SHORT CRACKS AND LIFETIME OF A NORMALIZED CARBON-STEEL UNDER CYCLIC BLOCK LOADING

The behaviour of short cracks and the lifetime of unnotched specimens of a normalized medium carbon steel is observed under constant cyclic loading and under cyclic block loading with changing mean stresses. Th e greatest part of the life is covered by the growth of short fatigue cracks. During cyclic block loading, the length of the loading blocks has a marked influence on the crack density and the lifetime of the sp ecimens. Short blocks lead to high crack densities and low lifetimes. This effect can be explained by a short crack growth model which rakes into account the crack arrest before microstructural barriers and cra ck linking in the case of high crack densities. The lifetimes for cycl ic block loading are calculated on the base of the constant level data and found to be in good agreement with the measured lifetimes.