Tension tests were performed on four layered rocks: a shale, a gneiss, and two sandstones. Cores 2.125 by 4.25 inches were prepared, with the layers dipping at 0°, 15°, 30°, 45°, 60°, 75°, and 90° to the short cylinder axis. All tests were carried to failure, and strain was measured by gauges mounted on the cores.
Rupture strength in tension is highly affected by the orientation of layering. In general, as the dip of the layers increases, the rupture strength increases correspondingly. For 0° to 60° cores, failure occurs along layering; for 75° and 90° cores, failure occurs across layering. Rupture strength in tension is lower if failure occurs along layering than if it occurs across it. Therefore, it is postulated that during rock deformation, if rocks are subjected to tension, it is more likely that failure will occur along pre-existing weakness planes, such as joints, faults, bedding, lamination, and foliation, than for failure to occur across layering and weakness planes.
The different theories of failure which assume that material is homogeneous and isotropic, whether in tension or in compression, are not applicable on rocks that are characterized by definite planes of weakness and anisotropy.