Investigation of Dynamic Fracture Behavior of Graphite

The strain-rate sensitivity of brittle materials, such as glass, ceramics or concrete-like materials, is usually easier to be performed in compression. However, also the tensile behavior, which affects phenomena such as spalling, scabbing and fragmentation, has to be investigated to achieve an exhaustive characterization. In this work, the spalling test has been performed to investigate the dynamic tensile behavior of graphite. This type of test, which should be used for the mechanical characterization of brittle materials, is based on propagation and reflection of elastic waves: the fracture for spalling occurs when, in the material, the tensile stress state, obtained by the reflection on a free surface of a compressive pulse, exceeds the strength limit. These conditions are usually reached using a SHPB setup with a long bar specimen free at one end. Due to the low impedance and high damping of the graphite, the specimens are instrumented with strain-gages, in order to measure their strain histories, and a numerical approach is used to extract the material strength model parameters.