Observations on the Prediction of Fatigue Crack Growth Propagation Under Variable-Amplitude Loading

The paper starts with a discussion on loads in service, after which a survey is given of various types of variable-amplitude loading as applied in test programs. The various phenomenological aspects of fatigue damage associated with fatigue cracks are indicated. Interaction effects between cycles of different magnitudes are defined. Methods for measuring interaction effects, examples of interaction effects, and possible explanations are reviewed. This includes both tests with simple types of variable-amplitude loading (overloads and step loading) and more complex load-time histories (program loading, random load, and flight-simulation loading). New evidence on crack closure is presented. Various types of prediction methods are discussed. The paper is primarily a survey of the present knowledge, with an analysis of the consequences for prediction techniques.