Composition of rock materials shows, that existing microcracks and heterogeneities in rock materials and concrete lead to formation of local stress concentrations which under certain conditions are the cause of formation of macrocracks and full disintegration of material. Paper describes and analyzes different models of cracks, based on which dependencies, denoting strength of materials and methods for calculation of crack propagation, leading to fracture of engineering structures, are developed.
INTRODUCTION
Fracture mechanics can be used in solutions of rock mechanics problems and first of all in problems of strength of rock materials. Recently large number of criteria of strength of materials is used in engineering practice. All of them postulate that fracture starts at the moment when combination of certain parameters (stresses, strains, temperatures, etc,) at some point reaches critical value. However, regardless that such criteria are widely used by engineers, they are applied only to certain materials and at certain conditions. None of these criteria explains the mechanism of fracture.
Numerous experimentshow that fracture of materials normally is determined by the process of formation and propagation of cracks. Investigation of this process is exactly the subject of fracture mechanics, which suggests different mathematical dependencies connecting crack propagation with stresses, strains and strength of materials. These dependencies are usually based on consideration of rock and concrete as linear elastic jointed media. Experimental research also shows, that nonlinear deformation of rocks and concrete is determined by the formation of microcracks, hence regarding fracture as a process of crack grow, fracture mechanics provides means of analyses of rock deformation. The presence of numerous microcracks and heterogeneities in rock materials and concrete causes local stress concentrations which under certain conditions lead to formation of macrocracks. If the propagation of macrocracks under given load is unstable it results in complete failure of material. Fracture mechanics takes into account this fact and offer mathematical models of this phenomenon.
It presents many dependencies between the growth of cracks and stress-strain state of materials. All of them are based on the idealization of rocks and concrete as elastic linear continuum containing numerous cracks.
