Infoscience

Thesis

High resolution eddy current inspection and eddy current testing for additive manufacturing

Non-destructive testing (NDT) is one of the key method developed for the safety mechanism of many industrial fields, e.g. aerospace, oil and gas and power industries. Among the NDT techniques, eddy current testing belongs to the well-established ones and has been used since the 1940s. Eddy current testing is widely used to detect defects in electrically conducting materials, especially surface breaking ones. Many devices and developments push the eddy current testing to a highly technical level. Eddy current testing applied to NDT is a normalized technique that is performed by certified inspectors. These have received a specific training to interpret the complex signals produced by the eddy current equipment. One key challenge of the interpretation of the eddy current signals is caused by variations of the conditions and material properties that affect the response. Nowadays, the inspectors need a deep understanding of physics and in some cases years of experience for an accurate defect identification. The field of rapid prototyping is one of the most growing and successful areas in the industrial world in the past years. News topics show that this technique is becoming more and more rapid manufacturing. Already nowadays, parts are built with rapid prototyping and used for demonstrations and functional concept studies. But the failure rate of built objects with rapid prototyping, especially metallic ones, is still high. There is therefore a demand for so called "quality machines" able to control the quality of the manufactured components, if possible in situ and to generate the corresponding documentation. Industries and universities are undertaking projects and research in that direction and the eddy current technique has been identified as a potential candidate to reach this goal. In this context, we identified three specific needs for the eddy current non-destructive testing. Firstly, for many applications the accurate size and dimensions of the defects are necessary to estimate the remaining lifetime of a subject. The crack depth of fatigue cracks that is not accessible with standard eddy current instruments should be estimated accurately. Another special need that we observe is the development of an assisting device tool that will help the operators to identify cracks and defects easier. Finally, additive manufacturing of metallic objects has a high potential for improvements and eddy current testing would be an added value for lower failure rates during the manufacturing. In this work, we will demonstrate how eddy current non-destructive testing meets the above mentioned needs. To this end, • we measure the local magnetic field of the induced eddy currents with a special sensor array. We measure and calibrate our system on artificial defects and use a model to estimate the crack depth of fatigue defects. Finally we open these defects and compare the estimation with the actual defect; • we show the development of a 2D hand scanning eddy current system which provides images, and we will give a practical approach towards scanning devices. This 2D hand scanning device does not change the operators’ hand scanning habits and provides; • we took into account delamination and porosity changes, which are two of the fundamental problems of additive manufactured parts, in order to demonstrate the performance of eddy current testing for additive manufacturing.

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