FATIGUE CRACK PROPAGATION BEHAVIOUR UNDER VARIABLE AMPLITUDE LOADING IN THE NEAR‐THRESHOLD REGION OF A HIGH‐STRENGTH LOW‐ALLOY STEEL

The near-threshold behaviour of a high-strength low-alloy steel used in helicopter rotors under variable amplitude loading is investigated in ambient air and in vacuum. Strong sequence effects occur, in particular a high decrease in crack growth rates after overloads of 40% on maximum load while keeping the amplitude constant. A detailed study of the retardation phenomenon due to the overloads provides an explanation for the different behaviour observed in air and in vacuum and has laid the basis for a method of predicting crack propagation under helicopter loading spectra.