Finite Element Modelling of Eddy Current Non-Destructive Evaluation in Probability of Detection Studies

Licentiate thesis, 2012

The goal of this work has been to evaluate the possibility to use mathematical modelling to characterize the capability and reliability of automated eddy current inspections. The safety in aerospace propulsion is critical. Components that are critical for the operation are therefore designed to withstand material degradation. This is verified through the use of non-destructive evaluation (NDE) methods. The NDE methods are characterized in a statistical manner by probability of detection (POD) assessments. The result will be different when evaluating different materials, geometries, defect types and also by the specified procedure settings. This is in principle leading to a costly and time consuming campaign for every new NDE application. Eddy current evaluation is one of the most applied methods for NDE of aero engine components and is studied within this work. The nature of the method is complex and there is therefore a need for deeper understanding that may be gained from mathematical models. Such models can have several objectives as for example procedure and equipment optimization or understanding of the method capability and reliability. This work focus on the model based estimation of method capability as the method is applied in a realistic and automated procedure. The finite element method is used for prediction of the eddy current interaction with defects. The work has shown that tight fatigue cracks can be modelled together with the variations of a realistic procedure with good accuracy. Bridging electrical contacts between the faces of the fatigue crack can be captured in a finite element model and is important for a relevant description of the flaw. The influencing procedure parameters with their variability can also be included in the mathematical description resulting in a prediction of the POD as a function of defect size. The model based POD approach has the potential to be an important part in characterization of NDE methods applied to evaluate the structural integrity of components in the future. Three papers are included in this thesis. The first presents the methodology in the set up of a 3D model of eddy current NDE. The second paper concerns the description of the realistic fatigue crack, evaluated in both models and in experiments. The third paper shows a comparison between an experimental and a model based POD assessment.