Abstract
A method of detecting fatigue crack initiation in some weak section of a machine element is examined using a holed specimen under random load conditions. Waveforms of two strain functions, that is h1 = εy1 + λεy2 and h2 = εy1 - λεy2, are successively observed. εy1 and εy2 are the strains in the vicinity of the hole and λ is the strain range ratio Δεy1/Δεy2. The function h1 can be used to substitute for the load parameter and the waveform of h2 changes its shape after the crack initiation due to the crack closure behavior. Therefore, we can detect the fatigue crack initiation at the hole by comparison of these two waveforms. Also, these waveforms are analyzed with FFT (Fast Fourier Transformation) to allow for a more effective detection of crack initiation. Power spectrum density of the waveform h2 varies immediately when the crack length measured from hole edge reaches about 0.5 mm. The figures showing the coherence of two waveforms h1 and h2 clearly vary at the same length 0.5 mm. Thus, the crack initiation of its length of 0.5 mm can be easily detected with the analysis of the strain waveform.
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