Synergistic Effects of Volumetric Defects and Notch Geometry on the Fatigue Behavior of Additively Manufactured Metallic Materials

Abstract

In this dissertation, the synergistic effects of volumetric defects and notch geometry on the fatigue behavior of additively manufactured (AM) metallic materials were investigated. Factors influencing fatigue crack initiation and short crack growth behavior were identified and assessed. Both experimental and numerical techniques were utilized to understand the notch fatigue behavior. During experimentation, AlSi10Mg and 17-4 precipitation hardening (PH) stainless steel (SS) specimens with varying geometry types (cylindrical and flat), notch root radii, ρ, ligament widths, w, and volumetric defect contents were used. These specimens were tested under uniaxial cyclic loading, and their fracture surfaces were analyzed to identify the crack initiation site and investigate the fatigue failure mechanisms in different notch configurations. Irrespective of the material or the geometry type, ρ0.1 (specimens with ρ of 0.1 mm) showed shorter fatigue lives with lower scatter compared to ρ5 and ρ50 specimens. For ρ0.1 specimens, significant cyclic plastic damage at the notch root, due to the high stress concentration, caused critical crack initiations. For cylindrical specimens, ρ5 and ρ50 showed similar fatigue lives for AlSi10Mg; however, for 17-4 PH SS, the latter showed longer fatigue lives. It was likely due to the influence of wider variation in the critical defect features in AlSi10Mg than 17-4 PH SS, on the crack initiation behavior. Notch geometry as well as the critical defect’s size and location (within the cross-section and height relative to the notch root plane) influenced the fatigue behavior of cylindrical notched specimens. In the case of flat specimens, there was an additional influence of microstructure on the fatigue behavior of near-defect free 17-4 PH SS specimens. Delta ferrites (δ-Fe) acted as weak points in the microstructure, resulting in the formation of microcracks, and eventually crystallographic facets as the crack initiation site. Larger w resulted in shorter fatigue lives for ρ0.1 and ρ5 AlSi10Mg and 17-4 PH SS flat specimens. Specimens with w of 10 mm induced higher cyclic plastic damage at the notch root than 5 mm, thus promoting crack initiation and early failure. In specimens with minimal plasticity at the notch root, i.e., ρ5 and ρ50 specimens, mode-I stress intensity factor (SIF) of critical features calculated based on linear elastic fracture mechanics (LEFM), i.e., Murakami’s approach, correlated well with experimental fatigue lives. However, in specimens with significant plasticity at the notch root, i.e., ρ0.1 specimens, equivalent plastic strain at the notch root correlated well with the experimental fatigue lives. Based on LEFM, fatigue criticality of volumetric defects in AlSi10Mg and 17-4 PH SS notched specimens were assessed using a non-destructive technique, i.e., X-ray computed tomography (XCT). Assuming a defect-crack equivalency and accounting for local stress fields using linear elastic finite element analysis, mode-I SIF of defects detected via XCT was calculated and used to represent their fatigue criticality. For validation, ρ5 and ρ50 cylindrical and flat specimens were XCT scanned and tested; all crack initiating defects fell within the 99.3 percentile of the highest SIF defects in the respective notched specimens for AlSi10Mg and 17-4 PH SS. The behavior of short cracks initiated from the volumetric defects, including their growth and arrest, under the influence of the notch stress fields, were believed to have a significant influence on the fatigue lives of notch members. Utilizing a numerical modeling technique, the short crack growth behavior of cracks initiating from volumetric defects in notched specimens was investigated. It utilized the effective SIF of cracks, put forth by El-Haddad, to assess the short crack growth behavior. Notch geometry, defect’s size, shape, and location influenced crack arrest behavior. The minimum effective SIF of cracks, at the crack arrest, was used to obtain the fatigue notch factor. Utilizing the fatigue notch factor-based framework, fatigue lives of flat notched specimens with varying ρ were predicted. This was validated using laser powder bed fused AlSi10Mg and 17-4 PH SS flat notched specimens with ρ of 5 mm and 50 mm. For AlSi10Mg, 95% of all fatigue life predictions fell within the scatter band of 3, and 100% for 17-4 PH SS.