大型锻件内部空洞缺陷修复的力学条件

介绍了大型锻件内部缺陷修复条件研究的现状,针对钢锭内部空洞缺陷的尺寸远远小于本身尺寸的特点,依据圣维南原理,将各种空洞缺陷的形状假设为数学上可处理的椭球形。由于大型锻件的锻造是在高温下进行的,材料具有粘性流动的特征。在上述分析假设的基础上,对于线性粘性材料模型,利用损伤力学中远场应力与物体内部微观损伤的力学关系,得到了锻造过程中外载荷与内部空洞体积变化的解析式,进而得到了空洞缺陷闭合的修复条件。条件表明应力状态影响空洞的闭合方式,三向压应力是修复空洞缺陷的最佳应力状态。该条件可直接根据锻造水压机的在线载荷与压下量计算锻件内部空洞缺陷的修复情况。不同边界应力状态下的试验表明,计算结果与试验结果接近。     

一种基于断层测量的反求工程

断层测量技术能同时测量物体的表面和内腔尺寸 ,在反求工程中具有较好的应用前景。现有方法如 CT和 MRI的测量精度较低 ,且成本很高。针对上述问题 ,研究了一种基于层去图像法的断层测量系统 ,并给出了它在反求工程中的应用     

Measurement accuracy in X-ray computed tomography metrology: Toward a systematic analysis of interference effects in tomographic imaging

In this paper an investigation of interference effects leading to limitations of metrological performance of X-ray computed tomography (CT) used as a coordinate measuring technique is presented. Using reconstruction data, image quality metrics, and calculations of artifact formation, a deeper understanding and explanation of the physical and technical limitations of CT used in dimensional metrology is given. This is demonstrated in a case study using a simple hollow cylinder made of steel as a test object and calibration measurements from a tactile coordinate measuring machine (CMM). Two different threshold determination strategies for surface computation are applied. Within the study it is also shown that CT image properties, threshold determination strategies, and systematic and random measurement errors must have a definite correlation. As a conclusion it is recommended to focus more strongly on the correlation of local CT image quality and data evaluation operations in order to reduce systematic errors in surface computation and to increase repeatability of dimensional CT measurements.     

Three dimensional computerized microtomography in the analysis of sculpture

The Alte Nationalgalerie, Staatliche Museen zu Berlin (SMB) and the Federal Institute for Materials Research and Testing (BAM) tested the accomplishment of the three dimensional computerized microtomography (3D-microCT)-a new flat panel detector computerized tomography (CT) system at the BAM with extended energy range, with high voltage X-ray tubes (330 and 225 kV), with micrometer focal spot size and micrometer resolution and enlarged object size (up to 70 cm diameter)-for examining plaster statues.The high spatial and density resolution of the tomograph enable detailed insights into the individual work processes of the investigated cast plaster statues. While initiated in support of the conservation process, computed tomography (CT) analysis has assisted in revealing relative chronologies within the series of the cast works of art, thus serving as a valuable tool in the art-historical appraisal of the oeuvres.The image-processing systems visualize the voids and cracks within and the cuts through the original cast works. Internal structures, armoring, sculptural reworking as well as restorative interventions are virtually reconstructed.The authors are currently employing the 3D-microCT systems at the BAM into the detection of defects in Carrara marble sculpture. Microcracks, fractures, and material flaws are visualized at spatial resolution down to 10 microm. Computerized reconstruction of ultrasound tomography is verified by analyzing correlations in the results obtained from the complementary application of these two non-destructive testing (NDT) methods of diagnosis.     

Material influence in coordinate measurement using X-ray CT

The X-ray computed tomography system (XCT) as a coordinate measuring system has been increasingly introduced into the manufacturing industry because of its capability of measuring internal features. However, the metrological performance evaluation method for XCT has not yet been standardized. One of the biggest reasons of this is the difficulty in evaluating the material influence. The previous research by the authors revealed that the material influence significantly affects the form of the object to be measured. ISO/TC 213/WG 10, which covers the ISO standards of coordinate measuring machines, proposes the introduction of an additional object, called an “obstructive body,” by which the material influence is enhanced. This study presents the experimental results and a software simulation, in which the material influence is produced in a specific area and not on the entire surface of the test object. Additionally, requirements for the obstructive body in the industrial standardization of performance evaluation methods for XCT are proposed.     

A computer simulation platform for the estimation of measurement uncertainties in dimensional X-ray computed tomography

The knowledge of measurement uncertainty is of great importance in conformance testing in production. The tolerance limit for production must be reduced by the amounts of measurement uncertainty to ensure that the parts are in fact within the tolerance. Over the last 5 years, industrial X-ray computed tomography (CT) has become an important technology for dimensional quality control. In this paper a computer simulation platform is presented which is able to investigate error sources in dimensional CT measurements. The typical workflow in industrial CT metrology is described and methods for estimating measurement uncertainties are briefly discussed. As we will show, the developed virtual CT (VCT) simulator can be adapted to various scanner systems, providing realistic CT data. Using the Monte Carlo method (MCM), measurement uncertainties for a given measuring task can be estimated, taking into account the main error sources for the measurement. This method has the potential to deal with all kinds of systematic and random errors that influence a dimensional CT measurement. A case study demonstrates the practical application of the VCT simulator using numerically generated CT data and statistical evaluation methods.     

Determining casting defects in near-net shape casting aluminum parts by computed tomography

Three types of near-net shape casting aluminum parts were investigated by computed tomography to determine casting defects and evaluate quality. The first, second, and third parts were produced by low-pressure die casting (Al-12Si-0.8Cu-0.5Fe-0.9Mg-0.7Ni-0.2Zn alloy), die casting (A356, Al-7Si-0.3Mg), and semi-solid casting (A356, Al-7Si-0.3Mg), respectively. Unlike die casting (second part), low-pressure die casting (first part) significantly reduced the formation of casting defects (i.e., porosity) due to its smooth filling and solidification under pressure. No significant casting defect was observed in the third part, and this absence of defects indicates that semi-solid casting could produce high-quality near-net shape casting aluminum parts. Moreover, casting defects were mostly distributed along the eutectic grain boundaries. This finding reveals that refinement of eutectic grains is necessary to optimize the distribution of casting defects and reduce their size. This investigation demonstrated that computed tomography is an efficient method to determine casting defects in near-net shape casting aluminum parts.     

工业CT检测中弹壁厚度对装药均匀性测量结果的影响

利用工业CT技术检测弹体轴向装药均匀性以及弹壁厚度变化对测量结果的影响, 对比了同一模拟装药样件装配在不同壁厚的模拟弹体内时CT相对密度测试结果的差异, 并分析了弹壁厚度对装药CT相对密度值的影响. 结果表明, 对于弹体外径约100 mm、 内径约90 mm的样品, 弹壁厚度每增加1 mm, 同一装填材料的CT值数据随之增加约1%左右. 因此, 为检测弹体轴向(即上段、中段、下段)装药密度均匀性, 在工业CT设备穿透能力范围内, 不用测出装药的绝对密度, 只需检测上、中、下各个部位的装药CT值, 并减除弹壁厚度变化量对CT值产生的影响量, 通过比较上、中、下各个部位装药最终的CT值就可获得弹体轴向装药的密度均匀性.     

大型工业CT在摇枕、侧架无损检测中的应用

摇枕、侧架作为列车转向架的重要组成部分,对列车行驶安全起到至关重要的作用。各生产厂家亟需具有高穿透力的检测设备对其进行大批量的检测。本文介绍了采用工业CT技术的一种新型的CT/DR扫描系统,能够对摇枕、侧架的内部结构及内部的气孔、砂眼、夹杂物、缩孔、疏松、冷隔、裂纹等铸造缺陷进行无损检测,能够很好地满足该行业的需求。     

A simple method for CT-Scanner calibration against effective photon energy

An original method for CT-scanner calibration against effective photon energy is presented. It is based on optimization of the CT-scanner output characteristic (the dependence of the measured linear attenuation coefficient on the calculated value). A significant gain in accuracy is achieved with respect to the calibration techniques which are conventionally used in commercial computed tomography. The method can be recommended for use in dual-energy computed tomography. As an example of its application, measurements of the effective atomic number and electron density of organic plastics are described. It is shown that the method provides acceptable results even for an increased noise level in CT images.     

Use of X-ray micro tomography to study the microstructure of loose-packed and compacted milk powders

The use of X-rays to evaluate food microstructure has developed considerably in recent years. In this work we used X-ray micro tomography (XMT) to visualize the microstructure of loose-packed and compacted samples of spray-dried skim milk powder (SMP) and whole milk powder (WMP) and to quantify the proportion of both interstitial and occluded air voids in each sample. Visual scrutiny of the acquired XMT images showed details of the microstructure of the milk powders such as the spherical morphology of the particles, the size of the particles and internal air voids of various sizes. Within loose-packed powders, the proportion of air voids was higher (13% average) in WMP than in SMP. This was reversed in the compacted powders, in which the proportion of air voids was higher in SMP. The disparity in the proportion of air voids in both loose-packed and compacted samples of SMP and WMP was attributed to the size, shape and surface properties of the particles.     

Study of void closure in hot radial forging process using 3D nonlinear finite element analysis

Hot radial forging is used to reduce porosity and increase strength for large-diameter billets. The goal of this research is to study void closure behavior in the hot radial forging process. A nonlinear coupled finite element model is developed to investigate the deformation mechanism of internal void defects during the hot radial forging process. The model is formulated in a three-dimensional frame and a viscoplastic material model has been used to describe the material behavior subjected to large deformation and high temperature. A global–local technique is employed to obtain accurate solutions around the void region. The effects of void location, mandrel, die shape, and the reduction of the tube thickness on the final void reduction are systematically investigated. The predicted reductions for central longitudinal voids in hot upsetting and hot rolling processes are in good agreement with experimental findings. The simulation results provide a valuable procedure for the design of porosity reduction during the hot radial forging process.     

Improved curvature-based registration methods for high-precision dimensional metrology

Multiple measurements using various data acquisition systems are generally required to substancially enhance measurement accuracy, reliability and holisticity of freeform shapes. The obtained multiple measurement data of the shape are transformed and fused into a common coordinate system within a registration technique involving coarse and fine alignments. Standardized methods have been established for fine registration such as Iterative Closest Points (ICP) and its variants. For coarse registration, no conventional method has been adopted yet despite a significant number of techniques which have been developed in the literature to supply an automatic rough matching between data sets.The work presented in this paper proposes an improvement of registration techniques by the consideration of new discrete curvature parameters. Two main issues are addressed in this paper: the coarse registration and the fine registration. For coarse registration, two novel automated methods based on the exploitation of discrete curvatures are presented: an enhanced Hough Transformation (HT) and an improved Ransac Transformation. The use of curvature features in both methods aims to reduce computational cost. For fine registration, a new variant of ICP method is proposed in order to reduce registration error using curvature parameters. A specific distance considering the curvature similarity is combined with Euclidean distance to define the distance criterion used for correspondences searching. Additionally, the objective function is improved by combining the point-to-point (P-P) minimization and the point-to-plane (P-Pl) minimization with automatic weights. The algorithms are applied on simulated and real data performed by a computed tomography (CT) system. The obtained results reveal the benefit of the proposed improved curvature-based registration methods.     

Measurement of temperature induced X-ray tube transmission target displacements for dimensional computed tomography

We characterised the displacement of a transmission X-ray tube target parallel to the beam axis due to thermal effects using interferometry. Displacements of up to 10 μm with a sensitivity of about 0.4 μm/W deposited power were measured relative to the X-ray tube's mechanical mounting position. They significantly influence the scale factor of dimensional computed tomography measurements by several 10⁻⁴ at high geometric magnifications. Finite element simulations revealed that the displacement is due to thermal deformation of the entire target assembly and helped to derive a linearised model that predicted the displacements within a few tenths of a micrometre. Radiographic measurements of a calibrated standard confirmed that the mechanical displacement of the target corresponded with the X-ray focal spot position. The findings help to increase the accuracy of dimensional X-ray computed tomography measurements and to ameliorate the design of transmission X-ray tubes.     

Sensitivity of X-ray computed tomography measurements of a gas-solid flow to variations in acquisition parameters

Due to its high spatial resolution and non-invasive nature, X-ray computed tomography has become a popular method for determining the flow characteristics of multiphase flows. However, because many of the X-ray computed tomography systems used for non-destructive imaging of multiphase flows provide the operator wide leeway in the selection of imaging parameters, the potential exists for errors to be introduced into the measurements if the algorithms are sensitive to these changes. In this paper, a representative multiphase flow (specifically, a fluidized bed) is imaged with a wide range of X-ray tube electrical potentials, currents, and detector exposure times and reconstructed with a wide range of centers of rotation. The results of these tests show that while the raw CT intensities are sensitive to these parameter variations, once the measurements are calibrated to reference images (in this case through a void fraction calculation), the final results are insensitive to most changes. In the extreme cases where there is some sensitivity to the parameter changes, the causes and practical implications are discussed.     

Three-dimensional image reconstructions of complex objects by an abrasive computed tomography apparatus

The process of capturing the shapes of objects through surface data sampling and generating a 3D CAD model is termed reverse engineering (RE) because the process is the reverse of the normal design and manufacturing sequence. The digitisation process can be achieved through spatial measurements taken by a coordinate measuring machine (CMM). The way of capturing data in a CMM can be divided into contact or non-contact methods. Diode-lasers or CCD cameras are commonly applied in non-contact measuring, while touch probes are applied in contact measuring. There are limitations in obtaining data for complex objects (for under-cut or inner structures) by means of the two abovementioned methods. Therefore, we have designed and assembled a novel device, called an abrasive computed tomography (ACT) apparatus, to overcome the abovementioned limitations. The ACT apparatus uses an abrasive method to remove the inlaid object layer by layer and to capture the cross-sectional image of each layer with a CCD camera. A numerical scheme is applied to obtain the Bezier curve of the boundary in each layer. The combination of all cross-sectional boundaries is used to reconstruct the 3D CAD model of the object. The 3D CAD model can then be transmitted to generate the tool paths in a CNC machine or produce rapid prototyping in a RP machine.     

Non-contact inspection of internal threads of machined parts

During the manufacturing process of mass-produced automotive parts, millions of internal threads are produced annually by each automaker. The manufacturers invest a lot of effort and resources to improve the quality of the threads by inspecting their features, mainly by using manual contact gages. The majority of manufactured threads are used in threaded joints that assemble two mechanical parts. However, there are threads that have additional functionality, for example, an automotive spark plug. Recently, automotive designers have found that the angular direction of the spark plug inside the combustion chamber affects the efficiency of the engine. Hence, the manufacturers are interested in measuring the exact position of the thread starting point, allowing them to position the plug in the right direction during the plug assembly process. Our technique allows precise measurement of the thread starting point and meets this requirement. In order to improve the quality of internal threads, it is essential to measure the threads dimensional features. It is also important to inspect its internal surface for defects such as the existence of burrs, thread discontinuities, or internal porosity defects. The paper introduces two novel non-contact electro-optical techniques for inspecting small-size internal threads. The first one enables in-process internal thread geometrical measurements using a high-precision laser sensor which utilizes a motorized periscope. The second technique is based on image processing and uses innovative machine vision; a line-scan technique which uses a CCD camera with a “sight pipe” panoramic optical device. The two methods are complementary; together they allow the practical inspection of all needed features of the internal thread. The methods were tested successfully.     

A novel depth-from-focus-based measurement system for the reconstruction of surface morphology with depth discontinuity

Errors due to magnification variations are caused in optical systems when three-dimensional shape measurements based on depth-from-focus (DFF) are performed. These magnification variations cause focus measures to be incorrectly interpreted. In this paper, a DFF-based system which considers the magnification variations has been developed to undertake accurate measurements of surface morphology with depth discontinuity. The image magnification can be represented by magnification factors, which are computed using the ratio of diagonal image lengths of a rectangular calibration block, according to optical system movements. The image calibration is performed using the magnification factors. The calibration makes the image size of the object features identical, in spite of magnification variations resulting from optical system movements. Therefore, the application of accurate and reliable focus measures is made in the calibrated images. The performance of our presented system has been verified through experiments with actual objects with depth discontinuity.     

激光位移传感器在自由曲面测量中的应用

以点激光位移传感器(HL-C211BE)为对象,研究它在自由曲面测量中的应用。针对激光位移传感器因测点倾角代入的测量误差,提出了一个可以量化的倾角误差模型。基于直射式点激光三角法原理,分析了激光光路的几何关系,从会聚光斑光能质心发生的偏移推导出倾角误差模型。随后,用高精度激光干涉仪和正弦规对激光位移传感器进行校对实验,并用误差模型对测量结果进行补偿。结果显示,补偿后激光位移传感器的测量精度得到明显提高。对一非球面凸透镜进行了实验测量,得到了自由曲面测点倾角的计算方法,并用倾角误差模型修正了测量数据。实验结果表明,量化的倾角误差模型可以将激光位移传感器的测量误差控制到小于10μm,满足激光位移传感器在自由曲面测量中应用的要求。