Spectrum load fatigue—underlying mechanisms and their significance in testing and analysis

The constant amplitude fatigue process is driven by cyclic slip and surface chemistry. These in turn are affected by crack geometry, crack closure and crack-tip stress state, including residual stress. The sensitivity of these factors to spectrum loading determines durability and residual life under service load conditions. Cyclic slip forms the basis for the Rainflow cycle-counting technique and is therefore an integral part of any spectrum load fatigue analysis or test. Assessment of the relative significance of operative mechanisms impacts quality and cost effectiveness of spectrum load tests and also, the reliability of life predictions.