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Reference object for evaluating the accuracy of porosity measurements by X-ray computed tomography
Get PDFInternal defects such as voids and porosity directly influence mechanical properties, durability, service life and other characteristics of industrial parts. There are several non-destructive and destructive methods for defects detection and evaluation. Recently, X-ray Computed Tomography (CT) has emerged as an effective tool for geometrical characterization of internal defects. 3D information about internal voids/porosity extracted from CT datasets can be utilized in many applications, such as production processes optimization and quality control. However, there are still challenges in using CT as a traceable method for internal voids dimensional measurements. In order to enhance the accuracy and reliability of CT porosity measurements, a metrological validation method is required.This study presents the application of a new reference object for accuracy evaluation of CT porosity measurements and discusses results obtained by using it. The reference object is made of aluminium and is composed of a cylindrical body and four cylindrical inserts with micro-milled hemispherical features of calibrated sizes resembling artificial flaws. The accuracy of porosity measurements is evaluated according to various characteristics (diameters and depths measurements errors) and repeatability of measurements. Design of experiments technique is used to investigate the influence of CT parameters settings on porosity measurement accuracy
Reference standards and methods for traceable X-ray computed tomography dimensional metrology
Get PDFX-ray computed tomography (CT) is an imaging technique, which originally found its application in the medical field and over years of research extended its use to industry, mainly to nondestructive testing. In the last years, CT has been also used as a tool for dimensional metrology, and is considered the third generation of measuring techniques in coordinate metrology, after tactile and optical coordinate measuring systems. The main advantage of CT over other measuring instruments is the fact that both internal and external geometries of the measured workpiece can be visualized as a 3D model, and analyzed without destruction.
CT measurement chain is affected by numerous and often complex influence factors. Furthermore, due to the fact that it is still relatively new technique for coordinate metrology, and because of the lack of research and standardized procedures, CT has not yet reached maturity as a dimensional measurement technology. Sources of CT measurement uncertainty are still not completely understood in some cases and, as a result, achieving CT measurement traceability is difficult.
Traceability of measurements is an important property ensuring that the measurement results are traceable to the SI unit through unbroken chains of calibrations. Reference standards are a typical means to test metrological systems and investigate various factors that influence the measurement results. In this thesis, several standards dedicated to different mainstays towards the establishment of measurement traceability – namely metrological performance verification according to international standards, metrological performance verification of CT specific applications, calibration, and evaluation of measurement uncertainty – were developed. Furthermore, as an output of experience and findings gained during the project, a good practice guide for developing reference standards for CT dimensional metrology was compiled.
The first standard proposed in this Ph.D. work was developed for the evaluation of accuracy of CT porosity measurements and for establishing their metrological traceability. The design of the standard contains hemispherical features resembling artificial internal porosity and its dismountable configuration ensures calibration by different measuring instruments. It was proved that not only the accuracy of CT porosity measurements can be evaluated using this standard, but the accuracy can also be improved by using the artifact. Moreover, a procedure for establishing the traceability of CT porosity measurements obtained from industrial parts was proposed.
Calibration of CT instrument geometry was achieved by using a standard developed during this project, namely the CT calibration tube (CT2), in combination with the so-called “minimization procedure” used for estimating CT geometrical parameters, and a Monte Carlo method for evaluating the measurement uncertainty. Furthermore, based on results from experimental implementations of the developed standard and method, it was proven that it is possible to align the CT system to its nearly ideal geometry. Moreover, a comprehensive CT instrument geometry alignment method was proposed, which is based on the use of the CT2 standard, the minimization procedure, and a novel advanced reconstruction algorithm (the so called FlexCT, developed by KU Leuven).
Multi-material aspects of CT dimensional measurements were also addressed in this Ph.D. thesis. A series of reference standards were developed for evaluating the multi-material effects on gap measurements. Different behavior between results obtained on mono- and multi-material samples confirmed the existence of the multi-material influence. Furthermore, an alternative dual-energy CT scanning approach was applied to enhance the measurement results
Porosity measurements by X-ray computed tomography: Accuracy evaluation using a calibrated object
No full textAccurate identification and measurement of internal voids and porosity is an important step towards improvement of production processes to obtain high quality materials and products. Recently, the importance of knowing the exact size, shape, volume and location of defects has become even higher as tighter requirements and new standards have been introduced in industry. There are several well-established methods for defects evaluation based on various principles (both destructive and non-destructive). However, all conventional methods have various deficiencies and the information about internal voids/porosity that can be extracted is limited. Most of these drawbacks can be overcome by using X-ray computed tomography (CT). Unlike other methods, CT provides full three-dimensional information about shape, size and distribution of internal voids and porosity; however, the accuracy of measurements is still under investigation. Hence, further evaluations on CT porosity measurements must be performed in order to consider X-ray computed tomography a reliable instrument for the assessment and detection of internal defects.
A reference object with artificial defects was used in this research work in order to evaluate the accuracy of porosity measurements by CT. The reference object was manufactured by ultra-precision micro milling. The object contains dismountable components with embedded internal hemispherical features that simulate internal porosity. The artificial porosity was micro-milled on top surfaces of dismountable cylindrical inserts. The hemispherical calottes were thereafter calibrated by traceable coordinate measuring systems and calibrated values were compared to actual values measured by a CT system. The accuracy of CT porosity measurements was then evaluated based on results obtained on various measurands, using different software tools and measuring procedures, comparing real scans to numerical simulations and investigating the influence of CT system parameters settings on measurement results
Multi-material gap measurements using dual-energy computed tomography
No full textX-ray computed tomography is a highly versatile investigation method with applications in a wide range of areas. One of the areas where the technique has seen an increased usage, and an increased interest from industry, is in dimensional metrology. X-ray computed tomography enables the measurement of features and dimensions that are difficult to inspect using other methods. However, there are issues with the method when it comes to measurements of objects that consist of several materials. In particular, it is difficult to obtain accurate computed tomography results for all materials when the attenuation of materials differs significantly. The aim of this work was to measure small air gaps between different materials using dual-energy X-ray computed tomography. The dual-energy method employed in this work uses two energy spectra and fuses the data in the projections space using non-linear fusion. The results from this study show that the dual-energy method used in this work was able to capture more measurements than regular absorption computed tomography in the case of specimens with highly different attenuation, enabling, in particular, the measurement of smaller gaps. The contrast-to-noise ratio was also increased significantly with the use of dual-energy
Establishment of metrological traceability in porosity measurements by X-ray computed tomography
No full textInternal porosity is an inherent phenomenon to many manufacturing processes, such as casting, additive manufacturing, and others. Since these defects cannot be completely avoided by improving production processes, it is important to have a reliable method to detect and evaluate them accurately. The accurate evaluation becomes even more important concerning current industrial trends to minimize size and weight of products on one side, and enhance their complexity and performance on the other. X-ray computed tomography (CT) has emerged as a promising instrument for holistic porosity measurements offering several advantages over equivalent methods already established in the detection of internal defects. The main shortcomings of the conventional techniques pertain to too general information about total porosity content (e.g. Archimedes method) or the destructive way of testing (e.g. microscopy of cross-sections). On the contrary, CT is a nondestructive technique providing complete information about size, shape and distribution of internal porosity. However, due to the lack of international standards and the fact that it is relatively a new measurement technique, CT as a measurement technology has not yet reached maturity. This study proposes a procedure for the establishment of measurement traceability in porosity measurements by CT including the necessary evaluation of measurement uncertainty. The traceability transfer is carried out through a novel reference standard calibrated by optical and tactile coordinate measuring systems. The measurement uncertainty is calculated following international standards and guidelines. In addition, the accuracy of porosity measurements by CT with the associated measurement uncertainty is evaluated using the reference standard
Experimental investigations on the accuracy of X-ray computed tomography for porosity measurements of additive manufactured parts
No full textRecent advances in Additive Manufacturing (AM) have shown a great potential in production of intricate structures due to its almost unlimited design freedom even for internal features. However, inherent imperfections such as internal porosity arise from the AM process. As these internal defects cannot be completely removed, knowing the information about the defects shape, size and distribution becomes crucial. Recently, X-ray computed tomography (CT) has emerged as an advanced tool for internal defects measurement; however, the errors of CT porosity measurements have not yet been thoroughly quantified. In this work, in a first step, CT porosity analysis conducted on Ti6Al4V specimens produced by AM is compared to results by Archimedes method, microscopic analysis and multisensor CMM. In a second step, a newly developed reference object is used for evaluating errors of CT porosity measurements
Experimental investigation of new multi-material gap reference standard for testing computed tomography systems
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