Abstract: Additively manufactured (AMed) alloys have been widely developed in recent years owing to their attractive properties. Due to the unique processing of fast manufacturing, however, extrinsic defects such as pores or unmelted particles seem to be unavoidable in AMed alloys. The effect of extrinsic defect on tensile strength can be estimated based on the notch strength criterion, while for ductile alloys, this criterion remains unestablished. In this work, we utilized notches to mimic the extrinsic defect and studied the notch effect on tensile strength. As a model ductile AMed alloy, the 17-4PH stainless steel made with a directed energy deposition technique was investigated. We found that the tensile strengths of both the as-printed and aged steels are notch-insensitive. With the increase of stress concentration factor, the notch strength ratio (NSR) firstly increases and then decreases. Furthermore, based on the notched tension experiments and the finite element analysis, a notch strength criterion for ductile alloys is proposed. Only when the von Mises stress at the center of the notch plane reaches the necking true stress, the notched specimen reaches its maximum nominal stress (i.e., the notch strength). This criterion, which was further validated with various other alloys, reveals the physical nature of the notch strength in ductile alloys. The notch strength criterion also enables a semi-quantitative assessment of how NSR varies with geometry and differs across materials.

Key words: Additive manufacture, Notch effect, 17-4PH Steel, Finite element analysis (FEA), Notch strength