Energy dissipation rate: An indicator of coal deformation and failure under static and dynamic compressive loads

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scopus – master journals – JCR

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۴٫۲۷۶ در سال ۲۰۲۰

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۲۶ در سال ۲۰۲۱

شاخص SJR

۰٫۹۰۱ در سال ۲۰۲۰

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Q1 در سال ۲۰۲۰

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Energy dissipation rate: An indicator of coal deformation and failure under static and dynamic compressive loads

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Energy dissipation rate: An indicator of coal deformation and failure under static and dynamic compressive loads


Dynamic disasters in  Chinese coal  mines pose a significant threat to coal  productivity. Thus, a thorough understanding of the deformation and failure processes of coal  is necessary. In this study, the energy dis- sipation rate is proposed as  a novel indicator of coal  deformation and failure under static and dynamic compressive loads. The  relationship between stress-strain, uniaxial compressive strength, displacement rate, loading rate, fractal dimension, and energy dissipation rate was investigated through experiments conducted using the MTS  C60  tests (static loads) and split Hopkinson pressure bar system (dynamic loads). The results show that the energy dissipation rate peaks are  associated with stress drop during coal deformation, and also positively related to the uniaxial compressive strength. A higher displacement rate of quasi-static loads leads to an  initial increase and then a decrease in energy dissipation rate, whereas a higher loading rate of dynamic loads results in  larger energy dissipation rate. Theoretical analysis indi- cates that a  sudden increase in  energy dissipation rate suggests partial fracture occurring within coal under both quasi-static and dynamic loads. Hence, the energy dissipation rate is an  essential indicator of partial fracture and final failure within coal,  as well as a prospective precursor for catastrophic failure in  coal  mine.

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  1. Introduction

China,  the world’s largest producer and consumer of coal,  pro- duced about 3.41  billion tons of coal and consumed nearly 2.70  bil- lion   tons  coal   in   ۲۰۱۶٫  However, dynamic  disasters,  such  as rockburst and coal  and gas  outburst, present a  serious threat to coal  productivity in Chinese coal  mines. With the increase in min- ing  depth in recent years, the number of accidents in Chinese coal mines has  been on the rise  and this has  drawn extensive attention from both government and research institutes. Consequently, sub- stantial efforts have been carried out  to investigate the mechanism underlying these accidents to  implement  effective measures  for their prevention and control [1,2].  In  general, dynamic disasters involve an  abrupt failure of  coal  under external loads generated by  various mining-induced activities, such as  roof  fall,  fault slip, and blasting. It  is  therefore of utmost importance to  understand the deformation and failure process of coal  under different types of external loads.

Theoretical and experimental studies have already indicated that the energy concepts play  a significant role  in  describing the deformation and failure process of  rock  materials. For  example, the  energy  concept  ‘‘specific   energy”,  defined  as   the  energy required to  excavate a  unit volume of rock,  was  considered as  a useful index to  identify the critical failure mode transition depth in  rock  cutting [3].  The  energy concept ‘‘dissipated energy” was closely related  to  the fatigue deformation of  rock   under  cyclic loading [4]. In addition, other types of energy involved in rock engi- neering  have  also   been  extensively  investigated.  The   seismic energy released during brittle rock  failure and rock  blasting results in  strong tremor, eventually causing serious destruction  to  the underground tunnel [5].  The  fragmentation energy is intrinsically associated with the breakage and the creation of new surfaces of rock  blocks [6].

Researches in  recent years have demonstrated that the defor- mation  and  failure of  coal   are   irreversible  processes  involving energy dissipation, which eventually cause continuous damage and material  deterioration  during the  loading process [7].  The mechanism of these processes has  already been proven in  many experimental studies on energy dissipation of coal  under different loads. Quasi-static experimental  studies  of  coal   under  uniaxial

compression conditions evaluate the positive relationship between dissipated energy and electromagnetic/acoustic energy emitted during coal  fracture and failure [8].  Additionally, the  results  of these studies also   confirm the link   between  energy dissipation and coal  deformation and failure under conventional triaxial com- pression conditions [9].

The  deformation and failure of  coal  under dynamic loads are usually conducted using the split Hopkinson pressure bar  (SHPB) system,  which  is  an   innovative technique  for   achieving high- strain-rate  loads in  dynamic compressive tests  [۱۰].   Significant progress has  been made in  rock  dynamic tests over  the past few years, a series of key  loading techniques has  been reported in the literature,  including the  pulse  shaping, momentum-trap  [۱۱], strain  controlling,  coupled  static-dynamic  loading,  and  axial- radial confining loading techniques  [۱۲].   Various measurement techniques have also  been introduced to modify the SHPB system, such as  the X-ray  micro- computed tomography [13],  laser gap gauge, and digital image correlation [14]. Moreover, many dynamic properties of rock  materials have been studied using the modified SHPB system, including the dynamic deformation and energy dis- sipation, the dynamic fracture toughness [15],  and the dynamic crack propagation [16].  The  experimental studies of coal  dynamic properties demonstrate  that  the  energy dissipation is   closely related to  coal  fragmentation caused by the dynamic loads [17].

However, the aforementioned energy dissipation refers to  the total energy dissipated during the whole loading process, rather than the instantaneous energy consumed at any  time during the loading. Thus,  the traditional energy concepts are  not  appropriate for quantitatively describing the instantaneous energy dissipation of coal.  Besides, the energy dissipation of coal  in quasi-static tests and in  dynamic tests  has   not   yet   been compared in  previous studies.

In this study, experiments of coal  under quasi-static loads and dynamic loads were conducted using the MTS C60 tests and SHPB systems,  respectively,   and  the   energy  dissipation  rate  was

proposed to quantitatively describe the energy dissipation process and the deformation process of  coal.   The  relationship between stress-strain,  uniaxial compressive strength, displacement  rate, loading rate, fractal dimension, and energy dissipation rate under both quasi-static and dynamic loads were experimentally investi- gated and theoretically discussed.

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