Slow failure of quasi-brittle solids

A new mesoscopic non-equilibrium thermodynamic approach is developed. The approach is based on the thermodynamic identity associated the first and second law of thermodynamics. In the framework of the approach different internal dissipative channels of energy are taken in account in an explicit form, namely, the thermal channel and channels of defect subsystems. The identity has a perfect differential form what permits to introduce an extended non-equilibrium state and use the good developed mathematical formalism of equilibrium and non-equilibrium thermodynamics. The evolution of non-equilibrium variables of a physical system are described by a Landau-based equation set expressed through internal or different kinds of free energy connected by means of the Legendre transforms. The accordance between the different kinds of energy is possible owing to introduction of some trends into the equation subset described the defect subsystems and having a nature of structural viscosity. The possibilities of the approach are illustrated on the example of quasibrittle solid damage and failure. Taking into account only one type of defects (viz., microcracks) and mechanical parameters in an expansion of free energy down to third powers in relative to average energy per microcrack, the description of destruction of quasi-brittle solids during long-term loading is considered. The consideration allows to find equilibrium and non-equilibrium values of the free energy. A qualitative behavior of the system on parameters of the theory is analyzed. The destruction of material is described from the uniform positions, both at uniform tension and uniaxial compression. Origins of the high stability of mine workings at small depths and their instability at large depths are explained.