![]()
ABSTRACT In this study, quasi-static and dynamic biaxial compression tests were conducted on cubic coal specimens to investigate mechanical properties and fracturing characteristics at different strain rates. In both tests, biaxial static pre-stress (σ1, σ2) were initially applied to simulate in-situ stress conditions. To understand the influence of stress changes from static and dynamic sources, the quasi-static load was achieved by using a true triaxial static loading apparatus, and a triaxial Hopkinson bar system was used to apply dynamic load, respectively. The real-time deformation and fracturing process were recorded by high-speed cameras. Experimental results show that at the same pre-stress conditions, the localised damages were observed on the free surface of the specimen under continuously static loading, and then followed by the ejection of the whole surface shortly. However, under dynamic loading, specimens are in a state of localised damage on the free surface at lower strain rates (equation), but nearly complete damage at higher strain rates (equation). Peak stress values remain approximately constant under static loads, but increase logarithmically with increasing strain rate under dynamic loads. The failure patterns under static loading are tensile splitting that is parallel to free faces, while under dynamic loading specimens changed from nearly intact, splitting failure to fragmentation with the increase of strain rate.
1. INTRODUCTION A better understanding of the mechanical properties and fracturing behaviour of coal is significant for support design and potential hazard prevention in underground coal mining (Salamon and Munro, 1967; Fama et al., 1995; Medhurst and Brown, 1998; Bräuner, 2017). The failure of coal materials is greatly related to the in-situ stress environment caused by geological discontinuities and mining activities. Currently, efforts were mostly made to explore coal failure mechanism under various quasi-static loading conditions in the laboratory (Hobbs, 1964; Bieniawski, 1968; Townsend et al., 1977; Peng et al., 2014; Gao and Kang, 2016; Liu et al., 2019). However, coal mass is actually unavoidably subjected to dynamic disturbances highlighted by blasting, roof breakage and fault slip under in-situ complex stress conditions (Hasegawa et al., 1989; Gibowicz and Lasocki, 2001; Alber et al., 2009; He et al., 2012). The split Hopkinson pressure bar (SHPB) has been widely used to investigate the dynamic properties of coal at high strain rates (> 10 s) (Hopkinson, 1914; Zhang and Zhao, 2014; Zhao et al. 2014; Liu et al., 2015; Yin et al., 2019). Experimental results indicated that dynamic strength and fragment degrees of coal specimens were obviously rate-dependent. It is worth noting that in previous studies, coal specimen was normally subjected to uniaxial (σ1 > σ2 = σ3=0) or traditional triaxial (σ1 > σ2 = σ3≠0) confinements. Moreover, the stability of coal mass in the accessible areas of the mine (e.g., tunnels and stope face) is important, which is inherently associated with the safety of miners and equipment, as well as the stability of support system. For underground coal mining, coal masses around these areas are majorly subjected to biaxial stress conditions (Zhang et al., 2017). To the best of our knowledge, few studies were conducted to examine coal failure behaviour under biaxial pre-stress conditions (Zhang et al., 2017a; Zhang et al., 2017b). Furthermore, these studies only investigated the mechanical properties of coal under quasi-static biaxial loading conditions, and the effect of dynamic loads was not considered. In this study, both quasi-static and dynamic tests were conducted on cubic coal specimens by using a true-triaxial loading testing apparatus and a triaxial Hopkinson bar system, respectively. Real-time deformation and fracturing process of coal were captured by high-speed cameras. The relationships among stress-strain response, peak stress, and failure patterns were explored and discussed in detail.
Site News