| Rock rheological mechanics has always been a difficult and hot spot in the research of rock mechanics theory.The study of rock rheological properties generally includes two aspects:one is to study the constitutive model of rock rheology,its main combined model consists of three basic rheological components,such as elastic components,viscous components and plastic components,which are composed of a combination of strings and parallels;the other is through rheological experiments,mainly including uniaxial compression rheological tests and triaxial compression rheological tests.Then by fitting the experimental data,or using neural networks and other methods to identify and invert the model parameters.Although there are many models to describe the rheological characteristics of rocks at present,these models are only useful for the first two stages of creep and are ineffective for non-linear accelerated creep stages.Even though many scholars have proposed improved models based on nonlinear viscous elements,fracture damage theory,or internal-time theory,the identification process is also difficult due to the large number of model parameters.Therefore,this thesis selects the two traditional representative modelsin the constitutive model of rock rheology,Nishihara model and bingham model.Based on the cyclic load to improve the two models,and proposed improved fatigue constitutive model,which can describe nonlinear rock rheology and characterize the accelerated creep phase of rock.The model parameters source of nonlinear fitting of rock creep test data was obtained from previous experiments,and then was analyzed.At the same time,we compared and analyzed the two improved models in the mechanical model structurethe,the model parameters,the rheological characteristics of rocks and the applicable scope of the two improved models.The research results obtained in this paper are as follows:(1)On the basis of reading numerous literature and the basic elements of the traditional rheological model,we summarized the composition of rock fatigue models under cyclic loading,which includes three basic fatigue elements:the elastic fatigue elements(HF),viscous fatigue elements(NF)and plastic fatigue elements(YF).Furthermore,we described the constitutive relations and combination rules of the three fatigue elements,and gave the expression of the equivalent stress function of the basic element under cyclic loading.Of course,we introduced the characteristics of rock fatigue deformation under cyclic loading and the composition of the deformation.(2)By replacing the linear viscous elements in the viscoplastic body of traditional Nishihara model with nonlinear viscous elements containing fatigue parameters,an improved Nishihara fatigue model under cyclic loading was established to represent the nonlinear rheological characteristics of rock under cyclic loading.,and deduced the constitutive equation of the improved fatigue model.By reading the literature and using the experimental data of the loading tests of various types of rock under the cyclic loading,which comes from previous researchers,we carried out the rationality fitting test of the constitutive equations,and fitted the model parameters.The fitting results showed that:compared with the results of existing rock fatigue tests,the improved Nishihara fatigue model can better describe the three fatigue deformation stages:deceleration,constant velocity and acceleration of rock under cyclic loading.The fitting correlation coefficient was above 0.994,even up to 0.997,and the model contained fewer parameters,indicating that the model can fully reflect the nonlinear deformation characteristics of rock under cyclic loading,thus verifying the rationality of the model.(3)By replacing the constant-valued viscous element in the traditional Bingham model with a coefficient-variable viscous element related to the number of cycles,and replacing the fixed-valued elastic element with a non-valued elastic element that decays with the number of cyclical load cycles increasing,we derived the Bingham fatigue constitutive model which can reflect the various strain laws of rocks under cyclic cyclic loading,and deduced the constitutive equation of the improved model.By reading the literature and using the experimental data of the loading tests of various types of rock under the cyclic loading,which comes from previous researchers,we carried out the rationality fitting test of the constitutive equations,and fitted the model parameters.The fitting results showed that:the established constitutive equations can better describe the strain characteristics of different rocks under cyclic loading,and the correlation coefficients of the strain curve fitting for all types of rocks are all above 0.868,which verifies the rationality of the model.In addition,in the same rock model,the initial value of the elastic element coefficient decreases with the increase of the dynamic stress amplitude of the cyclic load,and the acceleration strain rate parameter of the rock increases with the increase of the dynamic stress amplitude.(4)In this paper,we compared and analyzed the rheological properties of the Nishihara fatigue model and the Bingham fatigue model under the cyclic loads,and their mechanical models,model parameters,rheological properties,and applicable scopes of the models.The results indicated that by comparing and analyzing the mechanical model,the Bingham model under cyclic loading belongs to a three-element model and the Nishihara fatigue model under cyclic loading belongs to a five-element model.The comparative analysis of the model parameters shows that the constitutive equation of Nishihara fatigue model contains five parameters,however,the Bingham fatigue model contains four parameters,and the meaning of each parameter is explained.For the rheological properties of the established Nishihara fatigue model,when?(27)?_s,the model will not be infinitely deformed,nor will the stress be zero when relaxed;when???_s,the rheological properties of the established Nishihara fatigue model are Change,creep is metastable,creep leads to infinite deformation,slack is not complete.For the rheological properties of the established Bingham fatigue model,when?(27)?_s,the model has only the spring deformation,but no creep;when???_s,the model has the properties of instantaneous deformation,constant-speed creep and relaxation,belonging to the instability Creep.The established Nishihara fatigue model is suitable for describing the nonlinear elastic-viscoelastic-viscoplastic rheological properties of any rock.The Bingham fatigue model can fully reflect the nonlinear elastic-viscoplastic characteristic of soft rock. |
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