On the potential of supersonic particle deposition to repair simulated corrosion damage
Introduction
The Aloha accident [1] was one of the first incidents to highlight the potential problem of multiple mechanically fastened repairs to corrosion damage, see Fig. 1 which reveals that despite the presence of MSD in the fuselage lap joint a section of the failure ran from corrosion repair to corrosion repair. One common approach to corrosion damage in operational aircraft is to blend out the corrosion and rivet a mechanical doubler over the region, see Fig. 1. Unfortunately, if the aircraft is operated in an aggressive environment corrosion can occur over a (relatively) broad area and this can lead to a number of mechanical repairs that lie in relatively close proximity, see Fig. 1. This repair process involves drilling holes that act as stress concentrators in the base structure and, unless the operational environment changes, these holes now provide additional sites at which corrosion can develop and subsequently lead to additional cracking. As a consequence a repair methodology is needed whereby the structure is not further damaged and new sites at which corrosion and subsequent cracking can occur are not created.
In this context it has been shown that, the Rosebank Engineering patented supersonic particle deposition (SPD) process1, which uses aluminium alloy 7075 metal powder2 with particles sizes in the range of 30–50 μm, has the potential to address a range of problems associated with aircraft structural integrity [3], [4], [5]. Indeed, [5] presented the results of a full scale fatigue test on an F/A-18 centre barrell3, which had twelve SPD doublers, see Fig. 2, Fig. 3, applied to a range of features, which was subjected to a measured operational RAAF spectrum. These doublers were found to experience peak stresses in the spectrum of up to 250 MPa without, at 8500 simulated flight hours, any evidence of cracking or delamination, see [5] for more details. As such this test when taken in conjunction with laboratory test results also presented in [5] highlighted the fact that SPD can withstand representative load spectra with peak stresses greater than 200 MPa without failure.
Subsequent constant amplitude tests [5] revealed the potential of SPD scarf repairs to repair simulated corrosion damage without the need to install a mechanical doubler. The tests reported in [5] involved SPD scarf repairs4,5 to simulated corrosion damage in 2 mm thick, 400 mm long and 42 mm wide 7075-T6 aluminium alloy specimens, see Fig. 4, Fig. 5. The length of the scarf used in [5] was 50 mm and its depth was 0.3 mm, see Fig. 4, Fig. 5. In [5] the depth of the scarf was taken to correspond to the depth of the simulated corrosion. To simulate small corrosion pitting/damage that was not removed by the scarfing process a 0.2 mm deep notch was machined across the full width of the test specimens [5]. Specimens that also had the material in the “scarfed” section removed but did not have a SPD doubler were also tested. These later specimens will be termed “unrepaired” specimens [5]. The “unrepaired” specimens were subjected to constant amplitude cycling at a peak stress of 140 MPa and R (=σmin/σmax) = 0.1 at a test frequency of 5 Hz, whilst the SPD scarf repaired specimens were tested at both 140 MPa, which is slightly below the endurance limit (of ∼150 MPa) of the SPD as determined from uniaxial S–N tests on the SPD material alone, and 160 MPa at a test frequency of 5 Hz, at R = 0.1. The tests were performed at room temperature in laboratory conditions. The “unrepaired” specimens, i.e. those without an SPD doubler repair lasted an average of approximately 36,800 cycles. The tests on the SPD scarf repaired specimens tested at 140 MPa were stopped 15,000,000 cycles with no evidence of cracking in the SPD doubler or in the underlying specimen, see [5] for more details.
Two SPD repaired specimens were also tested at 160 MPa6. One failed at approximately 640,000 cycles and the other at 1,330,000 cycles. In both instances the failure was due to fatigue cracking that initiated from defects at the machined notch in the specimens, see [5]. In all cases the fatigue lives of the SPD repaired specimens were dramatically greater than those seen for the “unrepaired” specimens tested at 140 MPa. At this point it should be recalled that both the SPD scarf repaired specimens and the “unrepaired” specimens contained a 0.2 mm deep notch at the base of the scarf. As such the lives of these specimens are not directly comparable to that of un-notched specimens without a scarf. Since for many representative flight load spectra the majority of the loads are significantly beneath the peak load in the spectrum these, albeit limited, test results suggest that when tested under a representative load spectrum with a peak stress of 160 MPa then a SPD scarf repair should last a very large number of cycles. As such this paper presents the results of a numerical study into the potential of SPD scarf repairs to repair simulated corrosion damage in 2 mm and 6 mm thick skins which are subjected to a range of representative load spectra. Whilst the analysis is somewhat idealized the results of this study suggest that this approach has the potential to repair corrosion damage in structures subjected to load spectra representative of helicopters, fighter, maritime reconnaissance, and civil transport aircraft.
Section snippets
SPD repairs to corrosion reworks subjected to a range of representative load spectra
It should be noted that, to date, SPD has primarily been used to produce protective coatings [6], [7], [8], [9], [10], [11], [12], [13], [14]. More recent aerospace applications have seen SPD being used to restore damaged/worn geometry [3], [10], [11], [13], [14]. In this context a joint US Army/US Navy study [11] has shown how this technology can be used to protect magnesium helicopter components and [3], [10], [14] outlines how the technique is now widely used to rehabilitate damaged/worn
Conclusion
Previous constant amplitude laboratory tests have suggested that supersonic particle deposition (SPD) can be used to repair corrosion damage in aircraft aluminium alloys under constant amplitude loading. Whilst the present analysis is somewhat idealized and is limited to 2 mm and 6 mm thick sections the results presented in this study for SPD scarf repairs suggest that, for components where the peak stress in the spectra is less than 200 MPa, this approach has the potential to address problems
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On the role of the interface on the damage tolerance and durability of cold spray repairs to AA7075-T7351 aluminium alloy wing skins
2021, Applied Surface Science AdvancesCitation Excerpt :Cracking that initiates in the substructure at the interface between the cold spray coating and the substructure. This failure mechanism is discussed in [8,18,22-24, 28,32,33]. Cracking in the substrate immediately outboard of the repair [8,9. 19].
Review of additive manufacturing technologies and applications in the aerospace industry
2019, Additive Manufacturing for the Aerospace IndustryApplication of SPD to Enhance the Structural Integrity of Fuseage Skins and Centre Barells
2018, Aircraft Sustainment and RepairSaltwater corrosion behavior of cold sprayed AA7075 aluminum alloy coatings
2018, Corrosion ScienceCitation Excerpt :In more recent work, CS has been applied to the repair of load-bearing structures using AA7075. Jones et al. have reported that SPD can be used to repair corrosion damage in aircraft aluminum alloys, enhancing the fatigue lives of the repaired coupons and structures [4,7]. Despite these successes in using cold spray deposition for repair, the corrosion performance of the repair material itself remains an open question.
Nondestructive inspection of fatigue crack propagation beneath supersonic particle deposition coatings during fatigue testing
2017, International Journal of FatigueCitation Excerpt :Examples include an application to the Apache helicopter aluminium alloy (AA) 7075-T73 mast support [1] and AA 6061 deposition on the F/A-18 Hornet generator control unit tube flange [2]. SPD repair technology has also been demonstrated for the restoration of mechanical properties in military aircraft AA structure containing simulated corrosion [3,4]. Early applications of the technology have been mainly for the purposes of providing protective coatings to existing structure.
Additive metal solutions to aircraft skin corrosion
2020, Aeronautical Journal