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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reliable for detection of the flaws in ferrous steel. Several approaches can be noted as possible ways to suppress mentioned above specific noise. First way consists in the additional magnetization of the inspected area. But this approach can create some limitations due to ferrite core magnetization. Second way can be realized due to new selective EC probe development. In accordance with our experience, many specific ferrous steel inspection problems can be solved due to double differential EC probe development. Double differential type EC probes were developed some decades ago (Uchanin and Grabskij (1992); Uchanin (2001)). But main peculiarities of such probes were investigated not long ago (Uchanin et al. (2002); Mook et al. (2007)) and summarized in detail in the monograph (Uchanin (2013)). These probes were designed to obtain the second order differential signal response and can be characterized by specific quasi-absolute signal response with maximal amplitude for EC probe situated directly over crack. For local defects, these probes are characterized by specific four spots sensitivity points in signal response spatial distribution with two positive and two negative peaks. In presented double differential probes small size separated coils are used to obtain high sensitivity and penetration in combination with high spatial resolution. The main features of such probes can be characterized by:  High sensitivity and spatial resolution due small size coil application;  High sensitivity to surface and subsurface defects under coating and the possibility to detect defects with large clearance or dielectric layer between probe and inspected surface; Wide assortment of new double differential type EC probes, characterized by different operational frequencies and operational surface diameters from 5 to 33 mm, were designed last decades (Uchanin (2013)). Coil size, number of turns, penetration and spatial resolution can be optimized for specific application. Special methodology was developed for EC coil efficiency estimation needed for optimization of the EC probe coils mounted on ferrite cores (Uchanin (2014)). Due to our experience, double differential EC probes were proved as efficient tool for new inspection technologies development in aircraft, railway transport, chemical industry, power engineering and so on. Results obtained were presented on top-level international conferences (Lutcenko et al. (2012); Nardoni et al. (2014, 2017); Opanasenko et al. (2016); Uchanin et al. (2006, 2010, 2017)). The main advantages of these probes for ferrous steel components inspection are the possibility to detect cracks with high sensitivity for large air clearance between probe and inspected surface and during the inspection through dielectric and conductive (for example, aluminium) coatings. In this paper investigations concerned with new EC inspection technologies developed on the base of double differential type EC probes are presented.  High penetration for low frequency probes;  High lift-off and probe inclination suppression;  Wide inspected path with high spatial resolution. The possibility to detect defects through dielectric protective coating or air clearance with high enough sensitivity is needed in many applications. It is essential, for example, for inspection of gas turbine blades in fillet areas or when these blades are covered by ceramic coating. In this section small size MDF 0602 EC probe with 6 mm operational diameter was investigated. The investigations were carried out with special reference standard fabricated from ferrous steel 45, in which artificial 0.1 mm wide and different (0.1; 0.2; 0.5; 1.0 and 2.0 mm) deep slots were fabricated by electrical-discharge method. The EC probe signal responses were registered in the hand scanning mode and stored in PC memory in TIFF format. All mentioned above slots were detected with high signal-to-noise ratio. As example, Fig. 1 represent the signals obtained from shallowest 0.1 mm depth slot detected through 2.5 mm thick dielectric coating on operational frequency 40 kHz in impedance plane (left) and time-base (right) modes. Such thickness (2.5 mm) of coating was determined as ultimate thickness of dielectric coating, which doesn’t create the barrier for 0.1 mm deep crack detection. The cracks with depth more than 0.1 mm are detected through higher thickness of coating. Special diagrams were created to estimate ultimate thicknesses t U of dielectric coating for detection of cracks with different depths. To estimate sensitivity changes, when flaw is detected through dielectric coatings of different thickness, signals were registered for specimen covered by dielectric plates of different thickness (from 0.1 to 2.0 mm). For every slot depth a in the range from 0.1 to 2.0 mm the ultimate 2. Investigation of double differential type EC probes 2.1. Detection of cracks through dielectric coating or high clearance between EC probes and inspected surface