Modelling and Validating Ferrite-core Probes for GMR-eddy Current Testing in Metallic Plates

Abstract

Non Destructive Testing techniques are more and more exploited in order to quickly and cheaply recognize ∞aws into the inspected materials. Within this framework, modelling is a powerful tool for inspection improvements. It helps probe-coil designers to optimize sensors for each examination requirement, providing better understanding of the involved physics, supporting operator training and increasing defect analysis reliability. The efiect of the ferrite core is analyzed in order to optimize the design of probe-coils and study various conflgurations of inspection. Particularly, Finite Element based analyzes will be carried out into this path. Direct problem will be assessed, and direct model will be formulated, dependent by difierent parameters, e.g., coil shape, working frequencies and so forth. The model will be subsequently validated by in-lab experimentations. 1. INTRODUCTION Non Destructive Testing and Evaluation (NDT/E) of metallic materials ofiers an exciting and interesting challenge to both researchers and applied technologists. For this kind of materials, the major problem of interest is the detection, location, orienting and sizing of single cracks, nondestructive evaluation of homogenous materials is presently in a state of detecting a variety of damage modes. In plane words, the present state of knowledge concerning what damage mode in metallic materials is responsible for the flnal failure process is still unclear. Hence, it is still not possible to suggest to NDT/E personnel exactly what type of damage, size, orientation, etc. needs to be found in an inspection process. Hence, the challenge. The excitement for researchers in the area of NDT/E of metallic plates is in the fact that NDT/E is presently playing an important role in helping to identify damage mechanisms in homogenous materials and to characterize the role played by these damage mechanisms in the flnal failure process. A variety of NDT/E techniques has been applied extensively to the investigation and characterization of metallic materials. Generally, one flnds that a combination of complementary NDT/E techniques are appropriate, and often required, to obtain as complete information as possible on the damage state of the specimen. Eddy-current (EC) methods are commonly applied because of their relative ease of their use, and the relative amount of information that can be obtained from them. As far as metals are concerned, the EC techniques has been used for quite some time for varied applications such as the detection of cracks, porosities and inclusions; metal sorting, evaluation of plate or tubing thickness, measurement of coating thickness and the thickness of non-conducting fllms on metallic bases and so forth 1 When a coil carrying alternating current is brought near an electrically conducting material, eddy-currents are induced in the materials by electromagnetic induction. The magnitude of the induced eddy currents depends upon the magnitude and frequency of alternating current; the distance between the current-carrying coil and the material under test; the presence of defects or inhomogeneities in the material and the physical properties of the material. The induced eddy-currents modulate the impedance of the exciting coil or any secondary coil situated in the vicinity of the test material. The difierence between the original coil impedance and the modulated coil impedance (due to the presence of eddy currents) is monitored to obtain meaningful information regarding the presence of defects or changes in physical, chemical or microstructural properties. Typical testing conflgurations may consist of ferrite core coil probes, placed above a planar (or at least locally planar) conductive specimen and operating in the time-harmonic domain, at frequency depending on the problem (typically between a few (Hz) to a few (MHz)) (2). The aim of ferrite core is to focus the magnetic flelds into the specimen, in order to increase the probe sensitivity to the defect. For each application, the coil model as well as the operating frequencies are set according to the task. This work proposes an integrated approach starting from the design and implementation of a novel probe in order to optimize the sensor efiect and the drop-in suppression, the operating parameters of the frequency