A continuum theory for the mechanical response of materials to the thermodynamic stress of sintering

The microstructural material rearrangement that accompanies sintering is driven primarily by surface energies and tensions and depends to some extent upon the local geometry of the free surface. This article uses concepts from continuum mechanics to describe the macroscopic response of a body to the thermodynamic stress associated with these driving forces. This stress can be described as an equilibrium or sintering stress that is, as defined here, the stress that must be applied to prevent spontaneous shrinkage of a sinter body. An expression that quantifies the sintering stress is developed and is assumed to be dependent upon two microstructural parameters: the specific surface area and the mean curvature of the void-solid interface. The macroscopic response of the sinter body is modeled as a linear Newtonian fluid and the theory is used to analyze data from experiments on sintering copper powders.     

Measurement of parameters of simple lenses using digital holographic interferometry and a synthetic reference wave

The use of digital holographic intrerferometry in the testing of simple thin lenses is explored. Focal length, radius of curvature, and refractive index are the lens parameters that can be determined using this method. The digital holograms using the lens under test are recorded at various positions of the test lens using off-axis geometry. This is combined with a digitally computed plane wavefront to determine the curvature of the light beam emerging from the test lens. Focal length is the position of the test lens where a single fringe results. The radius of curvature of the test lens is also determined similarly using a long focal length lens to concentrate a collimated beam onto the test lens. The nonuniformities on the lens surface could also be found by using this method. The implementation of the method is shown by using computer simulations in the case of biconvex lenses. The method can be utilized to measure the parameters of plano-convex and concave lenses also.     

Terrain analysis from curvature profiles

This paper describes a new algorithm which uses Gaussian and mean curvature values of the terrain surface to extract feature points. These points are then used for recognition of particular subregions of the terrain and in estimating relative positions of these subregions in the terrain. The Gaussian and mean curvatures are chosen because they are invariant under rotation and translation. In the Gaussian and mean curvature images, the points of maximum and minimum curvatures are extracted and used for matching. The stability of the position of these points in the presence of noise and with resampling is investigated. The input for this algorithm is 3D digital terrain data. Curvature values are calculated from the data by fitting a quadratic surface over a square window and calculating directional derivatives of this surface. A method of surface fitting that is invariant to sensor-centered coordinate system transformation is suggested and implemented. Real terrain data are used in our experiments. The algorithm is tested with and without the presence of noise, and its performance is described.     

基于曲率驱动的类双线性图像快速插值方法

为实现保持图像边缘锐度的快速插值放大,提出了基于图像曲面曲率信息的类双线性插值方法.首先鉴于双线性插值的低通滤波固有特性,引入像素的值变化以构造类双线性插值模型;为获得相邻像素点的方向趋势,将灰度值模拟为地表高程并与二维坐标形成空间曲面;最后以指定方向的曲面的剖面曲率作为图像边缘等结构类型的判别依据,自适应优化两组两像...     

High-precision measurement of pixel positions in a charge-coupled device

The high level of spatial uniformity in modern CCD's makes them excellent devices for astrometric instruments. However, at the level of accuracy envisioned by the more ambitious projects such as the Astrometric Imaging Telescope, current technology produces CCD's with significant pixel registration errors. We describe a technique for making high-precision measurements of relative pixel positions. We measured CCD's manufactured for the Wide Field Planetary Camera II installed in the Hubble Space Telescope. These CCD's are shown to have significant step-and-repeat errors of 0.033 pixel along every 34th row, as well as a 0.003-pixel curvature along 34-pixel stripes. The source of these errors is described. Our experiments achieved a per-pixel accuracy of 0.011 pixel. The ultimate shot-noise limited precision of the method is less than 0.001 pixel.     

Reduction of Noise‐Induced Bias in Displacement Estimation by Linear Off‐Pixel Digital Image Correlation

Abstract: A linear digital image correlation algorithm is proposed to eliminate noise‐induced bias in one‐dimensional translation estimation using noisy images. The algorithm uses linear interpolation for both initial and current images at off‐pixel positions and solves directly the displacement parameter by minimizing a sum‐of‐squared‐differences coefficient. Both analytical results and numerical simulations using synthetic image sets show that there is indeed no noise‐induced bias in the displacement estimation using the proposed algorithm if the off‐pixel positions in both images are chosen properly according to the relative displacement between two images. When the displacement is only known initially within a range of ±0.5 pixels from the actual displacement, an iterative procedure using the algorithm is able to obtain the displacement estimation with a residual bias that converges to the noiseless subpixel approximation bias. A further refinement of the off‐pixel analysis algorithm will be needed so the remaining residual bias due to subpixel approximation can also be significantly reduced.     

Mechanical determination of internal stresses in as-quenched magnetic amorphous metallic ribbons

The layer removing method for determining the internal stress distribution through thickness in sheets has been applied to metallic glass ribbons. The internal stresses are obtained from the variation of the radius of curvature of the samples when the layers are removed. The experimental technique used is described here and the results for two ribbons, are presented. A general feature has been observed implying compressive stress near both surfaces (a strong one near the drum surface and a weak one near the air surface), and the central part under tensile stress. This is in agreement with the residual stress distribution shown by other materials after fast cooling procedures. This revised version was published online in November 2006 with corrections to the Cover Date.     

Subpixel precision of crack lip movements by Heaviside-based digital image correlation for a mixed-mode fracture

This study reports on the Heaviside-based digital image correlation (H-DIC) procedure and its application in fracture analysis. This improved DIC procedure was proposed to solve the uncertainty problems at the vicinity of the crack and to evaluate the opening and shear movements of crack lips and the orientations of cracks in the subset. Some tests were conducted to demonstrate the performance of the H-DIC algorithm. An application on argillaceous rock mass exhibiting multiple mixed-mode fractures is shown to validate the efficiency and robustness of the proposed method. This application consisted in processing images acquired from an experimental investigation performed in a gallery front submitted to climatic seasonal variations. The results illustrated how the H-DIC procedure enables to localise and to quantify the opening, shearing and orientation of subpixel cracks. A sensibility study performed on the opening and shear components demonstrated that the precision of crack aperture by H-DIC procedure is close to 0.14 pixel and the spatial resolution is equal to one pixel. Moreover, the crack area was calculated from local apertures on a monitoring duration of 1 year, and a maximum value of 595.8 mm² in winter was obtained.     

A finite element‐based level set method for fluid–elastic solid interaction with surface tension

A numerical method for simulating fluid–elastic solid interaction with surface tension is presented. A level set method is used to capture the interface between the solid bodies and the incompressible surrounding fluid, within an Eulerian approach. The mixed velocity–pressure variational formulation is established for the global coupled mechanical problem and discretized using a continuous linear approximation in both velocity and pressure. Three ways are investigated to reduce the spurious oscillations of the pressure that appear at the fluid–solid interface. First, two stabilized finite element methods are used: the MINI‐element and the algebraic subgrid method. Second, the surface integral corresponding to the surface tension term is treated either by the continuum surface force technique or by a surface local reconstruction algorithm. Finally, besides the direct evaluation method proposed by Bruchon et al., an alternative method is proposed to avoid the explicit computation of the surface curvature, which may be a source of difficulty. These different issues are addressed through various numerical examples, such as the two incompressible fluid flow, the elastic inclusion embedded into a Newtonian fluid, or the study of a granular packing. Copyright © 2012 John Wiley & Sons, Ltd.     

Phase retrieval in digital speckle pattern interferometry by use of a smoothed space-frequency distribution

We evaluate the use of a smoothed space-frequency distribution (SSFD) to retrieve optical phase maps in digital speckle pattern interferometry (DSPI). The performance of this method is tested by use of computer-simulated DSPI fringes. Phase gradients are found along a pixel path from a single DSPI image, and the phase map is finally determined by integration. This technique does not need the application of a phase unwrapping algorithm or the introduction of carrier fringes in the interferometer. It is shown that a Wigner-Ville distribution with a smoothing Gaussian kernel gives more-accurate results than methods based on the continuous wavelet transform. We also discuss the influence of filtering on smoothing of the DSPI fringes and some additional limitations that emerge when this technique is applied. The performance of the SSFD method for processing experimental data is then illustrated.     

A novel scheme for image sharpness using inflection points

It is useful to increase the sharpness in medical images. This improvement can help medical diagnoses and treatment outcomes for patients. Noise overshoot and oversharpening effects are common artifacts of conventional sharpening algorithms. In this work, we propose an adaptive sharpness algorithm that achieves better sharpening effects than standard methods. As the pixel value changes abruptly at an edge, a method that adequately emphasized sudden variations in pixel values is effective for use in the sharpening process. Measurement of the gradient norm value of each pixel is calculated and compared to a threshold value to produce a curve. The curve quickly drops at the initial stage and then decreased more slowly for higher norm values. To distinguish the edge, an inflection point is determined by taking the second derivative of the curve and identifying the turnover point (where the curvature changes sign). Norm values higher than the inflection point are identified as belonging to the edge, and a simple sharpness filter is adaptively applied to these points. The proposed approach yielded better results than global filtering and a conventional unsharp masking approach when evaluated using Pratt's figure of merit.     

High-fidelity numerical realization of multiple-step Fresnel propagation for the reconstruction of digital holograms

A cascaded Fresnel algorithm for the flexible reconstruction of digital holograms is proposed. Since the fast-Fourier-transform-based numerical realization of the Fresnel integral shows a dependency of its pixel resolution and its computation window size on the propagation distance different from that of the corresponding physical system, the computation window can be smaller than the actual physical diffraction field in the intermediate plane. Consequently, distortions in the final reconstruction may occur. A method is proposed to eliminate such distortion. The validity of this method is shown by both numerical simulations and experimental results.     

Automatic procedure for aberration compensation in digital holographic microscopy and applications to specimen shape compensation

We present a procedure that compensates for phase aberrations in digital holographic microscopy by computing a polynomial phase mask directly from the hologram. The phase-mask parameters are computed automatically without knowledge of physical values such as wave vectors, focal lengths, or distances. This method enables one to reconstruct correct and accurate phase distributions, even in the presence of strong and high-order aberrations. Examples of applications are shown for microlens imaging and for compensating for the deformations associated with a tilted thick plate. Finally we show that this method allows compensation for the curvature of the specimen, revealing its surface defects and roughness. Examples of applications are shown for microlenses and metallic sphere imaging.     

Simulation of spherical powder sintering by surface diffusion

The surface diffusion-controlled sintering of monosized spheres is studied by a computer simulation process. The simulation is used to determine the variations in neck size and surface area as functions of both sintering time and powder packing density. Both morphology parameters are shown to be complex functions of the sintering time, contrary to numerous models. This work shows that the exponent method is not sufficient for identifying the dominant sintering mechanism.This work was supported by the United States Energy Research and Development Administration.     

Measurement of curvature and twist of a deformed object using digital holography

Measurement of curvature and twist is an important aspect in the study of object deformation. In recent years, several methods have been proposed to determine curvature and twist of a deformed object using digital shearography. Here we propose a novel method to determine the curvature and twist of a deformed object using digital holography and a complex phasor. A sine/cosine transformation method and two-dimensional short time Fourier transform are proposed subsequently to process the wrapped phase maps. It is shown that high-quality phase maps corresponding to curvature and twist can be obtained. An experiment is conducted to demonstrate the validity of the proposed method.     

Full resolution of the Monte Carlo time scale demonstrated through the modelling of two-amorphous-particles sintering

Kinetics Monte Carlo approaches based on the discrete Potts model are shown to be limited for a complete resolution of the time scale for mesoscale problems. This is due to the inadequacy of a discrete modelling for a continuous physical problem. In the field of particle sintering involving viscous flow, it is shown how to overcome this limitation through the non-discrete Monte Carlo (NDMC) methodology, which is based on an energetic potential directly related to the energy scale of the system. In this paper is presented the last step in the complete resolution of the Monte Carlo time scale, counted in number of steps, into real time. The obtained conversion time scaling is defined as a function of materials physical properties and numerical parameters depending on the algorithm only. This resolution was based on the physical consistency of the sintering kinetics simulated using the NDMC. Actually, the NDMC kinetics was shown to perfectly match viscous sintering kinetics. This result was not straightforward since the sintering mechanism is not predetermined in the Monte Carlo methodology but implicitly generated as a function of mass transport probabilities.     

A finite deformation membrane based on inter-atomic potentials for the transverse mechanics of nanotubes

A finite deformation hyper-elastic membrane theory based on inter-atomic potentials for crystalline films composed of a single atomic layer is developed. For this purpose, an extension of the standard Born rule that exploits the differential geometry concept of the exponential map is proposed to deal with the curvature of surfaces. The exponential map is approximated locally and strain measures based on the stretch and the curvature of the membrane arise. The methodology is first particularized to atomic chains in two dimensions, and then to graphene sheets. A reduced model for the transverse mechanics of carbon nanotubes is developed in detail. This model is a hyper-elastic constrained membrane which fully exploits the symmetry of the transverse deformation. Additionally, a continuum version of the non-bonded interactions is provided. The continuum model is discretized using finite elements and very good agreement with molecular mechanics simulations is obtained. Finally, several simulations illustrate the strong effect of the van der Waals interactions in the transverse deformation of carbon nanotubes.     

几何重心法亚像素提取算法研究

研究了一种基于显微图像的几何重心法亚像素提取算法,用于实现超低转速的在线测量。首先根据显微图像法将超低转速的转动测量转换为平动测量; 然后利用提出的几何重心法的亚像素提取算法,解决了目前常用的曲线拟合法和梯度法的亚像素提取时间长、抗图像干扰能力弱的问题。几何重心法的核心是在整像素提取的基础上,在3×3的像素范围内,分别计算参考像素灰度值与提取的整像素灰度之差的指数值,在获取的3点指数值上,利用几何重心法提取出亚像素值。通过光栅基准平台微位移抓拍图像的验证,提出的几何重心法比曲面拟合法、梯度法用时短,平均用时<0.2s,提取的亚像素值的标准差<1/1000像素,满足了超低转速在线测量的基本要求。     

Estimation of wavelength difference using scale adjustment in two-wavelength digital holographic interferometry

We propose a method for an estimation of wavelength difference using scale adjustment in two-wavelength digital holographic interferometry. To estimate wavelength difference, two holograms recorded with different wavelengths are reconstructed on the basis of the Fresnel diffraction integral, and pixel sizes in the reconstruction plane, which depend on the wavelength in recording hologram, are analyzed. In the analysis, a zero-padding method and an intensity correlation function are used to adjust pixel sizes in the reconstruction plane and then obtain a wavelength difference given by a difference between the pixel sizes. Theoretical predictions and experimental results are shown to indicate the usefulness of the proposed method in this paper.     

基于引导因子与曲率惩罚模型的图像修复算法

目的 解决当前图像修复方法主要通过待修复像素点的法向量来确定修复过程,无法保证其修复顺序从破损区域的周边至中心进行,导致修复图像中存在块效应和不连续效应等问题。方法 将引导因子与曲率惩罚模型相结合,设计新的图像修复方法。利用破损区域的中心像素点与其他任意待修复像素点之间的距离来构造引导因子,并将其与置信度项以及数据项结合,形成优先权模型,用于选取优先修复块。利用待修复块的梯度特性对其平滑度进行判断,以明确该待修复块对应的最优匹配块的搜索范围,使其通过最小绝对差平方和（SSD）函数来搜索最优匹配块,从而将最优匹配块中像素点扩散填充至待修复块。最后,基于像素点间的等照度线曲率来建立曲率惩罚模型,以更新置信度项,从而实现图像的修复。结果 测试数据表明,与已有修复方案相比,所提算法可以更好地兼顾修复质量与效率。结论 所提方案具有较好的修复质量,可用于损坏面积较大图像的复原。