Advanced Engineering Show 2011: Adressing material shortcomings

Adressing material shortcomings
The Advanced Engineering show held at the National Exhibition Centre (NEC) served up a host of innovative materials for engineers to assess and critique. The sheer number of materials available on the market has never been more varied. But there is no perfect material and it is up to engineers to find the best fit for the product or application at hand.

Material selection in the early design phase dictates later engineering and manufacturing processes and increasingly lifecycle and disposal considerations. However, the material world is ever challenging traditional thinking and changing conventional wisdom.

The Metal Improvement Company based in Berkshire talked about advancemes in controlled shot peening. As the process maximises the life and properties of metals, it is an ideal process for design engineers looking to take weight out of structures or improve the reliability of key metallic components.

Many engineers may have forgotten about the process from their university days, but the metal surface treatment is well used in many industries, notably aerospace, to enhance fatigue performance by up to 1000% and therefore extend the life of critical components. It is also excellent at stopping and delaying cracks and crack propagation. A multitude of industries are increasingly turning to the technique from rail, medical and automotive down to components such as fasteners, valves, bearings and hydraulic fittings.

The process uses small, high-quality spherical steel, glass, or ceramic shots and fires them at a metal substrate. The impact causes a small dimple on the metal surface, which essentially carries out a very small and localised amount of work hardening. This process yields the material in tension with further movement restrained by the core resulting in surface residual compressive stress.

The process has also been advanced with the advent of laser peening. As the name suggests the process uses a laser to induce the same residual compressive stresses in metals. However, the laser is able to penetrate up to 10 times deeper than conventional cold working techniques with virtually no surface disruption. The process is clean, controllable and offers designers the ability to place residual compressive stress into key areas of components to retard crack initiation and growth, and increase fatigue strength.

The show was also an opportunity to find out from Composites UK, the trade body of the composites industry about the state of play of the advanced engineering materials sector in the UK. While it is clear that the overall use of the specialised engineering material has increased, its difficult manufacture still remains a confining factor to its use in lower volume niche applications.

The problem is that composite fabrication is still quite labour intensive and requires curing, often in large and expensive autoclave ovens. The manufacturing challenge is the major barrier to entry for many firms using carbon fibre. Despite the advantages of being extremely strong and lightweight, the cost and time of production puts many off.

The relationship, understanding and interaction between design and manufacturing engineers is something that some firms do better than others. But nowhere is the relationship more important than when working with composites. Engineers from both camps need to be more aware than ever of potential manufacturing principles and know just what is possible.

Out-of-autoclave technology is improving massively with the aim of offering firms considering the material a much lower capital investment cost. Derbyshire based Ebalta UK was showcasing its solution at the show that it hopes will enable higher volume and faster throughput of composite manufacture.

The process called Fibretemp allows the production of composite parts in either a resin infusion system, resin filled carbon fibre prepreg or standard wet layup procedures, without heating or post-curing in a large scale ovens or autoclaves.

Fibretemp uses the carbon fibre strands as electrical conductors. As the resistance of carbon fibres is much higher than metals, when a current is passed through it, it heats up. This phenomenon is exploited to heat and cure the material as current passes through the fibres.

Fibres positioned parallel to current flow act as conductors whereas transverse fibres distribute current all over the surface leading to uniform temperature distribution over the mould surfaces. A flexible copper strip is fed into each side of the mould which is connected to a control module, a low voltage transformer, allowing the heating to be precisely controlled.

Increasingly design engineers are being asked to consider the lifecycle of products and certainly many materials are bound by legislation that dictates their proper disposal. However, carbon fibre is notoriously difficult to recycle with a lot of products expected to go to landfill unless nothing is done.

As a result, an initiative by a group of UK companies to reuse and recycle carbon fibre material was launched at the show called Fibrecycle. It aims to recover carbon fibres from waste and develop reprocessing techniques while addressing material shortages, lower cost carbon fibre feedstock and develop novel property combinations.

The recycling processes that are to be developed and used include pyrolysis. This continuous or batch process burns off the resins with limited oxygen. The recycled fibres are then sold in milled, chopped or pelletised forms. Microwave pyrolysis is also under development in the UK, Germany and the US.

Another method developed by the University of Nottingham uses a fluidised bed. Cured and uncured composite materials are fed into a bed of sand where the fibre length is reduced to approximately 25mm. The sand is fluidised with a stream of hot air at 450-550°C. This forces the polymer to break down and vaporise, releasing the fibres and filler which are carried out in the gas stream. The resin is fully oxidised in a combustion chamber where the heat energy can be recovered.

The third method known as solvolysis, also known as a supercritical fluid process, sees the resin recovered as well as the fibres. High temperature and high pressure is required to reach a supercritical state with water, propanol or methanol used.

The resulting yarns and fabrics can then be blended with a carbon/PET and this can be sold at a lower cost than similar products currently available on the market. In common with other co-mingled and blended materials, the fabrics are simply placed in a mould tool under pressure and passed through a heating and cooling cycle so manufacture is also straightforward. Carbon fibre/PET composites offer 50% of the tensile strength and 90-100% of the tensile modulus of an equivalent composite based on virgin fibres.

Project manager, Dr Sophie Cozien-Cazuc of the Advanced Composite Group, says: "The materials that have been developed have a significantly lower environmental impact than virgin carbon fibre, because they divert materials from landfill and do not consume the energy needed to produce new fibres. The properties achieved mean that it is suitable for many applications especially in the automotive, aerospace, sports and leisure, medical and energy sectors."

Design Pointers
• Controlled shot peening maximises the life and properties of metals
• Laser peening is able to penetrate up to 10 times deeper than conventional cold working techniques
• An out-of-autoclave composite curing process called Fibretemp uses the carbon fibre strands as electrical conductors to heat and cure the material
• Disposal and recycling of carbon fibre is being developed and rolled out

Justin Cunningham

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