Structures regularly operate in environments that can cause 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 structure that are most susceptible to corrosion damage and which, after 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.
Simulation results showing the predicted change in the geometry due to corrosion
The severity of the corrosion damage varies 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 also vary with operational temperatures. Electrolytes produced by condensation, overflows and spillage may be of significant depth inside the aircraft, and may be contaminated with other chemicals. 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.
Using the results from the Corrosion Manager software, the material removed from the surface can be predicted (corresponding to corrosion occurring over a given exposure time), and cracks can be initiated in each potential problem area, to identify vulnerability to fatigue failure.
The geometry change caused by the corrosion mass loss can be used to perform a stress analysis of the structure, to determine the stress concentration in the component at the corresponding time in the life of the aircraft. It can then be assumed that cracks initiate, and the subsequent crack growth simulated. This crack growth simulation can take into account the corrosion damage, local stress concentration due to the damaged surface, and residual stresses. In the crack growth simulation, the full crack path and direction is determined along with the fatigue life.
Simulation results showing initial surface crack in the area of corrosion damage growing around the corner and then developing into a through crack
The fatigue crack growth simulation shown above assumed a single surface crack in the damaged area of the part where the stresses are highest. However, the stress concentrations indicate there is a line of high stress values across the damaged region. Therefore cracks are more likely to initiate along this a line than elsewhere in the part, so it is possible for multiple cracks to initiate in this region. The impact on the structural integrity and life of the airframe can be simulated for single and multiple crack scenarios.
Simulation results showing multiple cracks growing in the area of corrosion damage