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This paper focuses on the study of the stages IIa and IIb in Paris region of fatigue crack growth (FCG) for base metal and weld specimens of 316LN stainless steel (SS) by using acoustic emission (AE) technique. Meanwhile, the fatigue properties and AE characteristics were analyzed based on the K-means clustering method and fractographic observations. The results show that 316 SS weld exhibits a higher resistance to crack growth than that of base metal due to the deflection of crack path under mixed loading mode. The transition from stage IIa to IIb in the Paris region can be identified by the change of slope in AE cumulative count and cumulative energy vs. ΔK, which otherwise is not feasible from da/dN vs. ΔK plots. The AE signal characteristics of different source mechanisms such as ductile crack growth, plastic deformation within cyclic plastic zone and shear crack growth are significantly different in the two sub-stages during fatigue. The results suggest that AE technique is sensitive to the identification of the presences of stage IIa, stage IIb in the Paris region and the shear crack growth during FCG.
Chai et al. (Mon,) studied this question.