My research topics are mostly in the field of multi-physics and nonlinear soft solid mechanics. Utilizing a mix of theory, simulations, and experiments, I mainly focus on the dynamic behaviors of (soft) intelligent materials and structures as well as their stability. My works have been funded by grants awarded by the European Union, the Irish Research Council, the Chinese Science Council, Politecnico di Torino and Zhejiang University. The main research topics that I am specialized within are as follows: • Dynamics/Stability in Soft Solids • Tunable phononic crystals/metamaterials • Fluid-Structure Interaction • Refined Finite Element Methods • Elastodynamics in Smart Nanostructures
Dynamics/Stability in Soft Solids
• Linearized waves and vibrations in soft electro-active structures
Soft electro-active (SEA) materials with superior properties have found broad applications such as actuators, sensors, energy harvesters, biomedical devices and flexible electronics. However, some key issues still exist: (i) Biasing fields (e.g. pre-stretch and biasing electric field) affect the performance of a multi-functional system. How do we characterize the material properties and monitor the biasing field states in soft structures? (ii) Electric stimuli modify the effective material properties and geometrical configurations, and thus change the vibration and wave characteristics of SEA materials. Could we exploit the behaviors to design some useful devices?
Using the State Space Method, we have studied the effects of biasing fields and material properties on the elastic waves in SEA structures in three scientific articles: Use of circumferential waves for the structural health monitoring and for the self-sensing of actual states of SEA tube actuators; Utilization of axisymmetric waves to characterize the pressurized functionally graded elastomeric cylinders; Design of electrostatically tunable spherical resonant devices.
• Bending instability in soft electro-active structures
Applying the moments on the lateral surfaces, we can bent an elastic slab into a cylindrical sector, which is referred to as finite bending deformation. Experiments indicate that wrinkles and creases will appear on the compressed surface of a bent rubber slab, i.e., the so-called bending instability happens. Similarly, finite bending deformation commonly occurs in devices based on SEA materials, such as strain sensor, dielectric gripper and soft electronic robots. Thus, problems arise naturally: (i) How does the applied voltage affect the finite bending deformations of dielectric-elastomeric (DE) slabs? (ii) Under what conditions will the bending instability occur for the dielectric case? What is the instability pattern?
Employing the Surface Impedance Matrix Method, we have conducted the theoretical analyses of finite bending deformation and pattern evolution of the related instability for a DE slab and a DE bilayer subject to electric voltage and mechanical loads. We found that both circumferential and axial instabilities cohabit to create two-dimensional patterns on the inner surface of the bent sector. If required, the wrinkles can be designed to appear on either the inner or outer surface of the buckled bilayer.
Tunable PCs/MMs
Phononic crystals (PCs) and metamaterials (MMs) are artificial composites comprised of two or more constituent materials arranged in a periodic lattice. Unlike natural materials, PCs/MMs offer unique dynamic properties including band gaps, wave focusing, nonreciprocal wave propagation, wave guides, and topological protected wave modes; thus leading to promising applications such as acoustic filters, vibration and noise isolators, sensors, energy harvesters, and artificial lenses.
However, most conventional PCs/MMs operate at fixed frequency ranges once the material and geometric parameters are specified. Active and even smart control of the geometry and material properties of PCs/MMS by external stimuli can enable switchable dynamic behaviors of acoustic/elastic waveguides, which can broaden their application space in engineering.
Fluid–structure interaction (FSI) is fundamental to the flow characteristics induced by the structural motion in fluids and also is crucial to the performance of submerged structures. However, some interesting issues constitute our study motivations: (i) The solid physical model generalizes the isotropy of structures to the inhomogeneity and/or anisotropy; (ii) The fluid models are required to transform from Newtonian fluids to non-Newtonian cases: One of the driving forces is related to the situations where non-Newtonian fluids with high viscoelasticity are present, while another one deals with acoustic vibrations of nanoparticles or viruses with high resonant frequencies (GHz or even THz); (iii) The experimentally measured quality factors of different nanoparticles are considerably lower than the theoretical predictions, implying the presence of additional damping mechanisms, such as the intrinsic damping in the solid particles.
By introducing appropriate potential functions and employing the State Space Method, we have established a unified mathematical model for the multilayered hollow spheres submerged in complex fluids to account for the effects of various factors mentioned above on the FSI vibration behaviors. We theoretically proved the experimental results and our original scientific paper has been published in Journal of Fluid Mechanics.
Refined Finite Element Methods
Highly flexible composite structures, prone to suffering large-deflection and post-buckling, have been successfully used in a number of scenarios. Accurate predictions of stress distributions (especially the interlaminar stresses) in the geometrically nonlinear analysis are challenging but essential for their design and failure evaluation.
Within the framework of Carrera Unified Formulation (CUF) and based on a total Lagrangian approach, we have developed the unified formulations of geometrically nonlinear refined beam, plate and shellmodels for flexible composite structures. We have also derived the unified theory of slightly compressible elastomeric structures including geometrical and physical nonlinearities by using the well-known nonlinear hyperelasticity theory.
The geometrically nonlinear CUF models possess some unique advantages: (i) Accurate predictions of the nonlinear responses such as large-deflection, post-buckling and snap-through/snap-back; (ii) The nonlinear governing equations are expressed by the Fundamental Nuclei in a unified manner, enabling to automatically adjust the model efficiency/efficacy depending on the complexity of the problem; (iii) The higer-order model can accurately predict the nonlinear internal stress states of composite structures; (iv) Owing to the scalable behaviors, the full geometrically nonlinear CUF model can be conveniently harnessed to assess the effectiveness of different geometrically nonlinear strain approximations.
Elastodynamics ​in Smart Nanostructures​
Nanostructures have attracted much attention due to their applications in high-sensitive and high-frequency nanodevices such as MEMS/NEMS. In contrast to the macroscopic case, experiments and simulations indicate that the properties and responses of nanostructures are mostly size-dependent. One logical reasoning is the surface effect that considers the difference between the properties of a surface and its bulk. How could we mathematically model the surface effect in nanostructures?
Based on the state-space formalism and thin-layer model, we have developed the theory of surface/interface elasticity, which incorporates the coupling among multiple physical fields (e.g., elastic, electric, and magnetic). In addition, the residual fields are also included into the surface/interface elasticity. For isotropic materials, the present theory recovers the classical Gurtin-Murdoch surface elasticity if the thickness effect is dropped.