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

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

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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 re-gion of fatigue crack propagation was made for austenitic steel of Japan provenience. Related fa-tigue 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 chromatogra-phy 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 tough-ening induced by the austenitic microstructure are rather low when compared to those identified in ferritic- and ferritic-austenitic steels.

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 re-gion of fatigue crack propagation was made for austenitic steel of Japan provenience. Related fa-tigue 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 chromatogra-phy 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 tough-ening induced by the austenitic microstructure are rather low when compared to those identified in ferritic- and ferritic-austenitic steels.

Assessment, Extrinsic Crack Tip Shielding, Austenitic Steel, Fatigue Threshold

Assessment, Extrinsic Crack Tip Shielding, Austenitic Steel, Fatigue Threshold

POKLUDA, J.; KONDO, Y.; SLÁMEČKA, K.; ŠANDERA, P.; HORNÍKOVÁ, J.

2010

10.09.2008

TransTech Publication

Seoul

978-0-87849-369-2

Fracture and Damage Mechanics

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49

52

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