Effects of particle shape on dynamic mechanical behaviours of coral sand under one-dimensional compression

Coral sand is characterized by high porosity, high compressibility and high crushability mainly due to its irregular particle shape and internal pore structure. Single coral particles in a size fraction tend to have a variety of shapes from bulky to flaky, which emerges as one of the most pivotal factors affecting the macro-scale performances of coral sand. A series of confined one-dimensional dynamic compression tests were conducted on coral sand with four classified shapes (e.g., blocky, dendritic and rodlike, flaky particles, and coral debris) via a modified 37-mm-diameter split Hopkinson pressure bar (SHPB) apparatus. The effects of particle shape on the dynamic mechanical behaviours of coral sand at high strain rates were assessed in terms of the stress-strain response, compressibility, particle breakage and the change in particle shape measures. It is revealed that coral sand exhibits high small-strain stiffness, a low crushing strength, a notable strain hardening and a high compressibility behaviour as subjected to dynamic compression at high strain rates. In the low-stress level, it is the high interparticle friction and interlocking induced by the irregular shape form and rough surface texture that lead to a gentle descending tendency in the void ratio. While in the high-stress level, it is the particle crushing along with its rearrangement that results in the high compressibility of the coral sand assemblage. In general, the particle crushing strength tends to decrease as the particle shape shifts from bulky to elongated and to flaky. The large-strain stiffness decreases and the compressibility increases with an increase in the overall particle irregularity. Coral particles are prone to be more spherical and smoother as the average degrees of sphericity, convexity and aspect ratio approach to unity, due mainly to the intense asperity damage and primary fracture under dynamic tests. Furthermore, the percentage in the degrees of sphericity, convexity, aspect ratio and flatness increases with respect to the initial degrees of themselves. It can be further postulated that the extent of breakage could be quantified by the statistical change in the particle measures such as sphericity and convexity aside from particle dimensions for granular materials with irregular shapes.