Research On Three-dimensional Viscoelastic Constitutive Model And Numerical Mechanics Experiment Of Asphalt Mixture Based On Internal Microstructure

Asphalt mixture is a kind of complex particle composite material consisting of aggregate,asphalt binder and air voids.The effect of aggregates and asphalt mastic,the material properties of asphalt mastic and the spatial distribution of air voids in the mixture affect the mechanical properties of asphalt mixture.The traditional design method based on experience can not meet the needs of practical engineering because of the lack of theoretical basis.The Superpave design system using the volume composition of asphalt mixture as the design index can't directly obtain the internal volume constitutions of asphalt mixture due to the limitation of research methods.The Superpave design method is completely dependent on the experience,and has the same limitations as the traditional Marshall Design method.In order to obtain the mechanical response of asphalt mixture in micro or meso scale,the scientists tried to develop a numerical model based on internal microstructure that can accurately reflect the internal structure of asphalt mixture and then used the numerical analysis results of the model to study on the intrinsic relationship between the macroscopic response of asphalt mixture and the micro structural characteristics.Numerical model of asphalt mixture based on scanning CT slice obtaining with X-ray Computed Tomography and combined with digital image processing technology is an effective method to realize the target.Based on the current research achievements,there are some shortages in the internal structure finite element model of asphalt mixture based on CT X-ray technology,that is,model building,material properties and virtual experiment abundance and so on.Therefore,it is necessary to improve the basic work to realize the numerical simulation of the asphalt mixture structure.This paper mainly aimed at the shortcomings of the asphalt mixture in three dimensional numerical modeling and virtual experiment and drew the methods and achievements from interdiscipline,the internal structure numerical model of asphalt mixture is established based on CT slice image,and studied the secondary developme of three-dimensional viscoelastic constitutive model for asphalt mastic and methods of parameter acquisition.Numerical simulation of triaxial creep was carried out by using the numerical model.The first step in the application of CT slice images of asphalt mixture is to classify the materials in the image.According to the gradation distribution characteristics of CT slice image for asphalt mixture,there are existing problems in the segmentation of the material used in the common algorithm.An improved segmentation algorithm is proposed to segment the CT slice image for asphalt mixture.This paper tries to segment the adhesion of coarse aggregate from the perspective of morphological multi-scale and the segmentation method is evaluated through segmentation effect and segmentation accuracy.The results show that the improved segmentation algorithm can effectively reduce the beam hardening effect.This segmentation algorithm is simple and effective.In coarse aggregate segmentation,segmentation of the image with morphological multi-scale algorithm has a high precision.Three-dimensional microstructure numerical modeling reconstruction of asphalt mixture is realized by using the existing FE model building method based on CT slices with the combination of image segmentation and aggregate adhesion segmentation methods based on MATLAB platform.The ?Milky-2? super computer is applied to carry out the uniaxial creep tests for the micromechanical FE model of asphalt mixture with different sizes of element.This paper tries to quantitatively analyze the size effect of voxel in the micromechanical FE model on the test results by comparing the creep stiffness modulus of different voxel sizes.The analysis results show that for creep numerical simulation,the voxel size has little effect on the creep results when the body length is less than 1mm and the numerical model of 1mm voxle size is recommended as an analytical model.The incremental form of three-dimensional viscoelastic constitutive model of asphalt mastic is derived from the viscoelastic constitutive equations by using an incremental approach.The user material subroutine(UMAT)program is compiled by using FORTRAN programming platform.The viscoelastic parameters can be obtained indirectly by using simple and easy to realize creep tests to avoid direct determination of the Poisson's ratio of the viscoelastic material which depends on the time and temperature of the load.The asphalt mastic viscoelastic constitutive model is verified through the relevant experiments.The numerical simulation results of compression and bending tests under simple loading paths are compared with the experimental results.The analysis shows that the UAMT of the three-dimensional viscoelastic constitutive model of asphalt mastic and the method of determining the viscoelastic parameters are correct and effective.The model can accurately predict the mechanical behavior of asphalt mastic.According to the three-dimensional numerical model building method of asphalt mixture,numerical simulation triaxial creep test under 2 kinds of confining pressure and 3 kinds of axial compression boundary condition are carried out with the combination of the compiled UMAT program.The numerical model building method is verified to be effective and the scope of the use for this FE model is determined by comparing the experimental results with the numerical simulation results of triaxial creep test.