Airframe structures regularly operate in environments that allow high levels of corrosion damage, and this damage leads to stress concentrations within the structure and potential development of cracks. Even when only a thin film of electrolyte is present on the structure, this can still lead to an electrical field that causes surface damage.
Computation of this electrical field can be used to identify areas in the airframe structure that are most susceptible to corrosion damage and which, after possible fatigue crack initiation, may lead to structural failure. Corrosion simulation can be used to take account of the properties of the electrolyte as well as the structural materials, to determine the rate of material loss from the structure. Having removed material from the surface (corresponding to corrosion occurring over a given exposure time) the stress concentrations can be evaluated and, if required, cracks can be initiated in each potential problem area, to identify vulnerability to fatigue failure.
Different corrosion damage could arise when different electrolytes are present, for example airframes operating at or near the sea will be subjected to salt water spray, and the electrolyte properties may vary with operational temperatures. Electrolytes produced by condensation, overflows and spillage may be of significant depth inside the aircraft, and may not be pure water. External surfaces may be subject to de-icing chemicals.
All-metal airframes are vulnerable to general corrosion, and depending on construction, crevice corrosion and pitting may also occur. Composite airframes which combine aluminium alloy and carbon fibre are at greater risk of galvanic corrosion because of the significant difference between the positions of aluminium and carbon in the electrochemical series. These properties, as well as the effects of surface coatings, all need to be included in the corrosion simulation. Simulation of the galvanic effects leading to corrosion has been successfully used for many years to assist design of cathodic protection systems for marine vessels and offshore structures, and has more recently been applied to aircraft structures.
This paper will firstly use simulation to investigate the influence of parameters, including electrolyte thickness and conductivity, on the rate of corrosion for a galvanic cell caused by a metallic sample in contact with a more noble material. The two materials will be exposed to differing environmental conditions, including immersion in a deep layer of electrolyte, exposure to thick layers of electrolyte at junctions between the different materials, and to thinner layers of electrolyte which might be caused by condensation. The paper will reach conclusions regarding the type of environment that is likely to produce higher penetration rates for the assumed geometry and materials and where these higher rates are likely to occur The paper will determine the damage caused by corrosion mass loss over a given operational time, and investigate the effect of such geometry change on the electrical fields in the electrolyte and the resulting rate of corrosion.
The geometry change caused by corrosion mass loss will be used to perform stress analysis of the same structure, to determine the stress concentration in the component at the corresponding time in the life of the aircraft. It will then be assumed that cracks initiate, and the subsequent crack growth will be simulated. This crack growth will take into account the corrosion damage and will inherently include local stress concentration due to the damaged surface. In the crack growth simulation, the full crack path and direction will be determined along with the fatigue life.
This paper describes a methodology that can be used by design engineers, to identify potential problems in an airframe structure during initial design, giving scope for design changes to reduce potential failures and in-service repair costs. This methodology identifies areas of the structure that have the greatest risk of damage - which may not be obvious without combined corrosion and fracture simulation; and so provides more informed targeting of locations where “what if” fracture mechanics should be applied.
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