The influence of the environment and corrosion on the structural integrity of aircraft materials

Abstract

A B S T R A C T Fatigue results of 1 1 / 2 dog-bone jointed specimens manufactured from 7075-T6 aluminium alloy indicated that the application of corrosion preventative compounds (CPCs) at the faying surfaces slightly decreased the log mean fatigue life at 144 MPa, while the effect was not statistically significant at the higher stress level (210 MPa). The addition of the CPC also reduced fretting corrosion at the faying surfaces and shifted the fatigue initiation sites closer to the edge of fastener holes. Scatter in fatigue life was found to be associated with the location and size of the intermetallics at the initiation site. The presence of corrosion in the bores of the countersunk fastener holes reduced the fatigue life by up to one order of magnitude. Fatigue test results for dog-bone specimens manufactured from 7075-T651 and 2024-T351 aluminium alloys indicated that the presence of exfoliation corrosion reduced the fatigue life under dry conditions, with a greater reduction under humid conditions. The application of a CPC to the corroded region eliminated the influence humidity had on fatigue life.Many military aircraft are now operated well beyond their original design life, and age-related issues affecting aircraft structural integrity need to be addressed. The presence of corrosion in airframe structures can have a detrimental effect on the integrity of the aircraft structure by promoting fatigue crack initiation and accelerating crack growth. In addition, corrosion and fatigue cracks in ageing aircraft structures may have a significant effect on maintenance cost, aircraft availability and flight safety. It is increasingly likely in the future, that for reasons mentioned above, many components discovered with corrosion damage will be left in service, providing that the structural integrity of the component is not compromised. This approach will rely on knowledge of propagation rates for corrosion and fatigue under a variety of aircraft environments. This paper describes the results of work aimed at (i) gaining a better understanding of when and to what extent the environment degrades the fatigue strength of typical aircraft alloys, and (ii) identifying the effect of corrosion preventative compounds (CPCs) and corrosion on fatigue life.The 1 1 / 2 dog-bone fatigue specimen was identical to that used in a round robin testing AGARD program, 1 and consisted of aluminium alloy coupons assembled using a pair of cadmium-plated steel Hi-Lok interference fit fasteners (Fig. 1a). This configuration, developed by the Laboratorium fur Betreibsfestigkeit (LBF) in Germany, is a single-shear joint and simulates the load transfer and secondary bending moments commonly experienced at stiffener run-outs attached to the outer skin of airframe structures. 2 The alloy was 3.2-mm-thick 7075-T6 bare aluminium alloy plate and the coupons were machined with the longitudinal (rolling) direction parallel to the major specimen axis. All specimens were protected with a chromate conversion coating, applied to specification MIL-C-554...

A DIC-Based Study on Fatigue Damage Evolution in Pre-Corroded Aluminum Alloy 2024-T4

“…Similarly, bending fatigue experiments on pre-corroded AA7075-T6 showed that fatigue strength was reduced by about 60% due to the presence of corrosion pits [10]. Such observed significant degradation in fatigue properties is attributable to premature crack initiation and propagation originating from localized corrosion [11,12,13,14,15]. Fractographic observations on fatigue crack formation in pre-corroded AA2024-T3 alloy under fatigue loading showed that corrosion pits caused accelerated multiple-site crack initiation [16]; elsewhere, cracks in AA7075-T6511 were shown to initiate from pits clustered as a semi-eliptical surface micro notch, rather than the deepest pits [15].…”

The interaction between corrosion management and structural integrity of aging aircraft

“…Further investigation by Shankar and Dhamari 15 showed that the application of CICs increased the fatigue life of highly clamped bolted joints, but had a detrimental effect on bolted joints with low clamping force. Russo et al 16 found a significant fatigue life reduction in 1½ dog‐bone specimens when tested at low loads, but little change in fatigue life at high load level. This result was the opposite of those observed when ‘conventional’ joints (i.e.…”

Fretting Wear in Selected Elements of Rail Vehicles

“…Fretting wear also occurs in other fields of technology, where a pair of elements touching each other is subjected to oscillatory tangential displacement. It follows from the review of literature that fretting wear is found in, for example, aviation industry [5][6][7], in medicine [8][9][10] in orthopaedic items connecting fractured limbs and in artificial joints, as well as in the automotive industry [11].…”