Measurement of Fatigue Crack Opening and Closing Points in Thin Plate Specimen and It's Difficulties

Abstract

The measurement of fatigue crack tip opening and closing points in a machine element with complicated shape is expected to involve difficulties. In the present study, the measurement of crack opening and closing points in a center notched thin plate was carried out by the unloading elastic compliance method and the strain interference method, and the application of these methods to an actual machine equipment was discussed. The strain measurements were carried out at several positions around the notch and crack for the purpose of determining crack opening and closing points. When the bending of the specimen arises during a push-pull fatigue process, the measurement of crack opening and closing points becomes more difficult unless the proper measuring position is chosen. However, when the strain function consisting of strains measured at two diffent positions on the same longitudinal axis is used in the case of strain interference method, the crack opening point can easily be measured with sufficient accuracy.