超音波疲労試験を利用した介在物検査手法

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This report proposes a new inclusion inspection method using ultra-sonic fatigue tests at 20kHz. Fish-eye fractures occur in high-strength steels and the most harmful defect for fatigue in the risk volume of a specimen appears at the fracture site. In the other word, fatigue tests certainly specify the defect causing fatigue failure. The method was validated on the basis of fatigue test results for two low-alloy steels and a spring steel. Fatigue tests had a larger inspection volume and detected a larger inclusion than a conventional direct observation method on mirror surfaces. In case of a low-alloy steel in which about 10μm Al_2O_3 inclusions existed together with smaller TiN inclusions, the TiN inclusion and matrix cracks also caused fish-eye fracture. Even in a such case, fatigue tests certainly specify the most harmful defect for fatigue. In case of Al_2O_3 inclusions, the inclusion size could be measured on the fracture surface. For TiN inclusions, however, the sizes were difficult to measure. For matrix cracks, the size could be measured in the spring steel but was difficult to measure in a low-alloy steel. Although distinction of inclusions based on ASTM standard and measurement of TiN inclusion sizes were basically possible in fatigue test, the procedures took time and were costly.
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