PSI - Issue 16

Valentyn Uchanin et al. / Procedia Structural Integrity 16 (2019) 192–197 Valentyn Uchanin, Orest Ostash / Structural Integrity Procedia 00 (2019) 000 – 000

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numbers N. SEC measurements were executed by placing EC probe directly on skin protective coating due to high lift-off suppression in the range of coating thicknesses (0.15 – 0.3 mm).

Fig. 2. Distribution of SEC of upper wing and center section skins measured between wingtips of AN-12 type aircraft.

Material in wing tip area (marked by * in Fig. 2) has the smallest SEC due to insignificant operational loading. So, aluminum alloy in this area can be assumed as identical to material in as-received state and SEC in this area can be applied for comparative measurements for degradation monitoring in accordance with proposed methodology (Ostash et al. (2014a, 2018). Presented results show, that the AA degradation monitoring can be executed nondestructively by EC measurements of SEC through the dielectric protective coating and pure aluminum plating in in-service conditions. There are not numerous investigations concerned with application of EC method for estimation of mechanical parameters of the metal in welding areas. Only in few studies it was shown, that the distribution of SEC measured by EC method correlated with the distribution of hardness of AA welded joints. The character of SEC distribution is dependent on the probe size (as it is shown in Fig. 3, left) for pulse-arc welding of AMg5 type AA. As the FCGR characteristics of materials, the fatigue threshold Δ K th and cyclic fracture toughness Δ K fc were used (Fig.3, right). The correlations between SEC and welded metal strength were investigated also. 3. Characterization of the aluminum alloys in the welded joints

Fig. 3. (left) Distribution of SEC in welding measured by EC probes with diameters 1.2 mm (▲) and 0.75 мм (○) and (right) correlations between SEC and FCGR characteristics and ultimate strength of weld metal (АМg5) for different welding conditions  – U  ;  –  K th ;  –  K fc .

The mechanical parameter such as fatigue threshold  K th and cyclic fracture toughness  K fc are dependent on the welding conditions changed by space orientation of a joint (Labur et al. (2016)). These characteristics are more sensitive to changes in the structural and mechanical state of the metal of welded joints than its strength under static loading (Fig.3, right). These changes are accompanied by changes of the SEC. So, the evaluation of the metal properties in the welded joints can be performed nondestructively by EC method possible to measure SEC without the direct contact with inspected surface .