Near-threshold fatigue crack growth and fatigue strength of ER70S-6 steel processed by wire arc additive manufacturing

In this work, fatigue crack growth (FCG) tests were conducted to determine the threshold stress intensity factor (ΔKth) of ER70S-6 steel processed by wire arc additive manufacturing (WAAM). Samples were oriented with the crack direction (CD) both parallel (//) and normal (⊥) to build direction (BD). In addition, tests were conducted to obtain data of S–N fatigue strength and cycles to failure (Nf). The resulting S–N data, together with the measured defect sizes and ΔKth values, were subsequently analysed using the Kitagawa–Takahashi (K–T) diagram approach. Applying R = 0.1, the ΔKth value for CD//BD samples at ~ 4.5 MPa√m has been found to be slightly lower than for CD⊥BD samples at ~ 5.0 MPa√m. The effect of defect size up to 520 μm affecting Nf can be explained using the K–T diagram approach. The results also show that, when the effect of defects is excluded, the endurance limit of CD⊥BD samples is slightly higher than that of CD//BD samples, consistent with the observed ΔKth values.