On the Accuracy of the Johnson-Cook Constitutive Model for Metals

A critical assessment is made herein on the accuracy of the Johnson-Cook (JC) constitutive model by comparing the model predictions with the test data for 2024-T351 aluminum alloy, 6061-T6 aluminum alloy, OFHC copper, 4340 steel, Ti-6Al-4V alloys and Q235 mild steel. These materials are selected because their test data are more complete in terms of true stress-true strain relationships, strain rate effects, temperature effects and failure. To further assess its accuracy numerical results for the ballistic perforation of plates made of 2024-T351 aluminum alloy using the JC constitutive model are also presented and compared with corresponding test data. It transpires that the JC constitutive model is applicable to Mises materials at quasi-static to intermediate strain rates and low to moderate temperature. It also transpires that for non-Mises materials the agreement between the model predictions and the test results are poor in terms of shear stress-shear strain curve and fracture strain. Furthermore, the accuracy of the JC model decreases with increasing strain rate, temperature and, above all, it fails to produce consistent results at high strain rates when the experimentally obtained dynamic increase factors (DIF) are employed in the calculations implying the form of the model’s equation (namely, quasi-static stress-strain curve multiplied by DIF) may be inadequate at least for the scenarios where high strain rates are involved.