In-situ deformation and fracturing characteristics of geomaterials under dynamic loading: Insights from ultra-high-speed X-ray phase contrast imaging and DEM modelling

Dynamic loadings, such as earthquakes, blasts, and impacts, are commonly encountered in mining and underground infrastructures, and therefore predicting the failure of geomaterials, including cement, concrete and rock at high strain rates holds significant importance. The exploration of fracturing and deformation properties in materials under dynamic loadings is essential for comprehending the failure process and mechanisms. In this study, a made-to-order split-Hopkinson-pressure bar (SHPB) is integrated with ultra-high-speed X-ray phase-contrast-imaging (XPCI) and X-ray digital image correlation (XDIC) analysis. This combination allows for the observation and quantification of in-situ through-volume fracturing process and deformation fields of materials such as cement, concrete and basalt under dynamic compression tests featuring various impact velocities. Furthermore, a coupled continuum-discrete element modelling is undertaken to validate the experimental findings and offer insights into the damage processes and failure mechanisms at various levels. Scanning electron microscopy (SEM) is also employed to characterise fracture surface morphologies, unveiling distinct failure mechanisms influenced by crack patterns originating from a microscale. The outcomes indicate that the XDIC technique is applicable when the material microstructure provides sufficient texture. The deformation of geomaterials under high loading rates can also be validated through numerical models utilising the discrete element method (DEM). Both experimental and numerical results consistently demonstrate that as the impact velocity increases, fracturing patterns transfer from interfacial cracks to transgranular cracks. Additionally, fracture initiation positions transition from the specimen's edge towards the middle. In case of higher heterogeneity, crack patterns evolve from tensile-dominant cracks to mixed (tensile-shear) cracks due to the generation of interfacial cracks, further enhancing plastic deformation. SEM results reveal that distinct fracture morphologies, such as roughness and flatness, are influenced by heterogeneous microstructures like microcracks, pores, grain boundaries, and minerals.