RESEARCH
III. Fracture Mechanics of Soft Materials
The fracture of soft materials involves large deformations and crack blunting before the crack can propagate. Consequently, the stress field close to the crack tip is significantly different from the traditional formulation encountered in the linear elastic fracture mechanics. Therefore, fracture analysis for these applications requires special attention.
Past & current achievements
The projects that I participate in mainly focus on crack propagation in soft materials that have specific thermal-mechanical behaviors, such as temperature-dependent viscoelasticity, Mullins effect, and soft elasticity. The aim of the projects is to uncover the rules that govern crack growth and provide guidance for fracture resistance design, which usually needs to relate the fracture theory concerning cracks with the constitutive theory concerning materials. Even though the projects are ongoing, I have got some achievements:
Dynamic effect in the fatigue fracture of viscoelastic solids
Crack growth in viscoelastic solids under cyclic loading tends to be faster than that under static loading with the same amplitude. This phenomenon, known as the “dynamic effect”, is a key mechanism underlying the fatigue fracture of soft viscoelastic polymers, but its physical nature remains a mystery. I developed a scaling theory to delineate how viscoelastic dissipation associated with crack growth is governed by the coupling between three time-dependent processes: cyclic loading, crack growth, and viscoelastic creep. In the limit of slow crack growth and slow cyclic loading, a simple integral equation was derived to predict the crack growth velocity under different cyclic loading frequencies.
Programmable response of liquid crystal elastomers under thermo-electro-mechanical coupling: A numerical approach
By incorporating electromechanics and thermodynamics within the framework of continuum theory, I proposed a feasible numerical approach to study the programmable response of liquid crystal elastomers under thermo-electro-mechanical coupling. The numerical approach adopts a quasi-convexification of the free energy function, which effectively avoids multi-solution problems that may occur when dealing with potential microstructures related to the local soft elasticity of liquid crystal elastomers. In addition to giving some theoretical solutions, the numerical approach was implemented in a finite element program by coding a subroutine. Some simulations were carried out to verify the effectiveness and demonstrate the capacity of the calculation program. This work makes it possible to study the fracture mechanics of liquid crystal elastomers by means of numerical simulation.
Fracture and crack propagation in liquid crystal elastomers
Liquid crystal elastomers have been recently explored extensively to make diverse active structures and devices. The capability of predicting their rupture under different loading conditions is crucial for the applications. In this work, I measured experimentally the fracture toughness and the crack propagation rate by means of pure shear fracture tests. According to the results, multi-scale analyses were performed to reveal the fracture mechanisms involving the polydomain-to-monodomain transition and the mesogen direction rotation. Now, I am trying to formulate an analytical model, as well as a calculation approach, to predict crack propagation under static and fatigue loadings. Besides, some instabilities associated with random defects and rate-dependent effects were studied.
Future research plans
My future research will be conducted continually around the theme of revealing fracture mechanisms and guiding material and structure design. Several potential studies are listed below:
To reveal how stretch-induced crystallization affects crack propagation. Even if the fracture resistance has been verified, the mechanisms are still ambiguous. This study will focus on how the stretch-crystallization alters the local fields near the crack tip and enhances the intrinsic fracture toughness and how the two effects work together to restrict crack growth.
To develop a multi-scale calculation model to study the coupling between strain-induced damage and crack propagation. So far, all the studies consider the damage and fracture in a separative way. But it is evident that the two behaviors are simultaneous and coupled with each other. The calculation model will integrate the micromechanics-based constitutive theory and the phase field theory. The simulation study will help us to understand the damage effect on the fracture, especially when the strain-induced damage is anisotropic due to the orientation of polymer chains.
To explore optimized strategies for the design of high-toughness and fatigue-resistant composite materials. Several studies have shown that a composite of two stretchable materials with large modulus contrast and strong adhesion can achieve a much tougher performance with more fatigue resistance than the constituents. But from the viewpoint of structure design, there should be many strategies that can achieve the same objective, such as incorporating fillers or fibers and configuring the constituents with different viscosity or toughness.
To formulate a theory for the analysis of the fatigue crack propagation in the materials possessing both damage and viscosity. The dynamic effect in viscoelastic solids has been studied in my previous work. However, the Mullins effect, as a prevalent damage phenomenon, has not been introduced. This history-dependent effect should make a difference in the dynamic behavior of fatigue fracture since the damage-induced changes in material behaviors are irreversible and the mechanical response cannot be repeated under cyclic loading.
Simulation of fracture in vascular tissue. The fracture of vascular tissue, and load-bearing soft tissue in general, is relevant to various biomechanical and clinical applications, from the study of traumatic injury and disease to the design of medical devices and the optimization of patient treatment outcomes
