On the cyclic stress-strain curve evaluation in low cycle fatigue

Abstract

Constant strain amplitude tests, strain amplitude changes and short cut procedures for cyclic stress-strain curve determination in low carbon steel were analysed in order to understand the differences or similarities they show in the cyclic stress-strain response. These observations were supplemented by electron microscope observations of the resulting dislocation structure after constant amplitude loading and strain amplitude changes. It was found that the previous cyclic history considerably influences the cyclic stress-strain response. Constant amplitude cycling after an amplitude change has a tendency to bring both the stress-strain response and internal dislocation structure into a steady state identical with that reached by cycling the virgin specimen at the same (final) strain amplitude. The actual stress-strain response during variable amplitude loading depends on the cyclic history. The lower bound of this response is the cyclic stress-strain curve obtained from tests on companion specimens and the upper bound is the stabilised hysteresis loop at the highest strain level. Between these extremes lies the cyclic stress-strain curve obtained from the incremental step test. This cyclic stress-strain curve approximates reasonably well the stabilised cyclic stress-strain response in variable amplitude loading.