Constitutive material law improvements for metals that manifest an unstable behavior in dynamic condition

Numerical analysis is regularly used in engineering practice as useful mean to study component behavior in service, especially when an experimental campaign is too expensive or not practicable. Numerical software is becoming more and more powerful and integrated, leaving the correct use of a material model as a weak point. As some materials could manifest an unstable behavior if deformed at high strain rate, it is of primary importance to use an appropriate constitutive law to reproduce their behaviour. This paper shows the results of the inverse calibration of two material (Ti-4Al-6V and AA 7075-T6) constitutive laws in their classical definition, as present in the literature. Moreover, both of them have been modified by the authors in the attempt of improving the complex material behaviour. The calibrated models are Johnson-Cook (JC) and Nowak-Pecherski (NP) models. The improvement of these models showed that JC is not suitable for calibrate materials that show an unstable behavior also after its improvement; conversely NP provided a better matching with the experimental data.