Theoretical and experimental studies of plasticity over a wide range of strain rates and temperatures.

摘要

The theoretical and experimental studies of plasticity are presented. The experimental technique has been developed to test materials at a wide range of strain rates and temperatures. The torsional Hopkinson bar apparatus has been modified for testing at elevated temperatures. The test results have proved that this technique is useful to test materials at high stain rates and temperatures. An elastic-plastic finite element analysis of a tensile test in a split Hopkinson bar apparatus has been conducted. The relationship between stress state in the specimen as well as the adjacent bars and specimen's geometry has been investigated. The numerical results show that for a test run in the split Hopkinson bars, the ratio of length over diameter of the specimen should be larger than about 1.6 in order to get true material behavior. Two types of steel, 1151 and hot rolled 1020, have been tested as an extensively wide range of strain rates and temperatures. The results for 1151 steel show a relative high strength. At different combination of strain rates and temperatures, it is found that different mechanisms operate during the deformation process and different material structures are obtained. Significant effect of dynamic strain aging has been observed in 1020 steel. A theoretical study of constitutive model for 1020 steel has been presented. The model combines a theory of DSA with a theory of the thermally activated deformation process. The relationship between activation energy due to solute cloud and the change of atom concentration has been established. The model predicts the negative strain rate sensitivity and a local maximum stress with temperature when DSA is present, as well as the phenomena that the effective DSA range shifts to higher temperature and its effect decreases when strain rate increases. This model is applied numerically in the modeling of experiments.