Edgewise Flexural Testing of a Model, Laminated Carbon-Carbon Composite

Mechanical testing of carbon-carbon composites has inherent difficulties associated with the unusual properties of these materials. One of the simplest tests to perform is the flexure test. The test geometry is of great importance and testing standards stipulate geometries designed to induce the desired failure mode. Usually this is tensile/compressive failure and so the main stipulation is that the span to depth ratio (SDR) is large. This increases the ratio of the maximum tensile stress to maximum shear stress, and hence makes tensile failure more likely. However, due to the high tensile strength and low shear strength of carbon-carbon composites it is very difficult to avoid delamination failure when the conventional flatwise orientation of specimen is used. Indeed, delamination was found to be the predominant failure mechanism for a similar material to that investigated here, up to an SDR of 50:1. Clearly this is a major limitation of flatwise flexural testing. However if the samples are tested in the edgewise orientation, the coincidence of weak planes with high shear forces is avoided. Hence delamination failure is not observed even at an SDR of 6:1.