Mere Plastics? Welcome to the composithic age!
Composites are here to stay! The global demand for these materials will ramp up tremendously in the years to come. An unlikely career for “mere plastics”! This article outlines the pros and cons of composites and looks at EADS research in this field.
COMPOSITES @ SPAIN
· Spain is a world leader in research in the advanced industrial application of composites, its lab results making a valuable contribution to industry at all levels.
· Composite technology arrived in Spain in the 1960s, when CASA (the Spanish Aeronautical Manufacturing Company) started a programme with Northrop to build the Spanish version of the F-5 fighter. From that point onwards, Spain was committed to the future of composites in the aerospace sector.
· A huge step forward came with the development of the A320’s horizontal tailplane made from composites, as well as a number of space structures designed at CASA Space.
· To focus all its composite-related know-how in Spain, EADS decided in 2006 to create FIDAMC, the Spanish Composites Research, Development and Application Centre, as part of Innovation Works, the EADS corporate labs for advanced research projects. FIDAMC is a cooperation between EADS, the Spanish Ministry of Industry and the Regional Government of Madrid.
Have you ever heard of reaction wood? It’s a very smart way nature has found to teach us a great deal about material design! In short, reaction wood forms when a woody plant is under mechanical stress, such as frequent winds, snow or soil movement, and it helps the plant to keep an optimum position, upright for the main stem or horizontal for the branches.
Thus reaction wood will either grow on the upper side of a branch and contract longitudinally (tension wood) or on its lower side and expand (compression wood), in both cases to keep the branch in the correct position after continuous, heavy snowfalls. Reaction wood is a perfect example of a material optimised with respect to bearing loads. Just like composites … From a mere 5% of the A310-300 back in the early 80s to the challenging 53% of new-generation aircraft such as the A350 XWB, global demand for these materials in the aerospace sector will ramp up tremendously in the years to come.
But composites are used in many other fields, their applications ranging from aerospace and military equipment to sports, the automotive sector and civil engineering. The idea behind composites is to combine a matrix (which may be polymeric, ceramic or metallic) with internal reinforcement, normally in the form of fibres. The combined properties of the matrix and fibres yield an optimal material for applications with anisotropic loads, which run only along certain directions. Thus concrete pillars behave well under compression but can crack under tension, so something is needed to reinforce the material. Concrete reinforced with steel rods is the solution! Strictly speaking, composites may have been around for more than 10,000 years – let’s recall that Pharaoh decided to punish the Israelites by refusing to give them straw for their bricks!
 |
| ||||
Composites: Pros & Cons
 |
| ||||
Composites have consequently been used in the aerospace sector to build lighter aircraft with lower fuel consumption and environmental impact.
In a field where stiffness is the primary design requirement (even more important than strength), carbon fibre reinforced composites have proved competitive and offer a huge potential for future aircraft applications (glass fibre alone, with its poor stiffness, is not a good structural material for aircraft).
Composites in Airbus products
Other attractive features of composites include better fatigue behaviour than metallic materials, high potential for component integration and potentially lower energy consumption in manufacture than aluminium.
The possibility of engineering a composite, i.e. selecting an optimum combination of matrix and reinforcement for a particular application, is both an opportunity and a challenge for the material designer.
Although it paves the way for a new generation of lighter components with optimised properties, it also demands more extensive knowledge to cope with the inherent anisotropy of this type of material in order to create structures with the required mechanical strength in the direction of the load. This is currently at the heart of most of the problems faced by composite engineers: more and more experience is needed to reach the level of know-how already available for widespread alternatives such as aluminium.
FIDAMC’S ROLE WITHIN EADS
· FIDAMC started operating in 2007 and explores emerging technologies and research projects to demonstrate the feasibility of manufacturing new composite structures as well as their performance under realistic loads (the specimens designed at FIDAMC will be big enough to allow the study of phenomena that would also occur in full-size components – such as representative wing or tailplane structures and fuselage sections up to a total volume per specimen of around 5x5x8 m3).
· FIDAMC has one clear goal: to become the bridge between basic research and industrial applications. Results achieved by years of basic research must ultimately be developed into something that benefits society!
· As a clear link between basic research and industrial needs, FIDAMC will focus on faster application of research results (currently anything from 5 to 15 years). Cost reduction, safety and quality are also paramount!
Throughout the development of an aircraft with composite structural components, heavily loaded areas such as sections within the wing or the horizontal tailplane may often fail a test due to buckling, fracture, poor interlaminar properties and many other potential causes. Composites are anisotropic (metallic materials are isotropic) and components in highly stressed parts may comprise around 200 layers of less than 0.2mm thickness each, with different fibre orientations depending on the layer to cope with the various load directions. This makes simulation a real nightmare but also an exciting challenge!
Computers cannot yet simulate every fibre in a material (for a full size component), as each calculation would take too long. The latest multiscale models aim to estimate average properties for a real component from smaller models that simulate individual fibres. Composite computer models are built according to a number of underlying hypotheses that simplify the problem; however, this means that the models are less realistic. Moreover, manufacturing feasibility is more difficult to achieve in composites since they involve many more phenomena. The curing process (similar to a “baking” process) causes shrinkage in the component due to crosslinking (inside thermoset matrices), the chemical reaction itself is exothermic, the process temperature gradients add thermal deformations according to the material’s coefficient of thermal expansion and in-mould processes introduce residual stresses.
HOW COMPOSITES ARE MADE
· The name may be new, but composites are by no means a modern invention. The most primitive of them, a mixture of straw and clay used to build huts, is almost as old as humanity itself. Today, fibre-based composites play a growing role in the construction of aircraft, helicopters, missiles, rocket launchers and satellites.
· The most common composites used in aerospace are carbon fibre reinforced plastics (CFRP), a mix of 60 percent carbon fibre and 40 percent resin. Other types include sandwich and honeycomb structures, fibre/metal laminates and glass fibre materials, while ceramic carbon fibre composites are used in high-temperature applications such as rocket engines.
· Several techniques are used: the most common one uses rolls of carbon fibre which are pre-impregnated with resin, then cut and shaped as required and built up in a series of layers. Pressure is then applied and the part is baked at high temperatures in an autoclave. In other methods, the carbon fibre is moulded, sewn or braided before adding the liquid resin. This can be done in several ways, some of which involve placing the part in a vacuum and baking it at room temperature. Richard Kleebaur/wic
As it’s not that easy to predict everything that will happen to a real composite part, testing is an essential tool for finalising a component’s design, adding all the necessary modifications (which unfortunately means extra weight) to ensure it will pass all tests. It must have been pretty daunting to introduce a composite structural component in such a demanding environment as a commercial aeroplane, especially when it was first done for a part as crucial as the A320’s horizontal tailplane, which meant a huge step forward for the application of composites to commercial aircraft.
Composites also have a number of aspects that must be addressed to produce new designs in an efficient way: costs of manufacturing and raw material, impact resistance, failure modes, recycling (of thermoset matrix composites), environmental performance (temperature, moisture, corrosion, ultraviolet radiation), metal-to-composite joining, etc. Nor can designers forget other more subtle factors, such as microorganism growth, when dealing with composite parts in contact with fuel (microorganisms present in the fuel could proliferate by using a composite’s organic matrix, as well as the fibres’ chemical sizing, as a source of nutrition and energy – a protective paint is applied to avoid this).
 |
| ||||
COMPOSITES RESEARCH – AN OVERVIEW
· EADS has always played a leading role in composite research: thus cost-reducing manufacturing methods that dispense with an autoclave and increase the automation of processes such as tape lay-up, robotic fibre placement, resin transfer moulding and resin film infusion techniques.
· Considerable efforts are being made to develop more advanced methods of design, sophisticated structure analysis and digital simulation and to advance the state-of-the-art in nondestructive testing and inspection, structural health monitoring, quick repair and lightning-strike protection technologies.
· Smart structure technologies promise further weight reductions and less frequent maintenance. Current research programmes will gradually introduce self-adaptive, selfmonitoring and self-healing structures to future aircraft.
· Tomorrow’s smart structures may also include nano-technology, resulting not only in materials with greatly improved thermal or electrical conductivity and fire resistance, but also with sensor functions and structural elements capable of changing their dimensions, shape or function on command (morphing). Richard Kleebaur
Finally, it is important to highlight that, quite ironically, many material engineers frequently still approach composites with a somewhat metallic mentality. This is, of course, quite understandable, given the many years of design carried out with metallic alloys as the primary structural material.
However, it is important to remember the anisotropic nature of composites as well as their characteristic features outlined above. Just like knots in wooden furniture, certain “defects” in a modern composite will not necessarily impact a structural component’s performance, it all depends on their location (and dimensions, obviously).
To sum up, composites will prove a new step in material development and optimisation for a number of diverse applications, but their characteristics, both their limitations and their huge advantages, make them a very special kind of material indeed! Just as stone, iron or bronze have given us the paleolithic and neolithic ages…might we already have entered the composithic age?
Isaac Prada-Nogueira
 |
| ||||
The HighFlyer
Would you like to know more about relevant studies for the aerospace industry and your employment prospects? Then subscribe to The High Flyer A free newsletter brought to you by EADS
