Cracking mechanisms of fissure-hole specimens under compression-shear conditions: insights from sand 3D printing technology and DEM simulations

The fissure-hole composite structure significantly influences the mechanical properties and failure modes of rock masses. In order to figure out the interaction mechanisms between holes and fissures under compression-shear loading, such as the crack initiation mechanisms induced by stress concentration and influences of different fissure patterns on the interaction intensity, sand 3D printing (3DP) technology is employed to fabricate rock-like specimens containing various fissure-hole configurations. Compression-shear experiments, digital image correlation (DIC) full-field strain analysis, and discrete element method (DEM) numerical simulations are integrated to investigate the failure patterns. The cracks generated in the specimens are classified into five types: tensile mode crack (TC), shear mode crack (SC), tensile-dominated mode crack (TDC), shear-dominated mode crack (SDC), and mixed mode crack (MC). Among these, SC and SDC are primarily distributed between the hole and the fissures, TDC and TC mainly occur between the fissures and the specimen boundary, while MC appears in both regions. As the fissure vertical distance L_V and fissure horizontal distance L_H increase, the specimens tend to exhibit shear-dominated mixed failure; conversely, as the fissure dip angle α increases, the specimens tend to exhibit tensile-dominated mixed failure. The load–displacement curves of the specimens comprise four stages: the compaction stage, the elastic deformation stage, the stable crack propagation stage, and the unstable crack propagation stage. The peak strength first decreases and then increases as L_V increases, while it increases monotonically as α increases, and first increases then decreases as L_H increases. The evolution law of energy dissipation in the simulation is also monitored in this paper. After reaching the peak load, the boundary energy is rapidly transformed into dissipative energy, and the strength change trend is positively correlated with the size of boundary energy, but negatively correlated with the conversion rate of dissipative energy. Finally, the crack initiation mechanisms under different fissure configurations are discussed. All specimens developed stress concentration zones connecting the specimen boundary, fissures, and holes, guiding crack coalescence and altering the interactions between the holes and fissures. Increasing L_V, α, and L_H ultimately weakens the fissure-hole interactions via changing the stress concentration between holes and fissures, the guided failure between them is weakened and the stabilities of specimens are improved. The research results elucidate the interaction mechanisms of fissure-hole composite structures under compression-shear loading and can provide new insights for optimizing engineering design and preventing rock engineering disasters.