On the calibration strategies of the Johnson-Cook strength model: discussion and applications to experimental data

The present paper aims at assessing the various procedures adoptable for calibrating the parameters of the so-called Johnson-Cook strength model, expressing the deviatoric behavior of elastoplastic materials, with particular reference to the description of High Strain Rate (HSR) phenomena. The procedures rely on input experimental data corresponding to a set of hardening functions recorded at different equivalent plastic strain rates and temperatures.After a brief review of the main characteristics of the Johnson-Cook strength model, five different calibration strategies are framed and widely described. The assessment is implemented through a systematic application of each calibration strategy to three different real material cases, i.e. a DH-36 structural steel, a commercially pure niobium and an AL-6XN stainless steel. Experimental data available in the literature are considered. Results are presented in terms of plots showing the predicted Johnson-Cook hardening functions against the experimental trends, together with tables describing the fitting problematics which arise in each case, by assessing both lower yield stress and overall plastic flow introduced errors.The consequences determined by each calibration approach are then carefully compared and evaluated. A discussion on the positive and negative aspects of each strategy is presented and some suggestions on how to choose the best calibration approach are outlined, by considering the available experimental data and the objectives of the following modeling process. The proposed considerations should provide a useful guideline in the process of determining the best Johnson-Cook parameters in each specific situation in which the model is going to be adopted.A last section introduces some considerations about the calibration of the Johnson-Cook strength model through experimental data different from those consisting in a set of hardening functions relative to different equivalent plastic strain rates and temperatures. In particular, the opportunity of using experimental data coming from Taylor impact tests is assessed, together with an evaluation of the possibility of using other less popular and somehow innovative ways for obtaining the Johnson-Cook strength model parameters.