Experimental observation of fracture patterns in layered slate

The layered structure of slate rock induces strength anisotropy. The strength in the direction of the layers (schistosity) is considerably smaller than in any other direction. A series of loading tests on circular samples and another series of loading tests on rectangular samples are performed to examine fracture patterns in slate. The tests are monitored by visual recordings and by recording acoustic emission. The processing of this data results in localization. This allows identification and analysis of the occurrence and propagation of the individual fractures. It is shown that the strength anisotropy on mu-scale is the key factor behind the strength anisotropy on sample scale, as well as behind the deformation behaviour of the sample. In addition, it is observed that a small variability in the layer direction can affect the fracture pattern considerably.