A theoretical model on surface electronic behavior: Strain effect

Deformation from mechanical loading can affect surface electronic behavior. Surface deformation and electronic behavior can be quantitatively expressed using strain and work function, respectively, and their experimental relationship can be readily determined using the Kelvin probing technique. However, the theoretical correlation between work function and strain has been unclear. This study reports our theoretical exploration, for the first time, of the effect of strain on work function. We propose a simple electrostatic action model by considering the effect of a dislocation on work function of a one-dimensional lattice and further extend this model to the complex conditions for the effect of dislocation density. Based on this model, we established successfully a theoretical correlation between work function and strain.     

可用于建筑结构检测的分布式光纤形变片

本文在光时域反射计(OTDR)和光纤形变片的基础上,提出了一种分布式光纤传感器-分布式光纤形变片.该检测方法属于OTDR的损耗调制法,即通过测量粘贴于光纤形变片上的光纤弯曲损耗来获取测量点的应变量或位移量.作者分别在微位移架和悬臂梁进行了位移检测和应变检测,结果表明该分布式光纤形变片提供了一种可检测应变量与位移量的检测方法.值得注意的是,该分布式光纤形变片的传感网络采用时分复用的总线拓扑,能在一根光纤上同时检测多个检测点的变化.     

Y型熔锥光纤耦合器的应变响应

本文对已广泛应用于光纤通信或作为光纤传感系统中处理元件的Y型单模熔锥光纤耦合器对应变的响应进行了研究.在悬臂梁上进行的应变实验表明,该型耦合器的耦合比不但对应变敏感,而且有单调性.其中,值得注意的是Y型耦合器的应变响应灵敏度高于结构检测中常用的电阻应变片对应变的响应灵敏度.     

Investigation of a coated optical fiber strain sensor embedded in a linear strain matrix material

A theoretical model has been developed to study the mechanical behaviors of the interface between an embedded optical fiber with coating material and a linear strain matrix. The results show that the longitudinal stress and strain in the fiber optic sensor are different from that distributed in the host material and depend on the strain distribution and embedded length of the optical fiber as well as the material properties of the fiber coating. The distribution of interfacial shear strain between the coating and the glass fiber and the distribution of strain/stress of the glass fiber are given.     

Response Characteristics of Strain Sensors Based on Closely Spaced Nanocluster Films with Controlled Coverage

Flexible strain sensor devices were fabricated by depositing Pd nanoclusters on PET membranes patterned with interdigital electrodes. The sensors responded to the deformation of the PET membranes with the conductance changes of the nanocluster films and were characterized by both high gauge factor and wide detection range. The response characteristics of the strain sensors were found to depend strongly on the nanocluster coverage, which was attributed to the percolative nature of the electron transport in the closely spaced nanocluster arrays. By controlling the nanocluster deposition process, a strain sensor composed of nanocluster arrays with a coverage close to the effective percolation threshold was fabricated. The sensor device showed a linear response with a stable gauge factor of 55 for the applied strains from the lower detection limit up to 0.3%. At higher applied strains, a gauge factor as high as 200 was shown. The nanocluster films also demonstrated the ability to response to large deformations up to 8% applied strain, with an extremely high gauge factor of 3500.     

Strain effects on phase transitions in transition metal dichalcogenides

We perform density functional theory calculation to investigate the structural and electronic properties of various two-dimensional transition metal dichalcogenides, MX₂ (M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, or W, and X=S or Se), and their strain-induced phase transitions. We evaluate the relative stability and the activation barrier between the octahedral-T and the trigonal-H phases of each MX₂. It is found that the equilibrium and phase transition characteristics of MX₂ can be classified by the group to which its metal element M belongs in the periodic table. MX₂ with M in the group 4 (Ti, Zr, or Hf), forms an octahedral-T phase, while that with an M in the group 6 (Cr, Mo, or W) does a trigonal-H phase. On the other hand, MX₂ with M in the group 5 (V, Nb, or Ta), which is in-between the groups 4 and 6, may form either phase with a similar stability. It is also found that their electronic structures are strongly correlated to the structural configurations: mostly metallic in the T phase, while semiconducting in the H phase, although there are some exceptions. We also explore the effects of an applied stress and find for some MX₂ materials that the resultant strain, either tensile or compressive, may induce a structural phase transition by reducing the transition energy barrier, which is, in some cases, accompanied by its metal-insulator transition.     

Three-dimensional characterization of strain localization bands in high-resolution elastoplastic polycrystals

In crystalline materials, the experimental observation of the localization of plastic strains in particular directions is generally restricted to the surface of a sample containing some hundreds of grains, because of the difficulties underlying microstructural analysis. In these conditions, the determination of the morphological characteristics of localization can be limited by the poor statistical representativity of the domain of observation. The purpose of this work is to extend the analysis of localization – localization bands or else – to the 3D elastoplastic strain fields of a high-resolution representative volume element of a polycrystal.     

Formulation of strain gradient plasticity with interface energy in a consistent thermodynamic framework

In this work, the strain gradient formulation is used within the context of the thermodynamic principle, internal state variables, and thermodynamic and dissipation potentials. These in turn provide balance of momentum, boundary conditions, yield condition and flow rule, and free energy and dissipative energies. This new formulation contributes to the following important related issues: (i) the effects of interface energy that are incorporated into the formulation to address various boundary conditions, strengthening and formation of the boundary layers, (ii) nonlocal temperature effects that are crucial, for instance, for simulating the behavior of high speed machining for metallic materials using the strain gradient plasticity models, (iii) a new form of the nonlocal flow rule, (iv) physical bases of the length scale parameter and its identification using nano-indentation experiments and (v) a wide range of applications of the theory. Applications to thin films on thick substrates for various loading conditions and torsion of thin wires are investigated here along with the appropriate length scale effect on the behavior of these structures. Numerical issues of the theory are discussed and results are obtained using Matlab and Mathematica for the nonlinear ordinary differential equations (NODE) which constitute the boundary value problem.This study reveals that the micro-stress term has an important effect on the development of the boundary layers and hardening of the material at both hard and soft interface boundary conditions in thin films. The interface boundary conditions are described by the interfacial length scale and interfacial strength parameters. These parameters are important to control the size effect and hardening of the material. For more complex geometries the generalized form of the boundary value problem using the nonlocal finite element formulation is required to address the problems involved.     

应变-临界电流测量系统中超导线应变分布及临界电流分析

Based on Pacman device which is widely used to investigate the axial strain dependence of the critical current in superconductors, the finite element analysis method is employed to carry out the force analysis of the spring and the superconducting strand, thereby the axial and lateral strain distributions of the superconducting strand are obtained. According to the two extreme assumptions (low inter-filament resistance and high inter-filament resistance), the effects of the strain homogeneity at the cross section of the superconductor on the critical current is analyzed combined with the Nb₃Sn deviatoric strain-critical current scaling law.     

Analysis of axial strain in one-dimensional loading by different models

Different phenomenological equations based on plasticity, primary creep （as a viscoplastic mechanism）, secondary creep （as another viscoplastic mechanism） and different combinations of these equations are presented and used to describe the material inelastic deformation in uniaxial test. Agreement of the models with experimental results and with the theoretical concepts and physical realities is the criterion of choosing the most appropriate formulation for uniaxial test. A model is thus proposed in which plastic deformation, primary creep and secondary creep contribute to the inelastic deformation. However, it is believed that the hardening parameter is composed of plastic and primary creep parts. Accordingly, the axial plastic strain in a uniaxial test may no longer be considered as the hardening parameter. Therefore, a proportionality concept is proposed to calculate the plastic contribution of deformation.