Consititutive Model Algorithm Research On Rock Mass In Tensile-shear Region And Strain Softening Simulation

The strength characteristics of rock and post-peak mechanical behavior are the main concerns in geotechnical engineering.A single constitutive model can not accurately simulate the materials with complex mechanical properties such as rock and concrete.This thesis proposes a modified Drucker-Prager(D-P)model based on the Rankine criterion in accordance with the existing problems of D-P criterion,losing stress angle effect and overestimating stress value in tensile-shear region.The same form of D-P yield criterion is keeped in compressive-shear region,and the original conical surface is replaced by introducing the Rankine pyramid surface in tensile-shear region.This method consider the actual tensile strength on the basis of not changing the form of D-P compressive-shear region.At the algorithm implementation level,from the perspective of implicit integration,considering the linear softening/hardening case,combined with the easy determination of the plastic flow direction of the D-P model and the linearity of the Rankine criterion in the principal stress space,the constitutive integral algorithm of the model and the corresponding consistent tangent modulus are derived.The implicit integration algorithm is realized by using the user material subroutine(UMAT)interface provided by ABAQUS.The efficiency and accuracy of the algorithm is verified by the numerical simulation of the the triaxial mixed tensile compression test and the uniaxial tensile compression test of tuff.In accordance with the strain softening problem in geotechnical engineering and the difficult solution problem of the finite element numerical calculation due to the negative tangent stiffness of strain softening model,this thesis establishes an elastoplastic strain softening constitutive model of rock,considering confining pressure,based on D-P strength criterion,and proposes corresponding loading and unloading criterion.The model introduces the isotropic softening hypothesis,which simplifies the process of strain softening into a series of brittle plastic processes.The strength parameters,as the softening parameters change,are nonlinearly fitted with the exponential relationship.The constitutive integral algorithm uses precise implicit integration algorithm for unconditionally stable features(radial return mapping algorithm).The thesis details the simulation method of strain softening and the process of program implementation,and the Fortran program of this model(UMAT)is implemented successfully into the finite element software ABAQUS.The simulated results show that the established strain softening model is simple in parameters and clear in physical meaning,and can capture the post-peak softening characteristics of rock materials.