The effects of manufacturing parameters on geometrical and mechanical properties of copper foams produced by space holder technique

We describe a powder metallurgical space holder method to produce open-cell metallic foams. By changing the values of the main manufacturing parameters such as volume percentage and the particle size of the space holder agent, we produce different copper foam samples which cover a wide range of solid fraction, pore size and cell wall thickness. All the specimens were synthesized based on a series of designed experiments. We demonstrate how the foams’ density, cell size and specific surface area can be accurately controlled using two easily adjustable manufacturing parameters. The three-dimensional structure of these foams was investigated using X-ray micro tomography. The image quality is sufficient to measure local structure and connectivity of the foamed material, and the field of view large enough to calculate material properties. By combining the finite element method with the tomographic images, we calculate the mechanical response of the foams. We show that the foams’ bulk and shear moduli are strongly correlated to their cell size, cell wall thickness and specific surface area. These parameters can be easily controlled during manufacturing.     

Internal microstructure evolution of aluminum foams under compression

In this paper, the internal microstructure deformation of open-cell and closed-cell aluminum foams under compression was investigated by using synchrotron radiation X-ray computed tomography (SR-CT) technique and digital image analysis method. The reconstructed images were obtained by using filtered back projection algorithm based on the original images taken from SR-CT experiments. Several important parameters including cross-section porosity, total porosity and cross-section deformation were computed from the reconstructed images. The variation of these parameters provided useful evolution information of internal microstructure of aluminum foams under compression.     

Quantitative Analysis of X-ray Lithographic Pores by SEM Image Processing

Arrays of regular macropores in electronic, magnetic, photonic and sensing devices can be patterned by X-ray lithography. Such structures inevitably contain some irregularity and require time-consuming pattern inspections. In this work, a pattern inspection by intensity-based digital image processing procedure is proposed and tested on scanning electron microscopy images of porous SU-8 polymer resist. The Otsu’s thresholding converted grayscale to binary images and the closing morphology algorithm was applied to reduce noise in the images. The Canny edge detector was used to identify the contour of each pore by detecting abrupt intensity changes in the binary image. Pores were detected and their sizes were subsequently evaluated. The morphological distributions analyzed by this procedure are comparable to those carried out by the one-by-one human inspection.     

Poisson’s ratio of eTPU molded bead foams in compression via in situ synchrotron X-ray microtomography

In situ synchrotron X-ray microtomography was used to characterize the bulk deformation behavior by computing the Poisson’s ratio of expanded thermoplastic polyurethane (eTPU) molded bead foams used in footwear midsole during compression. Quantitative data on morphological characteristics were obtained using an iterative image processing workflow. Image correlation on the 4D datasets using DVC was performed to calculate the volumetric and axial strain to estimate the Poisson ratio. Strain maps from DVC showed the influence of variability in ligament thickness distribution on the global mechanical behavior exhibited which dominated the response seen in these bead foams. Finally, our results showed a strong correlation between Poisson ratio and distribution of ligament thickness in foams.

     

A mesoscale quantification method of cavitation in semicrystalline polymers using X-ray microtomography

The ability of X-ray microtomography to quantify the cavitation in highly strained semicrystalline polymers is investigated on polyvinylidene fluoride. The cavitation phenomenon is studied on Circumferentially Notched Round Bars tested in tension. The tensile deformation induces void formation as attested by the whitening of the notch and SEM observations. The aim of this paper is to highlight the interest of a non destructive method based on X-ray microtomography in order to quantify the developed porosity in the cavited area and to follow step by step after relaxation, or continuously the evolution of the porosity during a tensile test. As voids may have a submicrometric size, as attested by SEM images in PVF2, the X-ray microtomography does not intend to image all the voids but to quantify them as a whole, including the non resolved voids. The method is designed to be used either with a laboratory microtomograph or with a synchrotron microtomography device. The quantification uses the correlation between the attenuation coefficient of the material and the porosity developed in the material. The measure is first validated on synchrotron images. Then the method is adapted to the laboratory microtomography prone to beam hardening artefacts. The results obtained with both devices are compared to each other and compared to the results obtained from local SEM analysis. Finally, the analysis of notched tensile specimens with varying notch root radius is investigated after interrupted tests. The analyses of the in situ tensile tests and relaxation effects are described in another paper [16].     

Automatic segmentation system for liver tumors based on the multilevel thresholding and electromagnetism optimization algorithm

In this article, we propose an automated segmentation system for liver tumors using magnetic resonance imaging and computed tomography. The proposed system is based on the algorithm of multilevel thresholding with electromagnetism optimization (EMO). The system starts with visualizing a patient's digital communication in medicine (DICOM) abdominal data set in three views. Two-stage active contour segmentation methods that integrate region-based local and global techniques using the active geodesic contour technique are proposed to segment the liver. To increase the accuracy and speed of segmentation for liver images, we identify the optimal threshold of the image segmentation method based on EMO with Otsu and Kapur algorithms. EMO offers interesting search capabilities while keeping a low computational cost. The proposed system was tested using a set of five DICOM data sets. All images were of the same size and stored in JPEG format (512 × 512 pixels). Experimental results illustrate that the proposed system outperforms state-of-the-art methods such as the watershed algorithm. The average sensitivity, specificity, and accuracy of the segmented liver using the active contour model were 97.05%, 99.88%, and 98.47%, respectively. Moreover, the average sensitivity, specificity, and accuracy of the segmented liver tumor results were 94.15%, 99.57%, and 96.86%, respectively.     

Fabrication,properties,and applications of open-cell aluminum foams:A review

Open-cell metallic foams or porous metals have a distinctive combination of excellent structural performance and superior functional characteristics,such as their light weight,energy absorption,sound absorption,heat dissipation,and electromagnetic shielding.As a primary representative of metallic foams,aluminum foam has developed into a new engineering material with many unique applications in the fields of aerospace,automotive industry,petrochemical industry,building materials,and etc.This paper summarizes the fabrication methods,properties,and applications of open-cell aluminum foams.The current status and development trends are also introduced.     

Strain-rate effects in Ni/Al composite metal foams from quasi-static to low-velocity impact behaviour

Metal foams are used as absorbers for kinetic energy but predominantly, they have only been investigated under quasi-static load-conditions. Coating of open-cell metal foams improves the mechanical properties by forming of Ni/Al hybrid foam composites. The properties are governed by the microstructure, the strut material and geometry. In this study, the strain-rate effects in open-cell aluminium foams and new Ni/Al composite foams are investigated by quasi-static compression tests and low-velocity impact. For the first time, drop weight tests are reported on open-cell metal foams, especially Ni/Al composite foams. Furthermore, size-effects were evaluated. The microstructural deformation mechanism was analysed using a high-speed camera and digital image correlation. Whereas pure aluminium foams are only strain-rate sensitive in the plastic collapse stress, Ni/Al foams show a general strain-rate sensitivity based on microinertia effects and the rate-sensitive nano-nickel coating. Ni/Al foams are superior to aluminium foams and to artificial aluminium foams with equal density.     

Prediction of the effect of artificial aging heat treatment on the yield strength of an open-cell aluminum foam

The effect of artificial aging on the compression yield strength of an open-cell AA6101 foam is studied using both experimental and modeling approaches. Isothermal calorimetry is used to analyze the precipitation kinetics of the foam. The modeling work combines the established approaches for predicting the yield strength of open-cell metallic foams as a function of the relative density and normalized strength, as well as the age hardening behavior of AA6101 alloy. The foam yield strength is related to the evolution of precipitate content during aging and is modeled for artificial aging at 180 and 220 °C. It is shown that the model predictions match very well with the experimentally determined yield strength values. The overall results suggest that the presented analytical and modeling approaches can effectively be used to predict the precipitation hardening behavior and/or optimize processing and properties of AA6101 foams.     

In‐Situ Synchrotron X‐Ray Microtomography Studies of Microstructure and Damage Evolution in Engineering Materials

In materials science X‐ray microtomography has evolved as an increasingly utilized technique for characterizing the 3D microstructure of materials. The fundamentals of X‐ray microtomography experimental methods and the reconstruction and data evaluation processes are briefly described. A review of in‐situ synchrotron X‐ray microtomography studies in literature is given. Examples of recent work include in‐situ microtomography investiagtions of metallic foams, in‐situ studies of the sintering of copper particles, and in‐situ investigations of creep damage evolution in composites. Future perspectives of in‐situ X‐ray microtomography studies in materials science are outlined.     

开孔与闭孔泡沫铝的压缩力学行为

研究了开孔与闭孔两种胞孔结构不同、制备工艺不同的泡沫铝在准静态压缩载荷下的压缩响应曲线.结果表明:开孔与闭孔泡沫铝压缩应力-应变曲线均具有多孔泡沫材料明显的三阶段特征,即线弹性段、塑性屈服平台段及致密段;相对密度对泡沫材料的力学性能(如杨氏模量、屈服强度)有很大影响;在准静态下,开孔泡沫铝表现出明显的应变率效应,而闭孔泡沫不如开孔敏感;泡沫铝材料表现为弱的各向异性;胞孔结构影响两种泡沫材料的压缩响应曲线.     

织物疵点检测与识别的方法研究

将织物图像分成大小相同的局部窗口,选取局部熵最小的窗口为待研究的感兴趣区域,在此区域内分割出疵点图像并用数学形态学中的开运算滤除噪声,计算疵点形状因子等作为识别参数,研究表明此方法因能避免对整幅图像进行复杂运算,在提取特征参数时对图像的全局搜索,具有识别正确率高、检测速度快等优点．     

Creep of open-cell Voronoi foams

The power-law creep of open-cell Voronoi foams is calculated using finite element analysis. The results are used to determine the geometrical constants in the power-law creep model for an open-cell foam with a random microstructure. In some foams, individual struts may be missing, either through fracture, as is the case in some metallic foams, or through resorption, as is the case in osteoporotic trabecular bone. Analysis of the effect of random removal of struts within the foam on the creep rate indicates that it can have a dramatic effect: removal of only a few percent of the struts can increase the creep rate by one to two orders of magnitude.     

3D mechanical analysis of low-density wood-based fiberboards by X-ray microcomputed tomography and Digital Volume Correlation

X-ray microtomography and Digital Volume Correlation are used to characterize the compressive behavior of fibrous materials, composed of wood fibers and thermoplastic fibers. 9-mm height and 9-mm diameter specimens are compressed uniaxially up to 30 % compression rate with an increment of 5 %. The evolution of microstructure is followed at different compression states by X-ray microtomography at a spatial resolution of 6 μm per voxel. Digital Volume Correlation is applied on microtomographic images to obtain the 3D strain field at each loaded state. The studied material shows a heterogeneous local strain field which relates not only to the complex microstructure but also to its modifications under solicitations. Microstructural parameters such as distributions of local porosities and fiber diameters are computed at different states by mathematical morphology. Relations between morphological parameters and 3D strain field are established. In a first approach, we show that the local mechanical behavior is controlled by distributions of local porosities.     

Pore connectivity of aluminium foams: effect of production parameters

This work studies the effect of some production parameters on the pore connectivity grade (i.e., the open-cell content associated cracks and missing cell walls) of aluminium foams produced via powder metallurgy route. Two types of precursors, extruded and hot uniaxially compressed, were used to create a varied group of Al–Si and Al–Si-Mg alloy-based foams in a wide porosity range. The cellular structure and defects were characterized by gas pycnometry and X-ray tomography. The analysis performed points to a high pore connectivity in all foam specimens, despite these materials are classified as closed celled due to their appearance, and a significant dependence on all the parameters varied. These dependences and the related mechanisms are discussed in the paper in terms of (i) the dissimilar foam evolution at initial stages (effect of precursor processing technique), (ii) the solidification shrinkage of each alloy (effect of composition) and (iii) the cell wall thinning (effect of foam porosity and local drainage). In addition, it has been observed that the interconnections are preferably located in the central parts of the samples, thus suggesting the possible effect of the cooling conditions on defect generation.     

局部高亮干扰文本图像的二值化方法研究

本文提出一种新的基于Curvelet变换的文本图像二值化处理方法,以消除文本图像中局部高亮度区域对二值化图像质量的影响.首先对具有局部高亮度区域干扰的原始文本图像进行Curvelet变换,得到图像在曲波域的Curvelet系数集；然后根据各Curvelet系数所表征的图像特征,对Curvelet系数进行非线性增强,以优化文本图像的直方图分布；对增强的Curvelet系数集进行反变换,得到直方图优化后的时域图像,进而应用Otsu方法实现文本图像二值化.应用本文方法对具有带状及点状局部高亮度区域的文本图像进行二值化处理,并采用ABBYYFineReaderl0对二值图像进行OCR识别.实验结果表明,通过本文提出的处理方法所得到的二值化图像,其字符的OCR识别准确率最高可达94.81％,优于其他四种典型的图像二值化处理方法.     

基于机器视觉的二维图像质量缺陷检测研究进展

目的 机器视觉图像处理技术是近年在图像处理领域发展起来的一门新兴边缘交叉学科，二维图像的质量检测是印刷行业中必不可少的环节，分析基于机器视觉的二维图像质量缺陷检测流程，探索影响基于机器视觉的二维图像质量缺陷检测精度的相关因素，为后续研究印刷品的二维图像自动化检测和质量控制提供参考。方法 在此基础上，围绕图像预处理中的灰度转换、噪声过滤、固定阈值分割、自适应阈值分割、Otsu法及边缘检测，对图像配准中的基于灰度统计信息分布配准方法、基于特征的图像配准方法进行总结，然后归纳分析图像的缺陷提取和分类。结论 以实际例子对上述研究内容进行了提炼，通过图像预处理中的噪声过滤为后续缺陷提取提供清晰图像，减少伪影干扰；通过图像预处理中的灰度变换、阈值分割、感兴趣区域提取减少系统处理时间，为实现高效的缺陷检测奠定了坚实的基础；通过图像配准消除了机械振动引起的图像位置偏移，确保后续缺陷提取的准确性；通过图像缺陷提取和分类帮助印刷企业找出生产问题，提供有针对性的改进措施，可为生产高质量产品提供支持。     

Improving performance of wavelet-based image de-noising algorithm using complex diffusion process

Abstract

Image enhancement and de-noising is an essential pre-processing step in many image processing algorithms. In any image de-noising algorithm, the main concern is to keep the interesting structures of the image. Such interesting structures often correspond to the discontinuities (edges). In this paper, we present a new algorithm for image noise reduction based on the combination of complex diffusion process and wavelet thresholding. In the existing wavelet thresholding methods, the noise reduction is limited, because the approximate coefficients containing the main information of the image are kept unchanged. Since noise affects both the approximate and detail coefficients, the proposed algorithm for noise reduction applies the complex diffusion process on the approximation band in order to alleviate the deficiency of the existing wavelet thresholding methods. The algorithm has been examined using a variety of standard images and its performance has been compared against several de-noising algorithms known from the prior art. Experimental results show that the proposed algorithm preserves the edges better and in most cases, improves the measured visual quality of the de-noised images in comparison to the existing methods known from the literature. The improvement is obtained without excessive computational cost, and the algorithm works well on a wide range of different types of noise.     

A visual consistent adaptive image thresholding method

Image thresholding method is an important preprocessing step in many applications of image processing. In this paper, we present a visual consistent adaptive image thresholding method, by combining the thresholding process with the characteristics of human visual system. In the proposed algorithm, we first roughly classify the image pixels into two categories. Then two sub-images are constructed to retain the essential information of the original image. For each sub-image, a corresponding global optimal threshold is calculated by optimizing an objective function. Finally, a visual consistent binary image is produced by combining the results from the previous steps with the information of pixels in each category. The proposed method has been tested on some different images, and the results are also compared with a number of known algorithms in the literature. The experimental results demonstrate that the overall visual qualities of our method are more suitable for human visual perception.     

Equivalent Thermal Conductivity of Open-Cell Ceramic Foams at High Temperatures

At high temperature, heat transfer in open-cell foams occurs by thermal radiation through the whole medium as well as by conduction through the solid matrix and air filling the pores. This paper applies the body-centered cubic cell model to predict radiative properties and the thermal conductivity of the open-cell foams. The model is validated by comparing the results with previous published works. Effects of structural characteristic parameters (cell diameter and porosity) and optical properties of the solid matrix (reflectivity and specularity parameter) on extinction coefficients and the radiative conductivity are discussed. The influence of temperature on the thermal conductivities including the effective, radiative, and the equivalent conductivity of open-cell ceramic foams are analyzed.