AN INELASTIC MULTIPLE DISCRETE APERITIES MODEL FOR THE EFFECTS OF COMPRESSIVE UNDERLOADS IN FATIGUE-CRACK GROWTH

As a common practice, the compressive (negative load ratio) excursions are ignored when analyses of fatigue crack growth in metals are condu cted. However, recent experimental data on fatigue crack growth with i ntermittent compressive load excursions have shown that the use of thi s assumption leads in most cases to nonconservative predictions. This paper presents a model that is capable of explaining the observed beha vior, including the 'saturation' of the compressive overload effects, and the increase in the crack growth rate once the initial, positive l oad ratio profile is resumed, following a compressive excursion. The m odel is based on the plastic crushing of a single asperity or multiple asperities located on the crack face close to the crack tip and under dominantly plane strain conditions. A comparison of the behavior for one and for two asperities is made. Moreover, the effects of hardness and strain hardening are also examined.