Co-planar crack interaction in cleaved mica

We have performed a combined experimental and analytical study of cleavage in mica using a double cantilever beam geometry in which a crack induced by a wedge driven into one side of the specimen interacts with a pre-existing, coplanar, internal crack. The internal crack is produced by inserting a fiber into the sample on the cleavage plane. As the wedge-driven crack approaches the internal crack, its growth is retarded by the defect, producing an increase in the apparent fracture resistance. With continued loading, the two cracks coalesce. The experiment has been analyzed using a cohesive zone approach to represent the interlayer adhesion in mica. Analysis of the various stages of the experiment reveal scaling dependencies of the different cohesive zone parameters. The coalescence event has been found to depend on parameters other than the fracture resistance of the interface, making it useful for determining additional parameters in the cohesive description, such as the characteristic opening to failure or the cohesive stress. Analysis of the coalescence event is reproduced with finite element calculations. The interaction experiment allows multiple parameters to be determined in a single experiment using a single sample. In our experiments, we observe an increase in the apparent fracture resistance without introducing additional mechanisms for dissipation. Out results reveal the nature of this pinning mechanism and its strength in terms of cohesive fracture parameters.