Plastic Deformation in Cracked Plates of Low-Carbon Steel under Monotonic Tension and Cyclic Tension-Compression

Abstract

Specimens of 0.03%C steel with a crack of the length 1.5 or 3mm were subjected to the cyclic tension-compression of amplitude 10 or 13kg/mm2 or to the monotonic tensile stress 10 or 13kg/mm2. In each case, the following three parameters defining the plastic deformation ahead of the crack tips were measured by means of optical microscopy and the back-reflection X-ray microbeam diffraction method, i. e., (i) the depressed zone size, (ii) the slip band zone size and (iii) the distribution of the excess dislocation density in the vicinity of the crack tips. The plastic deformation in cracked plates was compared in terms of these microscopic parameters. The results obtained are summarized as follows:(1) The size of the depressed zone formed ahead of the crack tips under monotonic loads was accurately predicted by Dugdale's equation. The increase in the excess dislocation density was detected in the depressed region, the maximum value being reached at a small distance away from the crack tips. The maximum stress in the depressed region evaluated from the excess dislocation density was smaller than the lower yield strength of the material.(2) As to the plastic deformation due to cyclic tension-compression, the depressed zone was observed in the specimen under a high stress intensity factor, but not under low stress intensity factors. The antinucleation in the depressed zone under low stress intensity factors may be caused by the following reasons: (i) the strain rate in the cyclic tests is higher than that in the monotonic tests, (ii) the work-hardening of the material at the crack tip due to cyclic stress is higher than that due to monotonic stress, and (iii) the compressive residual stress exists near the tip of fatigue cracks. The excess dislocation density took the maximum value at the crack tip, and its value is several times as large as that under monotonic stress. The flow stress of the material at the crack tip under cyclic stress is nearly twice as high as that under monotonic stress.(3) Slip bands were not observed in the vicinity of the crack tips of the specimens that were deformed under monotonic loads, while they were observed near the tip of fatigue cracks. The zone of slip bands near the crack tips does not correspond to the monotonically deformed zone, but to the cyclically deformed zone.