Abstract
During loading and unloading test, various rocks manifest different stress values of elastic-plastic transformation. This study proposes to include axial pressure increment ratio in the conventional triaxial compression test to evaluate different variables (nominal elastic modulus, nominal Poisson’s ratio, strain, and energy). The relationships among various factors including variables, the stress level of initial confining stress and axial pressures, were analyzed by analyzing the stress–strain plot record obtained from testing various rocks. The extreme value point of the deformation parameter, also known as the elastic-plastic threshold, was analyzed. In addition, the elastic-plastic thresholds were later used as unloading points during the unloading tests. Under the same confining condition, different rocks demonstrated different unloading levels. Furthermore, a linear correlation was observed between unloading levels and changing confining pressures, and the gradient is mainly related to the types of rocks. During the unloading tests of rocks, the rational unloading level is recommended to be no higher than the stress level at the elastic-plastic threshold under the corresponding confining pressure.
Highlights
Rock mechanics studies have been extensively conducted in China and abroad, which cover a broad range, from uniaxial to triaxial compression, simple to multiple conditions, and single to complex paths [1,2,3,4,5,6]
According to the results obtained from conventional triaxial compression tests, Liu [7] and Li [8] determined that 85–95% of the peak strength is the unloading point under the corresponding confining pressure
This study combines the conventional triaxial compression test with axial pressure increment ratio to find out the critical inflection points by monitoring compressive deformations of the rock under different confining pressures [27]
Summary
Rock mechanics studies have been extensively conducted in China and abroad, which cover a broad range, from uniaxial to triaxial compression, simple to multiple conditions, and single to complex paths [1,2,3,4,5,6]. The initial stress level mainly includes the initial confining pressure and the initial axial pressure. During a triaxial unloading test, the initial axial pressure (i.e., unloading axial pressure or unloading level) is typically lower than the peak strength under the corresponding confining pressure. According to the results obtained from conventional triaxial compression tests, Liu [7] and Li [8] determined that 85–95% of the peak strength is the unloading point under the corresponding confining pressure. This study combines the conventional triaxial compression test with axial pressure increment ratio to find out the critical inflection points by monitoring compressive deformations of the rock under different confining pressures [27]. Conducting unloading tests of rocks under complex stress paths provides a theoretical reference for the selection of initial axial pressure
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