A comparative study of high resolution cone beam X-ray tomography and synchrotron tomography applied to Fe- and Al-alloys

Upper Austria University of Applied Sciences, Stelzhamerstrasse 23, 4600 Wels, Austria.
NDT & E international : independent nondestructive testing and evaluation 10/2010; 43(7-3):599-605. DOI: 10.1016/j.ndteint.2010.06.004
Source: PubMed


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.

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    • "Electromagnetic focusing of the electron beam allows generating X-ray beams with an emission spot diameter down to well below one µm which is essential for CT examination with voxels-sizes in the sub-micron range. These characteristics with respect to spatial resolution principally allow CT measurements which valuably complement many absorption contrast setups at synchrotron radiation facilities [Withers 2007; Brunke et al. 2008; Kastner et al. 2010]. "

    Full-text · Conference Paper · Feb 2012
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    • "wt.% Si and 0.6 wt.% Fe, which was prepared by gravity casting [17] [18]. The sample investigated was cylindrical with a diameter of 0.4 mm. "
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    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 {mu}m 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. We report on the possibilities of polychromatic cone-beam phase contrast tomography for non-destructive characterisation of materials. A carbon fibre-reinforced polymer and the Al-alloys AlMg5Si7 and AlSi18 were investigated with high resolution cone-beam X-ray computed tomography with a polychromatic tube source. Under certain conditions strong phase contrast resulting in an upward and downward overshooting of the grey values across edges was observed. The phase effects are much stronger for the polymer than for the Al-alloys. The influence on the phase contrast of the parameters, including source-detector distance, focal spot size and tube acceleration voltage is presented. Maximum phase contrast was observed for a maximum distance between the source and the detector, for a low voltage and a minimum focal spot size at the X-ray source. The detectability of the different phases is improved by the edge enhancement and the resulting improvement of sharpness. Thus, a better segmentation of the carbon fibres in the fibre-reinforced polymer and of the Mg{sub 2}Si-phase in the AlMg5Si7-alloy is achieved. Primary and eutectic Si cannot be detected by attenuation-based X-ray computed tomography due to the small difference in X-ray attenuation between the Si and the Al-matrix in AlSi18. However, phase contrast effects lead to a significant detectability and the large primary Si particles become clearly visible in the tomograms of AlSi18. - Highlights: Black-Right-Pointing-Pointer Phase contrast increases with increasing distance object-detector and with decreasing U, X-ray spot. Black-Right-Pointing-Pointer The detectability of details is improved by the phase contrast. Black-Right-Pointing-Pointer For the carbon fibre-reinforced polymer the visibility of the carbon fibres is improved. Black-Right-Pointing-Pointer For AlMg5Si7 the detectability of the phases Fe-aluminides and Mg{sub 2}Si particles is improved. Black-Right-Pointing-Pointer For AlSi18 primary Si particles with a size > 50 {mu}m become detectable because of the phase contrast.
    Preview · Article · Feb 2012 · Materials Characterization
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    • "SRµCT at ID19 beamline/ ESRF Grenoble [17] sub-µCT with new nanotom m State of the art sub-µCT by non-stabilities or faults of the detector pixels. In [17], it was also reported that both, state of the art sub-µCT as well as SRµCT, shows very comparable contrast and spatial resolution. But also some drawbacks of the laboratory system were shown: this is the signal-to-noise ratio and the edge sharpness in the volume data. "
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    ABSTRACT: Nowadays, X-ray tube-based high-resolution CT systems are widely used in scientific research and industrial applications. Compact XCT systems are available that can reach resolutions down to 1 µm and below. But the potential, convenience and economy of these lab systems is often underestimated. The present paper shows the comparison of sophisticated conventional µCT with synchrotron radiation-based µCT (SRµCT). The different aspects and characteristics of both approaches like spatial and density resolution, penetration depth, scanning time or sample size is described in detail. Beside this, the advances in technology of industrial high resolution CT systems are shown. The paper also presents recent advances in the area of industrial high resolution CT systems from phoenix product line of General Electric. All major parts are designed to make the system extremely stable during the data acquisition process. So, the system is equipped with granite base and very precise rotation unit. The unique nanofocus tube technology with build-in cooling system stabilizes the tube and at the same time the diamond based target allows high photon flux at very small focal spot sizes. The unique detector with excellent contrast resolution and SNR is also thermally stabilized. Also, the user friendliness is increased through the fully automated process chain starting with detector calibration and going through acquisition and data reconstruction process with automated volume data evaluation The application results of this new technology show its high potentials for usage of the state of the art laboratory systems in the industrial and scientific application fields of material research, metrology, petro-industry, etc. To compare the potentials of laboratory based CT with synchrotron based CT, different samples were used: e.g. a low-carbon steel sample, and an aluminium multi-phase sample (AlMg5Si7) and some other. Concerning measurement costs, scanning volume, accessibility and user-friendliness sub-µXCT has significant advantages in comparison to synchrotron-XCT.
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