Micromechanical Fracture and Acoustic Emission (AE) Characteristics of Bonded Granular Materials With Holes Under Confined and Unconfined Loading: An Experimental Study and Particle Flow Code (PFC) Analysis

ABSTRACT This article explores the micromechanical failure processes and acoustic emission (AE) characteristics of rock‐like specimens containing pre‐formed holes under spherical loading, both in confined and unconfined conditions. Gypsum, which serves as a granular rock‐like medium, was used to prepare specimens with two holes, in which stainless steel balls were placed. A uniaxial compression machine applied the loading, while complementary particle‐based numerical simulations were conducted using particle flow code (PFC2D) to analyze crack initiation, propagation, and coalescence at the particle level. This study investigates nine‐hole configurations (three‐hole diameters × three‐hole spacings) under unconfined conditions, in addition to several configurations under confined conditions. Two‐hole configurations were examined to assess the impact of hole spacing on fracture evolution and AE activity. Under unconfined loading, tensile cracks initiated at the boundaries of the holes and propagated inward, leading to the coalescence of adjacent holes, as well as outward until they reached the boundary of the specimen. This process was accompanied by a stress drop on the stress–displacement curve, reflecting the failure of the granular bridges between holes. The magnitude of this stress drop decreased as the spacing between the holes increased. In the case of confined loading, similar tensile crack initiation was observed, but diagonal and shear fractures became more prominent, particularly around the loading boundaries. Increased wall displacements led to more branching and heightened shear activity. Both experimental and particle‐based numerical analyses provide new insights into granular fracture mechanisms, hole interactions, and the AE response in bonded granular systems under different loading conditions.