Analysis of endurance limits under very high cycle fatigue using a unified damage approach

A critical evaluation of endurance limits under very high cycle (giga-cycle range) fatigue is presented. The available experimental results are analyzed using the unified damage approach developed earlier by the authors. It is shown that the experimental evidence supports the theory that endurance limits at very high cycle fatigue are related to thresholds for crack propagation of incipient cracks from stress-concentration sites. The crack propagation behavior of the incipient cracks is related to the accumulation of internal stresses that is needed to augment the preexisting stresses. These in turn cause the incipient cracks to initiate at stress concentrations which either grow or arrest depending on the magnitude and gradient of the internal stresses. Micro-deformations at localized stress concentrations, such as inclusions, can further augment the local internal stresses needed for the incipient crack to nucleate and propagate. The presence of trapped hydrogen at inclusions can reduce the resistance to deformation, and hence reduce crack initiation and growth process. Understanding the role of preexisting stress concentrations (such as notches, inclusions, and hard second phases) provides a bridge for understanding the fatigue damage evolution in a smooth specimen compared to that in a cracked specimen. Kitagawa diagram helps provide this necessary bridge. Several critical issues related to fatigue testing method; fracture surface analysis, load history and environment are raised in this paper and briefly discussed in a self-consistent manner based on our unified damage approach.