Investigation of fracture mechanisms and substructural changes under sequential fatigue cycling involving LCF and HCF loads in a type 316LN stainless steel at 923 K

Abstract

Cumulative fatigue damage under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated at 923K by conducting HCF tests on specimens subjected to prior LCF cycling at various strain amplitudes. Under such loading conditions, a critical damage marking the strongest LCF-HCF interaction was found to occur at specific pre-exposures of LCF depending inversely on the strain amplitude employed. Fracture surface examination of specimens failed in HCF with prior exposure in LCF showed formation of isolated striation pockets as the main manifestation of LCF-HCF interaction. Detailed Transmission Electron Microscopy (TEM) analysis revealed extensive formation of microtwins as prominent microstructural manifestations of LCF-HCF interaction. All these features appear within the window of effective LCF-HCF interaction and hence are a strong function of the applied strain amplitude as well as the degree of prior LCF exposure. It was found out that even though formation of major crack (critical damage) is a pre-requisite for microtwins to appear, there are several microscopic factors associated to the formation of such twins, which were discussed in detail.