Jian-Shan Wang

Twisting of nanowires induced by anisotropic surface stresses

Many natural and synthetic quasi-one-dimensional materials are of helical or twisting shape and understanding the physical mechanisms underlying the asymmetric shape is of both theoretical and technological significances. In this letter, we pointed out that anisotropic surface stresses present as a possible reason for the formation of some micro-/nanohelices. Using Gurtin’s theory of surface elasticity, we quantitatively investigated the twisting deformation of nanowires due to anisotropic surface stresses. The present model can also elucidate the formation of some other helical materials at micro- and nanoscales, e.g., twisting lamellae in polymer spherulites, spiraled bacteria, and flagella.

Symplectic model for piezoelectric wedges and its application in analysis of electroelastic singularities

In this paper, a symplectic model, based on the Hamiltonian system, is developed for analyzing singularities near the apex of a multi-dissimilar piezoelectric wedge under antiplane deformation. The derivation is based on a modified Hellinger–Reissner generalized variational principle or a differential equation approach. The study indicates that the order of singularity depends directly on the non-zero eigenvalue of the proposed Hamiltonian operator. Using the coordinate transformation technique and continuity conditions on the interface between two dissimilar materials, the orders of singularity for multi-dissimilar piezoelectric and piezoelectric–elastic composite wedges are determined. Numerical examples are considered to show potential applications and validity of the proposed method. It is found that the order of singularity also depends on the piezoelectric constant, in addition to the geometry and shear modulus.

Surface effects on the elasticity of nanosprings

Nanosprings made of metallic or semiconducting materials hold promise for a wide range of important applications. In this letter, we extend the classical Kirchhoff rod model by incorporating surface effects on the mechanical response of quasi–one-dimensional nanomaterials. The refined model is then employed to derive the elastic constants of a nanospring by accounting for the effect of surface elasticity and residual surface stresses. The results demonstrate that the stiffness of nanosprings may exhibit remarkable dependence on their cross-sections sizes. This study is helpful not only for understanding the size-dependent behavior of nanosprings but also for their applications in micro/nano-electro-mechanical systems.

Surface effects on the superelasticity of nanohelices

Helical nanomaterials with superelasticity have a wide range of promising applications in micro-/nanoelectromechanical systems. Based on the theory of surface elasticity, we present a nonlinear rod model to investigate the superelasticity of nanohelices. Our results demonstrate that the superelasticity of nanohelices exhibits a distinct size dependence due to the increased ratio of surface area to volume. The superelasticity can effectively enhance the efficiency of energy storage and retrieval of nanohelices. This study is helpful for the characterization of the mechanical properties of nanosized helical materials and the optimal design of nanohelix-based devices.

Twin-intersection-related nanotwinning in a heavily deformed gamma-TiAl-based alloy

Nanotwins related to the type-I twin intersection in gamma-TiAl have been investigated by high-resolution electron microscopy. A triangular striated region can be formed around the twin intersection. Some nanotwins were observed to nucleate through interfacial dislocation reactions on the incoherent twin boundary of the triangular striated region. The nanotwins, which propagate by a twinning dislocation homogeneous glide mechanism, are mostly limited to the triangular striated region.

A damage mechanics model for twisted carbon nanotube fibers

Carbon nanotube fibers can be fabricated by the chemical vapor deposition spinning process. They are promising for a wide range of applications such as the building blocks of high-performance composite materials and micro-electrochemical sensors. Mechanical twisting is an effective means of enhancing the mechanical properties of carbon nanotube fibers during fabrication or by post processing. However, the effects of twisting on the mechanical properties remain an unsolved issue. In this paper, we present a two-scale damage mechanics model to quantitatively investigate the effects of twisting on the mechanical properties of carbon nanotube fibers. The numerical results demonstrate that the developed damage mechanics model can effectively describe the elastic and the plastic-like behaviors of carbon nanotube fibers during the tension process. A definite range of twisting which can effectively enhance the mechanical properties of carbon nanotube fiber is given. The results can be used to guide the mechanical twisting of carbon nanotube fibers to improve their properties and help optimize the mechanical performance of carbon nanotube-based materials.

Debonding criterion for the piezoelectric fibre push-out test

A theoretical model of piezoelectric fibre push-out is developed within the framework of a modified shear-lag theory to study stress and electrical field transfer. Based on the model, a debonding criterion for the piezoelectric effect is presented utilizing a fracture-mechanics approach to investigate the debonding process of piezoelectric fibre in the push-out test for combined electrical and mechanical loading, and for mechanical loading only. A numerical example is considered to verify the proposed debonding criterion. The results show that stress and electrical fields can be controlled by the interface. The study also shows that interfacial properties and piezoelectric coefficients have a significant influence on the debonding process of piezoelectric fibre composites, and the energy-release rate can be enhanced or reduced depending on the direction of electrical loading, which shows that the energy-release rate can be used as the debonding criterion.

Helical fiber pull-out in biological materials

Many biological materials, such as wood and bone, possess helicoid microstructures at microscale, which can serve as reinforcing elements to transfer stress between crack surfaces and improve the fracture toughness of their composites. Failure processes, such as fiber/matrix interface debonding and sliding associated with pull-out of helical fibers, are responsible mainly for the high energy dissipation needed for the fracture toughness enhancement. Here we present systemic analyses of the pull-out behavior of a helical fiber from an elastic matrix via the finite element method (FEM) simulation, with implications regarding the underlying toughening mechanism of helicoid microstructures. We find that, through their uniform curvature and torsion, helical fibers can provide high pull-out force and large interface areas, resulting in high energy dissipation that accounts, to a large extent, for the high toughness of biological materials. The helicity of fiber shape in terms of the helical angle has significant effects on the force-displacement relationships as well as the corresponding energy dissipation during fiber pull-out.

Theoretical and computational modeling of clustering effect on effective thermal conductivity of cement composites filled with natural hemp fibers

This paper investigates the effects of clustering on the effective transverse thermal conductivity of unidirectional cement composites filled with natural hemp fibers. A typical clustering pattern with four hemp fibers embedded into cement matrix is designed as the representative two-dimensional unit cell, which is taken from the periodic cement composite under consideration, and a clustering degree parameter is introduced to adjust the distance between clustered fibers. For this heterogeneous two-component composite model, distributions of the heat flux component are obtained using finite element simulation for various clustering cases involving different global fiber volume concentrations, clustering degree parameters, and thermal conductivity of both fiber and matrix, to evaluate the effective thermal conductivity of the composite. To further reveal the effects caused by clustered fibers, a random cluster pattern of hemp fibers in the unit cell is considered for comparison with the present regular clustering pattern. Further, a simple theoretical model with specified flexible factor f is developed by matching the theoretical and numerical predictions.

Singularity Analysis of Electro-mechanical Fields in Angularly Inhomogeneous Piezoelectric Composites Wedges

An analytical solution is presented to study the singularity behavior of electroelastic fields in a wedge with angularly graded piezoelectric material (AGPM) and under anti-plane deformation. The analysis is based on the mixed-variable state space formulation developed in this paper. The characteristic equation containing the singular order is derived using the method of variable separation. The results presented demonstrate the effects of the angular variation of material properties on the singularities of the AGPM wedge systems. The analysis indicates that the material inhomogeneity degree η can be used to control the singularities of AGPM wedge systems.