Crystallographic effects on the fatigue fracture of copper- sapphire interfaces

Interfacial fatigue cracks were propagated in copper-sapphire bicrystals with the boundary perpendicular to the load axis and (1 10)_Cu║║(1010)A1₂O₃— [001]_Cu║║ [0001 ] A1₂O₃to study the effect of crystallography in the fracture process. Cylindrical samples with a circumferential notch were loaded in compression- compression and compact tension specimens in tension-tension. Three interfacial cracks in the cylindrical sample nucleated simultaneously at sites corresponding to the maximum slip length, under local single slip conditions, for three of the four slip vectors expected for the (110) loading axis in the copper crystal. These cracks arrested with continued cycling, while two new cracks nucleated at 0° and 180° from [110]_Cuwhich also self-arrested. Then another crack started at 90° from [110]_Cuand grew with an inclined front. Striations could be observed on the copper fracture surfaces; however, they did not coincide macroscopically with traces of (111) slip planes. Large areas were also relatively free of features. Elastic analysis of the anisotropic near-tip stress fields for the interfacial crack revealed that the dominant crack growth direction had the highest energy release rate, whereas the second crack direction had the minimum mode II mix. A model to account for the noncrystallographic striations is proposed.