A METHOD OF DETECTING THE FATIGUE CRACK INITIATION AND GROWTH IN A HOLED SPECIMEN BASED ON CRACK TIP OPENING AND CLOSING

A simple and new method for determining crack initiation at a notched section of a moving machine element is proposed and applied to the specimen with a blind hole or a through hole. In this experimental case, the fatigue crack tip opening and closing points can be measured by the relation between the following strain function h and cyclic stress sigma h = epsilon(y) + lambda-epsilon(x), lambda = DELTA-epsilon(y)/DELTA-epsilon(x) where epsilon(y) and epsilon(x) are the strains measured in the vicinity of the hole in the center axis, and DELTA-epsilon(y) and DELTA-epsilon(x) are the ranges of epsilon(y) and epsilon(x), respectively. Crack initiation at the hole edge can also be detected with the wave form of such a function h. Furthermore, in the case of the blind holed specimen, the crack growth behavior can be evaluated with the range of the subtractive function related to h. The detection method proposed can be expected to be applicable to the monitoring of crack growth in many real cases.