Study On Marble Failure Energy Evolution Mechanism And Its Constitutive Model Under Different Loading And Unloading Stress Paths

Rock engineering excavation is a complex process of loading and unloading, andthe deformation characteristics, mechanical parameters and failure mechanisms underunloading stress paths are significantly different from those under loading conditions.Strengthening research on rock failure mechanisms under loading and unloadingconditions is of theoretical and engineering significance to deepen our understanding ofrock failure mechanisms. According to the experimental results of marble loading andunloading tests under different stress paths, rock failure energy mechanism are carriedout. A constitutive model of energy nonlinear evolution is established. The main workcan be summarized as follows:(1) According to marble specimens loading and unloading tests, rock deformationcharacteristics, failure features and strength parameters under loading and unloadingstress paths are carried out. Axial deformation plays a dominant role in elastic stage ofloading test, while hoop deformation increases quickly after yield starting. The crackinitiation stress is gradually close to peak strength point with confining pressureincreasing. Rock failure is delayed by confining pressure. Rock peak strength decreaseswith unloading rate increasing and it increases with confining pressure increasing. Rockcircumferential deformation and volume deformation are more sensitive to lowunloading rate.(2) According to the generalized plastic mechanics theory, marble loading andunloading yield surface based on the experimental data is constructed. This methodensures that the yield function is unique and accuracy. Shear yield surface appears linearform under loading stress path. However, it appears quadratic parabola form underunloading stress path. Volume yield surface under loading stress path is divided into twolinear forms with its compression stage and dilation stage. And unloading failure isdivided into linear form and quadratic parabola form.(3) Marble unloading process can be divided into four stages according to thecharacteristics of energy evolution: compaction stage, elastic stage, crack expansionstage and post-peak stage. Before rock yielding, the absorbed energy is stored inside the rock sample in the form of elastic energy. During the yield point to the peak point, theelastic energy growth is slow. Howere the dissipated energy growth is quick. After thepeak strength, the stored elastic energy of rock specimen achieves energy storage limit.The elastic energy releases instantaneously and dissipated energy increase quickly. Theenergy storage limit of conventional triaxial compression is higher than that of uniaxialcompression. It is also higher than that of unloading test. The changing rates ofabsorbed total energy, elastic energy and dissipated energy decrease with unloading rateincreasing. With the unloading rate increasing, the needed energy for rock unloadingfailure becomes smaller. Rock is easily failure under high unloading rate. The releaseelastic energy mainly accumulates during loading stage. The energy stored duringloading process play a decisive role in rock unloading failure process.(4) According to marble failure energy evolution features, and combining with theecology competition mechanism, energy accumulation evolution model is established.Based on the relationship between energy iterative growth factor and axial strain inenergy model, rock failure crack initiation strain warning under different stress paths iscarried out. Uniaxial compression test accounted for77%of the peak strain;conventional triaxial test accounted for72-76%; and the unloading test accounted for72-81%.