BASIC ISSUES IN THE MECHANICS OF HIGH-CYCLE METAL FATIGUE

Mechanics issues related to the formation and growth of cracks ranging from subgrain dimension to up to the order of one mm are considered u nder high cycle fatigue (HCF) conditions for metallic materials. Furth er efforts to improve the accuracy of life estimation in the HCF regim e must consider various factors that are not presently addressed by tr aditional linear elastic fracture mechanics (LEFM) approaches, nor by conventional HCF design tools such as the S-N curve, modified Goodman diagram and fatigue limit. A fundamental consideration is that a thres hold level for Delta K for small/short cracks may be considerably lowe r than that for long cracks, leading to non-conservative Life predicti ons using the traditional LEFM approach. Extension of damage tolerance concepts to lower length scales and small cracks relies critically on deeper understanding of (a) small crack behavior including interactio ns with microstructure, (b) heterogeneity and anisotropy of cyclic sli p processes associated with the orientation distribution of grains, an d (c) development of reliable small crack monitoring techniques. The b asic technology is not yet sufficiently advanced in any of these areas to implement damage tolerant design for HCF. The lack of consistency of existing crack initiation and fracture mechanics approaches for HCF leads to significant reservations concerning application of existing technology to damage tolerant design of aircraft gas turbine engines, for example. The intent of this paper is to focus on various aspects o f the propagation of small cracks which merit further research to enha nce the accuracy of HCF life prediction. Predominant concern will rest with polycrystalline metals, and most of the issues pertain to wide c lasses of alloys.