X線による金属材料疲労破壊に関する研究

Abstract

In the previous papers, the authors investigated the change in residual stress and microstructure in relation to the change in half-value breadth of annealed carbon steel in fatigue process, adopting (310) diffraction line. Based on the results of the experiments, the following conclusions were derived. Slip traces appear as regularly arranged line in the special slip planes of certain crystal grains in the early stage of alternating stressing, and they grow longer and wider with the increase of the number of cycles. In this period, both half-value breadth and compressive residual stress increase rapidly. It was considered that the broadening of half-value breadth comes from the accumlation of dislocations that occurs predominantly on the slip plane {110} by cross slip mechanism. In other words, it may be assumed that the extrusion and intrusion phenomena occur and the fatigue damage proceeds mainly by Cottrell and Hulls mechanism, and this also causes compressive residual stress to generate. In the second stage of fatigue, the half-value breadth increases gradually, while compressive residual stress decreases steeply and thereafter no change cccurs till fracture. Micro-structure shows distinctly that slip bands spread in the limited zone in certain grains. We may assume that micro-cracks initiate in these active slip bands and they promote the development of the visible cracks.However, there are two sorts of ploblems regarding the above investigation made by the authors. The first is that; in order to obtain an experimental proof to support the previous discussion, direct observations of the micro crack, extrusion, intrusion and cross slip phenomena are to be made in fatigue processs for present material. The second is that; according to the extrusion and intrusion mechanism, it is expected that the X-ray diffraction plane dependency of the change in half-value breadth and that of the changes in residual stress are present. In these connections, the authors attempted to investigate the relation between changes in half-value breadth diffracted from several atomic planes (310), (211), (220), (200) and (110), and changes in residual stress obtained from different lines of (310), (211) and (220) atomic planes. Specimens used in these series of experiments are annealed and electro-polished 0.07% carbon steel plate. A reversed bending fatigue testing machine was used for the fatigue test, and the speed of repetitions was 1700 cycles per minute. Residual stress was measured from (310), (211) and (220) diffraction lines, using CoKα1 and CrKα1 radiations, adopting the well known two exposure method at vorious numbers of stress cycles. The CoKα radiation was used to measure the change in half-value breadth of (310), (220), (211), (200) and (110) diffraction lines due to stress repetitions by normal X-ray diffractometer arrangement.Obtained conclusions are as follows;(1) Cross slip traces, extrusion and intrusion phenomena appeared extinctly in the first stage of fatigue stressing in the certain grains, and intensified gradually with increasing number of stress repetitions.(2) Micro cracks initiate in active bands zone, namely, it is observed obviously by electronmicroscope at the bottom of the intrusion slip at the stage of transition point from first stage to second. This micro cracks developed into visible cracks which lead to fracture during the second stage of fatigue.(3) The changes in half-value breadth of (220) and (211) diffraction lines during stress repetitions are quite similar to that of the (310) diffraction line which has been pointed out in previous papers. However, (200) diffraction line width changes greatly and that of (110) is a little in fatigue process.