疲労き裂先端ひずみ変化の詳細観察

Abstract

It is well known that the crack opening/closure phenomenon found by Elber plays an important role in the fatigue crack propagation behavior under any kind of loading condition. On the other hand, the retensile plastic zone's generated load (RPG Load) was newly proposed by Toyosada as for a fatigue crack propagation parameter in place of the crack opening/closure load. However, the accuracy of usual measurements seems to be not sufficient to discuss the fatigue crack opening/closure phenomenon quantitatively.In this paper, for the purpose of proper understanding of the fatigue crack opening/closure phenomenon, a detailed observation of change in strain near the fatigue crack tip was carried out. And successful measurement of the hysteresis curve of load vs. strain (ε') with high accuracy was made. By introducing the curve of compliance (dε'/dp) and the change rate of compliance (d2ε'/dp2) during one cycle, the following conclusions have been obtained.1) The unloading part of the hysteresis curve is found to be “S” shape, and the point of inflection of the unloading part is considered to be the crack closure point as Nisitani pointed out. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of compliance, the crack closure point can be measured automatically as the maximum point of the unloading part of this curve.2) The loading part of the curve of compliance is found to be “S” shape, and the point of inflection of the loading part is considered to be the RPG point. So if the accuracy of measurement is high enough to be able to obtain a smooth curve of the change rate of compliance, the RPG point can be measured automatically as the minimum point of the loading part of this curve.3) Even though the accuracy of measurement is high, it seems to be difficult to measure automatically the crack opening point, because it shows no distinguished feature.