Tension and Shear Cracks Growth-Based Evolution Model for Creep Characteristics of Brittle Rocks

摘要

In brittle rocks, the synergistic expansion of tension and shear cracks has a great influence on the fracture strength under long-term constant compressive loads. Hence, a creep mechanical model based on subcritical crack propagation is proposed, and the effects of tension and shear cracks created by the creep deformation are also considered. Based on this mechanical model, the evolution equations of axial, circumferential, and volumetric strains under long-term constant loads are theoretically derived. By comparison, the theoretical results are consistent with the experimental results. By applying different compression stress conditions, the critical damage valuesD(1)andD(3)are calculated by using the theoretical model based on the creep deformation process. For the same research specimen, the critical damage valuesD(1)andD(3)of creep phases are less affected by the compression stress conditions when failure time is not greater than its critical valuet(f)= t(cf). The effects of differential stress on minimum strain rate, failure time, and minimum normalized stress intensity factor are investigated. By contrast, the investigated three characteristic parameters are more sensitive under the varying confining pressures with a constant axial pressure rather than a constant confining pressure with varying axial pressures. According to the curves of normalized stress intensity factor and damage during the evolution of crack length, it is determined that the center position of the critical damage valuesD(1)andD(3)is the average damage valueD(2). Finally, the quantitative equations of three creep characteristic parameters based on three representative damage values are also derived.