Composites based on continuous fibre prepreg sheet laminates are a mature technology - widely used in the aviation industry for key structural components, However, the future horizon for composite development now lies in providing lightweight thick-section composite parts aimed at replacing metal components predominantly within the automotive sector. High thermal tolerance, thick section composites that are tough and durable could now offer a viable metal replacement technology for an expanding range of sub-chassis applications, particularly wheels, suspension, braking systems gear casings, rotor shrouds and components within the engine compartment.
Historically, monolith-type, thick-section parts have typically been made from aluminium or steel, and exceptionally with thermoset composites - but these have fundamental drawbacks when used for thick-section moulding. Thermoplastic discontinuous fibre tapes offer a tantalising alternative to traditional thermosets. Thermoplastic composites (TPC) based on e.g. PEEK and high-performance Nylons have the potential to offer a viable lightweight aluminium replacement option, with superior toughness and fatigue performance - both critical considerations for both automotive and aviation applications. The excellent formability and high flow characteristics mean parts can be produced quickly and cheaply with part counts into the 100,000's, making this class of composites uniquely suited to the volume demanded by the automotive industry, whilst also being capable of being used in thick section mouldings .
The recent development of Polyether ether ketone (PEEK) carbon fibre moulding compounds at Exeter showed that this material achieves a bulk modulus of ~40GPa when hot-pressed, which, whilst short of the ~70GPa offered by aluminium, is a marked improvement over previous offerings. Recent advances in manufacturing approach pioneered by the University of Exeter have seen the achievable modulus reliably pushed above 70GPa - directly on par with Aluminium, and, most excitingly, a technique by which controlled, localised orientation might be achieved through the use of pre-consolidated charges, exploiting the high viscosity of the material during manufacture. This technique could revolutionise the TPC sector, allowing the simple manufacture of thick-section components with the optimised design properties previously found only in multiaxial ATL processes. The new "pre-charges" route being proposed, will simplify manufacture, and remove the barriers to rapid volume production, similar to the advent of prepregs and SMC in the 1970's, that made possible the controlled, mass-manufacture of high performance composites in the aviation and automotive industries.
A base line improvement in properties together with the removal of manufacturing barriers, could change the current emphasis on thermosets to thermoplastics, which is highly important environmentally. Recycling of most types of thermosets is not commercially viable, despite extensive research into the area. Thermoplastic based systems have the potential to solve the recycling issue, with the ability to melt and re-press components without performance implications greatly improving the recyclability of the material - a characteristic that has long eluded thermoset CFRP's. Moreover, this trait lends itself exceptionally well to in-situ repair and damage healing. The viability of remanufacture and remoulding of composites needs to be established for all of the most common TPC's available. The study will both consider the remanufacture of components (closed loop recycling), and also the viability of 'shape change' with TPC's, i.e. the extent to which materials can be reprocessed like metals through re-melting and reforming multiple times. The future vision is for manufacturers to include recycling/remanufacture instructions as part of standard materials datasheets.
|