Experimental Study of Loading System Stiffness Effects on Mechanical Characteristics and Kinetic Energy Calculation of Coal Specimens

The loading system stiffness (LSS) of a rock mechanics testing machine is important in laboratory testing because it dictates whether the coal or rock failure process is stable or unstable. It is also critical to ejection kinetic energy after coal specimen failure. When unstable failure accidents such as rock burst occur in rock engineering, the speed of ejected coal or rock may reach or even exceed 10 m/s. Such unstable failures, if unplanned, can result in injuries of workers, damage to equipment and delays in production in underground mines. To systematically study the influence of the LSS on mechanical characteristics and kinetic energy of coal specimens, tests are first carried out using an LSS-variable testing system named BGYD-600. PCI-2 acoustic emission (AE) test system, acceleration sensor and dynamic strain gages are used to monitor the AE signals of coal and deformation, displacement of test machine. And the kinetic energy is calculated by energy analysis method. The test results show that the LSS has influence on the stress–strain curves of coal specimens after the peak strength. The smaller the LSS, the steeper the post-peak curves. In addition, as the LSS decreases, the spalling of coal blocks increases considerably, and the powdery phase begins to appear, and the coal specimen is destroyed at a shorter time with more coal blocks being ejected. Moreover, the maximum AE energy occurred not at the peak strength using the BGYD-600, which is different from the large stiffness testing machines. Finally, considering the impact of the elastic deformation energy release of the testing machine on the coal specimens, the kinetic energy calculation formula when the coal specimen failure is derived. And the influence of LSS on the kinetic energy, elastic deformation energy, dissipation energy and total input energy is analyzed. As the LSS increases, the kinetic energy of the coal specimen shows a decreasing trend. The quantitative relationship between mechanical characteristics, kinetic energy and LSS given in this paper has certain reference value for mining and civil engineering projects.