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Optimised wavelet-based local tomography of thermal barrier coatings
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Optimised wavelet-based local tomography has been found to be a useful non-destructive evaluation tool for studying the microstructure in thermal barrier coatings. Two-dimensional wavelet-based local tomography produced high-resolution images of regions inside the investigated samples. The investigated samples were cur from two thermal shock tested jet engine burner cans. The inside of the burner cans was coated with a thick thermal barrier coating consisting of a top coating made of partially stabilised zirconia (PSZ) and a bond coating (BC) of NiCoCrAlY. The coatings were manufactured with plasma spraying with two different spraying parameters. This resulted in samples with and without a segmented crack network in the as-sprayed top coating.

The obtained wavelet-based local CT-imagcs of the investigated samples from the thermal shock tested burner cans reveal cracks and pores in the microstructure. The obtained pixel-sizes in the local CT-images were 4.0 μm and 3.1 μm respectively. Small or no improvements in resolution arc made when the pixel-size is in the vicintiy of the X-ray focal spot size, in this case 5 μm. A traditional CT-investigation, collecting global data that covers the whole cross-section of the sample, would have resulted in 2.6 and 6.8 times bigger pixel-sizes respectively. A CT-image reconstructed from global data would therefore not resolve all features seen in the wavelet-based local CT-images.

To obtain CT-images with a high image quality a mathematical optimisation procedure is used to find the optimal equipment settings for collecting the CT-data. The geometry-sensitivity/noise, density-sensitivity/noise and mass attenuation-sensitivity/noise ratios are maximised using the weighted minimax method. The sensitivity/noise ratios are calculated using a mathematical model of the X-ray imaging system. The image quality is improved if the sample completely shades the detector and optimised equipment settings for this case are used. The sensitivity/noise ratios in the local CT-investigation of one of the samples are increased with at least 20% compared to if air gaps would have been present.

A procedure to suppress ring artefacts in the reconstructed CT-images is presented. The procedure is found to give good results.

National Category
Engineering and Technology
URN: urn:nbn:se:liu:diva-88707OAI: diva2:605573
Available from: 2013-02-14 Created: 2013-02-14 Last updated: 2013-02-14
In thesis
1. Optimised performance of industrial high resolution computerised tomography
Open this publication in new window or tab >>Optimised performance of industrial high resolution computerised tomography
2000 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The purpose of non-descructive enluation (NDE) is to acquire knowledge of the investigated sample. Digital x-ray imaging techniques such as radiography or computerised tomography (CI) produce images of the interior of a sample. The obtained image quality determines the possibility of detecting sample, ·elated features, e.g. details and flaws. this thesis presents a method of optinllsing the performance of industrial X-ray equipment for the imaging task at issue in order to obtain images with high quality.

CT produces maps of the X-ray linear attenuation of the sample's interior. CT can produce two-dimensional cross-section images or three-dimensional images with volumetric information on the investigated sample. The image contrast and noise depend on both the investig-Ated sample and the equipment and settings used (X-ray tube potential, X-ray filtration, exposure time, etc.). Hence, it is vital to find the optimal equipment settings in order to obtain images of high quality.

To be able to mathematically optimise the image guality, it is necessary to have a model of the X-ray imaging system together with an appropriate measure of image quality. The optimisation is performed with a developed model for an X-ray image-intensifier-based radiography system. The model predicts the mean value and variance of the measured signal level in the collected radiographic images. The traditionally used measure of physical image guality is the signal-to-noise ratio (SNR). To calculate the signal-to-noise ratio, a well-defined detail (flaw) is required. It was found that maximising the SNR leads to ambiguities, the optimised settings found by maximising the SNR were dependent on the material in the detail. When CT is performed on irregular shaped samples containing density and compositional variations, it is difficult to define which SNR to use for optimisation. This difficulty is solved by the measures of physical image quality proposed here, the ratios geometry-sensitivity/ noise, density-sensitivity/noise, and mass attenuation-sensitivity/noise. With these measures, a meiliod is presented that finds the optimal eguipment settings, where no improvement can be made without worsening at least one other sensitivity/noise ratio.

This thesis includes modelling and verification of the sharpness of the CT system in terms of the modulation transfer function, MTF. Together with the limiting perception factor and the maximised SNR, the detectability limits for any specific contrasting detail in the centre of a cylindrical sample can be determined. It is also demonstrated that the model can be used to suppress beam hardening when collecting CT-data. When homogeneous samples are imaged, the model can in addition be used to make post-processing corrections for suppressing the beam hardening artefacts.

Wavelet-based local tomography has been found to produce images with good accuracy from projection data only from a small region in a sample. Tlus technique is demonstrated on thermal barrier coatings, which contain internal cracks. With optimised eguipment settings and geometrical magnification of a region in the sample, wavelet-based local tomography produced high-resolution images of excellent quality. The increased resolution reveals features in the microstructure that cannot be resolved wiili traditional CT. This technigue will be a useful tool for characterisation of the microstructure in advanced materials.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2000. 34 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 659
National Category
Engineering and Technology
urn:nbn:se:liu:diva-30060 (URN)15520 (Local ID)91-7219-887-7 (ISBN)15520 (Archive number)15520 (OAI)
Public defence
2000-12-05, Sal C3, Linköpings universitet, Linköping, 10:15 (Swedish)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-02-14

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