Ｘ線フラクトグラフィにおける圧縮残留応力

Abstract

X-ray fractography is a technique for analysing the cause and mechanism of fracture from the information obtained by X-ray irradiation on the fractured surface. It has been shown that a good correlation exists between the residual stress on the half value breadth of diffraction profile and the stress intensity factor that had caused the fracture. X-ray fractography has been successfully applied for the in-service fracture of many types of fracture. However, in some cases the residual stresses on the fatigue fractured surface in service are compressive, which have not been found in the laboratory experiments so far.In the present study, fatigue experiments were carried out on 0.5% carbon steel to investigate the stress condition that produces compressive residual stress on the fractured surface. The specimen was a center notched rectangular plate of 8mm thick, and a wide range of stress ratio R=σmin/σmax were applied from tensile to compressive, namely R=0.50, 0.25, 0.20, 0.00, -1.67, -2.33, -2.40 and -3.00. From the results of experiments, it was found that, when the stress ratio was -3.00 and the minimum stress was -150MPa, the residual stress on the fractured surface became compressive. Since the minimum stress is far smaller than the compressive yield stress, the cause of the compressive residual stress was considered to be the result of crack closure. In this case, the crack opening ratio U=(σmax-σop)/Δσ, where σop is the crack opening stress, was about 0.3 and almost constant through the crack length.