Iris Altpeter

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.

Minimalistic Devices and Sensors for Micromagnetic Materials Characterization

Micromagnetic materials characterization requires sensors which essentially consist of two critical elements: an electromagnet which introduces a well-defined magnetic field to the material, and a sensor system which detects the materials response to the applied magnetic field. The devices developed at Fraunhofer IZFP obtain a multiparametric “magnetic fingerprint” with these sensors by means of several methods. The magnetic fingerprints of calibration samples are used as input for pattern recognition or regression analysis, thus allowing the prediction of mechanical-technological material characteristics (hardness, yield strength, etc.) or residual stress. This approach is called micromagnetic multiparameter microstructure and stress analysis (3MA). The long-term stability and reproducibility of the sensor and device characteristics are crucial for the reliability of the measured results. Therefore, the measuring hardware should follow a minimalistic approach. In this paper, we propose a way of simplifying the measuring hardware by multiple use of sensor elements, reducing the analog signal processing chain and transferring most signal processing tasks to the PC.

Nondestructive Determination of Residual and Applied Stress by Micro-Magnetic and Ultrasonic Methods

Ultrasonic techniques can detect volume and surface stresses and they are not limited to the investigation of ferromagnetic materials as the micromagnetic methods. The magnetic nondestructive techniques are restricted by eddy current damping to surface near regions. All ferromagnetic and magnetoelastic quantities are sensitive to the mechanical stress and microstructural state of the test specimen, whereas the ultrasonic velocity is mainly influenced by changes of the stress level and the texture. The main problem is: how to separate the different influences in the measuring quantities which is required to realize a quantitative nondestructive stress-measuring device.

Micro-Magnetic Evaluation of Micro-Residual Stresses of the IInd and IIIrd Orders

Micro-residual stresses (MRS) of the IInd and IIIrd orders play an important role in the fracture mechanical analysis of thermally-cycled materials and thus in lifetime analysis of such affected components. In multi-phase materials there can exist two kinds of MRS: thermally-induced MRS of the IInd order and coherent MRS of the IIIrd order. The first appear when individual material phases exhibit different thermal expansion coefficients and the second occur when the lattice parameter of the second phase particles which are embedded coherently in the matrix and the lattice parameter of the matrix are different. The main emphasis of the presented research work is the development of a micro-magnetic non-destructive technique for quantitative characterization of MRS of the IInd and IIIrd orders in iron-based materials. Forthat goal Fe-Cu-(Ni-Mn) samples were manufactured and characterized by means of a non-destructive procedure based on the tensile load dependence of the maximum Barkhausen noise amplitude.

Non-Destructive Testing with Micro- and MM-Waves — Where We are — Where We Go

Test methods based on microwaves are well suited to characterize mainly electrically non-conducting materials and to detect and image defects in non-destructive and in many cases in contactless way. While in the past the often complicated and expensive microwave equipment prevented the widespread application of microwave methods this situation is now changing due to the increasing availability of low-cost high integrated microwave components. Microwave testing has found an important application in the field of security, e. g. to detect and image dangerous objects hidden under clothes. The principles of this technique can be applied in the domain of joining (e. g. welding of plastics or gluing plastic to plastic) for contactless detection of defects under cover layers.

NDE OF MATERIAL DEGRADATION BY EMBRITTLEMENT AND FATIGUE

This paper presents an overview on current activities and results concerning NDE of material degradation by embrittlement and fatigue. In general we differentiate between NDT methods for the characterization of embrittlement and fatigue which are either microstructure or crack sensitive. At the present time there is no reliable NDE technique known which can be used on technical components to detect the onset of failure during the early stages of embrittlement and fatigue. Therefore, microstructure sensitive methods are required which have potential for detecting degradation during the early stages and for infield capability. Such methods are micro‐magnetic methods, eddy current methods and thermal methods. These techniques, however, have only been tested under laboratory conditions to date. The micro‐magnetic method will be discussed concerning embrittlement. Results on eddy current testing by using GMR and thermal methods are documented concerning fatigue characterization.

Detection of Residual Stresses and Nodular Growth in Thin Ferromagnetic Layers with Barkhausen and Acoustic Microscopy

In order to develop and optimize thin ferromagnetic layers which are used in combined write-read heads it is necessary to evaluate the residual stress state and defects in microstructure of these layers nondestructively. Residual stresses in ferromagnetic thin layers are due to non-optimized process conditions, undesired phase transitions or insufficient ductile adaptation to the substrate. The ratio of signal and noise and the sensitivity of the write-read heads are influenced negatively by residual stresses. The bad ductile adaptation between the substrate and the ferromagnetic layer leads to insufficient workability of the write-read heads.

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.

Electromagnetic techniques for materials characterization

Abstract Electromagnetic techniques have a special advantage for nondestructive applications in materials characterization because magnetic and mechanical properties are influenced by the same microstructure parameters, ie, dislocation density, grain boundaries, precipitations, and so on. Different magnetic techniques based on different interaction mechanisms are described. The usability of magnetic techniques for determination of microstructure state, texture, and stress state, is demonstrated by several applications.

Hybrid methods for materials characterization

A nondestructive hybrid method combines two or more testing methods. Hybrid methods acquire several measuring quantities with different physical information content of the tested materials and different sensitivities against disturbing influences. After calibration procedures this allows a quantitative determination of material properties like residual stresses, hardness, hardening depth, yield strength, etc. This section will give an insight into combinations of micromagnetic, eddy current and ultrasonic methods, and it presents examples of applying such nondestructive hybrid methods.