Behaviour of the crystalline structure of brass under slow and rapid cyclic stresses

Abstract

During X-ray tests on an annealed brass (Cu 69.43%, Zn 30.54%) subjected to cyclic stressing it was noted that the behaviour of the crystalline structure of the grains depended on the speed of application of the stress. The structural changes occurring under a slow cycle could be different in kind from those produced by stress cycles applied at 2200 cyc./min., the frequency associated with the fatigue-testing machine employed. The difference was not due to changes in the final external deformation of the specimens because such changes in each case were reduced to the same negligible proportions by the use of cycles of reversed direct stress, that is, by cycles symmetrical in tension and compression. The effect was especially noticed in the brass because, in contrast with a mild steel previously the subject of X-ray tests under cyclic stressing (Gough and Wood 1936), it belongs to the class of materials in which the primitive yield point under static tensile loading is less than the fatigue limit under cycles of reversed direct stress (as given by the amplitude S of the limiting safe stress range ± S ); in the brass, therefore, the stress cycles of great physical interest, which are those where the stress range just exceeds the limiting safe range, are cycles in which the stress during each repetition passes through the values corresponding to the primitive yield points in tension and also in compression. The extent of the difference between the slow and rapid cycle appeared to be such that in the latter the main structural modifications shown by previous X-ray work to be associated with transition through a yield point were entirely suppressed. It was evident that an investigation of the conditions accompanying this suppression would give useful information on the response of the internal crystalline structure of the metal to an externally applied stress; also it would afford an opportunity for distinguishing the individual parts played by the various structural changes which have been brought out by recent X-ray work on the progressive deformation of metals and on metals in different states of hardness. In the present paper therefore the X-ray observations are compared with the static and cyclic stress/extension relationships as given by mechanical measurements.