PSI - Issue 16

Valentyn Uchanin et al. / Procedia Structural Integrity 16 (2019) 198–204 Valentyn Uchanin, Giuseppe Nardoni / Structural Integrity Procedia 00 (2019) 000 – 000

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thicknesses t U of dielectric coatings were estimated (slots were detected with sufficient signal-to-noise ratio (see Fig. 1). It is intelligible that the ultimate thickness t U of dielectric coating depends on the flaw depth and operational frequency. Information about the ultimate thickness t U for any selected EC probe and inspection parameter is needed for inspection procedure development. For MDF 0601 type EC probe the dependences of ultimate thickness t U on the flaw (slot) depth were obtained for operational frequencies 40 and 400 kHz (see Fig. 2).

Fig. 1. Signals obtained from shallowest 0.1 deep slot through 2.5 mm thick dielectric coating on operational frequencies 40 kHz in (left) complex plane and (right) time base mode.

Fig. 2. The dependences of ultimate dielectric coating thicknesses t U on the slot depths a for operational frequencies 40 (  ) and 400 kHz ( Δ ) Presented diagram (Fig. 2) reveals essentially better detectability of the crack under the dielectric coating for lower operational frequency (40 kHz). For this frequency the shallowest 0.1 mm defect is detected through 2.5 mm thick dielectric coating and deepest 2.0 mm defect is detected through the 5.0 mm thick coating. These results are useful to estimate the possibility to detect the flaws in fillet areas, when some clearance between the EC probe and inspected surfaces is created due to the inspected surface curvature. Obtained diagrams (Fig. 2) permit to estimate the sensitivity threshold of selected probes when the cracks are detected through dielectric coating with known thickness. In many practical applications, the flaw in ferrous material must be detected through protective aluminium coating. Such inspection conditions were simulated by few 0.5 mm thick aluminium plates and reference specimen with 0.5; 1.0 and 2.0 mm deep slots (see previous section). Lower 40 kHz operational frequency was selected for better penetration. In Fig. 3 the hodographs (in impedance plane) obtained from different depth slots covered by 0.5 mm (a), 1.0 mm (b) and 1.5 mm (c) thick aluminium plates are presented. Amplifications in Fig. 3b and Fig. 3c are 6 dB and 12 dB higher than the amplifications on Fig. 3a relatively. Presented results create the powerful background for detection of cracks in gas turbine blades covered by aluminum protective coating. 2.2. Detection of cracks through conductive nonmagnetic coating