| Summary: | The technique of rapid evaluation of fatigue limit using infrared thermography has been paid attention during the past 30 years. This technique is beneficial because it also makes possible to detect the location of fatigue damage in real structures. In this technique, the second harmonic of temperature variation during cyclic loading is often used as a measure of the temperature evolution to evaluate the fatigue limit. The source of the second harmonic has been already investigated quantitatively for the load amplitude above the fatigue limit but has not been investigated very well for the load amplitude below the fatigue limit. In this research, five factors (second harmonic caused by energy dissipation, applied load signal, photoelectric current of infrared sensor, quantization error and specimen movement) were examined quantitatively to examine the source of the second harmonic below the fatigue limit. Experiments were conducted for double edge notched specimens of type 304 stainless steel. As a result, it was found that the second harmonic of temperature variation below the fatigue limit is mainly caused by loading equipment. In conclusion, it is suggested that the fatigue limit should be evaluated by fitting curves considering the second harmonic proportional to the square of the load amplitude.
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