Fatique of Copper Polycrystals at Low Plastic Strain Amplitudes

Single crystals and polycrystals of pure copper were fatigued in tension-compression at constant low amplitudes of plastic strain and low cycling frequencies at room temperature in air. Surface patterns of persistent slip bands were quantitatively examined by optical microscopy. Bulk dislocation microstructures were examined by transmission electron microscopy. The microstructure and mechanical behaviour observed for the single crystals are in close quantitative agreement with comparable existing fatigue data. The cyclic stress-strain curve of the polycrystals shows a plateau in a linear plot of the saturation stress versus the plastic strain amplitude. The area fraction of PSB's on the polycrystals increases roughly linearly with the plastic strain amplitude. The dislocation microstructure in bulk grains consists of regular wall structures embedded in a matrix of less regular structures. A Sachs type model in which the small plastic strain incompatibility caused by bulk persistent slip is elastically accommodated is consistent with the experimental observations.