DAMAGE ACCUMULATION AND LIFETIME PREDICTION OF WOVEN GFRP UNDER CONSTANT TENSILE LOAD TEST IN HYDROCHLORIC ACID SOLUTION

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1 18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS DAMAGE ACCUMULATION AND LIFETIME PREDICTION OF WOVEN GFRP UNDER CONSTANT TENSILE LOAD TEST IN HYDROCHLORIC ACID SOLUTION M. Kotani 1 *, Y. Yamamoto 2, Y. Sato 2, R. Sato 2 and H. Kawada 1 1 Department o Applied Mechanics and Aerospace Engineering, Waseda University, Tokyo, Japan, 2 Department o Mechanics and Engineering, Graduate School o Waseda University, Tokyo, Japan * Corresponding author (robot_5ta2@toki.waseda.jp) Keywords: GFRP, Delayed Fracture, Durability, Hydrochloric Acid, Water Environment, SCC ABSTRACT Constant tensile load tests o woven glass iber reinorced plastics were conducted in air, in deionized water and in hydrochloric acid at 4 C. The delayed racture occurred in both solutions but did not occur in air in the range o this research. The racture time decreased with the increase in the applied stress and was shorter in hydrochloric acid than in deionized water. The resin adhesion on the glass iber surace decreased with the increasing test time and the racture surace o the glass iber lattened with the decrease o the applied stress. These transitions in the racture suraces exhibit the strength degradation o the glass iber and the iber/matrix interacial adhesion, i.e. the constituents o GFRP. The racture time o the woven GFRP was predicted based on the assumption o the global load sharing (GLS). 1 Introduction Glass-iber reinorced plastics (GFRP) have superior corrosive resistance compared to metal materials and has various applications such as wind turbine, industrial acility, marine structural materials. In act, GFRP operating in such services are based on damage tolerance design with excessive saety actor due to the lack o long-term reliability data. Unortunately, some ailure accidents o GFRP ater long-term operation under corrosive environments are reported [1]. Thus, signiicant research has been carried out dealing with the strength degradation and crack propagation properties o GFRP in corrosive environment to evaluate its long-term durability. It was clariied through these researches that such accidents occur due to the degradation o the materials properties and the stress corrosion cracking (SCC) due to the interaction between the applied stress and the environmental agents [2]-[3]. In addition, it was clariied that the applied stress progresses the degradations and the increasing temperature accelerates such degradations intensely [4]-[5]. However, there are a ew research which deals with the creep behavior o GFRP in corrosive environment. Jones has conducted a stress corrosion tests (creep test in H 2 SO 4 ) o unidirectional GFRP and inormed the transition o the racture surace and reported the possibility o the threshold stress o racture [6]. Kotani has conducted constant tensile load test o woven GFRP in hydrothermal environment and proposed the lietime prediction method based on the energy criterion [7]. The predicted results showed good agreement by considering the degradation o the GFRP induced by hydrothermal aging and its acceleration with increasing temperature. In act, there are GFRP constituents that possess high corrosion resistance and the ailure mechanism o corrosion resistant GFRP in corrosive environment is still uncertain. It is thereore required to clariy the mechanism o the delayed racture and to propose the lietime prediction method. There are signiicant researches proposing the prediction method on lietime and mechanical properties o composite materials. Turon has reported the possibility to calculate the mechanical properties o unidirectional composite by considering the degradation within its constituents rom Weibull parameters obtained rom the iber ragmentation test [8]. Kotani has predicted the strength degradation o unidirectional GFRP ater hydrothermal aging based on the assumption o global load sharing (GLS) and considering the strength degradation o the glass iber and the iber/matrix interacial adhesion [9]. The predicted results showed good agreement with the experimental data while considering the strength

2 degradations o the glass iber and the iber/matrix interacial adhesion which are obtained rom the iber ragmentation test. Present paper discusses the racture mechanism o corrosion resistant GFRP, consisted with NCR-glass iber and vinylester resin, in air, in deionized water and in hydrochloric acid. Firstly, static tensile load tests o woven GFRP were conducted in each environment in order to evaluate its mechanical properties. Constant tensile load tests were conducted in air, in deionized water and in hydrochloric acid to measure the racture time and the strain history. Finally, the racture time o the constant tensile load test was predicted using the strength o the constituents, i.e. glass iber, matrix resin and iber/matrix interacial adhesion. 2 Experimental 2.1 Specimen The specimen under study is the woven GFRP consisted by plain NCR-glass cloth or the iber reinorcement and vinylester resin or the matrix which both possess high corrosion resistance. The material properties o the constituents are summarized in Table 1. The GFRP plates were abricated by hot-press molding. The GFRP specimens were cut rom the plate, and the edges o each specimen were polished using emery paper in order to avoid edge eects. The specimen geometry or the static tensile tests and the constant tensile load tests was determined by the inite element analysis, and is shown in Figure 1. The average volume raction o the GFRP plate was approximately V =44%. Table 1: Material properties o GFRP constituents. Fiber reinorcement Resin matrix Material NCR-glass Vinylester Stiness E (GPa) Density (g/cm 3 ) Coeicient o thermal expansion (K -1 ) Static tensile test The static tensile tests o woven GFRP were conducted in air, in deionized water, and in one molar hydrochloric acid at 4 C in order to evaluate Fiber orientation t2. R its mechanical properties and to determine the stress levels o the constant tensile load tests in each experimental condition. The tensile loading was introduced by means o a 25-kN capacity testing machine. The testing speed was.5mm/min, and a strain gauge was bonded in the gauge section o the specimen to measure the axial strain during the test. The tests in both solutions, deionized water and hydrochloric acid, were conducted by attaching a vinyl bag and a silicon plug to the specimen, and illing the vinyl bag with each solution. The experimental setup or static tensile tests is shown in Figure Constant tensile load test 13 The constant tensile load tests o woven GFRP were conducted in air, in deionized water, and in one molar hydrochloric acid at 4 C in order to evaluate the strain history and the racture time. The constant tensile load tests were conducted using a creep testing machine. The tests in corrosive solutions; deionized water and hydrochloric acid, were conducted by attaching a vinyl bag which is illed 3 17 Figure 1: Specimen geometry or tensile test. 7 7 Figure 2: Experimental setup or static tensile tests in deionized water and hydrochloric acid.

3 DAMAGE ACCUMULATION AND LIFETIME PREDICTION OF WOVEN GFRP UNDER CONSTANT TENSILE LOAD TEST IN HYDROCHLORIC ACID SOLUTION with each solution to the specimen. A strain gauge was bonded in the gauge section o the specimen to measure the axial strain on the specimen during the test. The experimental setup is shown in Figure 3. loading direction. The relationship between both strengths is shown in the ollowing equation: GFRP UNI (1) The strength o the unidirectional GFRP will be calculated using the representative volume element (RVE) in present paper [9]. The RVE discussed in the present paper considers the eect o the interacial debonding length, which is shown in Figure 4. 2L d Figure 3: Experimental setup or constant tensile load tests in deionized water and hydrochloric acid. 3 Lietime prediction The racture time o GFRP under constant tensile load in deionized water and in hydrochloric acid was predicted based on the assumption o the global load sharing (GLS). This assumption is widely used in the strength calculation o the unidirectional composites which possess low iber/matrix interacial adhesion, or example C/C composites. Due to the weak iber/matrix interacial adhesion, the stress concentration in the vicinity o the broken iber and the adjacent ibers is ignored in the strength calculation. It is obvious that the iber/matrix interacial adhesion o GFRP ater immersion in deionized water and in hydrochloric acid decreases and leads to the assumption o the GLS. In act, residual strength o unidirectional GFRP ater hydrothermal aging, i.e. immersion in deionized water, was calculated by the assumption o GLS and showed a good agreement with the experimental data by considering the degradation o its constituents [9]. Besides, it is reported in previous researches that the racture o woven GFRP are dominant to the racture o the glass iber aligned toward the loading direction, so we also assumed that the strength o the woven GFRP GFRP would be proportional to the strength o the unidirectional GFRP UNI whose iber is aligned toward the First, the average iber stress stress o the broken iber Lr Figure 4: Representative volume element (RVE) o unidirectional GFRP. is expressed by the ave and the unbroken iber using the cumulative probability o iber ailure b P() as ollowing equation: ave 1 P( ) b P( ) (2) L' r Q P( ) 1 exp L where L r is the stress recovery length, L is the reerential length (=25mm), is the scale parameter and m is the shape parameter. The stress recovery length L r considers the eect o the interacial debonding length L d. The eective length considering is expressed in the ollowing equation [1]: L' r Lr L (3) d The interacial debonding length is expressed in ollowing equation [11]-[12]: i 1 H 2 m 4 L d ln 1 (4) z,swell H1 d where, and H 2 are constants, H 1 is the unction m o the iber stress in the longitudinal direction, z,swell is the swelling stress o the matrix resin in the longitudinal direction induced by water absorption, i is iber/matrix interacial shear stress and d is the iber diameter. H 1 and H 2 are: 3

4 H H 1 2 m m m E z,swell m m 1 E 1 E E m m 1 E 1 E m E (5) where is the coeicient o riction. Finally, the strength o the GFRP GFRP will be calculated using the rule o mixture as ollows: GFRP UNI avevuni m 1 VUNI (6) where V UNI is the iber volume raction o the composite in the loading direction (V UNI =V /2=22%). In the lietime prediction, the strength degradation o the glass iber ( and m) and the iber/matrix interacial adhesion ( i ) were substituted into the calculation to consider the degradation during the constant tensile load tests. 4 Results and Discussions 4.1 Mechanical properties o woven GFRP The stress-strain curves obtained rom the static tensile tests at various conditions are shown in Figure 5. It is described in the igure that the stressstrain curve shows a viscoelastic response. This is due to the mechanical properties o the matrix resin because that the GFRP has relatively small volume raction toward the loading direction. It is also shown in the igure that the mechanical properties o woven GFRP decrease in the deionized water and in hydrochloric acid. The mechanical properties o woven GFRP in various environments are listed in table 2. The ultimate tensile strength UTS and the rupture strain decrease 1.3% and 16.2% respectively in spite o with its high corrosion resistance. These decreases in the mechanical properties are induced by the damage accumulation which is generated by the solutions. Besides, the experimental condition, i.e. the stress level, o the Table 2: Mechanical properties o GFRP in various environments Environment Tensile strength Rupture strain Stiness UTS (MPa) (%) E (GPa) Deionized water HCl (1.mol/l) constant tensile load test was determined rom the results o the static tensile tests. Stress (MPa) 4.2 Delayed racture o GFRP in corrosive environment The strain curve obtained rom the constant tensile load tests in various solutions is shown in Figure 6. It is depicted in the igure that the strain is higher in the order o air, deionized water, and hydrochloric acid. It is suggested that such dierence is induced by the decrease in the stiness due to the immersion into each solution which can be ascertained in the results o the static tensile tests. Fracture occurred in deionized water and in hydrochloric acid but did not occur in air. In addition, rupture strain o the constant tensile load tests were lower than those Strain (%) Strain (%) Figure 5: Stress-strain curves o woven GFRP in various environments HCl (1.mol/l) Knee point HCl (1.mol/l) Time t (h) Figure 6: Strain behavior o woven GFRP in various environments ( app / UTS =.6).

5 DAMAGE ACCUMULATION AND LIFETIME PREDICTION OF WOVEN GFRP UNDER CONSTANT TENSILE LOAD TEST IN HYDROCHLORIC ACID SOLUTION obtained rom the static tensile tests. It is obvious that the strength o the woven GFRP is decreasing during the tests in both solutions. The relationship between the applied stress and the racture time is shown in Figure 7. The racture time in hydrochloric acid is shorter than that in deionized water. In addition, the racture time lengthens with the decrease in the applied stress and shows the possibility o the existence o the threshold stress o racture. Applied stress app (MPa) HCl (1.mol/l) Fracture time t (h) Figure 7: Relationship between the applied stress and racture time obtained rom constant tensile load test in various environments. The racture suraces o woven GFRP ater constant tensile load tests were observed using scanning electron microscope (SEM). The racture surace o woven GFRP in deionized water is shown in Figure 8 and in hydrochloric acid is shown in Figure 9. The SEM pictures in low magniication show the surace o the glass iber which is aligned in the transverse direction and pictures in high magniication shows the racture surace o the glass iber in the loading direction, respectively. It is obvious in the racture suraces that the resin adhesion on the iber surace decreases with the increase in the test time. This transition in the iber surace shows the decrease o the iber/matrix interacial adhesion. It is also ascertained in the racture suraces o the glass iber latten with the decrease o the applied stress. It is reported in previous researches that the glass iber strength is dominated by the surace law which is built up while manuacturing the glass iber and the surace law propagates by the stress corrosion generated by the applied stress and the solution. Thus the racture surace o the glass iber lattens as (a) app =24MPa, t =8.8hour (c) app =24MPa, (d) app =18MPa, t =8.8hour t =69hour Figure 8: Fracture surace o woven GFRP in deionized water. (a) app =24MPa, t =.33hour (b) app =18MPa, t =69hour Mirror zone (b) app =18MPa, t =5.16hour Mirror zone (c) app =24MPa, (d) app =18MPa, t =.33hour t =5.16hour Figure 9: Fracture surace o woven GFRP in hydrochloric acid. the surace law propagates. The delayed racture o GFRP occurs when the strength o GFRP decreases 5

6 and reaches the applied stress. In addition, the applied stress on the GFRP is mainly held by the glass iber aligned toward the loading direction. Thereater, the racture surace o the glass iber in equal applied stress in deionized water and in hydrochloric acid possess the similar latness. From these transitions in the racture suraces o GFRP, the decrease o the iber/matrix interacial adhesion and the glass iber strength were ascertained. 4.3 Lietime prediction based on GLS The lietime o woven GFRP under constant tensile load were predicted based on GLS and the predicted results are shown in Figure 1. The iber strength ( and m) and the iber/matrix interacial shear stress ( i ) used in the lietime prediction were obtained rom the iber ragmentation tests o the single iber composite. The strength degradation o the glass iber was predicted using the subcritical crack growth model by evaluating the strain history o the constant tensile load test [13]. The degradation o the interacial shear stress was evaluated by the elastic analysis considering the interacial debonding length. The predicted results showed good agreement with the experimental data and expresses the validity o the lietime prediction method proposed in the present paper. Applied stress app (MPa) HCl (1.mol/l) Fracture time t (h) Figure 1: Lietime prediction based on the GLS assumption. Conclusion In order to clariy the damage accumulation o the corrosion resistant GFRP corrosive environment, the constant tensile load test in air, in deionized water and in hydrochloric acid was conducted. The strain o woven GFRP under constant tensile load increased in the order o tests in air, deionized water and hydrochloric acid. Besides, the delayed racture occurred in deionized water and in hydrochloric acid but did not occur in air. The racture time was shorter in the hydrochloric acid than in deionized water. The decrease o the resin adhesion with the increase o the test time was conirmed, which certiies the degradation o the iber/matrix resin adhesion. The racture time was predicted by the assumptions o the global load sharing (GLS). The strength degradation o the glass iber and the iber/matrix interacial adhesion were substituted into the strength calculation o woven GFRP. The good agreement with the predicted results and the experimental data was obtained in both solutions. Acknowledgment This work was supported by Mizuho Foundation or the Promotion o Sciences. Reerences [1] Y. Fujii, Journal o the Society o Materials Science Vol. 54, No. 1, pp , 25. [2] B. A. Magid, S. Ziaee, K. Gass, M. Schneider, Composite Structures Vol. 71, pp , (25). [3] J. N. Price and D. Hull, Composites Science and Technology Vol. 28, pp , (1987). [4] S. N. Sapalidis, P. J. Hogg, and S. J. Youd, Journal o Materials Science Vol. 32, pp , (1997). [5] M. Kotani, Y. Yasuuku, Y. Tamaishi, and H. Kawada, JSME International Journal Vol. 4, No. 11, pp , (21). [6] F. R. Jones, J. W. Rock, and J. E. Bailey, Journal o Materials Science, Vol. 18, pp , [7] M. Kotani, Y. Yasuuku, N. Inoue, K. Kurihara and H. Kawada, DURACORSYS 21, in CD-ROM, (21) [8] A. Turon, J. Costa, P. Maimi, D. Trias and J. A. Mayugo, Composites Science and Technology Vol. 65, pp , (25). [9] M. Kotani, Y. Shibata and H. Kawada, Proceedings o ICCM-17, (29). [1] A. B. Morais, Composites Science and Technology Vol. 61, (21). [11] A. B. Morais, Composites Science and Technology Vol. 66, (26). [12] K. Liao and Y.M. Tan, Composites: Part B Vol. 32, (21). [13] M. Kotani and H. Kawada, ACCM-6, pp , (28).