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- 2012 (7) (entfernen)
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- Computed tomography (7) (entfernen)
Computer simulation of radiography can be used for different purposes in NDT, such as qualification of NDT systems, optimization of radiographic parameters, feasibility analysis, model-based data interpretation, and training of NDT/NDE personnel. BAM has been working on modeling in the field of radiographic testing for many years. With the gathered theoretical background and the familiarity with practical requirements of industrial application the simulation software aRTist has been developed. This analytical simulator includes a description of the radiation source, the interaction of radiation with test pieces and flaws, and the detection process with special focus on film and digital industrial radiology. It features high processing speed with nearinteractive frame rates and a high level of realism. Here we focus on the recent developments of the simulator, notably the release of aRTist version 2. Extended functionality regarding automated virtual computed tomography now allows for arbitrary scan paths. Another program extension supports reliability investigations and provides a user interface for planning automatic simulations with varying parameters and defects.
Mobile Tomographic Computer Aided Radiometry (TomoCAR) is based on the mechanical position control of an X-ray tube in front of an object and the application of a Digital Detector Array (DDA) behind it. Several hundred radiometric projections in small angle steps are acquired during the controlled movement of the X-ray tube along a preselected way. The tomographic or laminographic reconstruction allows the three-dimensional (3D) representation of flaws. A specially designed radiometric detector array with small internal unsharpness and high image contrast was used for visualization and sizing of planar and volumetric defects in austenitic welds in nuclear power plants. A German pilot study was successfully performed on the basis of the European network of inspection and qualification (ENIQ) guidelines. This was the precondition for several applications of TomoCAR in nuclear power stations in Germany and Switzerland.
A versatile computed tomography (CT) system was developed on basis of the TomoCAR design for in situ inspection of large aircraft components under production conditions. A gate based planar computed tomograph was constructed and tested for inspection of the integrity of CFRP stringers, embedded in aircraft fibre composite shells. A probability of crack visibility >90% could be determined.
Computed THz - tomography
(2012)
Available THz Time Domain Spectroscopy (THz-TDS) systems can be applied in a pulse echo mode for
tomographic application. In contrast to common transmission methods, a TDS reflection geometry was used to
inspect cylindrical polyethylene samples with internal voids of known shape. The geometry of the voids was
simulated and its shape was reconstructed. B-scan pulse echo images were acquired in the time domain, similar
to UT pulse echo techniques on basis of the numeric model. The results were used to verify the simulation model
and the reconstruction algorithm, based on a modified Synthetic Aperture Focusing Technique (SAFT). The
model was used to conclude recommendations on an optimized measurement setup in the reflection mode. Based
on that, the quality of the reconstructed tomograms was improved.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of
diffuse background signals ('backlighting') of 2D X-ray detectors is suggested. Depending on the detector
geometry the mechanism may be different, either based on the optical scattering of the fluorescent screen
materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components.
Consequently, these erroneous intensity portions result in locally different violations of Lambert Beers law in
single projections (radiographs) as function of the detector area coverage and the magnitude of the attenuation.
The absorption of multiple metal sheets is investigated by monochromatic synchrotron radiation, thus excluding
beam hardening. The proposed correction procedure simply requires the individual subtraction of one and the
same fraction of the primary and transmitted mean intensity, as a constant (non-local) scattering mechanism is
assumed.
Incomplete tomographic data sets such as limited view (missing wedge) data represent a well-known challenge
for reconstruction algorithms, since they unavoidably lead to substantial image artefacts. Such data sets may
occur in industrial computed tomography of limited access (e.g. extended components, fixed objects), directional
opacity, limited sample life time or laminographic set-up. We present strategies to effectively suppress the
typical elongation artefacts (e.g. lemon-like deformed pores) by our iterative algorithm DIRECTT which offers
the opportunity to vary the versatile reconstruction parameters within each cycle. Those strategies are applied to
experimental data obtained from metallic foams as well as model simulations. Comparison is drawn to state-ofthe-
art techniques (filtered backprojection and algebraic techniques). Further reference is made to reconstructions
of complete data sets serving as gold standards. For quantitative assessment of the reconstruction
quality adapted techniques based on spatial statistics are introduced.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of diffuse background signals ('backlighting') of 2D X-ray detectors is suggested. Depending on the detector geometry the mechanism may be different, either based on the optical scattering by the fluorescent screen materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components. Consequently, these erroneous intensity portions result in locally different violations of Lambert–Beer's law in single projections (radiographs). When used as input data for computed tomography these violations are directly observed via modulation of the projected mass as a function of the rotation phase and the samples aspect ratio (dynamics). The magnitude of the diffuse background signal depends on the detector area covered by the projected sample. They are more pronounced the smaller the shadowed area and the stronger the total attenuation. This implies that the reconstruction suffers from additional anisotropic artifacts caused by elongated sample structures. This issue is studied simply by absorption of flat plates in a conventional laboratory radiography set-up and at a synchrotron radiation facility. In the latter case beam hardening artifacts can be excluded due to the monochromatic radiation. The proposed correction procedure requires simple integral intensity offsets as a constant (non-local) light scattering mechanism is assumed.