Fracture of brittle solids under impact: The decisive role of stress waves

Measurement of the impact strength of brittle solids is traditionally conducted by split Hopkinson pressure bar (SHPB), where one stress wave is generated and loads on the specimen (uniaxial-unidirectional, UD). What if this single stress wave is split into two (or more) smaller pulses that load on the same specimen? Based on the classical one dimensional elastic stress wave theory, nothing different would happen. However, our experiments revealed the opposite results. Electromagnetic split Hopkinson pressure bar (ESHPB), a newly developed technique that can launch two stress pulses simultaneously in opposite directions along the coaxial bars (uniaxial-bidirectional, BD), was adopted to test the compressive strength of a glass. Results indicated that the loading stress waves largely determined the measured compressive strength, with all other conditions identical. Significant discrepancy (as large as 69.1%) was observed between UD and BD strength. Possible reason for this discrepancy was proposed by high-speed photography.