On the growth rate of bending induced edge cracks in acoustically excited panels

Parts of an aircraft structure may be made to vibrate as a result of acoustic waves generated by various aircraft noise sources impinging on the structure. The stresses associated with this acoustically induced vibration may be sufficiently large to result in fatigue failure of portions of the structure. If acoustically induced fatigue cracks occur in the stiffened skin structures widely used in aircraft construction they may initiate in the skin panels near the stiffener attachment points. The initiation and subsequent propagation of these cracks at the panel edges is primarily due to the bending stresses arising from the out-of-plane vibration of the individual skin panels.The emphasis of the work described in this paper is on examining the growth rate of edge cracks in acoustically excited panels. A single panel with an edge crack is considered and this structural element is modelled as a flat plate clamped on three edges and part of the fourth. The crack is represented by the unclamped part of the fourth edge. Fracture mechanics principles are used to predict the crack growth rates associated with the first two modes of vibration of the edge cracked panel. The crack tip stress intensity factors associated with these panel modes are estimated by a technique based on finding the nominal bending stresses at the crack tips. The nominal bending stresses are in turn found from mode shapes determined by the Rayleigh Principle. The validity of the various assumptions is assessed by comparing the predicted crack growth rates with measured growth rates in panels representative of those used in aircraft construction.     

Free vibration of polar orthotropic circular plates with elastic growth stresses and drying stresses

This study was performed to analyze effect of transverse growth stresses and drying stresses on natural frequencies of transverse free vibration of thin tree disk with free boundary condition. Using Rayleigh-Ritz method and finite element method, explicit closed form expression was derived for homogeneous orthotropic circular plates with internal stresses, which enables to predict both the effects of growth stresses, drying stresses, and combined growth and drying stresses on the natural frequency changes. This model can be used for predicting growth stresses from known natural frequencies. The same approach can also be undertaken to validate models visco-elastic drying stresses.     

Homogene mechanische Spannungen in amorphen und kristallinen Galliumschichten

The homogeneous mechanical stresses of vacuum deposited Gallium-films have been studied during deposition and annealing. The liquid-like phase which can be frozen in by condensation onto a substrate at He-temperatures is built up almost without any homogeneous stresses, whereas all films with a crystalline structure show clearly such stresses. A metastable crystalline Ga-modification grows with tensile stresses. Compressive stresses however have been observed for the wellknown Ga-modification stable at normal conditions. To understand this rather complex behaviour the so-called “Ostwald-Rule”, an empirical rule for the crystal growth, can be applied successfully.     

冲击波中横向、纵向应力计响应初步研究

 采用低阻值锰铜压阻应力计，通过一维平板撞击实验和理论分析对横向、纵向应力计在有机玻璃和陶瓷材料中的不同响应进行了研究。实验结果表明，在陶瓷等硬质材料中进行横向应力测量时，可用康铜计补偿的方法，直接用锰铜计纵向标定的压阻系数计算，而在有机玻璃中却不能简单地得到横向应力。     

Skin layer at the actin-gel surface: quenched protein membranes form flat, crumpled, and tubular morphologies

We report the discovery of a hierarchically structured skin layer formed at the surface of an isotropic gel of filamentous actin bundles at high molar ratios of alpha-actinin, an actin cross-linker, to globular actin. Confocal microscopy has elucidated the full, micron scale 3D structure. The protein skin layer, composed of a directed network of bundles, exhibits flat, crumpled, tubelike and pleated multitubular morphologies, resulting from stresses due to the underlying gel. The skin layer, which readily detaches, constitutes a model anisotropic solid membrane with stress-induced, quenched disorder.     

Thermal stresses in layered barium titanate-based semiconductor ceramics

Thermal stresses emerging in a barium titanate-based semiconducting ceramic during heating by electric current are studied using numerical methods. It is shown that the highest tensile stresses are formed in the plane equidistant from the electrodes. The values of these stresses can be as high as 70 MPa, which is commensurate with the critical stresses. A method is proposed for reducing stresses by developing thermistors with a layered structure.     

On the use of acoustic birefringence to determine components of plane stress

To assess the integrity of a structure containing a (known) flaw, it is necessary to know the stresses acting on the flaw. Many common structural elements (e.g. beams and plates) are subjected to either true plane stress or generalized plane stress. For either of these cases, there are three in-plane stresses (one shear stress and two normal stresses) to be determined in general.In this paper, we consider the application of the acoustic birefringence method to generalized plane stress states, where the thickness-averaged values of the shear and normal stresses are sought. It is shown that the times-of-flight are thickness-averaged effects, whereas the polarization directions depend upon local values of stress. Consequently, when the symmetric (axial) component of the stresses dominates the antisymmetric (bending) component, the acoustic birefringence method can be used to determine all three stresses, if the boundary conditions are known. For unknown boundary conditions, the normal stresses can be determined to within arbitrary functions.Problems arise in using the normal incidence technique when either the shear stress vanishes, or the symmetric (membrane) components of generalized plane stress do not dominate the bending components. In the former case, the thickness-averaged difference in normal stresses can be obtained, provided that the birefringence in the unstressed state is known. In the latter case, the (averaged) difference in normal stresses can be obtained by measuring time-of-flight differences of off-axis SH-waves propagating in the planes of material symmetry of the plate.     

The Design of Hermetically Double-Coated Optical Fibers to Minimize Hydrostatic Pressure-Induced Stresses

The design of hermetically double-coated optical fibers to minimize hydrostatic pressure-induced stresses is investigated. Several stresses are important in the hermetically double-coated optical fibers, and they would produce the excess bending losses or the failure of hermetic coatings. These stresses can be minimized by appropriately selecting material's properties of hermetic coatings and their thicknesses. The hydrostatic pressure-induced stresses in optical fibers with some selected metallic coatings are also considered. The results show that chromium has the best performance in the minimization of hydrostatic pressure-induced stresses. However, lead and indium are poor materials.     

Relation between residual stresses and microstructure evolution in Al–Si alloys based on different casting parameters

Most designers take into consideration the stresses that act on a material but despite safety considerations, failure may occur due to other factors that were neglected in their design. These factors can be a pre-existing flaw, microstructure deficiency or the presence of residual stresses. Depending on the stress type, residual stresses combined with applied stresses can aid or hinder failures. Consequently, reducing the amount of residual stress can have a promising effect on the life of the component. Different casting parameters can change the microstructure and residual stresses of castings. In this research, the relation between residual stresses and microstructure evolution under the influence of different casting parameters was investigated, using both Al–Si–Mg (Al-356) and Al–Si–Mg–Cu (Al-319) alloys. Solidification rate, quenching rate, aging temperature and aging time were the main parameters considered for this study. The results indicate that the magnitude of the residual stresses increases with increasing solidification rate and quenching rate. Also, the residual stress relieving is proportional to the aging temperature     

Delaying transition to turbulence by a passive mechanism

Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the underlying linearized operator, these perturbations are highly amplified and able to modify the mean velocity profiles at leading order. We report results of wind-tunnel experiments in which we implement this concept by using suitably designed roughness elements placed on the skin to enforce nearly optimal perturbations. We show that by using this passive control technique it is possible to sensibly delay transition to turbulence.     

透射式GaAs光电阴极AlGaAs/GaAs外延层内应力的种类及其表征与测量

本文介绍了透射式GaAs光电阴极AlGaAs窗层和GaAs光电发射外延层内应力的种类及其表征方法和测量方法,另外还给出了相应的实例.     

An electrochemomechanical theory of defects in ionic solids. I. Theory

Charged defects diffuse through an ionic solid under electrochemical driving forces. Such a diffusion process can be affected by mechanical stresses in the solid. A deviation of defect concentration from its stoichiometric value during diffusion can cause volumetric strains in the solid. Such strains will result in mechanical stresses if the ionic solid is under mechanical constraint, or if the defect distribution is non-uniform. We develop a framework to account for the coupling between mechanical stresses and diffusion of charged defects in ionic solids. The framework consists of a system of nonlinear differential/algebraic equations governing the defect concentrations, electrostatic potential and the mechanical stresses. It is believed that this framework is the first fully coupled theory for the interaction between mechanical stresses and electrochemical forces in ionic solids.     

The Design of Hermetically Double-Coated Optical Fibers to Minimize Hydrostatic Pressure–Induced Stresses

The design of hermetically double-coated optical fibers to minimize hydrostatic pressure–induced stresses is investigated. Several stresses are important in the hermetically double-coated optical fibers, and they would produce the excess bending losses or the failure of hermetic coatings. These stresses can be minimized by appropriately selecting material's properties of hermetic coatings and their thicknesses. The hydrostatic pressure–induced stresses in optical fibers with some selected metallic coatings are also considered. The results show that chromium has the best performance in the minimization of hydrostatic pressure–induced stresses. However, lead and indium are poor materials.     

The application of automatic fringe analysis in fracture mechanics

The likelihood of propagation of a crack in a metallic plate under load can be assessed by evaluating an energy-based contour integral, the J integral, for the loaded specimen. Calculation of the integral requires that the local stresses in the specimen are known. These stresses can be obtained practically by the use of strain gauges or by moiré fringe methods. This paper describes a semi-automatic, computer vision-based method of calculating the J integral when moiré fringe methods are used to evaluate the stresses in the specimen.     

Structure of Be films quenched from the vapour

The crystallite boundary mismatch in crystalline films leads to tensile stresses which act on the substrate. Due to these stresses the substrate is bended. In the dense random packing of atoms in an amorphous film local tensile- and compressive stresses compensate and thus the films seem to be free of stresses with regard to the substrate. From this point of view the measurement of mechanical stresses produced by vapour quenched films can be used for structural analysis. Thus an apparatus is described which allows in situ measurements of mechanical stresses and electrical resistivity of vapour quenched films from 1.2 K to 400 K.An investigation of beryllium films revealed that stresses are produced even by films with a superconducting transition temperature of 9.6 K. From this fact it can be concluded that vapour quenched Be is not completely amorphous. Furthermore, it is shown that inhomogeneous films exist with a two phase double layer structure which is responsible for some peculiarities reported in the literature.     

Theory of Thermal Fatigue Caused by RF Pulsed Heating

The normal conducting electron-positron Linear Collider projects imply that accelerating structures and other RF components will undergo an action of extremely high RF fields. Except for the RF breakdown threat, there is an effect of the copper surface being damage due to multi-pulse mechanical stress caused by Ohmic losses in the skin layer. In this paper we would like to introduce a new “grain” model of the processes responsible for the fatigue of the metal surface. This model is based on the quasi-elastic interaction between neighboring grains in the metal due to the thermal expansion of the skin layer. This mechanism of fatigue is compared with another, where stresses are generated by the temperature gradient towards the bulk of the material. With the proposed formalism one can estimate the total number of the RF pulses required to fracture the surface depending on the temperature rise, pulse length and steady state temperature. The parameters necessary to finalize the proposed approach were found through the comparison of experimental data obtained at 11.424 GHz.     

Measuring stresses in thin metal films by means of Raman microscopy using silicon as a strain gage material

Mechanical stresses in microelectronics and micro‐electromechanical systems may influence the reliability of applications and devices. The origin of the stresses can be because of the joining of dissimilar materials with regard to the thermal expansion coefficient, electromigration or the deposition process utilized. Stresses can lead to delamination, crack formation and stress migration and therefore to a failure of the device. Identifying the locations of highest stresses in a device is crucial for reliability improvement. Currently, both Laue X‐ray micro diffraction and convergent‐beam electron diffraction are able to locally determine the stresses in thin metal films. Here, we propose a modified method of indirect Raman microspectroscopy to measure stresses with a lateral resolution in the submicrometer range at a laboratory scale. The method encompasses the crystallization of an amorphous silicon layer by local laser annealing and its subsequent usage as a strain gage. Stresses in an aluminum thin film were determined as a function of temperature. In addition to the average stress, the stress distribution could be monitored. Copyright © 2009 John Wiley & Sons, Ltd.     

一种在线测量微机械薄膜残余应力的新结构

提出了一种利用临界屈曲法在线测量微机械薄膜残余应力的新结构，并采用表面微加工技术制作了两种测试样品.搭建了在线观测实验装置来实时监控释放过程中结构出现的临界屈曲变形模态，由此判断出结构内部的应力状态，同时在测得临界刻蚀深度的情况下，采用有限元方法计算出残余应力大小.借助有限元方法，先研究了多个参数对临界屈曲应力的影响，然后利用这种新结构对薄膜残余应力进行了实际测量，所得结果与微旋转结构的应力测量结果基本吻合.分析及实验表明，新结构在测量薄膜残余应力方面有许多优点，具有较高的实用价值，不仅能满足大量程的应力检测要求，而且只用一个结构就可以同时测量压应力和拉应力，从而极大提高了器件版图空间的利用率.     

Analysis by speckle interferometry of the dependency of yield stress on residual stress

We used electronic speckle pattern interferometry (ESPI) to measure in situ displacement fields nondestructively and with high resolution (10⁻² μm) by using the interferometry principle and the phase-shift technique. We measured the depth profile of the residual stress in steel pipe manufactured by thermomechanically controlled processing using a quantitative model, which explains the relationship between residual stress and displacement measured by ESPI in chemical etching. We analyzed the variation of yield stresses measured by the indentation technique and the residual stresses at various depths. The relationship between the residual stresses and the yield stresses was consistent with simulated results and can be used for indirect evaluation of the residual stresses from the yield stresses.     

Radial buckling of multi-walled carbon nanotubes under hydrostatic pressure

Radial buckling stresses of carbon nanotubes (CNTs) need to be studied in high-pressure resonance Raman scattering spectrum. In this work, the closed-form expression of the critical buckling stress of multi-walled carbon nanotubes (MWCNTs) under hydrostatic pressure is derived that can be conveniently employed. Using the derived formulae, the critical buckling stresses of single-walled carbon nanotubes and double-walled carbon nanotubes with different diameters are calculated. The results are in good agreement with other reported literatures. In addition, the critical buckling stresses of each layer of a quintuple-walled CNT in different buckling modes are predicted, showing that the buckling instability can occur not only in the outermost rolled layer, but also in other rolled layer of MWCNTs by considering different diameters and buckling modes.