Research On Failure Modes And Localized Progressive Failure Model Of The Cracked Media Under Static And Dynamic Stress Conditions

Aimed at the intermittent jointed rock mass (not intact rock sample) and the dynamic loading conditions, the failure modes and the mechanical properties of the Representative Volume Element (RVE) of the jointed rock mass are studied systematically through lots of experiment in meso-scope, the model and formula suitable for engineering practice are put forward in macro-scope.Firstly, regarding the RVE of the intermittent jointed rock mass as a "structural" material and based on the failure modes of the structural body (rock bridge) itself, the fracture propagation, coalescence are observed through the digital camera and video equipment and investigated by loading pre-fractured sandstone-like material, classified through a dimensional analysis, under static and dynamic (strain rate is 10-5~10-1 s-1) uni-axial compression. The specimens have two or three pre-existingcracks that are produced through pre-installed steel slice and arranged in different echelon and co-planar geometries. The modes of fracture propagation and coalescence, as well as the law of strength variation with the cracks collocation under static loading are summarized systematically, and the new exploration on the failure modes is obtained under dynamic loading. The new exploration makes up for the blankness of failure modes of the jointed rock mass under dynamic loading.Secondly, through the comparison of the propagation length, coalescence mode of the fractures and strength increase of the pre-cracked samples under static and dynamic loading, the crack number-dependence of rate effect is demonstrated, and the inertia effect of the fracture propagation is regarded as the main reason of the strength increase of the brittle material under slow to medium fast loading rate. The new explanation is rooted in experimental data, and more direct and reasonable than others. In virtue of the explanation, a second reason of the mode II shear fracture under earthquake is opened out.Thirdly, based on the experimental observation of the failure process about the cracked samples, and in virtue of the analysis by sliding crack model, the failure characteristics of the cracked samples are summarized. The failure characteristics are thought to be localized and progressive, and the progressive failure process can be abstracted as cohesion-weaken and friction-strengthen, moreover the cohesive and frictional strength components are not mobilized simultaneous.Fourthly, based on the cohesion-weaken and friction-strengthen of failure process in essential, the model which can simulate the softening and hardening process is established and validated. In this model, the elastic and localized zones are divided, and the heterogeneity of the material is considered. Moreover, by the aid of the real strength parameter evolution law developed by the model, the softening process after peak value of the brittle material can be simulatedsuccessfully in essential.Finally, from the engineering practice viewpoint, the equivalent elastic modulus of the jointed rock mass is derived, and in virtue of the relationship of the fracture surface angle and dilation angle with the cracks collocation, the localized progressive failure model aimed at jointed rock mass and the strength model in shear condition are derived in macro-scope.