Fatigue Characteristics of Coal Specimens under Cyclic Uniaxial Loading

To investigate the fatigue failure of coal under uniaxial stress, multilevel cyclic loading tests on cylindrical coal specimens were conducted in a laboratory using the MTS815.02 rock-mechanics test system (MTS Systems Corporation, Eden Prairie, MN). Specimens were tested for as many as 1,500 loading cycles under each cyclic loading session. From these tests, the following conclusions can be drawn. The peak stress of cyclic loading has a significant effect on the coal specimen's fatigue behavior. When the peak stress of cyclic loading is less than the coal fatigue strength, the coal specimen's deformation stops increasing after a certain number of loading cycles, and the number of acoustic emission (AE) counts is low. When the peak stress of cyclic loading exceeds the coal fatigue strength, the axial, circumferential, and volumetric strains and the energy dissipation exhibit the same progression through three phases: a primary phase, a steady phase, and an acceleration phase. The region division of the three phases of the strain and energy dissipation evolution is coincident. The AE activity also exhibits three phases: an active, a quiet, and a very active phase. In the very active phases, numerous AE signals exist when the stress is at its peak, but no AE signals exist near the stress minima. Coal specimens that are subjected to conventional uniaxial compression tests and specimens that are deformed by cyclic loading tests fail in very different ways. The former specimens fail mainly because of a single inclined shear plane, whereas the latter specimens are reduced to a mound of small fragments.