Extrusions and intrusions in the cyclic plastic zone of the growing fatigue crack in polycrystalline copper

Abstract

Surface relief in the tip and along the path of a growing fatigue crack in polycrystalline copper has been observed and analyzed. A crack started from a semicircular notch in a prismatic specimen cycled with constant strain amplitude. In the notch tip and along the crack path on both sides of the specimen the surface relief of the grains was characterized by a high density of the PSMs belonging to two, up to three slip systems. Extrusions and intrusions of these PSMs intersect forming specific surface relief. Similar surface relief, corresponding to the grain boundary surface, has arisen on the fracture surface. It means that in the persistent slip bands inside the crystal, the material is pushed towards the neighboring grains or removed from the grain boundary (analogically to extrusions and intrusions on the surface). This mass transfer locally increases stresses on the grain boundaries creates discontinuities and weakens grain boundaries. Such novel mechanism of intergranular fatigue crack growth is discussed. The suggestion of an intergranular crack growth mechanism agrees with the predominantly intergranular crack path of the growing fatigue crack.