Fracture mechanics and notch sensitivity

A diagram valid for the analysis of the fatigue limit of cracks and notches centred in an infinite plate was recently proposed by the authors of the present work with the aim to make explicit the bridging at the fatigue limit between defect sensitivity (correlated to the length parameter a(0), according to El Haddad-Topper-Smith's definition) and notch sensitivity (correlated to a*, where a* is a particular notch depth corresponding to the intersection between the DeltaK(th) and Deltasigma(0)/K-t curves). The expression A*=K-t(2).a(0) being valid, defect sensitivity and notch sensitivity were seen as two sides of the same medal. Such a diagram is now extended to finite size components by simply introducing the shape factor alpha commonly used in fracture mechanics. The obtained critical defect size is termed a(D), which is a material and geometry dependent parameter, in order to distinguish it from a(0), which is a material parameter. As a consequence the critical notch depth a(N) is introduced, such that . a(N)=K-t(2).2(D) This results in the proposal of a 'universal' diagram able to summarize experimental data related to different materials, geometry and loading conditions. The diagram, the validity of which is checked by means of several results available in the literature, is applied both to the interpretation of the scale effect and to the surface finishing effect.