RETARDATION IN FATIGUE CRACK-GROWTH IN AL-2024 AFTER BLOCKS OF UNDERLOADING

In contrast with overloads, small numbers of underloads lead to acceleration in crack growth rate after application of the underloads. Larger numbers of underloads, however, give rise to retardation effects comparable with the effects of overloads. The retardation effects are attributed to the development of shear lips. Irregular shear lips show a greater effect than regular (smooth) shear lips. Increasing numbers of underloads give rise to greater retardation. For high numbers of underloads at high DELTA-K(eff) there is even an increase in the total life of the fatigue specimen with respect to the situation without underloads. High numbers of underload at high DELTA-K(eff) cause crack arrest at one side of the centre-cracked tension specimen. The non-growing side is often in single shear, the growing side always in double shear.