| This research addresses difficulties encountered during high strain-rate testing of low-impedance materials using a polymeric split-Hopkinson pressure bar (SHPB). Low-impedance materials commonly have low-strengths and high compressibility. This makes them difficult to test for three reasons. First, for SHPB testing, it is important that the impedance of the bars match that of the specimen. When low-impedance materials are tested, this requires the use of low-impedance bars, for example, polymers. However, polymeric bars are viscoelastic. This complicates the analysis because viscoelastic wave propagation must be included. Second, there is usually an interest in the large strain behavior of these materials. Due to the arrangement of the conventional SHPB, there is a limitation on the duration of the test. Because of this, it is not always possible to achieve high strains. Third, for valid dynamic testing, it is important that a state of equilibrium exist within the specimen. The behavior of many types of low-impedance materials often makes this difficult to achieve.; These three issues are dealt with individually as follows. A polymeric SHPB is modified by placing velocity gages at each end of the specimen. Using this configuration, viscoelastic wave propagation may be neglected, that is, the bars may be treated as linear elastic. This configuration is easier to use and often yields more accurate results than the conventional viscoelastic SHPB.; A second modification of the polymeric SHPB is used to increase test duration, allowing large-strain tests to be performed. This is done by the single-point measurement of strain and velocity. This provides a wave separation technique that accounts for viscoelastic wave propagation. The test duration is extended indefinitely, allowing low-rate testing in an otherwise high-rate method.; Finally, specimen equilibrium issues are considered during the dynamic compression of foams. Because of the material behavior of foam, equilibrium conditions can be difficult to attain. SHPB tests on foams are validated by accepted methods. These tests are then re-evaluated by studying the variation of strain-rate throughout the specimen during the test. This is done by instrumentation of the specimen with internal velocity gages. The effect this strain-rate variation has on test validity is investigated. |
