The mechanism of compressive unloading failure in single crystal quartz and other brittle solids

The phenomenon called unloading failure, in which the initiation and growth of cracks occurs during the removal of compressive stresses, has been observed in both uniaxial compression testing and Hertzian indentation testing of brittle materials. We have conducted finite element simulations of uniaxial compression experiments on single crystal quartz (specimens in which unloading failure was observed) to determine: (1) the mechanism responsible for unloading failure in brittle materials, (2) the dependence of this mechanism on friction between the specimen and loading platens, (3) the cause for the selection of specific fracture planes on which the unloading cracks propagate. The simulation results indicate that a slip-stick mechanism which is highly dependent on interface friction is responsible for the generation of tensile stress states leading to failure during unloading. The results also show that elastic anisotropy of a single crystal specimen leads to the self-selection of preferential failure planes during unloading.