Based on the incident stress wave and reflected stress wave from the free surface, a new method together with experimental technique of determining spalling strength of rock under high strain rate is developed. It is found that the spalling strength of samples can be determined by only knowing the length of first spallation and the incident wave under specific loading waveform. Using this method with the improved Split Hopkinson Pressure Bar (SHPB) and high-speed camera and other experimental devices, the spalling strength of granite specimens under high strain rate is tested. Comparison with multiple experimental results shows that the new measuring method can accurately calculate the dynamic tensile strength of rock materials under high strain rate.
When the compressive stress wave caused by dynamic disturbance going from a medium of higher wave impedance to a medium of lower wave impedance, the compressive stress wave will be reflected to tensile stress wave through the contact surface and. When the tensile strength of superposition by both reflected wave and incident wave is higher than the dynamic tensile strength of a material, dynamic tensile fracture of the material is induced, i.e., spalling occurs. If the stress wave is slowly rising, the following spall is possible to occur after the first spallation. The spalling phenomenon was firstly observed by Hopkinson in 1914, thus, it is also called Hopkinson spallation. Rock and rock like materials are likely to have spalling failure under the impact load (Cho et al. 2003).
The spalling failure is caused from the dynamic tensile strength of materials, so the dynamic tensile strength of brittle materials like rocks and concrete are often determined by spalling experiments in the lab. Using Split Hopkinson Pressure Bar (SHPB) device, and according to the spalling features shown by incident pressure wave transforming to reflected tensile wave through free surface, Li et al. analysed the strain-rate effect on the tensile strength of various concrete-like materials (Li et al. 2009, Li et al. 2003, Weerheijm et al. 2007, Zhang et al. 2009). Erzar et al. (Erzar et al. 2010; Wu et al. 2005, Zhu 2008) measured the dynamic tensile strength of brittle materials under high strain rate. Kubota (Kubota et al. 2008) determined the dynamic tensile strength of sandstones by spalling experiments.