Correspondence Between Notched and Unnotched Specimen’s Fatigue Data: Fundamental Concept and Formulation of Equivalent Stress Ratio REQ

In the present study, a creative consideration is developed for the problem of the correspondence between notched and unnotched specimen’s fatigue data. The proposed concept is essentially different from that of volumetric damage, and it has the great advantage of being avoidable to encounter the fatally invalid point of the volumetric approach, such as determination of damage-volume sizes. Moreover, the fatigue criteria derived are very easy to be applied to practical notch problems. The approach is developed as follows; (1) A hypothesis of plastic adaptation in a surface layer is proposed and mechanical models reflecting the hypothesis are constructed. (2) From these models, equivalent stress ratios REQ are derived as the new parameter for the correspondence between the cyclic stress conditions of the notched and unnotched specimens. REQ is generalized for proportional multiaxial cyclic loading through the consideration for the case of torsional cyclic loading. Moreover, REQ is derived from the mechanical model for the stress field of the mode I fatigue crack tip. (3) Experimental data on fatigue strength and fatigue crack propagation rates are rearranged on the basis of REQ and are formulated. (4) Practical criteria are proposed for the fatigue strength and the fatigue crack propagation rates.