Multiscale Numerical Study Of Concrete Cracking Process Based On Lattice Model

Concrete is a composite of multiphase heterogeneous materials, its cracking process and internal structure are inseparable.For many years, a large number of domestic and foreign researchers are working on cracking process of cement-based materials, and have raised a lot of cracks model, but stayed at the macro-level research mostly, there is no depth to the material microstructure, and it is difficult to give a dynamic crack cracking process.In response to these issues, this paper will combine multiscale methods with lattice fracture model. First a Python program algorithm is developed to track the real-time dynamic process of crack width,completed the lattice fracture analysis of cement paste at micro-scale, and then the put and place model for meso-scale random aggregate,the author proposes that using HYMOSTRUC3D quickly generate a three-dimensional model of random spherical aggregates model to study the cracking process of mortar and concrete at meso-scale. The main contents are as follows:The concept of randomness is adopted to quantitative analysis the nature of irregular and non-homogeneous of cement-based materials, and the influence of simulations of lattice fracture caused by randomness is analysised. The results show that: the randomness reflect the material internal phase nonuniform degree. With randomness increases, the elastic modulus of overall lattice model, tensile strength and peak strain were reduced.on the basic of the theory of lattice fracture model, crack width theoretical formula of lattice model is deduced, MATLAB and Python program are written, which are implanted in the commercial finite element software ABAQUS, therefore the crack width value changes of lattice models can be tracked under different stress levels. The basic idea of the program algorithm is: for each step of lattice fracture calculations, the element with maximum ratio σ/ft is not directly removed from the lattice system, but is set to crack element, the crack element is the element that the value of the modulus of elasticity is negligible compared to other lattice, ie the ratio of the elastic modulus for two tends to0. By calculating the value of the incremental of nodal displacement,then the exact deformation of crack element can be obtained. If the deformation of all the crack element in the database is recalculated each time, then, the tracking calculations of the crack width under different stress levels can be realized. Numerical results show that the algorithm can effectively track the dynamic process of crack width.In terms of random aggregate model, we use MATLAB programming for digital image processing, and reference the methods that F. Bernard uses NIST’s CEMHYD3D code generate mortar structure model,the author proposed using HYMOSTRUC to generate a three-dimensional model of random spherical aggregate model quickly. This solves the inefficiencies of random aggregate put and place model of the numerical simulation of concrete presently.Based on information-passing multi-scale modeling methods, the paper uses HYMOSTRUC3D model to generate cement paste microstructure lattice image, and discretized into the two-dimensional voxel-based image which is used for building lattice network model. The lattice system of cement paste at micro-scale is calculated according line elastic finite element theory. The obtained constitutive relation of cement paste at microscopic scale is used as input parameters of meso-scale lattice analysis of mortar, The lattice system of mortar at meso-scale is calculated according non-linear finite element theory. This non-linear is obtained by reducing the elastic modulus and the tensile strength after multi-step iterative.The stress-strain curve, crack location distribution, distribution of crack width under different stress levels of concrete are obtained. In particular, for the distribution map of crack location, the first cracks generated around the initial porosity, and then gradually extend to the interior main crack, the crack width distribution always exhibit a superposition of two normal distributions, the first normal mean means the micro crack width, and the number of micro-cracks mostly accounts for about50%; second indicates large crack width, and the number of cracks is associated with the stress level. Crack distribution map shows that: the mortar and concrete cracks are first generated at the interface transition zone and then developes in cement paste or mortar matrix.