Effect of loading rate on the mechanical response and energy evolution of skarn rock subjected to constant-amplitude cyclic loading

Abstract

This work aims to reveal the mechanical responses and energy evolution characteristics of skarn rock under constant amplitude-varied frequency loading paths. Testing results show that the fatigue lifetime, stress−strain responses, deformation, energy dissipation and fracture morphology are all impacted by the loading rate. A pronounced influence of the loading rate on rock deformation is found, with slower loading rate eliciting enhanced strain development, alongside augmented energy absorption and dissipation. In addition, it is revealed that the loading rate and cyclic loading amplitude jointly influence the phase shift distribution, with accelerated rates leading to a narrower phase shift duration. It is suggested that lower loading rate leads to more significant energy dissipation. Finally, the tensile or shear failure modes were intrinsically linked to loading strategy, with cyclic loading predominantly instigating shear damage, as manifest in the increased presence of pulverized grain particles. This work would give new insights into the fortification of mining structures and the optimization of mining methodologies.

1. Introduction

The integrity of hard rock pillars is essential for maintaining the stability of roof enclosures and ensuring the safety of underground constructions [1], including deep laboratories [2], defensive structures [3], and tunnels [4]. Particularly in quarries undergoing underground mining [5], these refuge pillars not only bear the weight of the overlying rock mass but also endure dynamic loading from mining activities and other variable loads. The failure of these pillars could lead to widespread collapses within the mine, resulting in significant casualties, economic, and property losses [6, 7]. Additionally, the proximity of pillars within chambers, coupled with the effects of excavation-induced disturbances, leads to stress concentration, exacerbating the risk of rock spalling and collapse [8]. This situation hinders the early detection of risks and the implementation of effective risk mitigation measures [9]. Given the heterogeneous nature of rock and the mechanical contexts that influence stress distribution longitudinally within the pillars, it is vital to investigate the mechanical responses of rock under the influence of unidirectional cyclic stresses. This study aims to enhance our understanding of these mechanisms, contributing to the development of more effective engineering solutions to mitigate the risks associated with underground mining operations.

The vertical stress exerted on the upper and lower ends of a rock column by the overlying rock layer, in conjunction with the presence of free surfaces, can be conceptualized as a uniaxial loading model [10]. This model is further subjected to dynamic repetitive perturbations, such as those caused by earthquakes [11], blasting [12], and other dynamic disturbances common in extractive industries. This phenomenon has led many scholars to describe the resulting damage pattern as “rock fatigue” [13, 14], noting its mechanistic parallels to the creep effect [15]. This similarity has been encapsulated in models that represent the cumulative damage over time under sustained stress. In prior research within the field of rock mechanics, various cyclic loading tests [16] and numerical simulations [17] were conducted to explore these phenomena. These studies have meticulously categorized and analyzed the results, finding that factors such as stress magnitude, waveform characteristics, loading and unloading sequences, and the rate of fatigue significantly influence the observed outcomes [18 −21]. From these analyses, empirical formulas have been developed and refined to accurately predict the damage threshold of rock materials.