Near-Threshold Fatigue Crack Growth and Fatigue Strength of ER70S-6 Steel Processed by Wire Arc Additive Manufacturing

Abstract

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 orientated 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). Following S–N testing, the sizes of defects were determined. 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 the ΔKₜₕ value for CD⊥BD samples at ~ 5.0 MPa √m. Some elongated α-ferrite grains orientated close to BD have been identified, suggesting that the cementite containing pearlite colonies may be locally aligned with the BD, reducing fatigue crack resistance along BD and thereby lowering ΔKₜₕ. For both CD//BD and CD⊥BD samples, the effect of the defect size up to 520 μm affecting Nf can be explained using the K–T diagram approach. It is also shown 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 ΔKₜₕ (CD⊥BD samples) > ΔKₜₕ (CD//BD samples).