Effect of micro-defects on fatigue lifetime of additive manufactured 316L stainless steel under multiaxial loading

Abstract

The statistical distribution of the geometric characteristics of microscopic defects, stress concentration factor (SCF) caused by microscopic defects and multiaxial fatigue lifetime of additive manufactured 316L stainless steel were investigated. The samples were machined after manufactured by Selective Laser Melting. Based on the X-ray computer tomography of typical 316L stainless steel specimen, the distribution laws of the geometric characteristics of microscopic defects in the material were studied, and the results indicated that the size and shape characteristics distributions of defects were subjected to lognormal rules well. The 3D volumetric models of typical microscopic defects under uniaxial, pure torsional, proportional and 90° non-proportional loading were imported into a finite element software to calculate SCFs caused by defects. The degree of stress concentration located at the edge of the microscopic defect under uniaxial loading was more serious than that under pure torsional loading with same nominal Von Mises equivalent stress (VMES). Simultaneously, greater SCF was arisen at the edge of microscopic defect under non-proportional loading instead of under proportional loading. The above conclusions were proved to be correct through analyzing the experimental data of additive manufactured 316L stainless steel under multiaxial fatigue loading. It showed that the fatigue lifetime under uniaxial loading was shorter than that under pure torsional loading with same nominal VMES. Meanwhile, the specimens subjected non-proportional loading had shorter fatigue lifetime than that under proportional loading with same nominal VMES.