Mechanisms of failure under low cycle fatigue, high cycle fatigue and creep interactions in combined cycling in a type 316LN stainless steel

The present study investigates the effect of different damage modes like low cycle fatigue (LCF), high cycle fatigue (HCF), creep and their interactions during combined cycling at high temperature (923 K) in a 316LN stainless steel. The experiments were designed with multi-step load sequences where specific number of small amplitude HCF cycles (block) were introduced at the stabilized cyclic load under LCF for a given strain amplitude and repeated until failure. Cyclic life was found to decrease with the increase in HCF block size. However, the extent of decrease in cyclic life also depends on the LCF strain amplitude, which is attributed to the additional damage incurred by creep and ratcheting. Creep damage was found to be opposed by a strong compressive ratcheting originating from Bauschinger effect, resulting in net strain accumulation in tensile or compressive direction depending on whichever damage process of the two is dominant. Typically, fatigue fracture, intergranular creep fracture or creep rupture can be identified when failure was dictated by LCF or creep. However, failure life was actually found to be governed by multiple damage interactions between LCF, HCF and creep for specific loading conditions, as emphasized through detailed fracture surface investigations. HCF damage was found to act as a catalyst by joining small transgranular (LCF) or intergranular (creep) cracks, thus facilitating the crack propagation and final failure by the respective modes. This leads to strong synergistic LCF-HCF-creep or HCF-creep interaction, the regimes of which were suitably mapped as a function of LCF strain amplitude and block size.