Finite Boundary Effects in Problem of a Crack Perpendicular to and Terminating at a Bimaterial Interface

In this paper, the problem of a crack perpendicular to and terminating at an interface in bimaterial structure with finite boundaries is investigated. The dislocation simulation method and boundary collocation approach are used to derive and solve the basic equations. Two kinds of loading form are considered when the crack lies in a softer or a stiffer material, one is an ideal loading and the other one fits to the practical experiment loading. Complete solutions of the stress field including the T stress are obtained as well as the stress intensity factors. Influences of T stress on the stress field ahead of the crack tip are studied. Finite boundary effects on the stress intensity factors are emphasized. Comparisons with the problem presented by Chen et al. (Int. J. Solids and Structure, 2003, 40, 2731-2755) are discussed also.

Interface cracking and particulate size effect in particle-reinforced metal-matrix composites

The mechanical behaviors of the ceramic particle-reinforced metal matrix composites are modeled based on the conventional theory of mechanism-based strain gradient plasticity presented by Huang et al. Two cases of interface features with and without the effects of interface cracking will be analyzed, respectively. Through comparing the result based on the interface cracking model with experimental result, the effectiveness of the present model can be evaluated. Simultaneously, the length parameters included in the strain gradient plasticity theory can be obtained.

Studies of electron-transfer and charge-transfer coupling processes at a liquid/liquid interface by double-barrel micropipet technique

Investigation of a heterogeneous electron-transfer (ET) reaction at the water/1,2-dichloroethane interface employing a double-barrel micropipet technique is reported. The chosen system was the reaction between Fe(CN)(6)(3-) in the aqueous phase (W) and ferrocene in 1,2-dichloroethane (DCE). According to the generation and the collection currents as well as collection efficiency, the ET-ion-transfer (IT) coupling process at such an interface and competing reactions with the organic supporting electrolyte in the organic phase can be studied. In addition, this technique has been found to be an efficient method to distinguish and measure the charge-transfer coupling reaction between two ions (IT-IT) processes occurring simultaneously at a liquid/liquid interface. On this basis, the formal Gibbs energies of transfer of some ions across the W/DCE interface, such as NO3-, NO2-, Cl-, COO-, TBA(+), IPAs+, Cs+, Rb+, K+, Na+, and Li+, for which their direct transfers are usually difficult to obtain because of the IT-IT coupling processes, were quantitatively evaluated.

Effects of solution viscosity on heterogeneous electron transfer across a liquid/liquid interface

Scanning electrochemical microscopy (SECM) is employed to investigate the effect of solution viscosity on the rate constants of electron transfer (ET) reaction between potassium ferricyanide in water and 7,7,8,8-tetracyanoquinodimethane (TCNQ) in 1,2-dichloroethane. Either tetrabutylammonium (TBA(+)) or ClO4- is chosen as the common ion in both phases to control the interfacial potential drop. The rate constant of heterogeneous ET reaction between TCNQ and ferrocyanide produced in-situ, k(12), is evaluated by SECM and is inversely proportional to the viscosity of the aqueous solution and directly proportional to the diffusion coefficient of K4Fe(CN)(6) in water when the concentration of TCNQ in the DCE phase is in excess. The k(12) dependence on viscosity is explained in terms of the longitudinal relaxation time of the solution. The rate constant of the heterogeneous ET reaction between TCNQ and ferricyanide, k(21), is also obtained by SECM and these results cannot be explained by the same manner.

Charge transfer across the water/nitrobenzene interface by three-electrode system

A droplet of aqueous solution containing a certain molar ratio of redox couple is first attached onto a platinum electrode surface, then the resulting drop electrode is immersed into the organic solution containing very hydrophobic electrolyte. Combined with reference and counter electrodes, a classical three-electrode system has been constructed, Ion transfer (IT) and electron transfer (ET) are investigated systematically using three-electrode voltammetry. Potassium ion transfer and electron transfer between potassium ferricyanide in the aqueous phase and ferrocene in nitrobenzene are observed with potassium ferricyanide/potassium ferrocyanide as the redox couple. Meanwhile, the transfer reactions of lithium, sodium, potassium, proton and ammonium ions are obtained with ferric sulfate/ferrous sulfate as the redox couple. The formal transfer potentials and the standard Gibbs transfer energy of these ions are evaluated and consistent with the results obtained by a four-electrode system and other methods.

Study of electron-transfer reactions across an externally polarized water/1,2-dichloroethane interface by scanning electrochemical microscopy

A novel method to study electron-transfer (ET) reactions between ferrocene in 1,2-dichloroethane (DCE) and a redox couple of K3Fe(CN)(6) and K4Fe(CN)(6) in water using scanning electrochemical microscopy (SECM) with a three-electrode setup is reported. In this work, a water droplet that adheres to the Surface of a platinum disk electrode is immersed in a DCE solution. The aqueous redox couple serves both as a reference electrode on the platinum disk and as an electron donor/acceptor at the polarized liquid/liquid inter-face. With the present experimental approach, the liquid/liquid interface can be polarized externally, while the electron-transfer reactions between the two phases can be monitored independently by SECM. The apparent heterogeneous rate constants for the ET reactions were obtained by fitting the experimental approach curves to the theoretical values. These rate constants obey the Butler-Volmer theory i.e., them, are found to be potential dependent.

Study of electron transfer across the liquid/ice-like matrix interface by scanning electrochemical microscopy

In this work, we report the findings of a study on scanning electrochemical microscopy (SECM) to investigate the interfacial electron-transfer (ET) reaction between the 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ(.-)) in 1,2-dichloroethane and ferricyanide in an ice-like matrix (a mixture of insulting ice and conductive liquid) under low temperatures. Experimental results indicate that the formed liquid/ice-like matrix interface is superficially similar in electrochemical characteristics to a liquid/liquid interface at temperatures above -20 degreesC. Furthermore, imaging data show that the surface of the ice-like matrix is microscopically flat and physically stable and can be applied as either a conductive or an insulting substrate for SECM studies. Perchlorate ion was selected as the common ion in both phases, the concentrations of which controlled the interfacial potential difference. The effect of perchlorate concentration in the DCE phase on interfacial reactions has been studied in detail. The apparent heterogeneous rate constants for TCNQ(.-) oxidation by Fe(CN)(6)(3-) in another phase under different temperatures have been calculated by a best-fit analysis, where the experimental approach curves are compared with the theoretically derived relationships. Reaction rate data obey Butler-Volmer formulation before and after the freezing point, which is similar to most other known cases of ET reactions at liquid/liquid interfaces. However, there is a sharp change observed for heterogeneous rate constants around the freezing point of the aqueous phase, which reflects the phase transition. At temperatures below -20 degreesC, surface-confined voltammograms for the reduction of ferricyanide were obtained, and the ice-like matrix became an insulating one, which indicates that the aqueous phase is really a frozen phase.

Investigation of proton transfer across the water/1,2-dichloroethane interface by o-phenanthroline using glass micro/nano-pipets and cyclic voltammetry

Facilitated proton transfer across the water/1,2-dichloroethane (DCE) interface supported on the tips of micro- and nano-pipets by o-phenanthroline (Phen) was studied by using cyclic voltammetry. The formed micro- and nano-liquid/liquid interfaces functioned as micro- and nano-electrodes under certain experimental conditions. The dependence of the half-wave potentials on the aqueous solutions acidities was studied and the ratio of association constants between Phen and proton in the aqueous and DCE phases was calculated by the method proposed by Matsuda et al.. The standard rate constant (k(0)) and the transfer coefficient (alpha) evaluated by using nano-pipets were equal to 0.183 +/- 0.054 cm/s and 0.70 +/- 0.09, respectively.

Stress-strain fields and the effectiveness shear properties for three-phase composites with imperfect interface

Based on the 'average stress in the matrix' concept of Mori and Tanaka (:Mori, T., Tanaka, K., 1973. Average stress in matrix and average elastic energy of materials with misfitting inclusion. Acta Metall. 21, 571-580) a micromechanical model is presented for the prediction of the elastic fields in coated inclusion composites with imperfect interfaces. The solutions of the effective elastic moduli for this kind of composite are also obtained. In two kinds of composites with coated particulates and fibers, respectively, the interface imperfections are takes to the assumption that the interface displacement discontinues are linearly related to interface tractions like a spring layer of vanishing thickness. The resulting effective shear modulus for each material and the stress fields in the composite are presented under a transverse shear loading situation.

An experimental study on turbulent coherent structures near a sheared air-water interface

The turbulence structures near a sheared air-water interface were experimentally investigated with the hydrogen bubble visualization technique. Surface shear was imposed by an airflow over the water flow which was kept free from surface waves. Results show that the wind shear has the main influence on coherent structures under air-water interfaces. Low- and high- speed streaks form in the region close to the interface as a result of the imposed shear stress. When a certain airflow velocity is reached, "turbulent spots" appear randomly at low-speed streaks with some characteristics of hairpin vortices. At even higher shear rates, the flow near the interface is dominated primarily by intermittent bursting events. The coherent structures observed neat sheared air-water interfaces show qualitative similarities with those occurring in near-wall turbulence. However, a few distinctive phenomena were also observed, including the fluctuating thickness of the instantaneous boundary layer and vertical vortices in bursting processes, which appear to be associated with the characteristics of air-water interfaces.

Thermal-mechanical interface crack behaviour of a surface mount solder joint

The effect of thermal-mechanical loading on a surface mount assembly with interface cracks between the solder and the resistor and between the solder and the printed circuit board (PCB) was studied using a non-linear thermal finite element analysis. The thermal effect was taken as cooling from the solder eutectic temperature to room temperature. Mechanical loading at the ends of the PCB was also applied. The results showed that cooling had the effect of causing large residual shear displacement at the region near the interface cracks. The mechanical loading caused additional crack opening displacements. The analysis on the values of J-integral for the interface cracks showed that J-integral was approximately path independent, and that the effect of crack at the solder/PCB interface is much more serious than that between the component and solder.

Application of laser cladding technology to improve the interface of thermal barrier coatings

The present study is focused on improvement of the adhesion properties of the interface between plasma-sprayed coatings and substrates by laser cladding technology (LCT), Within the laser-clad layer there is a gradient distribution in chemical composition and mechanical properties that has been confirmed by SEM observation and microhardness measurement. The residual stress due to mismatches in thermal and mechanical properties between coatings and substrates can be markedly reduced and smoothed out. To examine the changes of microstructure and crack propagation in the coating and interface during loading, the three-point bending test has been carried out in SEM with a loading device. Analysis of the distribution of shear stress near the interface under loading has been made using the FEM code ANSYS, The experimental results show clearly that the interface adhesion can be improved with LCT pretreatment, and the capability of the interface to withstand the shear stress as well as to resist microcracking has been enhanced.

A Crack Perpendicular to the Bimaterial Interface in Finite Solid

The dislocation simulation method is used in this paper to derive the basic equations for a crack perpendicular to the bimaterial interface in a finite solid. The complete solutions to the problem, including the T stress and the stress intensity factors are obtained. The stress field characteristics are investigated in detail. It is found that when the crack is within a weaker material, the stress intensity factor is smaller than that in a homogeneous material and it decreases when the distance between the crack tip and interface decreases. When the crack is within a stiffer material, the stress intensity factor is larger than that in a homogeneous material and it increases when the distance between the crack tip and interface decreases. In both cases, the stress intensity factor will increase when the ratio of the size of a sample to the crack length decreases. A comparison of stress intensity factors between a finite problem and an infinite problem has been given also. The stress distribution ahead of the crack tip, which is near the interface, is shown in details and the T stress effect is considered.

Concentration distribution in solution crystal growth: effect of moving interface conditions

The linear diffusion-reaction theory with finite interface kinetics is employed to describe the dissolution and the growth processes. The results show that it is imperative to consider the effect of the moving interfaces on the concentration distribution at the growth interface for some cases. For small aspect ratio and small gravity magnitude, the dissolution and the growth interfaces must be treated as the moving boundaries within an angle range of 0 degrees < gamma < 50 degrees in this work. For large aspect ratio or large gravity magnitude, the effect of the moving interfaces on the concentration distribution at the growth interface can be neglected except for gamma < - 50 degrees.