Experimental Study On Generalized Stress Relaxation Of Rock Based On DIC Technology

There is a potential failure risk of rock engineering under long-term high stress activity,which is related to the gradual accumulation of damage internal structure of rock,until the damage exceeds a critical threshold.The mechanical properties of rock with time effects are usually explained by the concepts of creep and stress relaxation,which are mainly related to the evolution of stress and strain with time.The generalized stress relaxation theory assumes a general rheological phenomenon in which the stress and strain states of a rock change simultaneously over time.In this paper,sandstone and tuff are used as test objects.Based on 3D-DIC technology,experiments on generalized stress relaxation characteristics of rocks under different stress conditions under different water pressures are carried out.The evolution of axial and radial strains over time,and their characteristics affected by factors such as water pressure,stress levels,and lithology.The results of this research provide an assessment of the long-term stability of rock mass engineering in water-rich environments.Theoretical support.The main research results of this paper are as follows:(1)In generalized stress relaxation tests with different rheological direction coefficients,the evolution rules of stress and strain with time are similar to the laws of creep test phase such as primary creep phase,steady creep phase and accelerated creep phase.Similarity,the rate of evolution changes with increasing of water pressure and stress levels.(2)The evolution of different types of cloud diagrams of rocks during the process of generalized stress relaxation.The axial and radial strain cloud diagrams can better reflect the evolution characteristics of rock surface deformation.The axial strain field has a layered uniform distribution in the strain concentration area at the initial moment of rheology test.And there is minute strain concentration area in the radial strain field.In the subsequent evolution,the strain concentration areas of the axial and radial strain fields gradually exist around the locations of the macroscopic cracks would appear after rock failure,and the evolution difference of the concentrated area of the radial strain field is greater than the axial strain field.(3)With the increase of water pressure,the evolution rate of the axial and radial strain fields of sandstone and tuff during the rheological process is significantly accelerated.The axial and radial strain fields of tuff evolve over time as a strain concentration area.It is more concentrated and appears as plaque-shaped,and is not affected by the increase of water pressure as the strain field evolution of sandstone is significantly different.The average strain change rate of axial and radial strains,and the variance of strain at different locations increase with gradients such as water pressure.With the increase,the sandstone strain evolution is more exponentially related to the hydraulic pressure gradient,so that sandstone with larger strain increments under high water pressure have a tendency to failuer.However,the mean and variance of the tuff's strain gradually stabilized with time,and no obvious macro-cracks appeared during the rheological process.(4)With the increase of the axial stress level of rheological start stress,the evolution rate and strain concentration of axial and radial strains of sandstone are accelerated.When the strain state of the sandstone is in the elastic stage,the evolution rules of the axial and radial strains under different rheological direction coefficients are not affected by changes in water pressure.When the strain state of the rock is in the stage of stable and accelerated expansion of cracks,as the rheological direction coefficient changes from the unloading direction to the loading direction of strain-stress curve,the difference affected by the water pressure gradually becomes apparent,the rate of change of the overall mean and the variance of the strain values at different locations,with the increase of the water pressure,its evolution rate also increases significantly.When the rock is subjected to a stronger external load,the existence of water pressure will increase the tensile stress at the tip of the pores and cracks in the rock,leading to a tensile failure at the tip.The increase of water pressure will increase the pore pressure of the rock and increase the tensile stress on the tip of the pore and crack.