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PhD Candidate of Theoretical and Applied Mechanics

Northwestern University



2145 Sheridan Road
Tech A323
Evanston, IL 60208-3109

Email Weixin Li



Weixin worked on micromechanical modeling and experimental characterization of the fracture and damage of anisotropic quasi-brittle materials including shale rocks and carbon-polymer composites. He received Master's and Bachelor's degrees in Engineering Mechanics from Xi'an Jiaotong University in China.

Cusatis Group Research


Computational and Experimental Studies on the Anisotropic Mechanical Behaviors of Shale

Organic-rich shale and other shale-like rocks are highly complex, anisotropic, and heterogeneous materials that can be characterized by several levels of hierarchy. In this work, an integrated computational and experimental study was conducted to understand and characterize the anisotropic elasticity, fracture, and failure of shale. 

A micromechanical framework based on the Lattice Discrete Particle Model (LDPM) is formulated to capture the features of shale anisotropy and heterogeneity at multiple length scale. The model adopts an ‘a priori’ discretization and simulates shale at the level of the major heterogeneities (grains in this case). In addition, material anisotropy is captured by considering sources of anisotropy at different length scales, such as partial alignment of anisotropic clay minerals and naturally formed bedding planes. A multiscale framework is completed by utilizing a mathematical homogenization approach based on the asymptotic expansion of field variables to upscale the proposed micromechanical model.

A series of experiments are performed to investigate shale strength, deformability, and fracturability. Seismic velocity measurement, uniaxial compression, direct tension, and Brazilian tests are conducted on the Marcellus shale specimens in various bedding plane orientations to study the mechanical properties and their anisotropy. Finally, fracture tests are conducted on the specimens of increasing size and different principal notch orientations. It is found that there exists a significant size effect on the fracture properties calculated by using the Linear Elastic Fracture Mechanics (LEFM) theory. The fracture properties calculated by using Bazant's Size Effect Law are independent of the testing method and are found to be anisotropic.

Discrete Poromechanics and Fracture-Permeability Behaviors of Rocks

The modeling of porous materials like concrete and rock usually involves multiple, interacting physical processes (multiphysics), such as thermal-hydraulic-mechanical coupling. The Lattice Discrete Particle Model (LDPM) is reformulated in a poromechanics setting by adopting two coupled dual lattices simulating mechanical and transport behaviors, respectively. The proposed multiphysics modeling framework is capable of simulating complex poromechanics couplings in various applications including concrete durability analyses and rock hydraulic fracturing.

The permeability and mechanical behaviors of fracture-damaged shale is investigated numerically through the proposed multiphysics solver. The proposed numerical model for rock discretizes the equivalent two-phase porous medium into two networks of lattice elements: the former connects rock grains to mimic the solid phase and the latter represents the intergranular fluid network to reproduce the fluid flow conductivity. Fluid flow along intergranular pores and cracks is simulated at the length scale of a grain. As the model is capable of simulating crack and damage initiation, the transport properties are assumed to evolve with progressive damage locally. The numerical model is utilized to simulate the direct shear triaxial experiments conducted at LANL in order to understand the fracture-permeability behaviors of shale . 

Fracture and Failure of 3D Textile Carbon-Polymer Composites

Carbon-polymer textile composites are widely used in various industries due to their outstanding performance, such as high strength-to-weight ratio, superior fracture and impact resistances. It is essential to characterize and understand the mechanical behaviors, especially fracturing damage and failure behaviors as well as the scaling characteristics, of the materials to facilitate their engineering applications, and to develop numerical models in order to assist material design.

A thorough experimental campaign is conducted to characterize the elastic, inelastic, and fracture behaviors of 3D woven composites. Due to the complex mesostructure, their mechanical responses are complicated and are featured by orthotropic stiffness, pre-peak nonlinearity, complex failure envelopes, post-peak softening, and structural size effect. In particular, the fracture properties and the scaling of the mechanical properties associated of the quasi-brittle character are obtained from either direct testing of graduate postpeak softening of fracture specimens stabilized by proper design of grips or size effect method. In addition, a constitutive model based on microplane theory is formulated to capture these features of the materials. 

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