9.4 Split Hopkinson’s Pressure Bar In order to identify the behavior of the carbon composite at even higher strain rates, such as higher than 100/s, a conventional split Hopkinson’s pressure bar (SHPB) was used along with thick specimens. Figure 9.7 shows the experimental result. The forces in the incident bar and transmission bar were measured with the strain gauges attached to the bars while the strain in the specimen was measured from a strain gauge attached to it. Figure 9.7a shows the forces at the two ends of the specimen. Although they are not identical, they are relatively similar from an overall sense. The experimental results were used for further analyses. The corresponding strain history and strain rate history for Fig. 9.7a are given in Fig. 9.7b, c, respectively. Based on the near linearity between 0.03 and 0.1 ms in Fig. 9.7b and the plateau in Fig. 9.7c between 0.03 and 0.06 ms, there seems to exist a nearly constant strain rate of approximately 250/s between 0.03 and 0.06 ms. The corresponding stress–strain curve is given in Fig. 9.7d. The 250/s corresponds to a strain range between 0.004 and 0.006 and a modulus around 26.5 GPa. This result is also presented in Table 9.1 along with the results from the drop weight impact tests. The zero strain rate given in Table 9.1 represents static testing based on a hydraulic testing machine. Figure 9.8 shows all the stress–strain curves for comparison. Both the curves associated with strain rates of 200 and 250/s are presented by dash-dot lines. The valid range based on the nearly constant strain rates between 0.03 and 0.06 ms, however, are shown with solid lines. 9.5 Discussions 1. The microstructure of the fiber composite, if there is any, must be considered in the selection of specimen dimensions. 2. The thick specimens work better than the thin specimens for compressive testing as well as higher strain rate testing. 3. The large impacting mass is adequate for achieving nearly constant strain rates up to 125/s. 4. Trial-and-errors are required for identifying a suitable shaper material with an associated thickness for achieving force balancing. As the shaper becomes softer, the loading rate (strain rate) due to the same impacting force decreases. 5. Multiple tests are required for obtaining a reliable testing result. 6. It is a challenging task to achieve force equilibrium in the specimen by introducing a shaper without significantly reducing the strain rate. Fig. 9.7 Loading fixture for DWIT based tensile strain rate testing 78 G. Li and D. Liu
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