During forming processes, damage is caused in the microstructure of metals, which affects the mechanical properties of components under service conditions. So far, the influence of typical forming steps on the fatigue damage behavior was not investigated. Consequently, an evaluation of the damage caused during forming processes will enable an optimization of lightweight constructions.
In this study, the influence of the degree of deformation and the associated microstructural damage on the fatigue properties of 16MnCrS5 (1.7139) steels were investigated. The aim was to qualify a time-efficient testing procedure based on stepwise load increase tests (LIT). Thus, the construction of Woehler curves requiring a multitude of constant amplitude tests (CAT) can be avoided. The investigations allow a description of the interaction between the ductile pre-damage and the fatigue damage behavior. A detailed characterization of the cyclic damage behavior under CAT and LIT was performed by measurements of stress-strain-hysteresis, the change of deformation-induced temperature, and the change of the electrical resistance based on direct current (DC). The initial degree of the forming-induced damage, the development of damage during the cyclic loading, and the influence of microdefects were investigated by light and electron microscopy.
Initially, LIT were performed on samples without damage and then successfully varied by CAT. Within LIT, first significant material reactions can be detected at stress amplitudes exceeding 220 MPa. Here, the change of the deformation-induced temperature well correlates with the electrical resistance and the specimens’ deformation behavior. Therefore, LIT are well appropriate to announce microstructural changes and fatigue damage early at 71% of the fatigue life.