Ultra-low cycle fatigue characterization and ductile fracture models of additive manufactured 316L stainless steels

This study employs both experimental and numerical methods to examine the cyclic performance of additively manufactured (AM) 316L stainless steels, specifically focusing on their ultra-low cycle fatigue (ULCF) and ductile fracture characteristics across various stress states. To assess the dependency of ULCF on the stress states, four distinct specimen geometries were utilized: smooth round bars (SRBs), notched round bars (NRBs), flat grooved plates (FGPs), and shear plates (SPs). These specimens underwent cyclic loading with both constant and variable deformation ranges. Analysis of the fracture surfaces using a scanning electron microscope (SEM) indicated that under ULCF conditions, ductile fracture mechanisms of AM stainless steels are similar to those of the conventional steels. Furthermore, fabrication defects such as unmelted powder clusters and gas pores were identified as critical contributors to the degradation of ULCF performance. Experimental results revealed a strong correlation between fracture strain, ULCF life, and stress triaxiality, with elevated stress triaxiality markedly diminishing both fracture strain and ULCF life. An elastoplastic constitutive model for AM 316L stainless steel was calibrated using test data. Subsequently, the stress-weighted ductile fracture model (SWDFM), its modified version (CSWDFM), and the cyclic exponential Lode parameter void growth model (CELVGM) were employed to predict fracture initiation, with its parameters rigorously calibrated against experimental fracture responses. Results show that CELVGM provided the most accurate predictions for NRB and FGP specimens, with prediction errors of 0.067. However, its applicability is demonstrably limited for specimens experiencing complex multiaxial stress states like the SP configuration.