Notch And Defect Sensitivity Of ADI In Torsional Fatigue

This work presents an extension of the well known Kitagawa-Takahashi diagram valid for cracks subjected to mode I loading to the case of mode III loading. By considering the crack as a limit case of a notch characterised by a very small tip radius and taking advantage of the N-SIF stress parameter for describing the local stress field close to the notch tip, the diagram is extended in order to ecompass also U- and V-shaped notches. The notch opening angle and notch acuity, defined as the ratio between the notch depth and the notch tip radius, can be arbitrary. After presenting the adopted analytical expressions, the theoretical estimations are compared with the experimental results obtained from torsional fatigue tests on smooth and notches specimens in austempered ductile iron. An equation for fatigue strength estimations in presence of defect-like notches and crack-like notches having an arbitrary notch opening angle is proposed and validated by means of the available experimental results. Finally observations of the fracture surfaces and of the early fatigue crack propagation are presented.