Type-I to Type-II non-Masing behavior of 304L SS under low cycle fatigue: Material’s internal changes

Abstract

This study investigates the Type-I and Type-II non-Masing behavior of 304L austenitic stainless steel fatigue tested at room temperature for strain amplitudes ranging from ±0.25% to ±1.0% at a strain rate of 1 × 10-3 s−1. The material exhibited Type-I non-Masing behavior at lower life fractions (≤10%) and Type-II non-Masing behavior at higher life fractions (>10%). Type-I indicates that the master curve construction is possible, but in Type-II, the master curve can not be constructed. Further, low cycle fatigue tests conducted at ±0.25%, ±0.6%, and ±1.0% of strain amplitude were interrupted at life fractions of 8%, 30%, and 50% to investigate the internal material changes. At ±0.6% and ±1.0% of strain amplitude, the martensite content, dislocation density, and local misorientation increased with increasing life fraction, whereas the twins fraction decreased. Low martensite content and internal defects observed at low life fraction (8%) cause a minimal change in the strain-hardening rate behavior at different strain amplitudes, thereby, the Type-I non-Masing behavior. However, at higher life fractions (30% and 50%), high martensite content, high dislocation density, stacking faults, deformation twins, shear bands, dislocation walls, and cells have caused a significant change in the strain-hardening rate behavior, and thus, the Type-II non-Masing behavior.