CYCLIC DEFORMATION RESPONSE AND FATIGUE DAMAGE PROCESS OF NOTCHED COPPER SINGLE CRYSTALS

摘要

Cyclic slip and fatigue crack initiation-propagation behavior in notched copper single crystals of single slip orientation with two different notch geometries were investigated via both experimental tests and Finite Element Method (FEM) analysis. It was found that, unlike the case for an unnotched single-slip-oriented single crystal, the stress concentration at the notch region in the present notched crystals led to strong slip localization on the primary slip and on the highly stressed secondary slips as well. Accordingly, quite often cracks initiated not only from the primary Persistent Slip Bands (PSBs), but also at those PSBs formed on the secondary slip systems. Such a crack initiation feature strongly affected the fatigue life behavior in the crack initiation stage. Three phases of crack propagation, namely micro-crack or small crack growth, semi-steady state growth and rapid growth, were found in the curves of crack growth rate vs fatigue crack length. In the first phase, features of growth acceleration of small cracks was detected in these single crystals; in the second phase, the crack propagated alternatively on different slip planes with a Stage II mode and the crack growth rate was either independent of crack length or increased very slowly with increasing crack length as compared to that in the third phase. The above results were discussed in relation to the features of surface slip as well as to the stress intensity factor.