Lightening the load - shape memory alloy reinforced composites for aircraft
Charlotte Meeks, Senior Scientist, and Andy Foreman, Principal Scientist at QinetiQ, UK, discuss shape memory alloy reinforced composites in delivering reduced foreign object damage for aircraft.
The requirements for composite aircraft primary structures are challenging. Using technology to reduce mass and manufacturing cost is therefore increasingly important, because equipment manufacturers and airlines are striving to decrease emissions and lower cost (and price) per passenger mile. Fibre reinforced polymer (FRP) composites have the potential to wholly address these requirements, but several issues remain with this class of material. Of these, arguably the biggest relates to their relatively poor impact performance and damage tolerance.
Despite this, FRPs are still widely deployed in a variety of high-performance, safety-critical aerospace applications. Their use is facilitated by certification authority regulations that address foreign object impact (FOI) threats. Many components using FRPs, including wing skins, control surfaces and fan blades, operate in environments where the FOI threat is considerable. For example, it is estimated that one in every 1,000 UK flights is subjected to a bird strike. In such applications, post-impact structural integrity is clearly the critical design driver. This invariably necessitates far heavier FRP structures than would otherwise be necessary to carry simple structural or aerodynamic design loads.
It is therefore clear that poor impact performance causes a severe restriction on the cost, mass, and efficient use of FRPs in high-performance, safetycritical applications. In turn, this suggests that improving the impact performance of FRPs has the potential to significantly reduce the cost and mass of future designs.
Shape memory alloy reinforced composites
A recent programme of Ministry of Defence-funded work at QinetiQ has been addressing this issue via the development of a highly novel polymer composite reinforced with shape memory alloy (SMA). These alloys are a special class of functional metallics that have the ability to undergo large amounts of elastic and plastic deformation at high stresses. The SMA wire selection, geometry and architecture, as well as the weaving process (developed by Sigmatex UK Ltd), are all designed to ensure the high specific properties of the FRP are maintained, while the SMA phase provides dramatically enhanced potential to absorb energy before structural penetration. The result is a patented material solution that can potentially deliver a step change in several different aspects of the impact performance of FRPs.
Bird strikes remain a severe threat to aircraft because impact events generate severe and multiple impact scenarios. In order to characterise the performance of the SMA-reinforced FRP, a series of leading edge structures were assessed using a simulated bird strike test. The different leading edge structures contained varying volume fractions of SMA reinforcement. The volume was controlled by changing the number of wires placed in the warp and weft direction of the fabric, and by changing the number of plies containing SMA reinforcement in the structure.
The baseline leading edge structure consisted of a monolithic carbon fibre-reinforced plastic (CFRP). A number of leading edge structures containing various levels of SMA reinforcement were also manufactured so that the influence of SMA on impact performance could be compared, for equivalent mass and thickness.
The leading edge structures were subjected to foreign object damage representative impact tests (40-250ms-1), simulating both hard and soft body projectiles. The results of the study showed that the presence of 10% by volume SMA in the structure delivered up to 217% improvement in energy absorption per unit mass.
Top: Baseline carbon FRP (no SMA) – ball penetrates through skin.
Bottom: SMA-reinforced carbon FRP (10% SMA) – ball is repelled from surface
Composite structures often need to be protected against environmental effects such as ice build up, high intensity electro-magnetic radiation or lightning strike. Structures requiring this protection typically use a parasitic material added to the surface of the structure, incurring additional cost and mass. A structural material that can avoid the application of these parasitic layers offers further potential to reduce the cost, mass and complexity of traditional manufacturing techniques. The potential applications of SMA-reinforced composites for lightning strike protection and de-icing have therefore also been investigated.
De-icing an aircraft is crucial, because ice build-up on control surfaces can significantly affect air flow, reducing the ability of the wing to generate lift and increasing drag. Potentially, the embedded SMA wires could be used to heat the surface, removing or completely preventing ice build up. Tests have shown that, using an insulated layer, the SMA wires would perform equally as well as commercial heater mats that are traditionally used in leading edge applications.
Aircraft are vulnerable to lightning strike – they are typically struck one or two times a year. FRPs do not readily conduct electricity, and thus aircraft structures that use the materials are susceptible to damage following lightning strike. Typical possible damage includes vaporisation of metal control cables, welding of hinges on control surfaces and explosion of fuel vapours within the fuel tanks. As a result, composite aircraft require a specialist lightning strike protection system. These are typically parasitic layers or coatings of metallic foil or mesh added to the surface of the composite structure.
Lightning strike assessments, at highest threat level, showed that using SMA-reinforced material eliminates the need for these parasitic layers. In fact, lightning strike tests have shown that just 5% by volume SMA delivers protection to an equivalent level of commercially available lightning protection products. The SMA-reinforced composites therefore offer a potential reduction in the cost, mass and complexity of traditional techniques for lightning protection in composite structures.
Significant research effort has been targeted at mass, cost and performance optimisation of SMA-reinforced FRPs. It has been demonstrated that SMA-reinforced composite technology has great potential to act as an integrated system for both impact energy absorption and lightning strike protection.
Potentially the material could be used anywhere impact poses a significant threat. However, particular benefit can be delivered where structural mass is driven by the need to resist impact threats during service. For example, modelling suggests that many leading edge panels are significantly heavier than would be needed to carry basic structural loads, due entirely to the need to resist penetrative impact threats. Thus the mass of such a structure could be significantly reduced using SMA reinforcement without cost increase. Finally, the multifunctional properties of SMAs suggest further mass and cost benefit is possible compared to traditional parasitic solutions.
Charlotte Meeks, Senior Scientist, Air Engineering Group – Technology Insertion. QinetiQ, Cody Technology Park, Ively Road, Farnborough, Hampshire, GU14 0LX, UK. Email: firstname.lastname@example.org