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UBC Theses and Dissertations

The effect of the mean stress on the endurance limit Ukrainetz, Paul Ruvim


The effect of the mean stress on the endurance limit is a matter of considerable academic and practical importance. So many variable and uncertain factors are involved that it is not surprising that many different formulas and theories have been proposed since the start of investigations into this subject in about 1858. In this thesis, factual data obtained in the course of tests planned primarily for the purpose of determining the effect of the mean stress are presented and discussed. A standard fatigue testing machine vas used for all the tests. Various mean stresses, both for axial-load and torsional tests, were employed. A critical examination of the proposed theories of fatigue failure has been made. The theory which considered that the inception of fatigue resulted from alternating shearing stress and that the resistance to fatigue was influenced by the magnitude and sign of the steady normal stress was found to explain best the damage done to the material structure. It has been the opinion of a number of investigators that the fatigue strength under axial-load is decreased by an increasing tensile mean stress and in torsion the fatigue strength is unaffected by the mean stress. The experiments done here clearly indicate that this conclusion is true.

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