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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Uchanin (Ed) (2011)). Principal attentions taken into consideration were the possibility of selected method to estimate the residual life of the long-term exploited structures and usability in in-service conditions. Besides that, NDT can create the background for material optimization under the impact of engineering procedures (e.g., welding). In both cases, the fundamental difference of our approach is the searching of the correlations between the material properties possible to be measured nondestructively and FCGR characteristics needed for residual life estimation. In this study the evaluation of AA degradation in aging aircraft and the characteristics of the AA welded based on the SEC measurements are presented. EC meters for SEC measurements at the operational frequency 60 kHz in the range from 14.0 to 37.1 MSm/m with a measurement error of 0.1 МSm/m through the dielectric coating were developed (Uchanin et al. (2012)). High level of lift-off suppression (up to 0.5 mm) for local EC probe was achieved due to the new processing algorithm based on the EC probe signal phase processing (Uchanin (2012)). For ferrous steel structures, the local coercive force (CF) measurements by magnetic structurescope KRM-Ts supplied with attachable type probes were applied. Other important problem is the development of the NDT methods for noncontact measurements of the mechanical stresses in structural components. Innovative EC method based on the changes of MA was proposed for the measurements of such stresses in steel components. 2. Е valuation of aluminum alloys degradation in aging aircraft Conventional aircraft maintenance practices don’t take into consideration the phenomena of material degradation for residual life estimation. The specimens cut out from long exploited aircraft components were applied to determine the true characteristics of D16 grade and B95 grade AA (Al-Cu-Mg system, analog of 2024 AA and Al Zn-Mg-Cu system, analog of 7075 AA, respectively). It was shown that the most sensitive to AA degradation are the characteristics of plasticity (relative elongation δ ) and fatigue crack growth resistance (fatigue thresholds  K th and cyclic fracture toughness  K fc ) (Ostash et al, (2006)). These characteristics are needed for estimation of the fatigue life and relevant reducing correction factor needed for the residual life calculation taking into account the AA degradation (Ostash et al. (2007)). As NDT based method, the SEC (as structure sensitive characteristic) measured by EC method can be applied for AA degradation monitoring (Ostash et al. (2014)). Today such measurements are used for the AA heat treatment influence evaluation mainly. Aircraft components can be described as 3-layer structure: first – dielectric coating; second – about 0.5 mm aluminum plating; third – evaluated AA layer with changed SEC. The main demands to EC conductivity meter were: possibility to measure SEC through dielectric coating and aluminum plating (in 3-d layer) and high locality for inspection of aircraft components with large number of holes. EC probe with coils mounted on 1.2 mm diameter ferrite was applied to determine high locality. The data obtained for long-term exploited D16ATNV and V95T1 alloys (Fig. 1) show the 1.5 – 2.0 times reduction of the fatigue threshold  K th that is corresponded to 20 – 30% SEC ( σ ) growth. This degradation process depends on the level of operational stress  eqv in several areas of aircraft component.

Fig. 1. Dependences of elongation  , fatigue threshold  th and SEC σ of degraded D16 АТ NV (left) and V95 Т 1 (right) alloys on equivalent stresses in various areas of bottom and upper wing skins, respectively.

The SEP measurements in different wing skin areas of long- term exploited aircraft “ANTONOV - 12B” (produced in 1966) show that upper skin V95T1 material degradation occur more intensive between 9th and 2 nd ribs of wing (RW) areas and in area of center section between different stringers (St) (Fig. 2). In Fig. 2 all RW are marked by