Optimizing the combined isotropic/kinematic hardening parameters of pressure vessel materials and welds using the incremental elastic-limit approach

In this work, a novel optimization approach is introduced to extract combined hardening parameters from the cyclic stress-strain data obtained from the initial hardening cycles of isothermal, low-cycle fatigue tests. The incremental elastic-limit (IEL) concept is proposed due to the often-undiscernible elastic range of a stabilized stress-strain cycle, that increases the complexity of hardening parameters optimization. The optimization process is implemented by taking an iterative search for the elastic range by a fixed elastic limit increment, and the corresponding hardening parameters are obtained using the nonlinear fitting algorithms in the MATLAB™ Software. An implicit stress-update function is introduced to simulate the cyclic stress and strain with a given set of hardening parameters and yield strength. The fitness of the optimization is calculated based on the least square difference between the experimental and simulated stress-strain data. Furthermore, the IEL concept is incorporated to optimize the cyclic hardening parameters. In the final step, finite element (FE) analysis using the optimized hardening parameters is applied to demonstrate the effectiveness of the IEL approach. The proposed methodology is applied to pressure vessel steels and Ni-based weld metals.