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Continued...
Shaping components such as commercial aircraft wing skins is a
complex operation due to a number of factors, including the physical size of the
components. The Airbus A380 wing skins are up to 33 metres long and 2.8 metres
wide, with thicknesses that vary abruptly between 3mm and 28mm, due to the
minimum weight design criteria of the product. Furthermore, an assembly
tolerance of -0, +1mm is required over all areas of the wing skin panels. Given
the complexity of the double-curved aerodynamic shape required for these
components, this produces an extremely challenging forming operation.
A particular feature of the CAF process is that the initial
deformation experienced by the component during deflection to the tool surface
is predominantly elastic. This loading is maintained during the heat treatment
process (which may be completed using multiple temperatures and dwell periods),
which activates creep (stress relaxation) and ageing mechanisms in the component
material. When the clamping is removed at the end of the heat treatment cycle,
some permanent deformation remains in the component. The amount of elastic
deformation that is recovered during this unloading is termed spring-back.
As the temperature time profile used in creep age-forming is
solely specified to produce an improvement in mechanical properties of the
component material, any retained shape after creep age-forming may be viewed as
purely a by-product of a required heat treatment operation.
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