Mechanical response of dual phase steel at quasi-static and dynamic tensile loadings after initial fatigue loading

Abstract

This paper presents an analysis of the mechanical behavior of DP500 steel previously exposed to cyclic loadings with constant stress amplitude. Stress–strain curves were determined for a two strain rates using servo-hydraulic testing machine and tensile Hopkinson bar. Tests were accompanied by digital image correlation, which enabled us to estimate strain localization at various initial fatigue loading conditions. Independently of the pre-fatigue parameters, initial cyclic loadings induce increase of yield stress. Similar behavior is observed for the ultimate tensile strength (UTS) estimated during the Hopkinson bar test. For low strain rates, UTS remains almost constant. On the contrary, increasing the number of cycles lowers the value of elongation at fracture and decreases the strain corresponding to UTS. In every case discussed, higher initial cyclic loading stress induces stronger evolution of mechanical parameters of DP500 steel. Macroscopically estimated strain measured at quasi-static loading conditions is higher than dynamic ones. The opposite behavior was found during analysis of local strain, i.e. strain obtained during the Hopkinson bar experiment was higher than that obtained by using the servo-hydraulic testing machine.