Dynamic fracture mechanism of granite with different shape holes under high strain rates based on HFDEM

Abstract

The shape of underground chambers in deep mining varies due to their geological environment and intended use, which results in different failure modes under the influence of mining activities. However, the effect of chamber shape on the mechanism of structural integrity under dynamic load is still unclear. In this paper, granite samples with circular (C), rectangular (R), long ellipse (EL), and short ellipse (ES) holes were prepared. The dynamic mechanical response and cracking mechanism of granite were systematically analyzed using the split Hopkinson pressure bar (SHPB) test system and the hybrid finite and discrete element method (HFDEM). The results indicate that the dynamic strengths of granite with EL and ES represent the maximum and minimum values within the range of close strain rates, respectively. When EL granite is subjected to dynamic load, the axial stress concentration (in the load direction) is weak, and the transverse stress shows relative dispersion, which is the primary reason for its highest dynamic strength. The failure of granite with various holes primarily involves a tensile-shear mixed fracture, with relatively few pure type II cracks. The chamber’s transverse span is the primary factor influencing the distribution range of the fracture area.

1. Introduction

During deep mining, the complex geomechanical environment frequently leads to high-energy and large-scale engineering disasters. The mechanism of occurrence is unclear, complicating the prediction and effective control of disasters [1]. As one of the main structures in deep mining environments, the underground chamber is not uniform in shape to facilitate mining and transportation. Under the influence of dynamic disturbances, the surrounding rock of chambers with different shapes may fail, which poses a threat to mining safety, resulting in significant economic losses and casualties [2, 3]. Therefore, it is highly significant for the engineering design and disaster prevention of underground structures to thoroughly understand the failure behavior of rocks with openings of different shapes.

The influence of disturbance load on underground cavern structures in deep mining environments has been thoroughly examined. Uniaxial compression tests were conducted on sandstone specimens featuring double cracks and single round holes. It was observed that the peak strength, peak strain, and elastic modulus of the defective specimens were lower than those of the intact samples. These properties change nonlinearly with β ranging from 0° to 90° [4]. Additionally, the failure mode of a porous rock under a high loading rate shifts from axial splitting to crushing [5 −7]. Based on the theory of elastic-plastic mechanics, the analytical derivation of the characteristic radius in the non-uniform stress field is presented. The criteria for evaluating the plastic zone around the circular roadway, identifying potential dangers, and assessing the critical point of the dynamic disaster, are established. This provides a theoretical foundation for stability analysis, supports design decision-making, and guides disaster prevention and control [8]. The strength variation mechanism of sandstones with different hole shapes was studied. It was found that sandstone with square holes exhibited the lowest bearing capacity, while sandstone with diamond holes displayed the highest bearing capacity [9].

Additionally, defects such as holes significantly weaken coal strength, and larger hole sizes are negatively correlated with uniaxial compressive strength and elastic modulus [10]. For coal bodies with few or no holes, load failure primarily involves shear force. However, as hole size increases, tensile damage characteristics become increasingly evident. When subjected to a load, specimens with rectangular holes fail due to secondary cracks.