# Experimental Study on the Dynamic Characteristics of Fractured Coal Under Cumulative Impact
## 1 Introduction  
Coal resources have long played a pivotal role in China's energy system, with their safe and efficient extraction remaining a central issue in mining engineering [1, 2]. As mining operations progressively extend to deeper geological strata, the frequent occurrence of dynamic hazards poses significant threats to coal safety extraction [3, 4, 5]. Coal-rock masses in deep mining progress are frequently subjected to complex dynamic disturbances, such as blasting vibrations, mechanical excavation and so on, which typically exhibit repetitive and cyclic cumulative characteristics [6, 7, 8]. These disturbances induce progressive crack propagation within coal-rock masses [9] and generate cumulative damage to coal seams [10, 11]. The evolutionary pattern of major disasters induced by minor perturbationsand multiple cycles demonstrates distinct time-dependent cumulative effects, presenting critical challenges in dynamic hazard prevention within deep mining operations. Moreover, coal seams subjected to long-term geological movements and external disturbances contain numerous discontinuities, which include fractures, joints and faults. In situ coal strata typically exhibit complex fracture networks [12; 13]. These fracture networks significantly alter the mechanical and deformation properties of coal-rock masses [14], leading to more complex dynamic responses in fractured coal-rock systems. Under dynamic loading disturbances, such systems exhibit heightened susceptibility to triggering dynamic disasters, including rockbursts and coal-gas outbursts [15; 16; 17; 18]. Therefore, investigating the dynamic characteristics of fractured coal rock under cumulative impacts holds substantial scientific and engineering significance. Extensive research has been conducted on the mechanical properties of coal-rock masses, yielding substantial scientific achievements [19; 20; 21; 22; 23; 24; 25]. [PERSON] et al. [26] performed uniaxial compression testing of coal under controlled moisture conditions, revealing moisture-dependent degradation of critical mechanical parameters: peak stress, elastic modulus, strain-softening modulus and post-peak modulus exhibited inverse correlations with hydration levels. Thermal-mechanical characterization through uniaxial compression testing of coal specimens under varying thermal conditions revealed temperature-dependent fracture propagation dynamics and micro-to-macro failure mechanism transitions ([PERSON] et al. [27]). Through numerical simulations, [PERSON] et al. [28] demonstrated that the joint influence on shear failure diminishes with increasing load intensity in coal masses. The dynamic behavior of coal rock under impact loading has attracted significant research attention [29; 30; 31; 32]. The split-Hopkinson pressure bar (SHPB) device has become a standard experimental method for studying the dynamic responses of brittle geomaterials [33; 34; 35]. For example, a comprehensive research initiative has been conducted to decode cryogenic-conditioned dynamic mechanical responses of coal specimens under sub-zero thermal environments ([PERSON] et al. [36]). [PERSON] et al. [37] attempted to use the SHPB device to investigate the stress-strain behavior, dynamic peak stress, elastic modulus and energy distribution characteristics of coal-rock composites under varying loading rates. The experimental results revealed that bedding planes induce mechanical degradation in coal specimens, with the interfacial angle between bedding planes and loading direction demonstrating pronounced influence on mechanical behavior, particularly showing minimum compressive strength at 30\({}^{\circ}\)or 45\({}^{\circ}\) orientations ([PERSON] et al. [38]). [PERSON] et al. [39] utilized a modified SHPB device; the researchers conducted combined static-dynamic loading tests on coal specimens while systematically investigating the coupled effects of static axial pre-stress and loading rate on dynamic tensile strength and crack propagation behavior. [PERSON] et al.