Elastic-Plastic Threshold and Rational Unloading Level of Rocks

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. Appl. Sci. 2019, 9, 3164 2 of 15 and the unloading level has no significant effect on the energy evolution process. Zhang [11] and Zhu [12] considered that the unloading stress level mitigated possible damage within rocks during the unloading damage stage. Regarding the initial axial pressure of unloading, Liu et al. [13] selected 50% of the conventional triaxial compressive strength and the corresponding confining pressure, whereas other researchers often adopted 60% and 80% [14][15][16], 70% [17][18][19][20], 80-90% [21], and 15-90% [22]. Most researchers selected 80% of the peak strength as the loaded axial pressure in unloading tests [23][24][25][26]. Details of studies discussed above can be viewed in Table 1. However, mentioned studies failed to consider the effects of the initial axial pressure during exploring the effects of unloading on rock failures.

Inversion Method of the Young’s Modulus Field and Poisson’s Ratio Field for Rock and Its Test Application

“…For Poisson’s ratio, with the increase in stress level, the Poisson ratio increases gradually. The Poisson’s ratio of sandstone under axial stress of 99% σ c exceeds 0.5, indicating that the sandstone has already entered the plastic deformation stage, which is consistent with the research conclusions of the literature [ 30 , 31 ].…”

Transition Threshold of Granite Mechanical Characteristics at High Temperature

“…Between 400 and 600°C, the strength drop is more significant [21,22], as shown in Figure 4. [19,[23][24][25], namely, the ratio of the increase in strain to the increase in axial pressure, which is expressed as follows:…”

“…In view of the abovementioned problems, Ji [19] combined conventional triaxial compression tests to explore the critical value of rock damage transitions under triaxial compression [20]. Following on this work, we further study the mechanical properties of rock after high temperature under different stress conditions.…”

An Experimental Research on Surrounding Rock Unloading during Solid Coal Roadway Excavation

“…Roadway excavation is defined as an unloading process, which is essentially different from continuous loading failure in mechanical response, action mechanism, and deformation failure characteristics [4][5][6]. Recently, researchers have done a considerate amount of work on engineering excavation unloading [7][8][9]. Academician Qian Qihu [10] considered that the zone failure of roadway surrounding rock was led by dynamic unloading caused by excavation, and the practice also confirmed this conclusion.…”