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It has generally been accepted for many years that understressing has a beneficial effect on the fatigue properties of metals. Further, it has been thought that this improvement is the result of a mild cold-working effect produced by the cyclic stressing at or below the endurance limit of the material. This conclusion is based on the observation that previously under-stressed specimens will undergo a larger number of cycles to failure relative to the virgin life at some higher stress in the fracture range.
Unquestionably cold-work can have a beneficial effect on the fatigue properties; drawing and shot-peening produce marked improvements. Metallographic as well as other investigations have shown that deformation and cold-work are produced in the fatigue process. None of the previous studies on the understressing effect have, however, been made from the necessary statistical approach. It is now well established that both the fatigue life and endurance limit are statistical quantities. Lack of recognition of this fact can lead to misinterpretation of the data and erroneous conclusions.
The purpose of this investigation was to study the understressing effect from a statistical viewpoint. The experimental work was an extension of the research reported in a companion paper 4 and is being published separately to give it proper emphasis. It will be shown that the understressing effect can, in part if not wholly, be interpreted as a phenomenon based on selectivity and the statistical nature of the endurance limit; and that cold-work does not play a dominant role in this behavior.
Scientific Liaison Officer at the London Branch of the Office of Naval Research, London,
Mehl, R. F.
Director of Metals Research Laboratory and Head of the Department of Metallurgical Engineering, Carnegie Institute of Technology, Pittsburgh, Pa.