The increase in Young׳s modulus of rocks under uniaxial compression

Abstract

a b s t r a c tThe sources of nonlinear deformation in hard rock under short term uniaxial compression can be attributed to crack closure, sliding, compaction and crack generation. The common approach to finding Young's modulus of a "hard rock" is to determine the linear part of stress-strain curve. However, it is usually a difficult task, although several methods of resolving it have been proposed in the past. We believe that in some rock types there is no linear part as such and provide evidence to support the fact that nonlinear deformation contributed by crack closure, sliding and compaction in different degrees could co-exist within the same stress range. We demonstrate that the change in tangent modulus under different stress levels is attributable to the combination of crack closure, sliding and dilatancy. The difference in tangent modulus under different stress levels could reach more than 20 GPa in some rocks. We have also found that the nonlinear deformation contributed by irreversible sliding, compaction and even dilatancy increases the rock stiffness in the second loading. This phenomenon was found under loads ranging from 15% to 95% of UCS.

The rock stress memory unrecoverable by the Kaiser effect method

“…The events were randomly distributed throughout the low stress level in the 1st loading cycle and were of similar or lower amount compared with the events at the same stress level in the 2nd loading cycle (117 events). We believe that these events in both cycles might be a part of the acoustic noise from the environment or from compaction [41], because they had a very low rate (o1.5 events/MPa) and the number of events was similar in reloading. Hence, the observation is not compatible with the nature of Kaiser effect created by the in situ stress.…”

“…The acoustic emission from 2nd loading cycle would reveal whether the Kaiser effect could be observed. However, it is difficult to define the exact onset of dilatancy by the stress-strain curve or volumetric strain curve in the loading process, because the non-linear deformation mechanisms associated with the crack closure, crack sliding and crack propagation can occur simultaneously [41]. Hence, in addition to the sudden change in the tangent modulus, bulk modulus, and stress-strain curve as an indicator of dilatancy, we use acoustic emission activity as an additional indicator of dilatancy [39].…”

Impact on stability of boreholes in brown coal over time and changes to in situ stress

“…Past experiments show a strong correlation between micro crack development in rock with decrease in elastic moduli, increase in acoustic emission, strain and change in porosity. Hsieh et al (2014) state that the nonlinear deformations in rock under uniaxial compression can be attributed to crack closure, sliding, compaction and crack generation and also that these irreversible changes in the rock including dilatancy increase the stiffness of the rock when reloaded a second time.…”

Coal Strength Development with the Increase of Lateral Confinement

“…For rock under compressive loading, its deformation usually consists of elastic and plastic components, where the plastic deformation are contributed by crack closure, sliding, new crack development and propagation until massive cracks coalesce into a macro failure band. Hsieh et al [14] argued that the elastic and plastic deformation of rock under compressive loading co-occurs for many rocks, and Young's modulus increases during the deformation phase where crack closure dominates. Using Nuclear Magnetic Resonance, Shang et al [15] characterized rock damage with porosity increment at the loading levels of 0.1, 0.2, 0.4, 0.6 and 0.8 UCS, verifying the progressive failure process of rock under loading.…”