofthe residual quasi-static properties normalized to the as-received quasi-static properties is shown in Fig. 2 withthe corresponding standard deviation bars. The distri-bution of static strength properties does not seem to varysignificantly in shape. The dimensionless shape para-metera is constant for the three conditions, namely: as-received (dry), wet and salt water. Table 2 summarizesthe mechanical properties of the composite afterexposure to the different aging environments. Also listedin the table are theA andB allowables for the strengthof the composite. Fig. 3 shows the moisture weight gainduring the cyclic absorption–desorption experiment. Thenumbers on the plot serve as locators and are points atwhich samples were removed for testing. These numberswill be used to present data in this paper. The stiffness,strength and strain-to-failure for the cyclically agedmaterial are illustrated in Fig. 4. As seen in the chart,there is a significant difference between the as-deliveredand the cyclically aged materials. There is however alarger difference in strength than in stiffness, but, in bothcases, there appears to be a significant change. Theproperties of the thermally aged material do not appearto differ from those of the as-delivered material.

3.2. Enviro-mechanical fatigue performance

The fatigue curves for the as-delivered, fresh-water-aged and salt-water-aged materials are shown in Fig. 5.The curves for the fresh-water- and salt-water-agedmaterials are nearly identical. The slopes of allS–Ncurves appear to be similar, suggesting the existence ofa common damage mechanism. Fig. 6 shows the com-parativeS–N plot for samples tested dry at two differenttest frequencies. There seems to be some difference inthe degradation of the glass-fiber-reinforced compositeunder these conditions as evinced by the slope of thefits. Fig. 7 represents theS–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 appearto be slightly different. The material tested was, how-ever, in different states (i.e., dry after five cycles andwet after one cycle). It is seen that the effect of moistureon the slope of theS–N curve is negligible even thoughthe specimens were subjected to both isothermal as wellas cyclic aging conditions. Fig. 8(a), (b) and (c) showthe residual strength changes for the as-delivered, wetand salt-water-aged specimens.


Fig. 1. Experimental set-up showing the hygrothermal chamber and a schematic illustration of the fluid cell.

Fig. 2. Properties of as-delivered, fresh-water-aged and salt-water-aged materials along with theA andB allowables.

4. Discussion

4.1. Quasi-static performance following moistureaging and cycling

The results indicate that the quasi-static modulus inthe dry specimens underwent a change of 11% and thestrength a reduction of 32% following aging in saltwater. The Poisson’s ratio increased by 6%. Data from

the fresh-water-aged specimens indicate a decrease inmodulus of about 11% and 32% in ultimate tensilestrength, from dry to water-aged. There was no changein Poisson’s ratio for this case. The results indicate littledifference between the material properties of the water-and salt-solution-aged materials. The results of the cyclicmoisture ingression indicate that the quasi-static materialproperties are reduced significantly after the first con-ditioning cycle, but damage does not continue to


Table 2Mechanical properties along withA and B allowables for strength on as-delivered, tap-water-aged and salt-water-aged specimens (SD=standarddeviation)

Modulus (GPa) Poisson’s ratio Strength (MPa) A allowable B allowable

Mean SD Mean SD Mean SD Mean Mean

As-delivered 15.5 0.67 0.31 0.03 212 17.95 149.9 179.6Tap water 13.3 1.2 0.31 0.03 145.5 13.4 102.6 122.6Salt water 13.85 2 0.33 0.03 145.05 14.47 100.2 121.7

Fig. 3. 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.


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 of1% on the cyclically aged materials indicates that wemust accept the null hypothesis that the strength of theas-delivered material is different from that of the othermaterials tested except for the material subjected to ther-mal relaxation. The implications from this statement arethat all other material states are different from the as-delivered material, while the thermally relaxed materialis similar. Aging in water has significant effects on thematerial, and most of these effects are irreversible upondesorption. The test of means analysis also indicated thatthere is a significant reduction in strength from the first

moisture cycle, similar to what was observed by Schul-theisz et al. [13] for glass/epoxy composites, and furthercycling did not cause significant changes in materialproperties. The properties at the second and fifth cyclesare similar to those at the wet and dry stages. This con-firmed the idea that the initial damage is not recoverablewhen the material is allowed to return to its initial moist-ure content, suggesting that there is damage to either thematrix, interphase, fiber or a combination. Nevertheless,it is also true that samples that were dried after the firstcycle showed some signs of partial recovery in strength.This indicates that a part of the total drop in propertiescan be attributed to matrix plasticization. The remaining,


Fig. 7. S–N curves for as-delivered, one-cycle/wet and five-cycles/dry materials.

irrecoverable part is probably due to degradation of theinterface and fiber. It is also conceivable that residualstresses which develop in the composite during its pro-duction could also play a part in the mechanical propertychanges observed. However, Fig. 4 shows that that the‘thermal’ data are statistically similar to the initialproperties; thus, the degradation cannot be attributed tothermal relaxation of residual stresses.

Preliminary investigation using scanning electronmicroscopy (SEM) [Fig. 9(a) and (b)] indicates the exist-ence of damage to both the fiber itself and cracks aboutthe interface region in the fresh-water-aged sample,compared with the as-delivered. Weitsman [14] reportssimilar findings for graphite/epoxy composites. Theongoing study is focused on quantifying the damage inthe aged composites and relating the damage to aginghistory.

4.2. Enviro-mechanical fatigue performance

The relationship between the as-delivered and agedmaterials is the shift in the intercept of theS–N curve(Fig. 5), or the ultimate tensile strength (UTS). Whenthe data for each material state are normalized withrespect to their respective UTS, the curves collapse ontop of each other. Conducting a statistical test of coinci-dence [15] indicates that the slopes of the lines are simi-lar. The lines are normalized and plotted in Fig. 10. Thissimilarity in slope is independent of prior moistureexposure, moisture content or type of moist environ-ment. In other words, the aged and the unagedEXTREN underwent a similar fatigue-induced failuremechanism. The slopeB of the S–N curves for all testconditions ranges from 9.9 to 12.7%.

The fatigue frequency effect depicted on Fig. 6 showsa slight increase in damage at 2 Hz compared with thatat 10 Hz. This could be due to the statistical nature offatigue data or due to damage accumulation with increas-ing time spent at a given applied stress level, as detailedby McBagonluri [1].

The S–N curves for the as-delivered, one-cycle/wetand five-cycles/dry materials, normalized with respect totheir respectiveSult, are depicted in Fig. 11. Statisticalanalysis performed on the as-delivered and one-cycle/wet data indicated that the two normalizedS–Ncurves are the same. The five-cycles/dry material iswithin the 95% confidence bands of the dry material,but the curve is not similar to the as-delivered and one-cycle/wet curves. The slope of theS–N curve decreases,as does its intercept. Performing tests of means on theindividual data sets with a significance level of 1% indi-cates that the null hypothesis must be accepted that thecyclically aged wet material and cycled dry material aredifferent. Test of means also indicates that the wetcycled materials are similar and the dry cycled materialsare similar. This indicates the importance of the finalstate the material was in when it was tested. It does,however, also suggest that the number of cycles doesnot prove to be very critical, a conclusion which is con-sistent with the data as seen in Fig. 4.

The residual strength curves for the as-delivered, tap-water-aged and salt-water-aged specimens are shown inFig. 8(a), (b) and (c). The curves were generated by fit-ting the data with Eq. (1), a form of the residual strengthequation suggested by Broutman and Sahu [16]. Thisequation is derived by considering remaining strength tobe a measure of damage and postulating that normalizedremaining strength (referenced to the initial strength) is


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 argu-ments for the identified ‘critical element’ [17] andassuming a power-law form for the kinetics equation,the following equation is derived for a ‘critical element’in the laminate:

Sres

Sult

512S12Sa

SultDSn

NDa. (1)

In this equation,Sres is the residual strength,Sult is theultimate tensile strength of the material,Sa is the applied


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 cyclesapplied to a specimen, andN is the life of the specimen.The parametera is a characteristic of the materialincluded to account for the non-linearity of the strength-reduction curve and may be determined by curve-fittingthe data to Eq. (1). Curves of the form of Eq. (1) werefit to the data for each load level and average values fora range from 1.0 to 1.2.

The damage mechanisms for the moisture-cycledmaterial appear to be changing. One possible reasoncould be toughening of the matrix due to plasticization.Glass/vinyl ester fatigue data discussed here for variousconditions — namely, temperature, moisture type andfrequency effects — are shown with Mandell’s data inFig. 12. It is interesting to note that the slopesB on these

plots are between 10% and 11% UTS/decade, which isconsistent with the results obtained by Mandell [9].Mandell and co-workers investigated the fatigue effectdue to fiber orientation, fiber volume fraction, resin typeand glass fiber type in polymeric composites. Mandellet al. observed a similar trend in the fatigue slope forthe aforementioned variables, concluding thatthe failuremechanism could be attributed to a fiber-dominated pro-cess. In other words, glass-fiber composite fatigue failureseems to occur as a result of gradual deterioration of theload-bearing fibers, and is independent of fiber volumefraction, resin type, glass fiber type and fiber orientation.The results presented in this paper suggest that this processis also not influenced by short-term exposure to fresh andsalt water, frequency of testing and exposure temperature.


Fig. 10. NormalizedS–N curves for as-delivered, water-aged and 3.5% salt-solution-aged materials.

Fig. 11. NormalizedS–N curves for as-delivered, one-cycle/wet and five-cycles/dry materials.

Subsequent work by Sims and Gladman [18] on theeffect of pre-conditioning (boiled and unboiled), pre-induced mechanical damage and orientation ofreinforced material on glass fiber fatigue collaboratedMandell’s findings. Sims and Gladman concluded that,“Independent of pre-conditioning treatment, orientationof reinforcement or preloading-induced mechanical dam-age, Mandell’s stipulation lays credence to the conceptof a monotonic fatigue failure mechanism for glass fiber-reinforced composite”. These results seem to validateMandell’s postulate that a fiber-dominated mechanism isresponsible for final composite failure, and further

extended Mandell’s variables to include pre-condition-ing, preloading and fiber architecture (woven). Onecould also infer that the effect of temperature is implicitin the pre-conditioning aspect of Sims and Gladman’saffirmation.

5. Conclusions

A fiber-dominated mechanism has been established asthe dominant mechanism responsible for the failure ofEXTREN under dry, wet and simulated sea-water


Fig. 12. Mandell’s relationship between tensile strength and slope of theS–log N curve, ‘B’, for different materials. Values are included forEXTREN 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 saltwater, experiences a one-time drop in strength withmoisture ingress. Results obtained for the inverse slopesof the stress levels versus life diagrams are consistentwith results reported by Mandell and co-workers forglass reinforcement. The temperature effect on fatigueresponse did not depend on the presence or absence ofthe ambient fluid. Furthermore, fatigue damage evol-ution and subsequent failure of fiber-reinforced com-posites have been found to be independent of moisturecontent or moisture regime in the short term, althoughlong-term aging and moisture ingress appear to affectthe fatigue performance of the material.

Acknowledgements

The authors would like to thank the Building and FireResearch Laboratory at the National Institute of Stan-dards and Technology (NIST), the National ScienceFoundation (NSF) Career Award, Strongwell and 3M fortheir support.

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