Opening and Closing Behavior of Non-Propagating Micro-Cracks in Rotating Bending and Torsional Fatigue Tests of Carbon Steels

Abstract

Non-propagating microcracks are observed on the surfaces of smooth unbroken specimens loaded more than 107 cycles at or slightly below their fatigue limits under rotating bending fatigue tests of a prestrained 0.17%C steel and under torsional fatigue tests of an annealed 0.54%C steel. In this study, the crack opening displacement at various distances from the tip of a fatigue microcrack was measured accurately on the specimen surface by using electron microscopy, and the opening-and closing-behaviors of the non-propagating microcracks were compared with those of propagating microcracks, in order to investigate the mechanism of non-propagation of fatigue microcracks.The main results obtained are as follows;(1) The opening-and closing-behaviors of non-propagating microcracks observed under rotating bending fatigue is almost the same as the one observed under torsional fatigue.(2) The shape of the rim near the tip of the non-propagating microcrack on the surface of the specimen is very different from that of the propagating microcrack, at the maximum stress in one cycle of the reversed stress under which the respective cracks are formed. The shape of the non-propagating microcrack shows the “cusp” type, while that of the propagating microcrack shows the “blunt” type near the crack tip. The crack opening displacement near the tip of the non-propagating microcracks is much smaller than that of the propagating microcracks formed under the reversed stress whose amplitude is slightly higher than that of fatigue limit. Moreover, the non-propagating microcracks open rarely at or near their tips even under the maximum stress in one cycle of the reversed stress under which the cracks are formed.