MECHANICAL PROPERTIES OF FRP UNDER IMPACT LOADS. Impact is a sudden application of an impulsive force to a structure. Almost all civil engineering

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1 Chapter 5 MECHANICAL PROPERTIES OF FRP UNDER IMPACT LOADS 5.1 GENERAL Impact is a sudden application of an impulsive force to a structure. Almost all civil engineering structures are subjected to impact loads one way or another. It has been shown experimentally that the engineering materials behave differently under impact loading than they do under static loading (Harding et al., 1960). In most cases the strength of the material is higher under higher strain rate. Impact behaviour of structures has received considerable attention over the years because it is a critical design consideration in many practical cases (Hui and Jones 1989; Miyamoto, 1994; and Harding, 1998). For example, many civil, nuclear, and ocean engineering structures are subjected to dynamic loads: wave and wind loads, pile driving, earthquake, blast loading due to accidental explosions, etc. Besides, there are cases where post-impact safety of structures has to be determined. Knowledge of the mechanical properties of engineering material at the applicable strain rates is important for successful design and analysis of structures. Sayed Mukhtar (Muktar) Homam, Ph.D., P.Eng. Ph.D. Thesis (2005), University of Toronto 212

2 Response of FRP and its bonds subjected to high strain rate and impact loads is an area in civil engineering where the available information is very limited. The purpose of this study is to generate data on axial-tensile properties such as strength, stiffness, and ultimate strain of GFRP and CFRP and to determine the stress-strain relationship of these materials under impact loads. Also, the performance of the FRP-FRP bond and FRP-concrete bond under impact loads were evaluated. The specimens were described in Section 3.3, the test setup was illustrated in Section and the literature review was summarized in Section 2.5. The following sections present and discuss the results of testing of FRP coupons, FRP SLB specimens, and FRP bonded concrete prisms under two levels of impact loads. 5.2 FRP COUPONS General In this section the responses of GFRP and CFRP coupons tested under impact loads are discussed. The goal is to determine strength and elongation of the materials subjected to impact loads and to determine the stress-strain relationship of the materials based on the experimental results Results and Discussion Two batches from each type of material were subjected to impact loads: one to severe impact (4.4 m/s) and the other to moderate impact (3.1 m/s). The coupons for impact tests were instrumented with one strain gauge, in the fibre direction, at the mid-length of the specimens. 213

3 Figure 5.1: Typical response of CFRP (CCIMPTS9) coupon under impact loading Figure 5.2: Typical response of GFRP coupon under impact loading Fig. 5.1 and Fig. 5.2 show typical graphs obtained from the impact tests of the CFRP and GFRP coupons, respectively. The sensors output signals were recorded by oscilloscopes and converted 214

4 to force in kn or strain in milli-strain. The specimens failed in a variety of modes. The ideal mode of failure was that the specimens failed in tension, at first impact, during the first wave burst, and at the strain gauge location. However, some specimens failed during wave reflection, at grips, or did not fail at first impact. For the analysis, the load and strain values up to the first peak are used, where the amount of reflected disturbance is minimal. During the severe impact test of CFRP coupons, the first two specimens failed on first impact. However, some later specimens did not fail on the first impact and had to be subjected to impact forces for a second time after further tightening of the grips. Care had to be exercised not to rupture the specimens at grips by over tightening them. Specimen CCIMPTS3 and CCIMPTS5 to CCIMPTS7 did not fail on first impact. Slip at the grips was the main reason for specimens not failing under the impact loads. Even though, in most cases, the amount of slip was too small to be visible to the naked eye, the small amount of energy that would have caused the specimens failure was consumed overcoming the grip-tab friction. In the case of CCIMPTS7, the slip was visible and the force recorded by the load cell at the first impact was very small. Similar observations were made in testing of GFRP coupons. GCIMPTS1 and GCIMPTS2 failed on the first impact, however, specimen GCIMPTS3 did not. To avoid the unsuccessful loading trials and loss of strain gauges the drop height was increased to cm, the maximum possible height, and the drop weight was increased from 10.0 kg to 15.0 kg. After the adjustments, the specimens failed on first impact. Some consequential changes due to these adjustments were observed in some of the sensors output. The average failure load recorded by the load cell increased by about 16%. Increases were also observed in the Wave-Rod output, but no discernable change was observed in the values of strain at failure (Table 5.1). In the analysis and 215

5 for comparison, the highest stress values sustained by each specimen were used whether the Table 5.1: Summary of data from impact test of FRP coupons Specimen Drop Height (cm) Drop Mass (kg) Load cell (kn) Wave- Rod Top (kn) Wave- Rod Bot. (kn) Strain at Max. Stress (milli-strain) Max. Tensile Stress (MPa) Modulus of Elasticity (MPa) CCIMPTS * 77.6 CCIMPTS * 61.7 CCIMPTS * 79.8 CCIMPTS * - CCIMPTS * 86.0 CCIMPTS * 81.3 CCIMPTS * - CCIMPTS * 84.3 CCIMPTS * 90.0 CCIMPTS * Average GCIMPTS * 39.7 GCIMPTS * 39.4 GCIMPTS * 51.9 GCIMPTS * - GCIMPTS * 42.1 GCIMPTS * 42.5 GCIMPTS * 39.8 GCIMPTS * 39.0 GCIMPTS * 45.6 GCIMPTS * 65.5 Average CCIMPTM CCIMPTM CCIMPTM CCIMPTM CCIMPTM CCIMPTM Average GCIMPTM GCIMPTM GCIMPTM GCIMPTM GCIMPTM GCIMPTM Average

6 (G/C= GFRP/CFRP; C = coupons; IMPT = impact loading; S/M = severe/moderate; # = specimen number) Impact Loads * Specimens failed under impact load values were from the first impact or the second impact. The highest stress values for the specimens subjected to two impacts were very similar to the peak stress values of specimens that failed after one impact. Therefore, no major error was expected to arise from neglecting the number of impacts needed for specimen failure. This might not have been the case if they required multiple impacts to fail. The modulus of elasticity of the coupons was calculated from the data obtained at first impact. The specimens subjected to moderate impact were not expected to fail. After the application of the impact forces, the moderate impact test specimens were tested under static loads for their residual tensile strength. Impact forces were measured by the load cell and Wave-Rod (Fig. 3.22). The Wave-Rod was calibrated and tested before and after impact tests, to ensure that it was a reliable load cell. During the test the Wave-Rod was not only used to measure the incident force transmitted to the specimens but also to find the wave velocity. Table 5.1 shows that the load values measured by the Wave-Rod are much larger than those measured by the load cell. This is due to the position of each sensor and its proximity to the point of impact. The wave travelled through the Wave- Rod, the specimen, and the universal joint to the load cell. The geometry of the load cell, S-shape (Fig. 5.3), could also influence its response to impact loads. When GFRP coupons were subjected to severe impact, the average rate of loading (slope of the force-time curve between 30% and 70% of maximum force) recorded by the Wave-Rod, was about kn/sec whereas by the load cell it was about kn/sec. In the same 217

7 Figure 5.3: The shape of load cell installed on the impact test frame specimens, the average force recorded by the Wave-Rod was about 58.2 kn and by the load cell it was about 29.0 kn. The average rupture strain in the GFRP specimens was recorded to be about 28.6 x Using the static modulus of elasticity of 24 GPa, the force corresponding to the rupture strain is about 17.4 kn which is much less than the 29.0 kn recorded by the load cell. The actual strength, however, is somewhere between 29.0 kn and 58.2 kn, because the specimen is located between the Wave-Rod and the load cell, and the wave intensity after passing through the Wave-Rod is still high compared to what is experienced by the load cell. The fact that the coupons have higher strength for the same average strain indicates that the modulus of elasticity of GFRP under impact is not 24.0 GPa but is somewhere between 41.0 GPa and 82.7 GPa. The multi-component structure of the test setup makes it impossible to apply wave theories to calculate the force in the specimen at the location of the strain gauge. Most of the work conducted by other researchers (Kawata et al., 1981; Kawata et al., 1982; Harding and Welsh, 1982) who produced stress-strain curves for their test specimens used small specimens with about 10 mm gauge length. Those specimens were tested in setups such as split Hopkinson s pressure bar (SHPB), where wave propagation theories could be employed to develop stress- 218

8 strain graphs for each FRP material. Other researchers (Fujii and Miki, 1973; Beever and Ellis, 1984; Harris and Adams, 1985; Lammerant et al., 1991) who tested larger specimens did not produce stress-strain curves but presented stress-time and strain-time behaviour of the tested specimens in their reports. For the purpose of evaluating the effects of loading rate on the mechanical behaviour of specimens, the force measured by the load cell is used to represent the force in the specimen. It was observed that the force measured by the Wave-Rod, due to its position along the wave path, did not represent the strength of the specimen. For example, it measured almost the same force for both CFRP and GFRP coupons subjected to the same impact energy. Fig. 5.1 and Fig. 5.2 show that the peak strain in the FRP coupon and the peak force in the load cell did not happen at the same time. For example, in a typical CFRP coupon the peak strain was reached in 0.5 milli-second after the oscilloscope was triggered, and the peak load (load cell) was reached in about 1.2 milli-second. To facilitate the generation of the stress-strain plots, the time axis for strain and load was normalized by dividing the time by the respective time to peak values, i.e. the time interval between the start and peak stress. Fig. 5.4 shows the normalized plots. Regression analysis was then conducted to represent the curves for both data sets with equations. The strain was best-fitted using a fourth-degree polynomial. The load was best fitted using a combination of two fourth-degree polynomials and a straight line. Fig. 5.5 shows the best-fitted curves for the load and strain. Once this was done, load and strain values at same time increments were obtained. These values were used to develop the stress-strain plots for each specimen as shown in Fig The same procedure was carried out for the GFRP coupons. After developing the stress-strain curves, the average response for each set of test specimens was 219

9 developed as shown in Fig. 5.7 to Fig The level of stress sustained by the material under different types of loading is compared in Fig. 5.11(a). Specimens subjected to severe impact resisted higher stress levels at peak compared to the specimens tested under static loads. The CFRP and GFRP coupons under severe impact had tensile strength values equal to about 190% and 175% of the static strength of their control counterparts, respectively. The CFRP and GFRP coupons subjected to moderate impacts did not fail under impact loads. They were tested for their residual strength afterwards. Their post-impact static strengths are compared with the static strength of their control counterparts in Fig. 5.11(b). No degradation was observed in the post exposure static strength of CFRP coupons. The reduction in strength of GFRP coupons was about 20%. To calculate the modulus of elasticity of each specimen, a line was fitted through the data points corresponding to 30% and 70% of the maximum stress of each specimen. There seems to be a wide variation in the calculated modulus of elasticity as shown in Table 5.1 and Fig. 5.7 to The average values of modulus of elasticity of CFRP under severe and moderate impacts were, 83.5 GPa and 99.0 GPa. For GFRP the average values of modulus of elasticity under severe impact and moderate impacts were 45.1 GPa and 42.0 GPa, respectively. The average modulus of elasticity of CFRP under severe impact, contrary to expectation, is lower than the one under moderate impact; however, the difference is within the range of the observed scatter in the modulus values of CFRP coupons. The average stress-strain curves are represented with 3 rd degree polynomials. In the case of the GFRP coupons a quadratic function works as well as a 3 rd degree polynomial. These polynomials are used when the relation between stress and strain is needed in any analysis. However, for simplification, the stress-stress strain relationships can be assumed to be linear, without any significant loss of accuracy. The linear dynamic modulus of 220

10 elasticity of CFRP and GFRP can be taken to be about 92 GP and 43 GPa, respectively. Fig and Fig show the tested CFRP and GFRP coupons, respectively. Except No. 2, all CFRP coupons failed in the gauge length region. The GFRP coupons failed entirely within the gauge length: Based on the impact tests conducted on CFRP and GFRP at 4.4 m/s and 3.1 m/s, the following observations can be made. - Both CFRP and GFRP materials resist higher stress levels under impact loads compared to their static tensile strength. - The strain values at failure for impact and static loads were very similar. - The modulus of elasticity values of the materials under impact loads were higher than those under static loads. - The stress-strain behaviour under impact loads is non-linear and can be modeled by third degree polynomials. - For simplicity, linear stress-strain behaviour can be assumed under impact loads without significant loss of accuracy. 221

11 Figure 5.4: Normalized force and strain curves for a typical CFRP coupon Figure 5.5: Best-fit curves for the normalized force and strain curves of a typical CFRP coupon 222

12 Figure 5.6: Stress-strain behaviour of CFRP coupons subjected to impact and static loads Figure 5.7: Stress-strain behaviour of CFRP coupons subjected to severe impact loads and their average 223

13 Figure 5.8: Stress-strain behaviour of GFRP coupons subjected to severe impact loads and their average Figure 5.9: Stress-strain behaviour of CFRP coupons subjected to moderate impact loads and their average 224

14 Figure 5.10: Stress-strain behaviour of GFRP coupons subjected to moderate impact loads and their average Figure 5.11: Comparison (a) maximum stress and (b) post-exposure strength of GFRP and CFRP coupons subjected to static and impact loads 225

15 Impact Loads Figure 5.12: Failed CFRP coupons tested under severe impact loads Figure 5.13: Failed GFRP coupons tested under severe impact loads 226