Influence of surface porosity on fatigue life of additively manufactured ASTM A131 EH36 steel

Abstract

The influence of surface porosity on the fatigue life of additively manufactured (AMed) EH36 steel samples via selective laser melting (SLM) was studied in-depth by using a numerical method, which the pores were simulative AM 3D pores. The surface pores in both simplified semi-ellipsoid/spheroid and more realistic triangular shapes were evaluated in the first place. Stress concentration factor Kt analyzed by ABAQUS and fatigue life assessed by FE-SAFE were closely related to the AM 3D pore size, shape, position, orientation as well as their interactions, which confirmed their effects on the crack initiation stage during fatigue testing. The effectiveness of adding a surface finish factor Kt to a smooth surface fatigue model was subjected to internal pore geometry in terms of Kt values ranging from 1 to 5.5. Different algorithms were attempted with the stress-life method giving a higher estimation than a strain-life method while becoming invalid when the stress concentration situation was rather serious (around Kt = 2.5). There was no much difference between the uniaxial and multi-axial fatigue algorithms because the uniaxial tensile cyclic loading was applied to the model. Theoretically calculated total fatigue life (crack propagation) using linear elastic fracture mechanics (LEFM) fell in the same order as the experimental results for all the samples (within a factor of 2). AMed material parameters were used for the fatigue life prediction where the microstructure element was counted. AMed pores could deteriorate the mechanical properties of the AMed materials a lot, such as strength, Young’s modulus and fatigue lifetime, etc. The statistical analysis showed the scatter band of the SN curves at a factor of 8 with respect to different laser scanning speeds.