A Study of Fatigue (Cyclic Deformation) Behavior in FCC Metals Using Strain Rate Change Tests
Strain rate jump tests were performed during low cycle fatigue using plastic strain rate as the real time computed control variable. Test materials included OFE polycrystalline copper, AA7075-T6 aluminum, and 304 stainless steel. The evolution of dislocation interactions was observed by evaluating the activation area and true stress as a function of cumulative plastic strain. Activation area values for each of the three materials were evaluated from an initial state to saturation. All three materials exhibit a deviation from Cottrell-Stokes law during cyclic deformation. Tests performed on each of the three materials at saturation reveal a dependence of activation area on plastic strain amplitude for copper and aluminum but no such relationship for stainless steel. These results reflect a contrast between wavy slip for pure copper and 7075 aluminum versus planar slip for 304 stainless steel tested at room temperature. Dislocation motion in copper transitions from forest dislocation cutting [1-6] to increasing contributions of cross slip. Dislocation motion in 7075 aluminum and 304 stainless steel is controlled by obstacles that are characteristically more thermal than forest dislocations: obstacles in 7075-T6 aluminum are identified as solutes from re-dissolved particles; obstacles in 304 stainless steel are also solutes.
Dr. T. S. Srivatsan, FASM, FASME
G. C. Kaschner and J. C. Gibeling, "A Study of Fatigue (Cyclic Deformation) Behavior in FCC Metals Using Strain Rate Change Tests", Key Engineering Materials, Vols. 378-379, pp. 371-384, 2008