Effects of surface undulation on fatigue of wire arc additively manufactured ER70S-6 steel: Numerical and analytical models

This paper addresses the modelling of the static and fatigue behaviour of as-built and machined wire arc additively manufactured (WAAM) steel, with particular focus on the effects of surface undulations, which arise due to process-induced irregularities in heat input, material deposition, and melt pool dynamics. Based on 3D laser scans of WAAM coupons, finite element (FE) models are developed and validated against experimental results obtained using digital image correlation (DIC). The FE method, though accurate, is computationally expensive, since very fine meshes are required to model the as-built undulating surfaces of the WAAM coupons. Therefore, two simplified analytical models, one based on bending and the other on surface curvature, are proposed for the local stress analysis of the WAAM coupons, allowing for the influence of undulating surface and material thickness. The proposed methods are shown to predict the local stresses in the WAAM steel with reasonable accuracy, achieving errors as low as approximately 8–9 %, while maintaining high computational efficiency. The obtained local stresses are further used for the prediction of the fatigue crack initiation location and fatigue life of WAAM steel, achieving good agreement with the experimental results. Two fatigue design classes of FAT 145 and FAT 135 with endurance limits of 270 MPa and 250 MPa, respectively, are derived for WAAM ER70S-6 steel using the proposed models. • Effect of surface undulation as a result of manufacturing process on fatigue is studied. • Numerical simulations of static and fatigue behaviour of WAAM steel are conducted. • Two analytical models are proposed to predict local stresses in WAAM material. • Accuracy of models in fatigue life and crack initiation prediction is demonstrated. • Formation mechanisms of stress concentrations in WAAM material change with thickness.