Stress evolution and failure process of Brazilian disc under impact

To reveal stress distribution and crack propagation of Brazilian discs under impact loads, dynamic tests were conducted with SHPB (split Hopkinson pressure bar) device. Stress states of specimens were monitored with strain gauges on specimen surface and SHPB bars. The failure process of specimen was recorded by ultra speed camera FASTCAM SA1.1 (675 000 fps). Stress histories from strain gauges offer comprehensive information to evaluate the stress equilibrium of specimen in time and space. When a slowly rising load (with loading rates less than 1 200 N/s for d 50 mm bar) is applied, there is usually good stress equilibrium in specimen. The stress distribution after equilibrium is similar to its static counterpart. And the first crack initiates at the disc center and propagates along the load direction. But with the front of incident wave becoming steep, it is hard for specimens to get to stress equilibrium. The first crack may appear anywhere on the specimen together with multiple randomly distributed secondary cracks. For a valid dynamic Brazil test with stress equilibrium, the specimen will break into two halves neatly. While for tests with stress disequilibrium, missing strap may be found when broken halves of specimens are put together. For those specimens broken up neatly at center but having missing wedges at the loading areas, it is usually subjected to local buckling from SHPB bars.