Chapter 4 - LCF-HCF Interactions

Abstract

Of primary concern in high cycle fatigue (HCF) design is the material capability, after it is subjected to service conditions that may degrade capability over time and cycles. Low cycle fatigue (LCF) loading, while accounts for in design and not leading to failure during the design life, may degrade the capability of the material regarding its HCF resistance. One other form of potential damage, which is due to transient stresses above the fatigue limit during HCF loading, must also be considered. A practical problem arises when, in designing against HCF, the occurrence of stress transients above the endurance limits (FLS) becomes a possibility. Both stress transients above the FLS and prior or combined loading, under conditions where part of the loading is above the HCF limit, constitute conditions that are referred to as LCF–HCF loading. The LCF portion of the loading is designated as such irrespective of the frequency of loading and is not necessarily restricted to true LCF conditions that normally involve strain-control testing, a low number of cycles to failure, and inelastic deformation in a typical load-displacement loop. It is simply used to designate loading conditions that are different from the HCF conditions being evaluated.