PSI - Issue 59

M. Karuskevich et al. / Procedia Structural Integrity 59 (2024) 175–181 M. Karuskevich et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Water Displacing Hard Film, Non Water Displacing Soft Film, Non Water Displacing Hard Film as shown at corrosion-doctors.org. Excellent protective properties of CPCs have been proved by numerous standard and special tests. No doubts exist regarding their possibility to protect metal structures against aggressive surrounding. At the same time results of some experiments with aircraft components covered by the CPCs reveal possibility of negative side effects, these are both reduction of the riveted joints fatigue life (O'Neill and Smith, 1975, Schijve et al., 1977, Kolkman, 1982, Jaya et al. 2010a,b), and increase of the fatigue cracks propagation rate (Purry et al., 2003, Kuhlman et al., 2003). Some conceptual explanations for these phenomena have been proposed, but high level of the requirements to the aircraft reliability defines the demand for more experimental data accumulation. The described below experiments prove actuality of the problem for the aviation industry, throw light on the fatigue behavior of the typical aircraft structure in the presence of CPC, target the future research activity aimed on the development of the methodology for grounded selection CPCs for aircraft structures. 2. Aircraft parts protected by CPCs It is known that certain zones of aircraft structures are prone to corrosion in the biggest extend. These are: the battery compartments, bilge areas, bulkheads, wheel wells and landing gear, water entrapment areas, wing flap and spoiler recesses, areas hit by exhaust stream, and cooling air vents (aviation-safety-bureau.com). Parts of the aircraft structure where additional treatment by the water displacing CPCs is possible and currently used are described by well-known CPCs manufacture ARDROX (chemetall.com). Bilge area of the fuselage is one of the most vulnerable for the action of the aggressive environment, essentially liquid accumulated at the bottom of the structure. Structural joints of skin and first of all riveted elements are of the special interest because crevice corrosion of these elements combined with cyclical loading is able to lead to the fatigue crack nucleation, then propagation and finally lost of structural integrity (Aircraft Accident Report, 1988). Pure aluminum cladding layer doesn’t solv e the problem completely due to the destruction of this layer at holes for rivets. The same can be said regarding the protection by oxidizing, primer and paints which are vulnerable to the mechanical damage and environment influence. Analysis of the Aloha flight 243 accident, as well as analysis of the aircraft zones prone to corrosion, draws attention to critical zones of the aircraft. It was found (Aircraft Accident Report, 1988), that the fuselage failure initiated in the lap joint; the failure mechanism was a result of multiple site fatigue cracking of the skin adjacent to rivet holes along the lap joint upper rivet row and tear strap disbond which negated the fail-safe characteristics of the fuselage. The fatigue cracking initiated from the knife edge associated with the countersunk lap joint rivet holes; the knife edge concentrated stresses that were transferred through the rivets because of lap joint disbonding (Fig. 1) (Aircraft Accident Report, 1988).

Fig. 1. Fatigue crack location in lap joint (Aircraft Accident Report, 1988).

This analysis as well as analysis of contemporary aircraft fuselage structures have allowed correct selection of the specimens design for fatigue tests.