THE EFFECT OF AN OVERLOAD ON THE RATE OF FATIGUE-CRACK PROPAGATION UNDER PLANE-STRESS CONDITIONS

It has been shown that the retardation in the rate of fatigue crack gr owth following an overload is largely the result of surface-related, p lane-stress plastic deformation. In the present article, in order to i solate the plane-stress behavior, the effect of an overload on the sub sequent rate of fatigue crack growth of 0.3-mm-thick specimens of 9Cr- 1Mo steel has been investigated and compared to results obtained using 6.35-mm-thick specimens. It was found that for the 0.3-mm thickness, as with thicker specimens, two opening load levels were associated wit h the overload process. The upper opening load is associated with plan e-stress deformation in the overload plastic zone, and this opening pr ocess is more clearly observed with thin as compared to thicker specim ens. Based upon the determined level of the upper opening load, a semi empirical analysis is developed for calculating the number of delay cy cles due to an overload as a function of thickness.