Designs on composites

Composites have quickly become a hot topic in the aerospace industry, replacing various metallic materials in many new aircraft applications. On the Boeing 787 for example, composites account for 50% of the aircraft's structural weight, and with the only visible metal on the leading edges and engine pylons, the aircraft has paved the way for future programmes.

Constructing an entire composite fuselage from nose to tail involves multiple sets of design and structural specifications, utilising a variety of manufacturing methods. As a result, Morson Projects stresses that design engineers need to ensure they have a full understanding of the manufacturing process before undertaking any design projects.

One aspect is the significant reduction in part count. While a typical metal fuselage requires over 100 metallic sheets compared with around three carbon fibre cloths for a comparable structure, manufacture is still just as labour intensive as the design process inevitably becomes much more complex due to the way that carbon fibre material behaves during the manufacturing process.

There are several key things to consider. Carbon fibre cloth is soft and pliable when in use, meaning it is prone to creasing, rippling and bridging, particularly when laying complex boundaries such as corner treatments and around doors and windows. This wrinkling effect may affect the strength, stiffness and fatigue resistance of the final product, so it is essential that fibre placement on large fuselage panels or wing skins undergoes a comprehensive simulation early in the design stage to avoid fibre deformation wherever possible. Failure to do this properly can be extremely costly as failures often do not come to light until physical testing.

Carbon fibre can also be prone to absorbing moisture which can lead to degradation of the material as layers come apart, resulting in costly repair and maintenance work. Therefore, it is important to identify any points on the structure that may suffer from moisture in the environment during the design stage so that these sections can be reinforced to avoid any unnecessary damage.

A further shortfall with composites is that they can be more susceptible to impact damage than metal structures, and repairing carbon fibre sections is a much more complex process. The most common type of damage to civil aircraft is from forklift trucks while the aircraft is stationary, with debris on the runway also a major issue. This issue is compounded by the fact that damage to composite components is not always visible to the naked eye, and that damage to carbon fibre can appear as a small dent on an exterior surface, but be much more extensive on the interior.

Such damage results in complex repair work, where the best course of action is always to use the original fibres and fabrics as to meet all the original design requirements for the structure. As such, it is of paramount importance that repair schemes are considered in parallel to the initial design of the structure to ensure that any damage that may occur can be catered for.

Finally, while composite manufacturing is currently still an expensive investment, lifecycle costs are now recognised as far more important than initial manufacturing costs. Aircraft need to be in service for 25 years so it is important to select the most effective manufacturing process that will see structures retain their performance and strength over this period.

Inevitably though, as the volume of composite manufacturing grows, production costs will decrease. This in turn will mean that the size and complexity of composite aerostructures will increase and continue to challenge composite engineers long into the future.

www.morson-projects.co.uk