Crystallographic effects on the fatigue fracture of copper-sapphire interfaces

Interfacial fatigue cracks were propagated in copper-sapphire bicrystals wi th the boundary perpendicular to the load axis and (110)(Cu)parallel to(10( 1) over bar 0)(Al2O3) - [001](Cu)parallel to[0001](Al2O3) to study the effe ct of crystallography in the fracture process. Cylindrical samples with a c ircumferential notch were loaded in compression-compression and compact ten sion specimens in tension-tension, Three interfacial cracks in the cylindri cal sample nucleated simultaneously at sites corresponding to the maximum s lip length, under local single slip conditions, for three of the four slip vectors expected for the [110] loading axis in the copper crystal. These cr acks arrested with continued cycling, while two new cracks nucleated at 0 d egrees and 180 degrees from [<1(1)over bar>0](Cu), which also self-arrested . Then another crack started at 90 degrees from [<1(1)over bar>0](Cu) and g rew with an inclined front. Striations could be observed on the copper frac ture surfaces; however, they did not coincide macroscopically with traces o f {111} slip planes. Large areas were also relatively free of features. Ela stic analysis of the anisotropic near-tip stress fields for the interfacial crack revealed that the dominant crack growth direction had the highest en ergy release rate, whereas the second crack direction had the minimum mode II mix. A model to account for the noncrystallographic striations is propos ed.