Some experiments on the resistance of metals to fatigue under combined stresses

Abstract

The basic object of the investigation was an experimental study of the fatigue resistance of metals to combined flexural and torsional stresses to which many engineering components; particularly crankshafts, are subjected in practice. Four independent variables have been employed; reversed bending stresses, reversed torsional stresses, static bending stresses and static torsional stresses superimposed on these cyclic stresses. Supplementary static, impact and simple fatigue tests, with chemical analysis and metallurgical examination, have been made on all the test materials. The investigation falls naturally into two parts, in which form it is reported. Part I of the report describes a comprehensive research into the fatigue behaviour of a selection of engineering steels and two alloy cast irons under combined bending and torsional stresses alternating in phase, the mean stress of both types of stress cycle being zero in every case; thus, two cyclic variables only have been explored in this part of the investigation. A special form of highspeed testing machine, referred to as the No. 1 Combined Stress Fatigue Testing Machine, was developed for this part of the research and is described in detail. Part II of the report describes a research into the resistance of a Ni. Cr. Mo. Va. aircraft steel to combined fatigue stresses in which the four independent variables (see above) have been investigated. A second new type of machine, referred to as the No. 2 Combined Stress Fatigue Testing Machine, was specially designed for this purpose and is described in detail. The test material, to British Standards Specification S.65A, was investigated in the hardened and tempered condition, having a tensile strength of 65 t/in.L The programme of combined stress fatigue tests consisted essentially of (a) tests on solid specimens of circular cross section, and (b) tests on three forms of hollow specimen each containing one type of discontinuity unavoidably associated with practical crankshafts ; a radial oil-hole, a transition tillet of small radius, a splined shaft having six splines; the external diameters, at the test section, of the solid specimen, the specimen with an oil-hole and the specimen with small fillet, was 0.500 in. in each case; the splined-shaft specimens measured 0.4913 in. over the crest diameter.