On the application of fracture fatigue entropy to multiaxial loading

Abstract

An experimental and theoretical analysis of low carbon steel 1018 subjected to multiaxial loading is presented. Different loading conditions, including tension–compression, torsion, in-phase, and out-of-phase, are applied to investigate the effect of loading type on fatigue life. The fracture fatigue entropy (FFE) framework is then used to predict life using both hysteresis loops and thermography. The results show that the out-of-phase loading leads to the minimum fatigue life, while the torsion exhibit the maximum life at a similar equivalent strain. The FFE predictions using thermography successfully captured the specimens' fatigue life subjected to the multiaxial fatigue loading independent of the loading condition. Furthermore, the results show that the strain energy obtained from the hysteresis loop is a good measure of dissipation energy in the cases of tension–compression, torsion, and in-phase loading conditions. Accordingly, the predicted life using the hysteresis loop agrees well with the thermographic and experimental results. However, in the case of out-of-phase loadings, the plastic strain energy obtained from the hysteresis loop deviates from the dissipation energy because the stored energy originated from the internal state variable variation. Accordingly, the predicted life using the thermography is more accurate than the hysteresis loop, which agrees well with the experimental results.