A moiré micro strain gauge

We have built a new strain gauge based on the moiré technique. This strain gauge mainly consists of two frames that can move with respect to each other. Displacements are recorded by using the moiré technique. We use a pair of similar gratings attached to the frames. The gratings are installed in parallel without physical contact and their lines making a small angle with one another. A moiré pattern is formed due to superimposing of the gratings. A diode laser light passes through the moiré pattern and a narrow slit, and hits on a light sensor. In response to external stress, one of the gratings is displaced and, as a result, the moiré fringes move in front of the slit. Due to the fringes movements, the light intensity on the detector varies and is recorded as voltage. The voltage output can be used to measure the strain. This instrument can detect displacements of the order of micron. In this paper we show the experimental results of our instrument.     

The flexural vibration of welded rectangular plates

A theoretical and experimental study of the effect of weld runs on the flexural vibrational characteristics of the common structural element, the rectangular plate, is described. A finite difference technique is utilized for the determination of the in-plane residual stress pattern due to the weld(s) and the Rayleigh-Ritz method, with beam characteristic functions, is used for the out-of-plane vibration analysis. The theoretical approach presented is applicable to rectangular plates of any practical aspect ratio, having any combination of out-of-plane boundary conditions for which beam functions may reasonably be used and subject to one or more weld runs parallel to any edge. Theoretical and experimental results for a number of specific plates are presented, demonstrating the effects of welding on the plate vibration and the capability and accuracy of the analytical approach in predicting these effects. Included is a study of the effect of using the full residual stress pattern as derived from the finite difference analysis, the effect of neglecting certain stress components and the effect of using simplified stress patterns developed primarily for the stress and buckling analysis of long plates.     

Residual stresses deriving from holographic interferometry data on a base of inverse problem solution

A transition model, which is capable of obtaining both membrane and bending residual stress components from initial experimental information, is developed for thin-walled plane structures. The determination of residual stresses is based on the combined implementing of the hole-drilling method and reflection hologram interferometry. Required input data are obtained by simultaneous measurements on through hole distortions in two principal strain directions on opposite sides of thin plane specimen. These sides are faces of the drill entrance and exit. Superimposed residual stresses field, which consists of both membrane and bending components, is a reason for the various deviations of each specific fringe pattern from an ideal form. This fact is a clear experimental indication of the bending stress contribution in a total stress field. Two ways of decomposition of superimposed residual stresses field are proposed and analysed in detail. Emphasis is laid on a careful quantitative formulation of the inverse problem needed for an accurate deriving both membrane and bending residual stress components. It is shown that an availability of two-side initial data is both an essential and necessary condition of such a formulation. Detailed analysis of an accuracy of the results obtained is performed. This analysis is based on a wide set of both actual interferograms and analogous reference fringe patterns related to superimposed residual stress field under study. Comparing residual stress values obtained proceeding from one-side and two-side data are presented for different types of superimposed field of interest.     

Two degrees-of-freedom Lloyd-mirror interferometer for superior pattern coverage area

In this study, we have developed a tunable Lloyd-mirror interferometer with two degrees of freedom, in contrast to a traditional system with one degree of freedom. This new Lloyd-mirror interferometer allows an angular rotation of the mirror independently from that of a sample stage, resulting in an increased pattern coverage area with tunable pattern periodicity. Both theoretical and experimental results verify that the tunable characteristic of the modified configuration enhances the nanopatterning capabilities of the Lloyd-mirror interference lithography system especially in achieving greater pattern coverage area for larger pattern periodicities.     

Spontaneous wrinkling pattern of a constrained thin film membrane

The wrinkling morphology of an inhomogeneously constrained thin film membrane is explored using finite element simulations, where a small circular area in a thin sheet undergoes expansion, and buckles emerge due to differential in-plane deformation. The local wrinkling patterns are characterized by the normalized circular (inner) area size, the modulus mismatch between the inner and outer regions, and the normalized stress in the inner area. As the stress increases, the morphology transits from ripple-like to petal-like and finally to a branched pattern. Through parametric studies, the effect of the governing variables on the pattern evolution, wave number, and maximum deflection is discussed. The model is used to qualitatively explain the delamination/blister morphology observed in thin film/substrate systems. The study has the potential of inspiring new fabrication techniques based on mechanical self-assembly.     

仿视觉过程的相位展开方法

在所有基于条纹分析的光学测量中,相位展开问题是难以避免的问题。所有传统的相位展开方法的共同点都是在相主值图中通过邻域内相主值差和寻找无噪声的相位展开路径来完成的。如果不利用整幅图像像素间的联系,则误差会沿展开路径传播。根据人眼识别栅线图周期的过程提出了一种与以往完全不同的相位展开方法。经过初步提取周期分界线、修补分界线和根据分界线确定周期三个步骤即可完成相位展开过程。该算法的原理简单明了,实现起来相对简单,只需单幅条纹图像即可得到周期,因而可用于动态测量。实验结果验证了该算法的可行性。     

Nanostructures made by mixing Rh atoms on N-adsorbed Cu(001) surface

Nanostructures and nanopattern formation by mixing Rh atoms were studied on the N-saturated Cu(001) surface using scanning tunneling microscopy and X-ray photoemission spectroscopy. Most of the Rh atoms are in the topmost layer of the surface, and make a line structure of two atomic width in the <100> direction. Each Rh atom in the line is bonded to one N atom. The density of trench-like structures at the N-saturated surface decreases with increasing the density of the lines. This indicates that the line structure reduces the surface stress induced by N adsorption. When the Rh atoms cover more than a quarter of surface on average, the lines form a rectangular pattern at the surface. The average separation decreases with the increase of the Rh density, and is 4 nm for the 30% Rh coverage. The pattern formation is attributed to the efficient reduction of lattice strain as the grid pattern on the partially N-adsorbed Cu(001) surface.     

Photoelasticity stress analysis using carrier fringe and FFT techniques

A novel carrier fringe technique for photoelasticity stress analysis is described and verified experimentally. Linear carrier fringes generated by using a quartz wedge are superimposed on fringes formed by the stressed model. The resultant fringe pattern is then captured using a charge coupled device camera and stored in a digital frame buffer. A fast Fourier transform method is then used to process the complete photoelastic fringe image over the whole surface of the model. The experiment also uses a minimum spanning tree phase unwrapping strategy to create a continuous map of the whole stress field. Finally, the whole principal stress difference field has been calculated and plotted from one single exposure showing the methods' potential for use as a reflection analysis system suitable for application to nonstationary objects and complicated stress patterns.     

An adaptive scanning scheme for effective whole field stress separation in digital photoelasticity

In photoelasticity, the method of obtaining the individual values of principal stresses/normal stresses separately is referred to as stress separation. Shear difference method is one of the widely used techniques for stress separation in digital photoelasticity. Normally a simple raster scanning approach is used in shear difference method in which stress separation is carried out for all the lines within the model domain by either row-wise horizontally or column-wise vertically starting from the boundary pixels. This requires the presence of a free boundary to start the integration scheme for every row of interest, which is not always possible in most of the practical problems. In order to overcome this, in this paper, an adaptive scanning scheme is proposed so that stress separation can be carried out even if there is only one free boundary pixel available in the model. The new scanning scheme is validated using the theoretically generated data for the problem of a ring subjected to internal pressure. Later, the applicability of this scheme is demonstrated by using two other example problems.     

A general finite element model for numerical simulation of structure dynamics

A finite element model used to simulate the dynamics with continuum and discontinuum is presented. This new approach is conducted by constructing the general contact model. The conventional discrete element is treated as a standard finite element with one node in this new method. The one-node element has the same features as other finite elements, such as element stress and strain. Thus, a general finite element model that is consistent with the existed finite element model is set up. This new model is simple in mathematical concept and is straightforward to be combined into the existing standard finite element code. Numerical example demonstrates that this new approach is more effective to perform the dynamic process analysis in which the interactions among a large number of discrete bodies and continuum objects are included.     

In situ measurements of InAs and InP (0 0 1) surface stress changes induced by surface reconstruction transitions

The anisotropic surface stress changes associated with the transition between different surface reconstructions of InAs and InP (0 0 1) surfaces are measured in situ and in real time in a molecular beam epitaxy (MBE) system. Reflectivity anisotropy of the surface measured at 1.96 eV, together with reflection high energy electron diffraction (RHEED) pattern, are used in order to identify the surface reconstructions, and the monitoring of the substrate curvature evolution to determine the variations in surface stress. Our results show the important contribution to the surface stress of the dimers present in these reconstructed surfaces. Furthermore, we provide for the first time quantitative values of the surface stress changes due to the transition between surface reconstructions for these III-V semiconductors compounds. We obtain values for these changes up to 0.7 Nm⁻¹, that is, of the same magnitude as the stress induced by deposition of one monolayer during growth of lattice-mismatched III-V semiconductor heteroepitaxial systems. This points out the great importance of surface stress evolution in this kind of processes.     

New applications of the method of caustics

The method of caustics is a very powerful technique for measuring the value of the stress-intensity factor. This method, however, cannot be applied to the three-dimensional stress analysis technique of stress-freezing, and it is rather difficult to apply to materials which exhibit small deformations such as ceramics.

In this paper, several new application techniques for caustics are shown. These new applications are: (i) the analysis of the individual values of the optical constants using a simple method based on the theory of caustics, (ii) the analysis of the reason why the caustic pattern cannot be obtained in the frozen-stress model in the immersion liquid by the conventional caustics technique using those optical constants, and the proposition of a new technique to apply the caustic method on such a frozen-stress model, (iii) the caustic coating technique, (iv) the multiplication technique of the caustic, and (v) some examples of the application of caustics to ceramics.     

Speckle interferometry

Illumination of a rough surface by a coherent monochromatic wave creates a grainy structure in space termed a speckle pattern. It was considered a special kind of noise and was the bane of holographers. However, its information-carrying property was soon discovered and the phenomenon was used for metrological applications. The realization that a speckle pattern carried information led to a new measurement technique known as speckle interferometry (SI). Although the speckle phenomenon in itself is a consequence of interference among numerous randomly dephased waves, a reference wave is required in SI. Further, it employs an imaging geometry. Initially SI was performed mostly by using silver emulsions as the recording media. The double-exposure specklegram was filtered to extract the desired information. Since SI can be configured so as to be sensitive to the in-plane displacement component, the out-of-plane displacement component or their derivatives, the interferograms corresponding to these were extracted from the specklegram for further analysis. Since the speckle size can be controlled by the F number of the imaging lens, it was soon realized that SI could be performed with electronic detection, thereby increasing its accuracy and speed of measurement. Furthermore, a phase-shifting technique can also be incorporated. This technique came to be known as electronic speckle pattern interferometry (ESPI). It employed the same experimental configurations as SI. ESPI found many industrial applications as it supplements holographic interferometry. We present three examples covering diverse areas. In one application it has been used to measure residual stress in a blank recordable compact disk. In another application, microscopic ESPI has been used to study the influence of relative humidity on paint-coated figurines and also the effect of a conservation agent applied on top of this. The final application is to find the defects in pipes. These diverse applications demonstrate the power of this technique.     

Observation of dark dot splitting pattern in terbium gallium garnet

We have observed a new polarisation pattern in terbium gallium garnet which is both laser excited and mechanically stressed. The pattern can be characterized by a splitting from a “one dot” structure to a “two dots” structure. We have found that the new pattern is very sensitive to the orientation and the strength of the external forces applied to the sample. Thanks to this sensitivity and to the image simplicity, this new effect may be used in a vectorial force sensor and actuator. Received: 25 March 1999 / Accepted: 20 April 1999 / Published online: 25 August 1999     

Scales of metal fatigue cracking

A new paradigm is proposed for considering metal fatigue cracking based on the principles of synergetics and physical mesomechanics. Fatigue cracking is described as a three-stage process. Metal evolution is studied with stress growth from the micro- (ultrahigh cycle fatigue) to meso- (high cycle fatigue) and then macroscale (low cycle fatigue). The notion of two effective stress concentration factors on the metal surface and in its bulk is introduced; their variation pattern with stress growth is discussed. In the general case, the propagation of through-the-thickness cracks is shown to also occur in three stages—on the micro- (shear), meso- (rotation with the formation of triangular fatigue striations) and macroscale (rotation plus shear which lead to the formation of fatigue striations of complex shape), consecutively.     

CCD camera automatic calibration technology and ellipse recognition algorithm

A novel two-dimensional (2D) pattern used in camera calibration is presented. With one feature circle located at the center, an array of circles is photo-etched on this pattern. An ellipse recognition algorithm is proposed to implement the acquisition of interest calibration points without human intervention. According to the circle arrangement of the pattern, the relation between three-dimensional (3D) and 2D coordinates of these points can be established automatically and accurately. These calibration points are computed for intrinsic parameters calibration of charge-coupled device (CCD) camera with Tsai method. A series of experiments have shown that the algorithm is robust and reliable with the calibration error less than 0.4 pixel. This new calibration pattern and ellipse recognition algorithm can be widely used in computer vision.     

Prestressed FRP and interface slip effect on interfacial stresses analysis: the new FRP sheets rigidity model

This paper presents an improved bi-material beam theory with adhesive interface and the new prestressed-fiber reinforced polymer (FRP) – model, which has been applied to the study the problem of interfacial stresses. This work explicitly considers the interfacial slip effect on the structural performance by including both the effect of adherend shear deformations and the fiber volume fraction of the prestressed laminates. This new method needs only one differential equation to determine both shear and normal interfacial stress, which is one aspect that has not been taken into account by the previous studies in the literature. A parametrical study is carried out to show the effects of some design variables, e.g., stiffness and thickness of adhesive layer and FRP plate.     

Predicting lepton flavor mixing from Δ(48) and generalized CP symmetries

We propose to understand the mixing angles and CP-violating phases from the Δ(48) family symmetry combined with the generalized CP symmetry. A model-independent analysis is performed by scanning all the possible symmetry breaking chains. We find a new mixing pattern with only one free parameter, excellent agreement with the observed mixing angles can be achieved and all the CP-violating phases are predicted to take nontrivial values. This mixing pattern is testable in the near future neutrino oscillation and neutrinoless double-beta decay experiments. Finally, a flavor model is constructed to realize this mixing pattern.     

On the transformation of mass and momentum densities in special relativity

By considering the mass and momentum densities of a point mass moving at uniform velocity, the known transformation of these densities from a representation in one inertial system to another is easily derived. The transformation is not linear in mass and momentum density, but the introduction of a dyadic stress density tensor gives a linear relation. The transformation is shown to hold for a general continuous mass distribution in which mass and momentum are conserved, provided a specific choice is made for the stress density tensor. This result contrasts with the particle viewpoint of matter in which only the divergence of the stress density tensor need be fixed so far as the transformation is concerned. A change of functions is made which greatly simplifies the transformations. The new functions are shown to represent a conserved fluid.Research supported in part by a grant from the California State University, Long Beach Foundation.