Deformation approach to the evaluation of the period of initiation and growth of fatigue macrocracks

We performed experimental investigation of the opening displacements of the contours of stress concentrators (notches and cracks) for various amplitudes of cyclic loading. On the basis of experimental results, we propose a new deformation parameter Delta epsilon(t) which is a function of the notch (crack) tip opening displacement Delta delta, namely, Delta epsilon(t) similar to Delta delta/pi(rho + d*), where rho is the radius of the tip of the notch and d* is the characteristic size of the prefracture zone. It is shown that this parameter uniquely determines the number of cycles N-1 to the initiation of a fatigue macrocrack independently of the geometry of the specimens and stress concentrators in elastic and elastoplastic materials, i.e., the dependence of Delta epsilon(t) on N-1 is a characteristic of the material. It is experimentally demonstrated that this dependence enables one to quantitatively describe the process of fatigue fracture both in the stage of initiation of macrocracks and their propagation.