is short, numerous wave propagations will overlap one another and result in a largely distorted wave form which is very different from the initially impact-induced one. With a special geometry design in the rod, it is possible to reduce the strain wave with less rebounding back to the nose. The initial strain wave caused by impact can then be largely isolated. The instrumented projectile (IP), based on geometrical manipulation, has been developed recently by the authors [1]. The short IP (about 6-in. long) consists of a solid neck and a hollow body. A pair of RSG’s is mounted on the opposite sides of the neck. A data acquisition board and a battery have also been installed inside the IP’s body. The IP has been used for impact testing. Experimental results have shown the high similarity of the wave propagation from the short IP and that from a 6-ft long cylindrical bar (which can separate the incoming wave from the rebounding one) when the IP and the long bar collide with each other. The wave propagation in a short rod without a special geometry is completely different from that in a long cylindrical bar when the two collide with each other. However, the wave propagation in a short rod with a special geometry is almost identical to that in a long cylindrical bar when the two collide with each other. The ability to eliminate the wave rebounding based on the special geometry design is thus demonstrated. Accordingly, the initial strain wave which bears the most important information of impact-induce force history can be clearly identified by the short instrumented projectile. 24.3 Research for Improving Experimental Accuracy Based on the innovation of the wave elimination technique and the success of the IP design, more applications based on the IP to improve engineering dynamic measurements have been explored. Some tasks are listed below and in this research investigation. 1. Drop-weight impact tester (DWIT) has been commonly used in both academia and industries due to its simplicity. However, the issue concerning wave propagation has been completely neglected in the design of the DWIT currently available in the market. As a task of this research study, it is to evaluate the effect of wave propagation involved in the DWIT by comparing the result from the DWIT with that from the IP (which preserves the initial impact-induced wave). The goal of this study is to demonstrate the importance of eliminating the wave propagation in correctly measuring dynamic forces. 2. Split Hopkinson’s pressure bar (SHPB) – SHPB has been commonly used for characterizing dynamic constitutive relations of materials at high strain rates which are required for modeling the performance of materials and structures subjected to high-velocity loading. SHPB is usually organized by aligning two long bars (such as 1.5 m each and mounted with strain gages) with a gas gun while the specimen to be tested is situated between the two long bars. When an impactor is shot out of the gas gun and collides onto the first long bar, a strain wave will be generated. The wave will then propagate through the bar into the specimen before entering the second bar. Based on the wave propagations in the two long bars measured by electrical resistance strain gages mounted on them, the constitutive relation of the specimen material tested can be established for the high strain rate performed. Since the IP can record the incoming strain wave without the rebounding one, it can be used to replace the second long bar. The length of the SHPB can then be reduced to about half. The shorter SHPB should prove to be more convenient for laboratory operations. Besides advancing the dynamic testing techniques given above, this research program can also be extended to other applications of the IP. (a) A more accurate drop-weight impact tester – The ones currently used in engineering and research institutes neglect the wave propagation involved in the DWIT testing. (b) An instrumented ballistic impactor – The ones currently used are non-instrumented and are not useful for scientific investigations. (c) A shorter Hopkinson’s pressure bar – The ones currently used are twice longer than the proposed one and are not convenient for labs with restricted space. (d) An instrumented Taylor’s impactor – The ones currently used are based on post-test measurements instead of in situ measurements. 172 G. Li et al.
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