An Investigation of Low Cycle Fatigue Life of Wide Plate with a Notch

Abstract

In the previous paper, a pulsating tension test was carried out using wide plate specimens of mild steel with a central notch of stress concentration factor Kt to be 5 and 10. By simulating the cyclic behaviors of the strain at the tip of a notch to hour-glass shaped small specimens, the low-cycle fatigue crack initiation life of a plate specimen was compared with that of a small bar specimen. The results showed good correlation between the wide plate and the small bar specimen, but the life of the plate specimen with a notch of 5 in Kt was slightly longer than that of small bar specimen, while that for notch of 10 in Kt was slightly shorter. This fact shows that the crack initiation life depends on not only the cyclic behavior of strain but also the configulation of a notch.In this paper, a same test was carried out using wide plate specimens and small bar specimens of mild steel and 50 kg/mm2 high tensile steel, and the crack initiation life was investigated more precisely based on the present and previous results. It was revealed that dominant factors affecting the crack initiation at the tip of notch in the pulsating tension test are maximum strain at the first loading, strain amplitude, increasing mean strain and restraint of deformation at the tip of a notch. Since the influence of the restraint of deformation at the tip of notch can not be realized in the strain controlled test simulating the cyclic behavior of strain at the tip of a notch, the larger Kt of a notch is, the shorter the crack initiation life of the plate specimen is than that of simulating test on the small bar specimen. By comparing with the results of completely reversed strain-cycling test of small bar specimens, it was found that mean strain and its increase show supper posed effect on the crack initiation life when the life of the plate specimen is shorter than about 700 cycles. As for the effect of the increasing mean strain, an effect of the increasing mean strain was found to be appeared only when the increasing rate of mean strain is larger than about 0.1%/cycle, from the results of comparing test of linearly increasing mean strain with that of constant mean strain.In this paper, two cumulative damage rules in low-cycle fatigue are proposed taking into consideration of four factors affecting the crack initiation. One is based on the cyclic behavior of strain, and the other on the cyclic behavior of notch opening displacement. These two rules are able to estimate the fatigue life of the plate specimen within a small extent of deviation.