Assessment of Extrinsic Crack Tip Shielding in Austenitic Steel near Fatigue Threshold

Asymmetric arrangements of stored crack-wake dislocations and low values of the size ratio SR, the plastic zone size / the characteristic microstructural distance, were found to be of basic importance for the shear misfit of crack flanks causing the roughness-induced crack closure in case of plain strain conditions. The crack wake dislocations produce also the plasticity induced crack closure as a result of a near-tip mismatch perpendicular to crack flanks. According to a recently published theoretical concept, an estimation of these extrinsic shielding effects in the threshold region of fatigue crack propagation was made for austenitic steel of Japan provenience. Related fatigue experiments were based on a standard load shedding technique associated with monitoring of the crack closure level. The surface roughness was analysed by means of the optical chromatography that enables a 3D reconstruction of fracture morphology. Calculated and measured effective threshold values of about 2.2 MPa.m1/2 are practically identical. Total levels of the extrinsic toughening induced by the austenitic microstructure are rather low when compared to those identified in ferritic- and ferritic-austenitic steels.