Volume 21, Issue 5 (September 1993)
A Detection Method of Fatigue Crack Initiation by Analyzing Strain Waveform
A detection method of fatigue crack initiation at some weak section of machine equipment during operation is examined using a partial-notched specimen. The waveform of the strain function composed of strains in the vicinity of the partial notch was observed successionally. It is found that such strain waveform changes its figure at the time when the crack length is 1 mm, due to the crack closure behavior. Therefore, in this experiment, the crack initiation could be detected when the crack length is about 1 mm. Since the engineering crack initiation size is frequently taken to be 1 mm, the present method of detecting crack initiation can be applied to real machine equipment. Also, this waveform was analyzed with FFT (Fast Fourier Transformation). Power spectrum density of the waveform varies immediately after the crack length reaches 1 mm. Furthermore, the figure of the coherence of two waveforms composed of the same strains clearly varies at the same crack length of 1 mm. Thus, it is thought that the engineering size crack initiation can be detected by the analysis of the strain waveform.