Characterization of surface irregularities and fatigue strength evaluation of wire arc additive manufactured high strength steel specimens

• Results confirm the presence of secondary crack-like defects at the notch-like valleys of WAAM specimens. • A novel surface characterization method separates waviness and roughness without distorting features, addressing prior limitations. • Microhardness variations stem from repeated changes during the manufacturing process, forming softer phases in interlayer regions at notches. • The area model with corrected stress intensity evaluates WAAM fatigue strength, considering surface feature and microstructure interaction. This paper aims to study the effects of surface topography on the fatigue strength of wire-arc additive manufacturing (WAAM) components, paying particular attention to the interaction of roughness, waviness, and microstructure. For this purpose, first, a novel surface topography characterization approach is proposed to separate waviness and roughness without distorting the surface features, as smaller-scale irregularities, called secondary crack-like defects, have been confirmed to exist at notch-like valleys of WAAM specimens. This novel approach can practically be employed for any surface data obtained by different measuring methods, and it is not limited to WAAM specimens. Then, Murakami’s area model with a corrected stress intensity factor is introduced to evaluate the fatigue strength of WAAM specimens by considering the interaction between surface features (roughness and waviness) and their simultaneous effect. This approach can also particularly predict the fatigue strength of other engineering components once a small secondary crack or defect exists at the notch tip without entailing fatigue tests or intricate analysis. Moreover, microstructure changes due to the back-and-forth transformations in the microstructure and the formation of a soft phase in the interlayer area at the notches during the manufacturing process with lower hardness values have been considered using this model.