Rolf Kern

Robust solutions of inverse problems in electromagnetic non-destructive evaluation

Non-destructive testing (NDT) is the application of physical measurement technology based on energy interaction with the material and its nonconformities. The materials response is sensed by transducers and sensors which?in most cases?scan the component and document the results in inspection images. However, NDT measures a physically defined quantity or even an intrinsic property. The difference between non-destructive evaluation (NDE) and NDT is in the interpretation of the inspection data. NDE has to discuss the inspection results in terms of quality elements and characteristics which are relevant to describe the fitness of the material for use. In the case of macroscopic defects these are the kind of defect (cracklike, globular) and its size and orientation to the main stress directions; in the case of material property determination the parameters are mainly mechanical properties. Therefore, in NDE one has to solve inverse problems. The solution of inverse problems based on mathematical procedures such as integral equations is a strong developing discipline and most of the articles prepared for this special issue of the journal have the objective of discussing the latest state of the art in that field. However, practical NDE needs robust and quick solutions which are to be applied mainly online. Therefore, we present here inversion procedures based on multiple linear regression algorithms applied to inspection data. We describe the calibration procedure to fit the free parameters of the model functions and give examples of practical applications in industry.

Quantitative Hardening-Depth-Measurements Up to 4 mm by Means of Micro-Magnetic Microstructure Multiparameter Analysis (3MA)

Micromagnetic techniques since years have been used to characterize the microstructure and to analyse residual stress states in magnetizable materials, i.e. steels [1,2]. Applying a dynamic sinusoidal magnetization in the frequency range 50 mHz ≦ f ≦ 110 Hz with field strength maxima up to 150 A/cm irreversible (magnetic Barkhausen noise) and reversible (incremental permeability) micromagnetic processes (Bloch-wall-jumps, rotations) give independent nd-quantities together with a derived coercivity. The question arises as to whether these techniques can be adapted for the characterization of surface-hardened materials for the estimation of the hardening depth.

Industrial Integration of Residual Stress Measuring NDT‐Systems Based on Ultrasonics and Micro‐Magnetics

For years, Fraunhofer‐IZFP has developed NDT methods for the characterization of residual stresses, that have been introduced into industrial practice. These NDT methods are based on micromagnetic and ultrasonic principles. Ultrasonic systems are well integrated in the quality surveillance strategies of the European railway companies. The micro‐magnetic systems find their application mainly in the field of the machine‐building and the car supplying industry. This contribution reports on these applications and documents the benefits for the industrial user, which are mainly in the context of safety and lifetime.