X線による二相合金の疲労破壊に関する研究

銅-亜鉛合金について

抄録

In the field of mechanical engineering, the need for a simple but effective way of evaluating the fatigue strength and life of structural materials is an important problem with which the design engineers have faced. Accordingly, a considerable amount of investigations have been made in this field. However, the basic nature of fatigue damage and the conditions which lead to the initiation and propagation of fatigue cracks are not sufficiently understood. Nor any satisfactory method of assessing the exact state of fatigue damage has yet been found.
In order to approach the problem of understanding the response of materials to various external forces, in this connection, the authors, considering it to be important to combine many fields of information, have investigated the fatigue damage of metals under various conditions of stress by employing the X-ray technique. Namely, the authors have studied the mechanism of fatigue of metals by observing the changes in half-value breadth, residual stress and micro-structure under constant and varying stress amplitudes with or without mean stresses. Some investigations^1)∼3) have been performed to study the relation between the changes in half-value breadth and the number of stress cycles. They offered a certain interesting knowledge on the behaviour of materials under cyclic stresses and a reliable means for predicting the fatigue life in principle. In the other series of experiments, ^4)∼6) the authors have made discussions on the changes in half-value breadth in relation to the basic mechanism of fatigue, and also suggested a noticeable information to attack the fatigue problem in the future.
It is clear at present, however, that the values of half-value breadth and residual stress are affected by the type of internal stress state seriously.^7)∼9) For example, in steel, the iron carbide (Fe₃C) lamella in ferrite may be in equilibrium with the matrix under a certain condition, that is, when it is formed or annealed, but upon cooling to room temperature or by plastic deformation, extremely high-locked stresses develop in the surrounding matrix because of the difference in some elastic and plastic properties or in thermal expansion coefficient. Really, the presence of the so-called “Gefügespannungen”has been pointed out, and it is considered to be due to the difference in the yield stresses of the various phases coexisting in an alloy.^10)∼12) Therefore, if we consider that the “Gefügespannungen”are caused by the progress of fatigue slips and the growth of micro-cracks in certain grains of the phase, they may contribute to the behaviour of line broadening and the state of residual stress in the fatigue process.
For these reasons, in the present paper, the authors attemped to investigate the fatigue mechanism of metals containing the α- and the β-phases (6-4 brass) in relation to the“Gefügespannungen”, and carried out some experiments on the changes in the half-value breadth and residual stress due to stress repetitions using the (α+β) brass. The conclusions obtained are as follows:
(1) The general feature of the changes in the half-value breadth of the α- and the β-phases during the stress repetitions is quite similar to that of the (310) diffraction lines, which was pointed out previously on annealed carbon steel.¹³⁾ However, the (310) and (321) diffraction line widths of the β-phase show changes greater than any others, while those of (400), (420) and (331) diffraction lines of the α-phase are very small in the fatigue process.