Estimating fatigue cracks, from the onset of loading, in smooth AISI 4340 specimens, under cyclic stresses

Abstract

High-cycle fatigue tests of polished specimens, as well as the treatment of smooth parts, are currently evaluated by the cumulative damage approach, using Miner's rule. It is known that, at some stage of the process, a crack develops, and the very well established LEFM crack propagation relations can then be used to estimate the residual life, with a good accuracy. The transition between crack initiation and propagation is not yet quite clear physically, nor is it defined. Therefore the crack propagation relations are often not easy to use, as the size of the transitional crack length is not defined. In this paper, which basically describes a curve fitting study, a relation for smooth specimens is proposed, which uses the parameters of crack length throughout all the fatigue stages. It uses the superposition of two expressions, one from fracture mechanics and the other from the cumulative damage approach, each in its own domain of existence. There is no need to define the transition between crack initiation and propagation. The calculated predictions show a good fit to the test results from smooth specimens of AISI 4340 steel. A much wider range of applications is indicated, and a general diagram for the classification of fatigue regimes in cyclic loading is proposed.