Examination of materials by electromagnetic methods

The present work deals with developing a high-sensitive electromagnetic probe for non-destructive testing of metals and metal components by the eddy current method. Metals are known to pertain to prevalent constructive materials. Metal parts and structures are often exposed to high loading and stressful environmental conditions, that usually result in reducing their lifetime. In many applications (for instance, at nuclear power plants or aircraft industry) the integrity and reliability of constructions are closely related to the safety matter. The economical factor has also great significance, concerning minimum overdesigning of components and maximum serviceability before their repair or replacement. To meet these demands the application of effective testing procedures and instruments is particularly necessary. Eddy current (EC) inspection provides appreciable resources for determination and evaluation of a wide variety of physical, structural and metallurgical conditions in metals and metal parts. The advantages of the method also comprise noncontact character, capability to examine subsurface areas of a test piece and adaptability to high-speed automated inspection. Conventional surface-scanning eddy current transducers such as pancake coils induce eddy currents in a relatively large volume of the material tested, with the current strength decreasing exponentially as the depth increases. Such a distribution of eddy currents results in low spatial resolution and low sensitivity to small subsurface flaws, which do not cause significant interruption of the current flow. The spatial resolution and sensitivity of inspection may be improved, beside adjusting proper electrical characteristics of a probe, also by generating eddy currents in the close vicinity of a defect. Therefore this work was aimed at developing an eddy current probe producing local EC excitation. For focusing the excitation field the probe employs a double-aperture core made of magnetically soft ferrite furnished with two air gaps. An excitation coil and two pick-up coils connected differentially are wound on the core. The differential arrangement and the application of magnetic shielding enable to reduce influence of background variations on a defect response. The probe demonstrates high sensitivity to small subsurface flaws. Using phase-sensitive detection, artificial cracks of depth as small as 10% located on the reverse side of 1.25 mm thick Inconel sample can be detected with 30 dB signal-to-noise ratio. Analysis of the probe performance is presented in the work as well. The dependencies of response solution on the excitation frequency and current magnitude are established. Examination of the probe by means of mapping using magneto-optic sensors and Finite Elements modelling is empoyed in order to evaluate distribution of the excitation field around the gapsAvailable from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi