PURPOSE:To forecast exactly in a short time a fatigue life and a fatigue crack generating position of a spot welded joint by sticking a strain gauge to the surface of an electrode indentation of a spot weld zone, and processing its output. CONSTITUTION:Eight strain gauges 5 are stuck to the surface of an electrode indentation 3 of a spot weld zone by placing them radially at equal intervals. A fatigue life and a fatigue crack generating position of a spot welded joint are forecast in a short time and exactly by processing a phase between each repeated load waveform output from each strain gauge 5, and a strain amplitude.
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