Method of Detecting Fatigue Crack Initiation through Analysis of Strain Waveform

A method of detecting fatigue crack initiation in some weak section in an actual working machine is examined using a partially notched specimen. The waveform of the strain function composed of strains in the vicinity of the partial notch is successionally observed. It is found that such a strain waveform changes its shape at crack length 1mm due to the crack closure behavior. Therefore, in this experiment, the crack initiation can be detected at about crack length 1mm. Since the engineering crack initiation size is frequently taken to be 1mm, the present method for detection of crack initiation can be applied to actual machine equipment. Also, this waveform is analyzed with FFT. The power spectrum density of the waveform changes immediately after the crack length reaches 1mm. Furthermore, the shape of the coherence of two waveforms composed of same strains clearly varies at the same length, 1mm. Thus, the engineering size crack initiation is expected to be easily detected through the analysis of the strain waveform.