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A cumulative damage procedure based on smooth-specimen tests of 2024-T4 aluminum alloy and aircraft quality SAE 4340 steel is formulated in which the effects of prestrain and mean stress on fatigue life are investigated separately. It is assumed that life reductions due to prestrain or mean stress at a given strain level may be separately incorporated into a simple cycle-ratio damage summation.
Small plastic prestrains are shown to cause considerable life reductions. Larger prestrains appear to have little additional adverse effect. It was found possible adequately to correlate results with and without mean stress by a parametric representation of mean stress and strain amplitude.
Analysis of arbitrary stress-strain sequences indicates that the proposed damage-summation procedure gives adequate life predictions. Cycle-ratio summations were close to unity for both stress-controlled and strain-controlled tests in which sequence, number of blocks, fraction of life at mean stress, and the life fraction at which plastic straining occurred were all varied.
fatigue, damage summation, life prediction, prestrain, plastic strain, mean stress, life reduction, aluminum, steel, straining sequences, stress control, strain control
Associate Professor, University of Waterloo, Waterloo, Ontario
Assistant Professor, University of Wisconsin, Madison, Wisc.