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Title:Influence of fluctuations in stress amplitude on fatigue of metals part 2
Author(s):Work, C.E.; Dolan, Thomas J.
Subject(s):Stress Amplitudes
Fatigue Of Metals
Abstract:The material contained in this report covers a continuation of the exploratory investigation presented in the Seventh Progress Report* on this contract, dealing with the fatigue life of metals subjected to repeated stresses that fluctuate between two different predetermined amplitudes. Small high speed rotating cantilever beam fatigue testing machines applied to the specimens the two general types of loading cycles studied, in which: (1) the amplitude of stress for each test was varied gradually and continuously between two predetermined limits, and (2) the stress amplitude was alternated abruptly at regular intervals between two predetermined values. The present report covers a study of three different metals, SAE 1045 steel, SAE 2340 steel, and 75S-T aluminum alloy, with the greatest emphasis on tests employing a loading cycles of the second type, in which 1000 cycles of a high stress amplitude were followed by 9000 cycles of a lower stress amplitude. The range of values for the minor stress amplitude has been extended to include several levels of overstress as well as of understress. It was found that the effect on fatigue life produced by intermittently applied blocks of an understress depended not only on the magnitude of the understress but also on the magnitude of the major stress alternated with the understress. When applied intermittently with a major stress amplitude that was only slightly above the original fatigue limit, the marked increase in total fatigue life that resulted for both notch and unnotched specimens of SAE 1045 steel and of unnotched specimens of SAE 2340 steel, was equivalent to (or interpreted as) an increase in the fatigue limit. For cases in which repetitions of a stress having an amplitude below the original fatigue limit were periodically alternated with repetitions of a somewhat greater overstress, the cycles of understress reduced the fatigue life of unnotched specimens of SAE 1045 steel, but no similar damaging effect was developed by the cycles of understress in tests of notched or unnotched specimens of SAE 2340 steel. In tests of unnotched specimens of SAE 2340 steel with a minor stress amplitude above the fatigue limit and the magnitude of the major stress amplitude only slightly greater than the minor stress amplitude, the fatigue life was practically the same as though all stresses had been applied at the minor stress level. For higher levels of the major stress amplitude the fatigue life was shortened and was influenced by the levels of both the major and minor stress amplitudes. Except in the cases cited above, failure occurred before the cumulative number of cycles at either the major or the minor stress amplitude reached the ordinary S-N curve.
Issue Date:1948-09
Publisher:Department of Theoretical and Applied Mechanics. College of Engineering. University of Illinois at Urbana-Champaign
Series/Report:TAM R 9
Genre:Technical Report
Sponsor:Office of Naval Research, U.S. Navy, Contract N6-ori-71, Task Order IV
Rights Information:Copyright 1948 Board of Trustees of the University of Illinois
Date Available in IDEALS:2021-11-04

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  • Technical Reports - Theoretical and Applied Mechanics (TAM)
    TAM technical reports include manuscripts intended for publication, theses judged to have general interest, notes prepared for short courses, symposia compiled from outstanding undergraduate projects, and reports prepared for research-sponsoring agencies.

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