DIAGNOSTICS OF DEGRADATIVE CHANGES IN PARAMAGNETIC ALLOYS WITH THE USE OF LOW FREQUENCY IMPEDANCE SPECTROSCOPY (original) (raw)

In the article theoretical bases of the low frequency impedance spectroscopy method and possibilities of its use in the diagnostics of paramagnetic alloys used in aeronautics are explained. The main focus is put on description of interaction of electromagnetic radiation with the material, eddy currents excited in the material (carrier of diagnostic data), modelling of the material and a given diagnostic problem with an equivalent RLC circuit, method of exciting and observation of eddy currents and bases of qualitative and quantitative analysis of the test signal. The knowledge necessary to consciously use eddy currents in NDT and SHM tests directed on the identification of an early phase of material degradation (the phase preceding an open crack) is also of particular importance in the article. The applied measurement instrumentation and sample results of in-house and other research centres' tests are presented. The in-house tests were performed on objects made of the ASTM 289 class C austenitic steel and ALSi13Mg1CuNi aluminium alloy and on paramagnetic materials used in transport and power industry, whose values of magnetic susceptibility are similar, but their composition, microstructure and other mechanisms of the early phase of fatigue degradation are different. Taking them as an example, the need of taking into consideration the specificity of aeronautical materials and diagnostic problem being solved by the selection of the frequency of electromagnetic radiation, methodology applied in preparation of test methods and diagnostic criteria is highlighted.