Johnson-Cook parameter evaluation from ballistic impact data via iterative FEM modelling

A methodology is presented for evaluating a strain rate sensitivity parameter for plastic deformation of bulk metallic materials. It involves ballistic impact with a hard spherical projectile, followed by repeated FEM modelling, with predicted outcomes (displacement-time plots and/or residual indent shapes) being systematically compared with experiment. The “correct” parameter value is found by seeking to maximise the value of a “goodness of fit” parameter (g) characterizing the agreement between experimental and predicted outcomes. Input for the FEM model includes data characterizing the (temperature-dependent) quasi-static plasticity. Since the strain rate sensitivity is characterised by a single parameter value (C in the Johnson-Cook formulation), convergence on its optimum value is straightforward, although a parameter characterizing interfacial friction is also required. Using experimental data from (both work-hardened and annealed) copper samples, this procedure has been carried out and best-fit values of C (∼0.016 and ∼0.030) have been obtained. The strain rates operative during these experiments were ∼104-106 s−1. Software packages allowing automated extraction of such values from sets of experimental data are currently under development.