Zhiwei Qian

3D MODELING OF FRACTURE IN CEMENT-BASED MATERIALS

In this article, a 3D lattice model is presented to simulate fracture in cement-based materials. In the paper, two applications are shown. The first application is modeling heterogeneous materials containing particle embedded in a matrix. A method is shown for coupling 3D information on the material structure obtained with CT-scanning to the material properties in the model. In the second application, fracture in fiber cement-based materials is modeled. Fibers are explicitly implemented as separate elements connected to the cement matrix via special interface elements. With the model, multiple cracking and ductile global behavior are simulated of the composite material. Variables in the model are the fiber dimensions and properties, the fiber volume in the composite, the bond behavior of fibers and matrix, and the cement matrix properties. These properties can be obtained by testing. Some examples of tests are given in the paper. The model can be used as a design tool for creating fiber (cement-based) composites with any desired mechanical behavior.

3D Lattice Fracture Model: Application to Cement Paste at Microscale

The fracture processes in cement paste at microscale are simulated by the 3D lattice fracture model based on the microstructure of hydrating cement paste. The uniaxial tensile test simulation is carried out to obtain the load-displacement diagram and microcracks propagation for a Portland cement paste specimen in the size of 100×100×100 µm3 at the degree of hydration 69%. The Youngs modulus, tensile strength, strain at peak load and fracture energy are computed on the basis of the load-displacement diagram.

3D Lattice Fracture Model: Theory and Computer Implementation

The lattice fracture model is presented in this paper, which is intended to simulate the fracture processes in multiphase materials to obtain the mechanical behavior in terms of load-displacement diagram and the cracks propagation. The basic procedures of lattice fracture analysis is that imposing a prescribed displacement on a lattice structure, finding the critical lattice element with the highest stress/strength ratio, removing it from the system and repeating until the system fails globally. One of the challenges in computer implementation of 3D lattice fracture model is the huge demand for computer memory. Matrix free technique is adopted to solve this problem.

Modeling Framework for Fracture in Multiscale Cement-Based Material Structures

Multiscale modeling for cement-based materials, such as concrete, is a relatively young subject, but there are already a number of different approaches to study different aspects of these classical materials. In this paper, the parameter-passing multiscale modeling scheme is established and applied to address the multiscale modeling problem for the integrated system of cement paste, mortar, and concrete. The block-by-block technique is employed to solve the length scale overlap challenge between the mortar level (0.1–10 mm) and the concrete level (1–40 mm). The microstructures of cement paste are simulated by the HYMOSTRUC3D model, and the material structures of mortar and concrete are simulated by the Anm material model. Afterwards the 3D lattice fracture model is used to evaluate their mechanical performance by simulating a uniaxial tensile test. The simulated output properties at a lower scale are passed to the next higher scale to serve as input local properties. A three-level multiscale lattice fracture analysis is demonstrated, including cement paste at the micrometer scale, mortar at the millimeter scale, and concrete at centimeter scale.

Numerical Modelling of the Effect of Filler/Matrix Interfacial Strength on the Fracture of Cementitious Composites

The interface between filler and hydration products can have a significant effect on the mechanical properties of the cement paste system. With different adhesion properties between filler and hydration products, the effect of microstructural features (size, shape, surface roughness), particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system is supposed to be different, as well. In order to understand the effect of the microstructural features, particle distribution and area fraction of filler on the fracture behavior of a blended cement paste system with either strong or weak filler-matrix interface, microscale simulations with a lattice model are carried out. The results show that the strength of the filler-matrix interface plays a more important role than the microstructural features, particle distribution and area fraction of filler in the crack propagation and the strength of blended cement paste. The knowledge acquired here provides a clue, or direction, for improving the performance of existing fillers. To improve the performance of fillers in cement paste in terms of strength, priority should be given to improving the bond strength between filler particles and matrix, not to modifying the microstructural features (i.e., shape and surface roughness) of the filler.