A New Procedure to Evaluate Parameters of Johnson-Cook Elastic-Plastic Material Model from Varying Strain Rate Split Hopkinson Pressure Bar Tests

The objective of this work is to develop a new procedure which can be used to estimate the parameters of the elastic-plastic material model from varying strain rate tests, such as split Hopkinson pressure bar tests. A major limitation of this technique is the varying nature of strain rate during the test. In order to correlate the strain rate with mechanical properties and evaluate the parameters of a material model, most of the studies in the literature assume that the strain rate is constant during the test with a given impact velocity. The conventional approach has limitations, and hence, a new method which incorporates the varying nature of strain rate in the algorithm to evaluate material parameters has been developed. This method has been used to evaluate the parameters of Johnson-Cook material model using results from compression split Hopkinson pressure bar tests. Impact velocities ranging from 4 to 21 m/s have been used in these tests. From the results, it was observed that the stress-strain curve evaluated by the new method lies between the two extreme bounds representing maximum and minimum values of strain rate as experienced by the specimen. The new model, when used in conjunction with varying strain rate data as measured during the test, was shown to predict the experimentally obtained stress-strain curves accurately for a wide range of impact velocities. This novel procedure shall be very useful to researchers to evaluate rate-dependent material properties for a wide class of materials subjected to different loading environments.