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Non-destructive characterisation of polymers and Al-alloys by polychromatic cone-beam phase contrast tomography
Abstract
X-ray computed tomography (XCT) has become a very important tool for the non-destructive characterisation of materials. Continuous improvements in the quality and performance of X-ray tubes and detectors have led to cone-beam XCT systems that can now achieve spatial resolutions down to 1 μm and even below. Since not only the amplitude but also the phase of an X-ray beam is altered while passing through an object, phase contrast effects can occur even for polychromatic sources when the spatial coherence due to a small focal spot size is high enough. This can lead to significant improvements over conventional attenuation-based X-ray computed tomography. Phase contrast can increase by edge enhancement the visibility of small structures and of features which are only slightly different in attenuation.
... The reconstructed data were processed and visualised with the programme VGStudio MAX 2.1. Further details can be found in [4] and [5]. Figure 1 shows a cross-sectional XCT-image and a three-dimensional visualisation of a short glass-fibre reinforced polymer. Due to the rather large density difference between the fibres and the polymer matrix, individual fibres are clearly visible in the XCT-data and can be segmented 3-dimensionally. ...
... The right picture of Figure 3 shows an XCT cross-sectional picture of talcum-filled polypropylene. Due to the high resolution and high coherence phase contrast, effects occur at the edges of the particles and polymer [4] resulting in an upward and downward overshooting of the grey values. The individual talcum particles can be clearly seen in Figure 3. ...
... It can also be seen that the grey value at the polymer-air interface is much higher than within the polymer. This can be explained by a rather strong phase contrast effect caused by the small focal spot size and the high resolution of the XCT-measurement [4,7]. Figure 6 shows a cross-sectional XCT picture of polypropylene with cellulose fibres. ...
... X -ray computed tomography (XCT) with cone beam geometry fulfils the needs of materials science to a great extent. XCT systems with nano-focus X-ray tubes in combination with flat panel matrix detectors can achieve voxel sizes down to (0.3 µm) 3 [1,2]. The measurement principle of XCT with cone beam emitted from an X-ray tube is as follows: A specimen is placed on a rotary stage between the matrix detector and the X-ray tube producing a polychromatic cone beam. ...
... A 3D-image can be reconstructed from these several hundred of projections. Recently, phase contrast laboratory X-ray computed tomography was utilized for materials science [3]. Since not only the amplitude but also the phase of an X-ray beam is altered while passing through an object, phase contrast effects can occur even for polychromatic sources when the spatial coherence due to a small focal spot size and a big distance between the source and the detector is high enough. ...
... Voltages between 50 and 100 kV were used. The voxel size used was between (0.5 µm) 3 and (1.5 µm) 3 . These parameters required measurement periods between 60 and 600 min. ...
High strength aluminium alloys are widely used in the aerospace and automotive industries because of their high specific strength, high specific stiffness, resistance to corrosion, toughness and fatigue durability. Usually, the as-cast microstructure consists basically of α-Al dendrites and intermetallic phases segregated in the interdendritic regions during the last part of the solidification. On the basis of selected examples of AlCuMg (AlCu4Mg1) and AlZnMgCu (AlZn7.7Mg2.3Cu1.5 + Sc, Zr) alloys, the possibilities and limits of high resolution cone beam X -ray computed tomography (XCT) with a transmission target yielding a minimum voxel size of (0.5 µm) 3 are presented. The 3D arrangement and morphology of the dendritic structure is analyzed within this contribution. Phase identification is performed by metallography and energy dispersive X-ray analysis. The evolution of the volume fraction of the different phases (eutectic phase and pores) during heat treatment is determined from XCT-measurements as a function of temperature and time for the AlCu4Mg1-alloy. Results for two different heat treatment temperatures are presented. For the AlZnMgCu alloys we show a method for the discrimination of two different types of segregations, which appear in the XCT-data as higher-absorbing features in comparison to the Al–matrix. We demonstrate that Al 3 (Sc,Zr) particles >5 µm can be distinguished from interdendritic segregations by their sphericity. Furthermore, we present the possibilities to detect primary Si-particles within an Al-matrix in the cast alloys AlSi18 and AlSi17Cu4 by X-ray phase contrast tomography.
... While not subject to human error, these new processes require a better understanding of the materials' microstructure in order to fully exploit the mechanical properties. For example, much work has been dedicated to the quantification of porosity using non-destructive evaluation techniques such as high resolution computed tomography (CT), ultrasonic testing or active thermography [3][4][5][6]. ...
... The contrast between the bundles running in plane and out of plane is due to phase effects at the edges of single fibres, which occur when the spatial coherence of the X-ray beam is sufficient (e.g. small focal spot size and large distance between the object and the X-ray detector) [4]. When comparing Fig. 4(c) and (d), there is an excellent correlation between the two images. ...
... This contrast enhancement relies on the use of very small focal spots (1-5 mm) and large magnification factors. Pushing the high resolution CT to its limits, Kastner et al. showed that even single fibres could be individually resolved and segmented [4,32]. However, the contrast enhancement is gained at the expense of the size of the field of view, which is restricted to centimetre-sized samples. ...
... Most relevant PEs to be considered were detector and photon noise to provide realistic noise contribution and image blur caused by the scintillator [19]. Propagation-based phase contrast effects [20] are not implemented in the SimCT tool. ...
... By ignoring the results from Specimen 2, because the SA/V ratio was at 0.17 µm −1 instead of 0.4 µm −1 from the reference part, for the remaining five specimens, a relative error of less than 3% for the (5 µm) 3 and less than 10% for (10 µm) 3 simulations was determined. In addition, it should be noted that in real CT scans, propagation-based phase contrast effects also occur [20,24], which can significantly increase the visibility of micro-voids with certain CT systems, resulting in lower ISOxx threshold values. ...
In this study new insights regarding quantitative porosity determination using X-ray computed tomography (CT) could gained by using CT simulation tools to generated synthetical 3D data with a well known nominal porosity content. Therefore 1,988 typical microstructure elements such as individual carbon fibres, needle like micro voids as well some macro voids embeded in a polymer matrix (epoxy resin & polyamid 6) was used as geometrical imput. The results of the threshold-based segmentation methods in this paper are in good agreement with similar experimental results previously published by the authors.
... Most relevant PEs to be considered were detector and photon noise to provide realistic noise contribution and image blur caused by the scintillator [19]. Propagation-based phase contrast effects [20] are not implemented in the SimCT tool. ...
... By ignoring the results from Specimen 2, because the SA/V ratio was at 0.17 µm −1 instead of 0.4 µm −1 from the reference part, for the remaining five specimens, a relative error of less than 3% for the (5 µm) 3 and less than 10% for (10 µm) 3 simulations was determined. In addition, it should be noted that in real CT scans, propagation-based phase contrast effects also occur [20,24], which can significantly increase the visibility of micro-voids with certain CT systems, resulting in lower ISOxx threshold values. ...
The fast and correct determination of porosity in carbon fibre–reinforced polymers and other polymeric material is an important application area of X-ray computed tomography (CT). In this CT simulation study, microstructures such as individual carbon fibres, micro-voids, as well as the polymer matrix including mesoscale voids, were taken into account to generate CT data synthetically with well-known porosity. It was found that the ratio of total surface area (SA) of the voids divided by the total volume (V) of the voids is suitable to describe and differentiate individual porosity samples. Our investigations revealed that the ratio SA/V can be used to estimate a minimal necessary voxel size for proper porosity segmentation by a simple ISO50 threshold. Under certain conditions, using an adapted ISOxx threshold value at a resolution of (10 µm)³ voxel size, the porosity in a specimen can be determined with an average measurement error below 10%. As long as the CT resolution is not high enough to completely resolve all void structures, using global threshold segmentation is always a compromise between over-segmentation of the macro-voids and under-segmentation of micro-voids.
... The formation of interference fringes depends on the degree of X-ray coherence [17], [23]. Amount and appearance of the fringes is influenced by the detector position [24]. At the same time, the intensity of X-rays emerging from focal spot follows the inverse-square law: I(x, y) ∝ 1/z 2 p for the propagation distance z 2 p . ...
... The edge enhancement was evaluated on an averaged line profile on the edge of air and polymer (five pixels above and below the line were averaged to reduce noise). Following parameters, signal to noise ratio SNR, absorption contrast value C ABS and phase-contrast value C PHC , were used as a figure of merit [24]: ...
X-ray phase contrast imaging (PCI) is sensitive to phase shift of X-rays induced by sample. This is advantageous for low X-ray absorption samples such as polymers, biomaterials, tissues, scaffolds etc. We show propagation-based phase-contrast X-ray computed tomography imaging (PBI) in a specific configuration with the wide-cone angle (>50∘), polychromatic X-ray source and a flat panel detector. We demonstrate PBI on measurements with a polymer composite. The experiments show a trade-off between high signal-to-noise ratio (SNR) acquisition at very large cone-angles and low-SNR acquisition at large propagation distances. The degree of data quality improvement by phase retrieval increases for high propagation distances. We show the application of PBI on macroporous synthetic hydrogels, which represent an important type of materials with a complex 3D morphology in the field of polymer science. With the use of the above described experimental configuration, it is possible to visualize the hydrogels and segment the structure of the sample in tomographic data. The segmented sample can be used for morphology characterization, such as the description of internal space or determination of specific surface area.
... Development of materials, precise mechanics, and electron focusing systems in laboratory X-ray sources made the propagation-based imaging available on many CT devices. Examples of edge enhancement via PBI with a laboratory, polychromatic X-ray source have been reported on various examples of fibres in the polymer matrix [33]- [35], mouse lungs [36], liver tissue [37] or cochlea [38] or leaves and insects [39]. ...
... The edge enhancement was evaluated on an averaged line profile (pixels in 8 μm area above and below the line were averaged to reduce noise). It was specifically evaluated on the edge of air and polymer matrix by calculating absorption contrast value C ABS and phase contrast value C PHC according to [33]: ...
Phase contrast imaging (PCI) is used to extend Xray computed nanotomography (nCT) technique for analyzing samples with a low X-ray contrast, such as polymeric structures or soft tissues. Although this technique is used in many variations at synchrotrons, along with the development of X-ray tubes and X-ray detectors, a phase contrast imaging becomes available also for laboratory systems. This work is focused on determining the conditions for propagation-based PCI in laboratory nCT systems based on three criteria. It is mostly employed in near-field imaging regime, which is quantified via Fresnel number. X-rays must reach certain degree of coherence to form edge-enhancement. Finally, setup of every CT measurement has to avoid geometrical unsharpness due to the finite focal spot size. These conditions are evaluated and discussed in terms of different properties and settings of CT machine. Propagation-based PCI is tested on a sample of carbon fibres reinforced polyethylene and the implementation of Paganin phase retrieval algorithm on the CT data is shown.
... Recently, novel X-ray imaging methods based on the phase shift of the X-ray beam inside the object have been developed, which offer a significant contrast enhancement. Indeed, the phase φ of an X-ray beam is altered while passing through a material according to the following relation [1]: ...
... The reconstructed data were processed and visualized with the program VGStudio MAX 2.1. Further details can be found in [1] and [2]. ...
Currently the basis for standard X-ray computed tomography (CT) is absorption. A volumetric map of a specimen in three dimensions is generated from a set of absorption radiographs. The contrast of details strongly depends on a difference in absorption coefficient between the detail and the environment. However, when the absorption difference is low, sufficient contrast for a good quality X-ray image can be not obtained. During the last decade, a number of novel X-ray imaging methods have been developed, which are based on the phase shift of the X-ray beam passing through a material. This contrast mechanism allows for a significant increase of the contrast enhancing the interface. X-ray computed tomography phase imaging methods can be classified into (1) interferometric methods, (2) techniques using an analyser (or diffraction enhanced imaging) and (3) free space propagation based methods (also called in-line based methods). In this presentation, we compare an interferometric CT method based on the Talbot-Lau interferometer with propagation based phase contrast CT-method and discuss the possibilities, advantages and restrictions in the field of non-destructive testing and evaluation.
... However, this is not a straightforward task since conventional imagining techniques, such as scanning electron microscopy (SEM), provide only meaningful information on the cell size and thickness of the solid phase [15][16][17][18]. In this context, non-destructive 3D imaging techniques such as X-ray micro-computed tomography (μCT) are suitable for studying the 3D morphology of polymer foams [19][20][21]. ...
The three-dimensional (3D) microstructure of sustainable and biodegradable protein foam absorbents is correlated to their liquid absorption characteristics using X-ray microtomography. The physicochemical relationships between the protein material pore size and liquid penetration and distribution allow for understanding how the pores' interconnectivity impacts the absorption, particularly considering capillary-driven transport phenomena within the thin nano cell walls. The foams were made via lyophilization of protein solutions containing cellulose nanofibers to emphasize the impact of the processing on the foam microstructure. The results show gaseous and solid phases of the foams covering ca. 1000 pores, providing information that cannot be obtained using traditional 2D analysis (SEM). A correlation with the channel tortuosity was established based on the statistical accuracy of the diameter and number of pores per mm³. The relationship between absorption kinetics and physical parameters enables the designing of biofoams with functionality also resembling commercial synthetic products that currently generate a high amount of nondegradable waste in our society.
... The dark-field indicates small-angle scattering within the sample, giving an insight to structural material properties on a sub-pixel scale. Owing to these capabilities, grating-based phase-contrast and dark-field imaging are of great interest in the fields of medical imaging [33][34][35][36][37][38][39][40][41] or non-destructive testing [42][43][44], which includes studies on paper material [45] or archaeological findings [46]. ...
If ancient documents are too fragile to be opened, X-ray imaging can be used to recover the content non-destructively. As an extension to conventional attenuation imaging, dark-field imaging provides access to microscopic structural object information, which can be especially advantageous for materials with weak attenuation contrast, such as certain metal-free inks in paper. With cotton paper and different self-made inks based on authentic recipes, we produced test samples for attenuation and dark-field imaging at a metal-jet X-ray source. The resulting images show letters written in metal-free ink that were recovered via grating-based dark-field imaging. Without the need for synchrotron-like beam quality, these results set the ground for a mobile dark-field imaging setup that could be brought to a library for document scanning, avoiding long transport routes for valuable historic documents.
... Since composite materials are considered as heterogeneous media, this technique is widely used in structural investigations to verify porosity levels, fiber concentration and orientation for instance. Additionally, it is also used in numerous fields such as NDT operations to detect damage mechanisms mainly delamination, fiber/matrix debonding and the matrix cracking (Kastner, Plank, and Requena 2012;Palka et al. 2016). Radiography technique has proven its efficiency in the NDT investigations and present a variety of advantages. ...
La détection et la caractérisation des endommagements dans des composantes automobiles fabriquées en matériaux composites restent un souci majeur que les constructeurs automobiles cherchent à confronter. Le projet de thèse s'inscrit dans cette thématique et a comme but l'analyse qualitative et quantitative des endommagements induits par différentes sollicitations dans un composite à matrice polyamide 66/6 renforcé de fibres de verre tissées sergé 2/2. Pour cela, des essais d’impact à faible vitesse à différents niveaux d’énergie, des essais de fatigue en traction-traction ainsi que des essais d’impact post-fatigue, sont réalisés. Une méthodologie expérimentale qui repose sur l'utilisation des méthodes de contrôle non-destructif est établit et a pour but l’évaluation de l’endommagement induit par différents chargements. Différents outils sont utilisés dans cette étude, à savoir : la microscopie optique, la microscopie électronique, la tomographie aux rayons X, les ondes guidées (les ondes de Lamb), l'imagerie C-Scan par propagation des ondes de volumes et d'autres outils ultrasonores. En effet, les mécanismes d’endommagement liés aux multiples sollicitations sont observés et leurs degrés de propagation en fonction du type et du niveau de chargement sont examinés. Les propriétés résiduelles sont estimées par le biais de la vitesse de propagation des ondes de volume. En outre, une investigation basée sur la propagation des ondes de Lamb est menée. A l'issu de cette étude, des indicateurs d'endommagement, permettant de quantifier l'état d'endommagement induit dans chaque échantillon inspecté, sont proposés. Finalement, des essais de traction quasi-statique sont réalisés pour la prédiction des propriétés résiduelles post-chargement.
... With conventional XCT methods, spatial resolutions up to (0.5 µm) 3 voxel size are possible [26]. Additional phase contrast effects can increase by edge enhancement the visibility of small structures and of features that are only slightly different in attenuation [29]. Several experiments have been done on Al-Si or Al-Si-Mg alloys using the XCT method for micro-pore detection [30][31][32]. ...
The formation of an inhomogeneous structure with central segregation occurs in commercial twin-roll cast AA 3003 aluminum alloy. The segregations form as manganese, iron, and silicon-rich channels spread in the rolling direction. Diffusion of silicon occurs during annealing at 450 °C, and the formation and growth of voids due to the Kirkendall effect occur. The evolution of pores studied by scanning electron microscopy and X-ray computed tomography shows that pores are formed near original pure silicon clusters. Their coalescence and the formation of large voids in the central zone of the strip occur at longer annealing times.
... It was shown that most of the fibres had a weighted average length of about 101 µm and an average fibre length of 86 µm in all examined samples ( Table 2). It is worthwhile noting that the fibre lengths obtained are rough guide values, because the fibres shorter than 50 µm were not taken into account to avoid possible misssegmentations of phase contrast edge effects along every void occurring in the high-resolution µCT [39]. For a small minority of fibre interacts, the algorithm was not able to track the entire fibre length, because of virtual fibre breakage. ...
Selective laser sintering (SLS) is an additive manufacturing process which nowadays receives abundant attention from industry sectors. However, the number of materials which can be processed by SLS is still very limited and requires further research. The present work aims to contribute to this topic by investigating the mechanical properties of neat and short carbon fibre reinforced polyamide 1212 processed by SLS. The specimens were built in different spatial alignments to obtain ample details on the tensile behaviour. The detailed examinations of the fractured specimens were performed by means of optical microscopy, scanning electron microscopy and X-ray computed tomography. The comprehensive analysis revealed that most of the fibres (85 – 95%) were oriented in the plane of the powder layer and here, the majority along the direction of the moving roller coater, which distributes the powder on the powder bed of the SLS machine. It was shown that this effect has a direct impact on the strength and stiffness of the printed tensile bars and thus on the mechanical behaviour of SLS printed parts. Furthermore, the analysis results indicate the possibility to control this mechanical anisotropy through a systematic alignment of the components in the powder cake.
... Sample number 7 was chosen to measure the threedimensional pores to achieve more precision using 3D x-ray CT. X-ray CT with cone-beam geometry has been used for non-destructive characterization and evaluation of materials [37]. X-ray transmission investigated the specimen using the same type of x-ray tube as in the micro-focused x-ray transmission imaging system [8]. ...
Simulations of the welding process for butt joints using finite element analysis (FEA) of the effect of porosity are presented. The metal used was aluminium alloy (grade 2024), and the filler material was alloy ER5356. The simulations were performed using the commercial software ANSYS, considering a double ellipsoid heat source, temperature-dependent material properties, material deposits, mechanical analysis, transient heat transfer, and defects (porosity). In this study, the FEA simulations were constructed for two types of heat source (single- and double-ellipsoid) used in gas tungsten arc welding (GTAW), and the calculated residual stress results were compared with the experimental values. Two double ellipsoid models were constructed for cases with and without porosity. The porosity was measured by three-dimensional (3D) computed tomography (CT), and the size and location of pores were mapped onto the weld bead created by the birth-and-death technique.
... Phase contrast is due to gradients of the effective phase shift, i.e., the real part of refraction index times projected thickness [1]. In contrast to these long range changes (with respect to the pixel size), dark field contrast refers to refractive ultra-small angle scattering by microscopic sample inhomogeneities namely particles, pores and fibers [8][9][10][11][12]. These three signals are directly related to instrument properties, such as mean transmitted intensity, initial phase position and the amplitude of oscillations of the interference patterns (without sample). ...
The performance of grating interferometers coming up now for imaging interfaces within materials depends on the efficiency (visibility) of their main component, namely the phase grating. Therefore, experiments with monochromatic synchrotron radiation and corresponding simulations are carried out. The visibility of a phase grating is optimized by different photon energies, varying detector to grating distances and continuous rotation of the phase grating about the grid lines. Such kind of rotation changes the projected grating shapes, and thereby the distribution profiles of phase shifts. This yields higher visibilities than derived from ideal rectangular shapes. By continuous grating rotation and variation of the propagation distance, we achieve 2D visibility maps. Such maps provide the visibility for a certain combination of grating orientation and detector position. Optimum visibilities occur at considerably smaller distances than in the standard setup.
... The technique suffers from depth limitation, as the attenuation of the modulated thermal diffusion process is high [14]. Radiography is widely used in defect and discontinuity detection in composite materials [15,16]. However, special considerations should be taken when scanning low atomic weight elements (such as polymers) with low X-ray attenuation coefficients. ...
This paper studies various manufacturing defects in glass fiber/Polypropylene (PP) composite parts and their methods of detection. Foreign Object Inclusion (FOI) of different shapes, sizes, and materials were placed in a glass fiber/PP panel made by compression molding. The paper aims to characterize the fiber orientation and fiber related defects such as fiber waviness in the composite specimen. Comprehensive investigation for different Non Destructive Evaluation (NDE) techniques, namely X-ray radiography and Ultrasonic Testing (UT) techniques to trace and characterize the embedded defects and the composite texture are presented. Conventional X-ray radiography successfully identified the fiber orientation in two dimension (2-D) plane; however, information for the sample depth was not captured. The radiography techniques showed low relative errors for the defect size measurements (maximum error was below 9.5%) when compared to the ultrasonic techniques. Ultrasonic techniques were able to map all the embedded artificial defects. Phase Array (PA) ultrasonic technique was able to precisely locate the FOI in the glass fiber/PP specimen. Nerveless, the shape and size of the defects were not accurately determined due to the high signal attenuation and distortion characteristics of the E-glass fiber.
... X-ray computed tomography (XCT) with cone-beam geometry has been used for non-destructive characterization and evaluation of materials since the 1980s [5]. XCT is a nondestructive test method that provides information about the spatial distribution of X-ray absorption in the analyzed structures. ...
Industrial tomography (XCT) is a nondestructive test method that provides information about spatial distribution of X-ray absorption in the analyzed structures. The aim of this paper was to examine the possibility and accuracy of application of XCT method for discontinuity and porosity detection in parts made of 316L stainless steel powder produced by Selective Laser Melting technology. Analysis conducted on three produced test samples showed that the application of XCT as a method of quality control of specimens produced with an additive manufacturing technology offers a wide range of possibilities to detect porosity within materials. Parameters such as the amount of porosity, pore size and pore shape are presented. Accuracy of XCT method strongly depends on the size of the samples analyzed, but the possibility of obtaining information in 3D nondestructively shows considerable advantages of XCT method over traditional metallographic cross-sectional analysis.
... The basic idea of DPCI consists in performing phasesensitive x-ray measurements (rather than imaging the absorption coefficient) and the reconstruction of the refraction coefficient f : R 2 → R. The mathematical model describes the relationship between the phase change and the refraction coefficient f in the following way, cf. [1], [2], [3], ...
Differential phase contrast imaging (DPCI) enables the visualization of soft tissue contrast using X-rays. In this work we introduce a reconstruction framework based on curvelet expansion and sparse regularization for DPCI. We will show that curvelets provide a suitable data representation for DPCI reconstruction that allows preservation of edges as well as an exact analytic representation of the system matrix. As a first evaluation, we show results using simulated phantom data.
... Several phase-contrast methodologies have been developed, but they are largely restricted to synchrotron sources (5) or low energies in the case of laboratory X-ray sources. However, results on metallic materials have recently been reported (8). Finally, although we do not cover them, other contrast mechanisms such as fluorescence (9) and diffraction (10) can also be used for X-ray tomographic investigations. ...
The thermomechanical properties of multiphase metals are determined by a combination of the properties of the microstructural phases and the internal architecture formed by them. The latter must be described using three-dimensional techniques in cases where the phases are distributed nonuniformly, have complex morphologies, form interconnected structures, and present contiguity between those structures. Furthermore, all these morphological aspects may change during service exposure. One of these techniques is X-ray tomography, which has experienced an increased interest from materials scientists during the past decade owing to the advances in spatial and time resolution to reveal nondestructively the internal structure of materials. The present review summarizes the main features of this technique in regard to its capabilities to image metal-based engineering materials three dimensionally. Special emphasis is put on the contributions of X-ray tomography to understand the relationships between architecture...
The 316 L stainless-steel samples were prepared by laser powder bed fusion (LPBF). The effects of processing parameters on the density and defects of 316 L stainless steel were studied through an orthogonal experiment. The density of the samples was measured by the Archimedes method, optical microscopy (OM) and X-ray Computed Tomography (XCT). The microstructures and defects under different LPBF parameters were studied by OM and SEM. The results show that the energy density has a significant effect on the defect and density of the structure. When the energy density is lower than 35.19 J/mm3, the density increases significantly with the increase of energy density. However, when the energy density is larger than this value, the density remains relatively stable. The process parameter with the greatest influence on energy density is the hatch distance D, followed by laser power P, scanning speed V and rotation angle θ. In this paper, the optimum parameters consist of P = 260 W, V = 1700 mm, D = 0.05 mm and θ = 67°, in which the density is as high as 98.5%. In addition, the possibility and accuracy of the XCT method in detecting the discontinuity and porosity of 316 L stainless steel were discussed. The results show that XCT can provide the whole size and variation trend of pores in the different producing direction of LPBF.
Talbot-Lau X-ray imaging provides additional information about the inner structure of materials by using X-ray gratings. It extends conventional X-ray imaging by two additional images, the differential phase-contrast and the dark-field image. This work focuses on the development and optimisation of this interferometric method with regard to a standardised use in various fields of application from medicine and non-destructive testing to the application in laboratory astrophysics. Examples of applications are presented for the field of defect detection in carbon fibre reinforced plastics and the investigation of archaeological findings. The technical requirements of the extended X-ray method in the field of lung imaging are presented by comparing the properties of different grating interferometers. Another aspect considered is the easy handling of such an X-ray system. For this purpose an alignment method was developed, which is based on a look-up table called moiré map. For experiments in laboratory astrophysics at an X-ray backlighter, this alignment method is mandatory. The implementation of a Talbot-Lau scanner enlarges the field of view which is typically limited by the gratings. Finally, this work presents a novel continuous phase-sampling scanning method together with an optimisation of the established reconstruction method. This method allows the extension of a Talbot-Lau scanner to a Talbot-Lau helical computed tomography scanner. In summary, the individual optimisations form an important basis for the transfer of interferometric X-ray imaging from research to clinics and industry.
X-ray dark-field imaging is a promising technique for lung diagnosis. Due to the alveolar structure of lung tissue, a higher contrast is obtained by the dark-field image compared to the attenuation image. Animal studies indicate an enhancement regarding the detection of lung diseases in early stages. In this publication, we focus on the influence of different Talbot-Lau interferometer specifications while maintaining the x-ray source, sample magnification and detector system. By imaging the same porcine lung with three different grating sets, we analyze the contrast-to-noise ratio of the obtained dark-field images with respect to visibility and correlation length. We demonstrate that relatively large grating periods of the phase and of the analyzer grating are sufficient for high quality lung imaging at reasonable dose levels.
To construct an oblique-view CT, oblique projection data are captured by angling the phantom bed relative to the plane of the X-ray beam. The projected image can be calculated from the absorption signals detected by a 2D array detector. In general, the scanning axis of the X-ray source and detector is approximately parallel to the long axis of the phantom and perpendicular to the plane of the X-ray source. In this setup, the image projection is a series of axial cross-sections through the phantom. This axial projection image depends on the thickness of the phantom. Generally, the image has a relatively high resolution for a thick phantom (>5 mm thick) and poor resolution when the phantom is thin. Moreover, mechanical and other practical considerations may limit the tilt and swivel angles to no more than approximately 30° between the horizontal phantom bed and the X-ray beam plane. In such cases, an oblique approach is a more effective method. In this paper, we propose high-speed reconstruction using a graphic processor unit (GPU)-based acceleration technique. This oblique CT system obtains projected images from the rotating phantom using a fixed-mounted X-ray source and detector and then reconstructs 3D voxels using the filtered back-projection (FBP) technique. In the experiments, we acquired 400 rotational projection images for the oblique-view phantoms and then reconstructed the aligned 3D view of the phantom. The results showed that the GPU technique achieved 7 giga update per second (GUPS) processing performance for 5123 volumetric reconstructions. The oblique-view CT was able to obtain the 3D inner shape of the phantom with the oblique rotational axis.
Computed tomography (CT) for non-clinical applications is constantly evolving in many different fields, from industry, laboratory and metrology applications. The Technological Center AIMEN, in collaboration with the University of Santiago de Compostela, has implemented a dual-detector computed tomography system. A specific visualisation tool has been developed including 3D rendering and image reconstruction, following the modular capabilities of the CT system. Software capabilities include CT reconstruction for different geometries, special filtering, higher computer memory requirements, basic knowledge extraction, etc., resulting in flexible software with possibilities for further applications development. This new tool is adapted to the emerging needs of an open CT system which is built in a modular configuration to answer new challenges, as those for dimensional metrology (improving its spatial resolution and image quality) and specific analysis for non-destructive testing.
The continuing development of X–ray light sources, optical devices and image analysis technologies enables us to nondestructively analyze internal structures of materials by using 3D imaging with a spatial resolution of micron, even dozens of nanometers. Innovations in materials science and technology will benefit from the new achievements of the X–ray tomography with higher resolution. This article devotes to review the origination and development of the X–ray tomography techniques for 3D imaging and introduce the principles and specifications of three imaging methods,including absorption imaging, phase contrast imaging and holographic imaging. The differences between synchrotron–based and laboratory–based X–ray tomography are also discussed. To explore new opportunities of the high resolution X–ray tomography in the development of material science and technology, particular emphasis is laid on the applications for traditional materials with a 3D view,such as the characterizations of holes, cracks, corrosion, composites, and in situ testing etc..
The porosity of carbon fibre-reinforced polymers is a very important topic for the practical applications of this material since there is a direct correlation between porosity and mechanical properties, such as shear strength. Therefore the porosity values of carbon fibre-reinforced composites for practical use in aircraft and cars must be lower than a certain level, usually 2.5-5 %. The most common non-destructive method for measuring the porosity is ultrasonic testing, since there is a mostly linear correlation between ultrasonic attenuation and porosity. However, the ultrasonic attenuation coefficient depends not only on the porosity, but also on the shape and distribution of the pores and the presence of other material inhomogeneities. This can lead to significant errors in the determination of the porosity. Acid digestion and materialography are also used for porosity measurement, but both methods are destructive. This paper deals with the application of X-ray computed tomography for the characterization of carbon fibre-reinforced composites, in particular for the quantitative determination of porosity with a reasonable degree of accuracy. In order to attain this, different segmentation methods were applied, systematic computed tomography investigations on a broad variety of composite samples with different measurement parameters were performed and the results are compared with results obtained by standard porosity measurement methods, such as ultrasonic testing and acid digestion. In addition, the possibility of characterizing the size, shape and position of all individual pores and other material inhomogeneities in three dimensions are demonstrated.
In materials science, various techniques for three-dimensional reconstruction of microstructures have been applied successfully for decades, such as X-ray tomography and mechanical sectioning. However, in the last decade the family tree of methods has grown significantly. This is partly through advances in instrumentation. The introduction of the focused ion beam microscope and the transformation of transmission electron microscopy into a multipurpose analytical and structural tool have made major impacts. The main driving force for progress is perhaps the advent of nanotechnology with the need to achieve nanometer-scale resolution and the desire to get a real three-dimensional view of the nanoscale world.
Since Roentgen's discovery of X-rays, just over a century a go, the vast majority of radiographs have been acquired and interpreted on the basis of absorption contras t. The recent development of X-ray phase contrast imaging techniques opens up a new opportunity especial ly for weakly-absorbing objects. Spatial and temporal coherence of X-ray synchrotron radiation permits to realize phase contrast imaging with ideal conditions. But it is possible to set up phase con trast imaging in laboratory. Laboratory phase contrast imaging can provide new solutions in an indust rial context. The aim of this paper is to present the different solutions to perform phase contrast imaging in lab oratory: - classical micro focus X-ray tube, - X-ray ultra microscope (XuM) developed by an Australian team - table top synchrotron developed by a Japanese team, - multilayer, - polycapillary.
IN conventional radiography, X-rays which pass through an object along
different paths are differentially absorbed, and the intensity pattern
of the emerging beam records the distribution of absorbing materials
within the sample. An alternative approach is phase-contrast
radiography, which instead records variations of the phase of the
emerging radiation. Such an approach offers improved contrast
sensitivity, especially when imaging weakly absorbing samples.
Unfortunately, current phase-contrast imaging techniques1-11
generally require highly monochromatic plane-wave radiation and
sophisticated X-ray optics, so their use is greatly restricted. Here we
describe and demonstrate a simplified scheme for phase-contrast imaging
based on an X-ray source having high spatial (but essentially no
chromatic) coherence. The method is compatible with conventional
polychromatic micro-focus X-ray tube sources, is well suited to large
areas of irradiation, can operate with a lower absorbed dose than
traditional X-ray imaging techniques, and should find broad application
in clinical, biological and industrial settings.
Phase-contrast x-ray imaging has been studied intensively in the past, mainly with applications to low density biological materials. In this work experimental and theoretical aspects of phase-contrast microfocus x-ray imaging of structural materials metals and polymers are considered. The diffraction field in an object from a point x-ray source is simulated and the effects of geometrical and material parameters on the x-ray phase-contrast image are obtained. It is shown that higher-order terms should be retained in the asymptotic expansion of the Green function in the image plane to accurately image discontinuities in structural materials. Experimental phase-contrast images of small defects are obtained with a 5 m microfocal x-ray source and compared with those from computer simulation as a function of geometrical magnification and photon energy. The phase-contrast x-ray imaging provides enhanced image contrast and improved edge definition and is important for further development of nondestructive evaluation of structural materials. © 2006 American Institute of Physics.
A convolution-backprojection formula is deduced for direct reconstruction of a three-dimensional density function from a set of two-dimensional projections. The formula is approximate but has useful properties, including errors that are relatively small in many practical instances and a form that leads to convenient computation. It reduces to the standard fan-beam formula in the plane that is perpendicular to the axis of rotation and contains the point source. The algorithm is applied to a mathematical phantom as an example of its performance.
X-ray computed tomography (XCT) has become a very important method for non-destructive 3D-characterization and evaluation of materials. Due to measurement speed and quality, XCT systems with cone beam geometry and matrix detectors have gained general acceptance. Continuous improvements in the quality and performance of X-ray tubes and XCT devices have led to cone beam CT systems that can now achieve spatial resolutions down to 1 μm and even below. However, the polychromatic nature of the source, limited photon flux and cone beam artefacts mean that there are limits to the quality of the CT-data achievable; these limits are particularly pronounced with materials of higher density like metals. Synchrotron radiation offers significant advantages by its monochromatic and parallel beam of high brilliance. These advantages usually cause fewer artefacts, improved contrast and resolution.
Tomography data of a steel sample and of two multi-phase Al-samples (AlSi12Ni1, AlMg5Si7) are recorded by advanced cone beam XCT-systems with a μ-focus (μXCT) and a sub-μm (nano-focus, sub-μXCT) X-ray source with voxel dimensions between 0.4 and 3.5 μm and are compared with synchrotron computed tomography (sXCT) with 0.3 μm/voxel. CT data features like beam hardening and ring artefacts, detection of details, sharpness, contrast, signal-to-noise ratio and the grey value histogram are systematically compared. In all cases μXCT displayed the lowest performance. Sub-μXCT gives excellent results in the detection of details, spatial and contrast resolution, which are comparable to synchrotron-XCT recordings. The signal-to-noise ratio is usually significantly lower for sub-μXCT compared with the two other methods. With regard to measurement costs “for industrial users”, scanning volume, accessibility and user-friendliness sub-μXCT has significant advantages in comparison to synchrotron-XCT.
Phase-contrast x-ray computed tomography (CT) is an emerging imaging technique that can be implemented at third-generation synchrotron radiation sources or by using a microfocus x-ray source. Promising results have recently been obtained in materials science and medicine. At the same time, the lack of a mathematical theory comparable with that of conventional CT limits the progress in this field. Such a theory is now suggested, establishing a fundamental relation between the three-dimensional Radon transform of the object function and the two-dimensional Radon transform of the phase-contrast projection. A reconstruction algorithm is derived in the form of a filtered backprojection. The filter function is given in the space and spatial-frequency domains. The theory suggested enables one to quantitatively determine the refractive index of a weakly absorbing medium from x-ray intensity data measured in the near-field region. The results of computer simulations are discussed.
Practical aspects of microfocal X-ray image enhancement utilizing phase contrast effects are considered. Optimization of the experimental technique to achieve phase contrast is described for application to the nondestructive testing of materials. Experimental phase contrast images of porosity and corrosion pits obtained with a 5 μm (2 × 10 -4 in.) microfocal X-ray source are compared with attenuation contrast images demonstrating significant enhancement of image quality. The phase contrast X-ray imaging provides enhanced image contrast, improved edge definition and X-ray phase information on the material.
X-ray computed tomography (XCT) has become a very important method for non-destructive 3D-characterisation of materials. XCT systems with cone beam geometry, micro- or nano-focus tubes and matrix detectors are increasingly used in research and non-destructive testing. Spatial resolutions down to 1μm can be reached with such XCT-systems for heterogeneities in metals with high absorption contrast. High resolution cone beam XCT is applied to five different Al-alloys: AlMg5Si7, AlCu4Mg1, AlZn6Mg2Cu2, AlZn8Mg2Cu2 and AlSi12Ni1. Up to four different types of inhomogeneities are segmented in one alloy using voxel sizes between (0.4μm)3 and (2.3μm)3. Target metallography and elemental analysis by energy dispersive X-ray analysis are used to identify the inhomogeneities. The possibilities and restrictions of XCT applied to Al-alloys are discussed.AlMg5Si7 XCT-data with a voxel size of (0.4μm)3 show inhomogeneities with brighter grey-values than the Al-matrix identified as elongated Fe-aluminides, and those with lower grey-values identified as pores and Mg2Si-particles with a “Chinese script-like” structure. Higher-absorbing interdendritic Al–Al2Cu-eutectic regions appear brighter than the Al-dendrites in the CT-data of AlCu4Mg1 with (1.1μm)³/voxel, whereas pores>4μm appear darker than the Al-matrix. The size and the 3D-structure of the α-Al dendrite arms with a diameter of 50–100μm are determined in samples from chill cast billets of AlCu4Mg1 and AlZn6Mg2Cu2 alloys. The irregular interdendritic regions containing eutectic segregations with Cu- and Zn-rich phases are >5μm wide. Equally absorbing primary equi-axed Al3(Sc, Zr) particles >5μm are distinguished in the centres of the dendrites by the level of sphericity values. The distribution of Ni- and Fe-aluminides in a squeeze cast AlSi12Ni1-alloy is imaged with (0.4μm)³/voxel, but the Si-phase cannot be segmented.
A method is presented for quantitative phase-contrast tomography using unfiltered radiation from a small polychromatic source. The three-dimensional distribution of complex refractive index in a monomorphous object is reconstructed given a single projection image per view angle. The reconstruction algorithm is achromatic and stable with respect to high-spatial-frequency noise, in contrast to conventional tomography. The density distribution in a test sample was accurately reconstructed from polychromatic phase-contrast data collected with a point-projection x-ray microscope.
We examine the usefulness and credibility of analyst recommendations by focusing on their behavior surrounding tender offer announcements. For our 1998-2001 sample, we find analysts did not identify takeover targets through their recommendations nor did they distinguish between wealth-increasing and wealth-decreasing tender offers. We find some evidence of conflicts of interest in analyst recommendations, but it is confined to the 1999-2000 dot-com period. However, the long-run performance following recommendations suggests that these conflicts have little ultimate cost to investors. 2007 The Southern Finance Association and the Southwestern Finance Association.
In this report we clarify two aspects for in-line phase-sensitive x-ray imaging, which includes phase-contrast imaging and phase imaging. First, we point out that there is confusion in the literature about the lateral coherence length, which is widely adopted as the coherence criteria for implementing phase-sensitive imaging. The confusion exaggerates the coherence requirement for clinical implementation of in-line phase-sensitive imaging. Instead we show that the ratio of the phase-space shearing length to lateral coherence length is a good measure for gauging the partial coherence realized in a specific image setting. Second, based on the general intensity equation for in-line phase-sensitive imaging, we discuss the differences between the phase-contrast imaging and phase imaging in terms of the physics mechanism, image acquisition approaches, computation algorithm development, and the potentials for tissue quantitative characterization.
A significant improvement over conventional attenuation-based X-ray imaging, which lacks contrast in small objects and soft biological tissues, is obtained by introducing phase-contrast imaging. As recently demonstrated, phase-contrast imaging is characterized by its extraordinary image quality, greatly enhanced contrast, and good soft tissue discrimination with very high spatial resolution down to the micron and even the sub-micron region. The rapid development of compact X-ray sources of high brightness, tuneability, and monochromaticity as well as high-resolution X-ray detectors with high quantum efficiency and improved computational methods is stimulating the development of a new generation of X-ray imaging systems for medical applications. The present paper reviews some intrinsic mechanisms, recent technical developments and potential medical applications of two-, three- and four-dimensional phase-contrast X-ray imaging. Challenging issues in current phase-contrast imaging techniques and key clinical applications are discussed and possible developments of future high-contrast and high spatial and temporal resolution medical X-ray imaging systems are outlined.
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