Abstract
Mudstone interlayer is a weak layer in rock engineering. When it is subjected to continuous stress higher than its damage threshold, due to the dislocation of particles in mudstone crystals and the expansion of cracks, mudstone strength is gradually damaged and deteriorated and the strain gradually increases, thus accelerating the phenomenon of creep damage. In order to describe the characteristics of the whole process of mudstone aging deformation, based on the damage evolution of strength parameters (cohesion and internal friction coefficient) with stress and time in mudstone creep tests, a novel damage nonlinear viscoelastoplastic body (D-NVPB) is proposed through improving traditional plastic element. D-NVPB describes the nonlinear characteristics of the accelerated creep stage of mudstone. With the element combination method, D-NVPB is connected with the Burgers model in series to form a new nonlinear damage creep model (D-NVEP model). The analysis results of creep characteristics theoretically verified the rationality of the model in describing the instantaneous elasticity, viscoelasticity, and nonlinear viscoplastic characteristics of the complete creep curve of mudstone. With the data obtained in the uniaxial compression creep test of mudstone under the action of a stress level of 14 MPa, based on the Levenberg-Marquardt nonlinear least squares method, the fitting calculation was performed through piecewise fitting and overall fitting. The correlation coefficient was 0.9909, which verified the applicability of the model. The obtained model parameters by the identification were used to predict the mudstone creep curve under the stress levels of 13 MPa and 15 MPa. The good prediction results further verified the feasibility of the model. Compared with the traditional creep model, the D-NVEP model can better describe the nonlinear characteristics of the accelerated creep stage and quantitatively display the strength damage evolution process of rock in the creep failure process.
Highlights
In the study, based on previous studies [17,18,19], through mudstone creep tests, from the perspective of the deterioration of rock mechanical parameters with stress and time, the damage characteristics of two mudstone parameters in the accelerated creep stage are explored in order to quantitatively reveal the rock strength damage evolution in the failure process
According to the above analysis results of creep damage mechanism, in the creep test, as new damage occurred inside the rock, the cohesive force and internal friction coefficient decreased with the increase in the overstress threshold difference and the creep time
In order to verify the applicability of the D-NVEP creep model in describing the nonlinear characteristics of the accelerated creep stage, with the uniaxial creep test data of mudstone under the stress level of 14 MPa in the previous study [16], the fitting calculation is performed through piecewise fitting and overall fitting
Summary
A novel nonlinear creep model based on damage characteristics of mudstone strength parameters engineering. The non-linear characteristics of rock mass creep were seldom explored from the perspective of the deterioration of rock shear strength parameters (cohesion and internal friction coefficient) in the accelerated creep phase (failure phase). In the study, based on previous studies [17,18,19], through mudstone creep tests, from the perspective of the deterioration of rock mechanical parameters with stress and time, the damage characteristics of two mudstone parameters (cohesion and internal friction coefficient) in the accelerated creep stage are explored in order to quantitatively reveal the rock strength damage evolution in the failure process. Under the continuous action of stress, pores and cracks in rocks are gradually compacted and the deformation increases, but the strain rate gradually decreases and enters the attenuation creep stage. Under the continuous action of stress, cracks rapidly expand, cross each other, form main cracks along the direction of the principal stress, and result in continuous and irreversible viscoplastic strain and even shear failure [27]
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