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Simple calculations for estimating extremes in fatigue life of metals, with or without accounting for cycle-dependent hardening, softening, and mean stress relaxation, are presented. These calculations aid in assessing the importance of cyclic deformation properties in cumulative damage procedures. An SAE 1045 quenched and tempered steel is considered for illustration. In general, cycle-dependent deformation phenomena may be important only in intermediate and long life situations. The sensitivity of predicted life to cyclic hardening or softening depends on the degree of hardening or softening a material exhibits, whereas in the case of mean stress relaxation it depends on the shape of the cyclic stress-strain curve or the strain hardening exponent. However, the need for a proper accounting of cyclic history, recognized as the “cycle counting problem” in fatigue literature, is the most important requisite of a cumulative damage analysis. Two alternative approaches of dealing with the cycle counting problem are discussed.
Visiting assistant professor, University of Illinois, Urbana, Ill.
Stock #: JTE10173J