PSI - Issue 66

Aditya Khanna et al. / Procedia Structural Integrity 66 (2024) 370–380 Author name / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Recent advances in Wire-Arc Additive Manufacturing (WAAM) technologies have made feasible the near net shape fabrication of large-scale loading bearing structural steel components for use in challenging operating environments (Kumar et al., 2022; Kumar and Manikandan, 2021; Kazanas et al., 2012; Pan et al., 2018; Williams et al., 2016). At present, the material characterisation studies of Additively Manufactured (AM) alloys are predominantly limited to basic material properties, such as strength, modulus, toughness, and S-N (stress-life) diagrams under cyclic loading. However, contemporary design concepts and maintenance strategies, such as the damage tolerance concept, also require the knowledge of fatigue crack growth (FCG) rates (Sales et al, 2024). This contrasts with conventionally fabricated structural alloys, for which extensive fatigue crack growth rate databases have been compiled over the past 50+ years. A thorough understanding of fatigue crack growth in AM materials is required prior to widespread application of these materials in load-bearing structural components working under cyclic loading. Differences in fatigue crack growth rates between AM and other manufacturing processes can be due to microstructure, the severity of manufacturing defects, and residual stress distribution. The last of these parameters, i.e., residual stress distribution, is the focus of the present work and has also been investigated in other recent publications (Sales et al, 2024; Smudde et al., 2024, 2023a, 2023b, 2022). In general, residual stresses develop after material processing and subsequently can evolve under applied loading. AM components are extremely vulnerable to defect formation as well as to a high level of residual stresses, which can significantly influence their fatigue life. According to the damage tolerance concept, it is possible to maintain safe and efficient operation with periodic defect inspections. The time interval between inspections must be determined by the residual fatigue life assessments, which require fatigue properties of the material. These fatigue properties are normally obtained using fatigue test specimens and observing and studying the fatigue crack growth in these specimens under various loading conditions. However, fabrication process of test specimens can change the residual stress field. Therefore, it is important to evaluate the intrinsic fatigue properties of the material, which are not affected, in particular, by residual stress field, which can be very different in the test specimens and in the structural component (Khanna and Kotousov, 2020). The present work, and other recent studies (e.g. Smudde et al., 2024) evaluate the Stress Intensity Factor (SIF) due to the residual stress field using the same experimental setup as a typical fatigue crack growth rate test. It is known that advancing a narrow cut in an elastic body with residual stresses leads to measurable changes in the displacement (and strain) fields. If a cut is made in a standard fatigue test specimen along the anticipated crack path, the change in the load line displacement or back face strain with cut length can be used to evaluate the residual stress intensity factor as well as the residual stress field perpendicular to the cut path using the “slitting” or “cut compliance” method. This method was developed by Finnie and co-workers in the 1990s (e.g. Schindler et al., 1997; Cheng and Finnie, 1986). Technical notes on the laboratory application of the slitting method are presented by Hill (2013). Lados et al. (2007) demonstrated that this destructive testing method can also be adapted to fatigue crack growth testing since the cyclic crack growth essentially represents successive extensions of a cut. The latter method is referred to as the “online crack compliance” method. Lados et al. demonstrated the online crack compliance method using crack mouth opening displacement measurements and recently, Smudde et al. (2023) demonstrated this method using the back-face strain measurements. In Section 2, the compliance-based method for residual SIF evaluation is reviewed. In later sections, this method is applied to recently published results (Sales et al., 2024) for fatigue crack growth testing of WAAM Super Duplex Stainless Steel (SDSS) specimens. The full details of AM process parameters, specimen preparation, instrumentation and signal processing, and loading conditions applied during the fatigue crack growth rate test are presented in the companion paper (Sales et al., 2024). The details pertinent to the present work are briefly repeated in Section 3. Section 4 provides key results for the experimentally measured crack tip opening load ratio vs. stress ratio and crack length and demonstrates the significant difference in the crack closure level measured for crack growth longitudinal to and transverse to the weld deposition direction. It must be noted that the crack tip opening loads measured using the ASTM E647 compliance-offset method is “global” in nature, i.e., the relative contribution of various mechanisms of crack closure, such as Plasticity-Induced Crack Closure (PICC), residual stress effect, roughness-induced closure, etc., cannot be separated using the standard method. Further analysis, as per the methods described in Section 2, is