Masing and Non-Masing Behaviors of Materials under Low Cycle Fatigue Loading

Abstract

Understanding the material's Masing/non-Masing behavior has immense significance in the plastic strain energy density-based fatigue life prediction methods, constitutive model development, and design/selection of advanced materials for extreme environmental conditions. Low cycle fatigue (LCF) tests were performed on 304L SS at room temperature, applying strain amplitude ranging from ±0.25% to ±1.0% at a strain rate of 1×10-3 s-1. The Masing/non-Masing behavior has been investigated at 5%, 8%, 10%, 12%, 15%, 20%, 30%, 40%, and 50% of fatigue life. It was observed that the master curve could be constructed with the hysteresis loops at lower fractions of fatigue life (up to 10%); however, the master curve could not be constructed at life fractions higher than 10%. Therefore, the non-Masing behavior of the material has been classified into two types, viz. Type-I non-Masing behavior (i.e., when construction of the master curve is possible) and Type-II non-Masing behavior (i.e., when construction of the master curve is not possible). Further, a model has been proposed to quantify and predict the cyclic plastic strain energy density (CPSED) and fatigue life for Type-II non-Masing behavior, which can also be used for Type-I non-Masing and Masing behavior. This model has been validated with 13 different types of materials and could predict the CPSED and fatigue life within a scatter band of 1.2 and 2, respectively. The interrupted LCF tests were conducted at the strain amplitudes of ±0.25%, ±0.6%, and ±1.0% for life fractions up to 8%, 30%, and 50%, and detailed microstructural investigations were done to explore the underlying cause of the Type-I and Type-II non-Masing behavior. The dislocation tangles, deformation twins, less local misorientation, and minimal martensite were observed at a low life fraction (8%), causing Type-I non-Masing behavior. However, at higher life fractions (30% and 50%), a high amount of martensite, high dislocation density, high local misorientation, stacking faults, deformation twins, shear bands, and cells have caused the Type-II non-Masing behavior. Further, these microstructural variables have been qualitatively correlated with the proportional stress limit and strain hardening rate behavior. It is concluded that the proportional stress limit changes in Type-I, whereas for Type-II, the proportional stress limit and the strain hardening rate behavior change with strain amplitude. However, neither the proportional stress limit nor the strain hardening rate behavior changes with strain amplitude for Masing behavior. The effect of the martensite on the cyclic plastic deformation behavior has been analyzed using 2D numerical simulation in the finite element analysis tool ABAQUS. The strain localization, equivalent plastic strain, and effective stress increase with the increase in martensite content in 304L SS. This type of plastic strain localization would promote more martensitic transformation and, thereby, changes in the material's strain hardening rate and mechanical properties, leading to non-Masing behavior. This study contributes fundamental understanding toward developing the microstructural-based robust phenomenological model to predict the non-Masing behavior of materials under low cycle fatigue.