FURTHER EXPLORATION ON CHARACTERISTIC REGION OF CRACK INITIATION FOR VERY-HIGH-CYCLE FATIGUE

With regard to very-high-cycle fatigue (VHCF) of high-strength metallic materials, we previously proposed the concept of crack initiation characteristic region and the related characteristic parameter (IJFatigue 2014, 58: 144-151), and proposed the numerous cyclic pressing (NCP) model to reveal the formation mechanism of this characteristic region (IJFatigue 2016, 89: 108-118). This crack initiation characteristic region is so-called fine granular area (FGA) on fracture surface for high-strength steels or rough area (RA) on fracture surface for titanium alloys. In recent years, the investigators in fatigue research field have paid great attention to the topic of crack initiation of VHCF for high-strength metallic materials and obtained new results. Therefore, several issues on this topic are of great interests and are necessary to be clearly addressed. These include: Does the microstructure refinement as well as nanograin formation in crack initiation characteristic region happen before or after crack initiation? What is the correlation between applied stress ratio and the formation of crack initiation characteristic region? What are the details of refined microstructure in crack initiation characteristic region including the thickness and the distribution of nanograins? Is vacuum environment the necessary condition for the formation of crack initiation characteristic region? What are the features of crack initiation characteristic region in different materials or with different loading modes? This article will clarify such issues by the comprehensive review of the recent results in the literature. This article will also briefly describe the important implications of the crack initiation characteristic region concept and the NCP model, which include: the assessment and prediction of VHCF properties for high-strength metallic materials, the approach to improve the VHCF properties of additively made metallic materials, and the possibility of manufacturing thin film metallic materials with nanograin microstructure. Specially, this article is dedicated to the memory of my supervisor Prof. Che-Min Cheng who passed away on August 25, 2021.