単一過大ひずみ負荷による塑性疲労表面き裂の加速・遅延挙動

Abstract

The surface crack propagation behavior after single overstraining during constant amplitude tests in low-cycle fatigue was studied by using the small holed and through-crack specimens of low carbon steel. The tests were carried out with various kinds of overstrain waveforms and ratios.The results obtained are summarized as follows;(1) During the subsequent crack growth after overstraining the retardation-after-acceleration phenomenon was observed on the surface fatigue crack specimen in the both tension and compressiontension waveforms as well as on the through-crack specimen, and the duration of acceleration for the small holed specimens was longer than that of the through-crack one. The acceleration-after-retardation phenomenon was also observed in the both compression and tension-compression waveforms.(2) The durations during acceleration and retardation increased linearly with increasing overstrain ratio.(3) The stress amplitude after overstraining differed in the tension side and the compression side in such a way that the compression side was larger when the final peak of overstrain was in the tension side, whereas the tension side was larger when in the compression side. But the difference between the stress gradually decreased with increasing cycling, and approached to the saturation value of the constant amplitude tests. In the case of the tension waveform, the decreasing duration was almost the same as the duration during accelation of crack propagation.(4) The value of (ΔJhmax/2σy), evaluated by the J-integral range at the tension side overstraining, was related to the affected retardation and acceleration crack lengths as given by ld/2, la/2=α(ΔJhmax/2σy).