The Study Of The Rock RHT Model And To Determine The Values Of Main Parameters

The brittle materials, rock and concrete, have the similar material defect of grain boundary, holes and fissures, and enjoy the same features like strain-hardening, damage softening and strain-rate effect, which give the possibility to describe the mechanical behavior of brittle-rock materials under the dynamic and static load based on the constitutive relation of concrete. The RHT constitutive model of concrete has been widely using in the numerical simulation analysis of explosion and penetration, etc, and gave positive results.The RHT model includes 34 parameters. The values of some parameters can be acquired accurately by experiment and theoretical derivation, and some are given by the model, but there are some are hard to be determined due to the complicated experimental methods. In practice, for these hard-acquired parameters, no matter what kind of rock material is studied by RHT model, the values of them are normally quoted directly from the relevant parameters of concrete or the simply modified values of concrete parameters. For the different mechanical properties of different rock materials, the results of numerical simulation often show much deviation. Therefore, in this thesis, to get the more accurate values of parameters, study the RHT model by employing the research methods of theoretical analysis, static tests, wave velocity tests, SHPB impacting tests and numerical simulation.(1) For the p-a equation of state, failure surface equation, elastic limit surface equation, liner strain-hardening section equation and damage softening section equation of the RHT model, the interaction relation and the meaning of parameters of the above different equations were studied and analyzed, meanwhile the whole 34 parameters of the RHT model were classified.(2) The tests of uniaxial compression uniaxial tension and triaxial compression under the condition of same strain rate and the tests of uniaxial compression and uniaxial tension under the condition of different strain rate were conducted using LS-DYNA software and the parameters of concrete RHT model. It is found that the RHT model can describe the mechanical behavior of materials well under the compressive stress condition but not well under the tensile stress condition, by comparing the simulated curves and the classical experimental curves of concrete.(3) The 19 parameters of RHT model were analyzed broadly and for analyzing the sensitivity about the remaining 15 parameters, the uniaxial compression (under low strain rate and high strain rate) and triaxial compression tests of limestone were simulated by LS-DYNA. It is found that, when change only one value of the parameters of B, gt*, N, pcomp, ft*, fs*, A, n, Q0, gc*, ξ, D1, εpm, Af or nf at a time, the lode angle stress-strain curve; the relative tensile strength ft*, relative shear strength fs* and initial damage parameter D1 had different levels of impact on the curve shape between the failure stress surface and residual stress surface of the damage softening equation; the lode angle dependence factor Q0 and the minimum damaged residual strain εpm had some effects on the curve shape between the failure stress surface and residual stress surface of the damage softening equation under the uniaxial compression condition but the value variation of Q0 and εpm had no influence on the stress-strain curve under the triaxial compression; the compressive yield surface parameter gc* had an significant influence on elastic section, liner hardening section and the curve shape between the failure stress surface and residual stress surface under the uniaxial compression condition, whereas the value variation of gc* had no influence on the stress-strain curve under the condition of triaxial compression; the porosity exponent N, compaction pressure pcomp, failure surface parameter A, failure surface index n and the shear modulus reduction factor ξ had different levels of effect on the liner hardening section and the curve shape between the failure stress surface and residual stress surface; the residual stress parameter Af and residual stress index nf had an important influence on the curve shape of damage softening section. In order to verify the sensibility of parameters B or gt* in the RHT mode ahout multiple materials, the uniaxial compression、 uniaxial tension and triaxial compression tests of concrete, diorite and limestone were simulated respectively by changing the value of B or gt* at a time and then the variation of stress-strain curves of the three materials were observed. As the result, the value variation of parameters of B or gt* had no influence on the stress-strain curve of the RHT model. The values of B or gt* can be fixed. In this paper recommend the values of parameters of concrete B=0.0105 and gt*=0.7 due to the hard relevant experiment.(4) The methods to determine the 21 parameters of RHT model were analyzed detailedly and it was confirmed that A, n,fs*,ft*, Q0, gc*,ξ, D1,εpm, Af, nf, pcomp and N were the hard-acquired parameters of RHT model. This thesis proposed a new way to determine the parameters of A, n,fs*,ft*, Q0, gc*,ξ, D1, εpm, Af, nf, pcomp and N, which is based on the static tests, acoustic tests, SHPB impacting tests, LS-DYNA numerical simulation and orthogonal experiments.(5) According to the above new method proposed in this thesis, it was achieved in acquiring the appropriate parameters of RHT model of granite, marble and red sandstone. Furthermore, the projectile penetrating granite targe test was simulated by the LS-DYNA base on the determined parameters of RHT model of granite in this paper. As a result, the simulated results were consistent with the experimental results, which illustrated that the values of the parameters of the material model were reasonable and proved that the new method proposed in this thesis to determine the parameters of RHT model was correct and practical.