Single Bunch X-Ray Phase-Contrast Imaging of Dynamic Tensile Failure in Geomaterials

Dynamic tensile fracturing of two quasi-brittle geomaterials during high strain rate spalling as observed in situ utilizing ultra-high speed single bunch X-ray phase-contrast radiography is presented (image acquisition rate of 1.4 MHz with 32 μm pixel size). A spalling experimental set-up, based on a single Hopkinson bar, has been developed to apply indirect dynamic tension with strain rates up-to 1000 s-1. Experiments were performed on a pink granite (low porosity content and large quantity of pre-existing cracks) and an ultra-high strength concrete (comparatively high porosity content and low quantity of pre-existing cracks). The in situ results show that the two tested materials exhibit notable differences in tensile failure patterns. In parallel, point-wise measurements of the rear face velocity have been obtained showing a pronounced difference in the velocity pull-back response. X-ray micro-tomography inspection has been performed on the samples post-mortem. The effects on the microstructure and the initial quantity of defaults on the observed dynamic tensile failure are discussed.