Cycle dependent stress strain behavior of metals

Forty five papers on the subject of cyclic deformation and fracture behavior of metals are reviewed in an Appendix. Cyclic changes in the deformation resistance of metals make the monotonic stress-strain curve inadequate for characterizing the mechanical resistance of a metal during fatigue. Instead, a cyclic stress-strain curve is needed which relates the cyclic stress and strain after the metal has adjusted to the cyclically stable steady-state condition. Cyclic stress-strain results are reported for annealed, cold worked and partially annealed OF HC copper. Specimens are strained between completely reversed limits and the stress range required to enforce the controlled strain range is measured. The cyclic stress is found to increase as much as a factor of five for the annealed copper and to decrease as much as a factor of two for the initially cold worked condition. After the initial transient adjustments in the deformation resistance, a nearly stable hysteresis loop develops. The locus of the tips of a set of stable hysteresis loops is defined as the cyclic stress-strain curve. Cold worked metals cyclically soften so that the cyclic stress-strain curve is below the monotonic stress strain curve. Annealed metals harden and the cyclic strain-strain curve is above the monotonic. Several experimental techniques for quickly obtaining the cyclic stress-strain curve from only a few specimens are explored. These include two step, multiple step, and incremental step straining, and monotonic tension after cycling. An analytical expression is presented for the cyclic stress-strain curve