By Dr R Ian Mair AM FTSE
1 INTRODUCTION
Surface vehicles are employed for the purpose of transporting goods and for the facilitation of personal movement. Manufacture and operation of the transport vehicles, and construction of the infrastructure necessary for their utilisation, impacts on almost every aspect of economic endeavour. Encompassing road, rail, shipping and aerospace activities, the transport sector also represents a substantial user of fabricated materials.
In Australia cars and light commercial vehicles account for 82% of all passenger kilometres1 with buses accounting for a further 7%. Rail and air provide approximately 4 and 6% respectively of total passenger kilometres. Road vehicles, because of their numbers and their use throughout the main population centres, are the primary means of transport. Table 1 lists the numbers of vehicles employed in 1995 and 1984.
Worldwide it is estimated that there are now more than 500 million passenger vehicles in use. Approximately 1400 kg of materials are required in an average passenger car, distributed generally as shown in Table 2. With around 50 million new cars produced annually the requirement for structural and functional materials is of the order 70 million tonnes which has to be produced, uses energy to move, and must finally be recycled or disposed.
With 50,000 employees in Australia and gross turnover of $13.6 billion, the automotive industry accounts for 7% of manufacturing turnover. It produced 325,000 cars in 1997 and exported cars and components worth $2.7 billion.
Driving the evaluation of alternative materials is the push for cost-effective enhancements that reduce fuel consumption, reduce emissions and enhance safety, always with an eye on future recyclability. In line with global trends the increased demands for return on capital and development of niche product opportunities have widened the search for alternative manufacturing routes.
Fibre reinforced polymer (FRP) composite materials provide the automotive industry with a new range of performance attributes and processing routes.
2 PERFORMANCE CHALLENGES AND ISSUES
Notwithstanding their attractiveness, FRP composites still have relatively low penetration in the automotive sector. The factors behind this have been noted by Warren3: 'The major obstacles to automotive industry implementation of polymer based composites stem from a variety of factors including industry inexperience with the materials, undeveloped high production rate processes, the need for new joining techniques, lack of knowledge about the material response to automotive environments, lack of crash models, immature recycling technologies and a small supplier base'. Research continues to challenge these constraints.
2.1 Achieving lower mass
Lightweight construction, resulting from the demand for reductions in fuel consumption and emissions, has been a common factor in the move to composites for all surface transport modes. For instance, USA studies have estimated that a 25% reduction in current automobile mass would save 750,000 barrels of oil per day, reducing yearly domestic fuel consumption by 13% and prevent around 100 million tonnes of CO2 emissions annually.3
But the introduction of any new design concepts must recognise the ever-present competitive production cost pressures. For mainstream automobiles there is an increasing trend towards fewer platforms, each with higher volumes that now exceed one million units. Vehicle manufacturers will seek to differentiate their brands as much as possible within these platforms.
Materials will be required that economically overcome the limitations of current forming processes, accommodate high levels of surface finish and give flexibility for a wide variety of shapes. Composites have the potential to suit this new 'platform' paradigm better than other materials and challenge
the view that they are ill-suited to conventional, mass-production automobile manufacture.4
For low to medium volume and niche or emerging markets in the order of 10,000 † 100,000, on the other hand, the emphasis will be on time to market, profitability at low volume, design skills specific to the niche and brand equity.5 This embraces the production volumes being considered for regional manufacture within emerging Asian markets, and also for the more specialist alternative-energy vehicles now under development.
The introduction of plastic materials and composites in automobiles has been increasing steadily, particularly in Europe. However, for composites to succeed in achieving widespread acceptance it will be necessary to take a total systems approach to replacing existing metal assemblies. Part-by-part substitution will be too expensive.
To date most attention has been addressed at finding low mass replacements for secondary structural elements, such as bolt-on body panels.
The primary requirements are that the material must be cheap and satisfy low processing cycle times. For closure panels and semi-structural components the use of SMC (sheet moulding compound) and GMT (glass mat thermoplastics) and like materials based on non-oriented, discontinuous glass mat reinforcement with fibre volume contents up to around 30% provides a proven solution with weight advantages over steel.
Low-density SMC formulations are also under development utilising hollow glass sphere filler materials to be competitive with aluminium in less demanding uses such as inner panels.
Future requirements must also look to reducing mass in the load carrying elements, with space frame structures receiving keen attention for volume production. Composites with oriented long fibre reinforcements and high fibre volume content (greater than 50%) have shown significant potential to achieve the desired goals where structural performance is required.
The aggressive mass reduction targets for future automobiles have focused longer-term interest on carbon fibre as a prospective prime candidate reinforcement material. Currently, relatively high costs pose a significant hurdle. The high precursor costs (45-60% of production costs) and high capital equipment costs (20-35%) are the major contributors. Accordingly effort is being directed at overcoming these barriers. Research is being targeted at new classes of very low cost precursors and on providing the means for scaling up precursor volumes. Thermoplastic advanced composites offer significant potential with their increased fibre volumes compared with GMT, high toughness and fast, clean processing.
2.2 Manufacture of structural elements
For all but the lowest production volumes there is a need to introduce automation into the composites manufacturing process to reduce both costs and cycle times. This often requires a fundamental rethinking of the traditional approaches that have evolved from hand lay-up of dry fabrics and resin impregnated fabrics (prepregs). Within the automotive sector automation becomes an imperative. This has lead to a range of materials and processes for secondary structural and appearance elements that are compatible with existing automotive plant and methodologies, the most notable of which are SMC and GMT. However, as noted above, application to the primary load carrying structure often points to the use of long fibre reinforcements.
Textile-based thermoplastic prepreg sheets have emerged as a natural extension of GMT-like materials capable of similar processing approaches. Non-isothermal stamping with its shorter cycle times, coupled with commingled yarns and highly drapable non-crimp fabrics, has found early component applications in areas such as floorpans, skidplates and bumper beams using polypropylene matrix materials. A constraint on the use of continuous high performance fibres is the depth of draw that can be accommodated. With recent advances in fibre structures, such as stretch-broken carbon fibre reinforcements, the limitations posed by the low extensibility of conventional systems could be overcome for the production of more complex high performance shapes. It is claimed that the average length of the carbon fibres in stretch broken materials of around 80mm allows elongations in formed parts of up to 25-30%.
To be able to use long fibre thermoset composites economically, on the other hand, and to improve damage resistance, emphasis has been placed on near-net shaped integral fibre preforms combined with liquid resin moulding.
The range of manufacturing options open to manufacturers continues to increase as efforts are made to find a process that achieves the aims of placing the reinforcement quickly and accurately in readiness for subsequent resin infusion. The selected process will be dependent on component size as much as production volume. However, no one preform process or resin infusion approach can be expected to satisfy the objective of economically viable production for all automotive applications.
An alternative to continuous fibre mat in less structurally demanding applications is the introduction of oriented discontinuous long fibres selectively placed to achieve maximum performance. Attracting most interest has been the Programmable Powdered Preform Process (P4) introduced by Owens Corning in 1993 (now known as the OCTM Preformable System) and adopted by the USA Automotive Composites Consortium as the basis of investigations into automated preform production systems. The system combines a proprietary preformable glass with unique binder technology and uses computer controlled fibre placement robots to build up complex preform shapes ready for infusion. The unique benefit of the system lies in its use of chopped fibres that are oriented and cut to variable lengths (10-130 mm) on the fly. The potential of the system has attracted interest from the US Air Force which is supporting development of the process for the preparation of preforms from discontinuous carbon fibre reinforcements which is also of interest to the automotive industry.
2.3 Design/Safety/Crashworthiness
The push to reduce fuel usage and emissions must not be at the expense of reduced passenger safety. Lower mass designs and the adoption of lighter materials must meet the demanding performance expectations of those they replace. Crashworthiness is the ability of a vehicle to prevent occupant injuries in the event of an accident. The high kinetic energy absorption per unit mass of advanced composites makes them an attractive material option, but, they are unlikely to find their way into primary load bearing components until their performance is adequately demonstrated.
Tests under the USA 'Partnership for a New Generation of Vehicles (PNGV)' initiative have demonstrated that composites can perform adequately when correctly designed and manufactured. Indeed, the wide adoption of carbon fibre composites in Formula 1 racing cars attests to that observation! Still, there is a need to generate readily available design data that accounts for the alternative materials available and the variety of manufacturing approaches that may be adopted. Collaborative programs in Europe, the USA and elsewhere are working towards such a goal, with an emphasis on the design, analysis and impact energy absorbing characteristics of high volume production technologies for continuous load bearing automotive structures.
Development of crash models is progressing in parallel. At this stage only approximate analysis and modelling is practical since there is considerable uncertainty and a lack of reliable data for dynamic loading conditions such that the computational requirements for predictive modelling are prohibitive.
2.4 Durability
Durability as applied to automotive composites covers the performance of the materials under conditions of low energy impacts, creep, fatigue, exposure to automotive fluids and temperature extremes typical of the operating environment to which they will be subjected. As serious interest has increased in the application of automotive composites so has the activity aimed at resolving the durability issues.
The database on performance is increasing as service experience brings to light the full scope of the operating conditions to be endured. This experience has prompted a comprehensive program supported by the USA National Institute for Standards and Technology to understand the essential science base for composites performance leading to the establishment of test methods, reference data and standard materials. Similar programs are underway in Europe, such as that coordinated by the Swedish Institute for Composites, into the long term properties of fibre reinforced thermoplastic components.
2.5 Recycling
The long-delayed 'Directive for End-of-Life Vehicles' came into effect in Europe in August 1999. The European Union mandated that, beginning in 2006, carmakers will be responsible for dismantling and recycling the vehicles they manufacture and the industry has been experimenting for some years with alternative recycling schemes. With its 95% by weight reuse and recovery target for all new models introduced after 2005, automotive plastics will be evaluated along with all other materials on their suitability, both in design for recycling and for introduction into future waste collection and reprocessing schemes.
There are a number of hurdles still to be overcome to make plastic and composites recycling viable. Identifying the plastic is difficult and any valuable plastics in the vehicle may be hard to remove. Once they are removed and identified there is still the need to have sufficient volume to be economically processed. A fluidised bed approach for the recovery of glass fibre composites has been demonstrated at the University of Nottingham as part of a UK study, but quantities in excess of 10,000 tonnes per year of scrap composite would be needed in order for it to be commercially viable. However, it was shown that a carbon fibre recycling plant has the potential to be economic at much lower annual throughputs and may be more viable in the short term.
The legislative requirements for recycling are less stringent in the USA where adequate availability of landfill sites and low car buyer demand for recyclability is viewed as reducing the urgency. However, awareness of the need to address the issues posed by changing community expectations underpins ongoing research activity. Efforts on present and future automotive composite materials continue to evaluate the technical and economic feasibility of various recycling options for thermoplastic and thermoset composites.3 Further, the global consolidation and integration of much of the automotive industry will accelerate the transfer of design approaches and recycling technologies and methodologies between markets such that they will eventually find their way into all new vehicle designs.
3 CLOSURE
Constant pressure for improved performance at ever decreasing costs is now unquestioned across all transport sectors. These pressures for change are stimulating the search for new and innovative approaches to satisfying apparently conflicting expectations, notably in the automotive industry. Advanced polymer composites have demonstrated their potential over many years as reliable engineering materials. They are now being aggressively researched and developed to bring them into the mainstream of applications in high volume markets as a way to meet these new demands. The next decade will see dramatic changes in the level of their acceptance and the diversity of their use, as engineers master the necessary design skills and wider adoption makes them more cost effective.
4 REFERENCES
- Bureau of Transport and Communications Economics (1996) "Transport and Greenhouse Costs and Options for Reducing Emissions", Report 94, AGPS, Canberra.
- Australian Academy of Technological Sciences and Engineering (1997) "Urban Air Pollution in Australia", Melbourne (ISBN 1 875618 37 6).
- Warren, C D (1999) "Present and Future Automotive Composite Materials Research Efforts at DOE", ICCM-12, Paris, July.
- Brylawski, M (1999) "Uncommon Knowledge: Automotive Platform Sharing's Potential Impact on Advanced Technologies", SAMPE-ACCE-DOE Advanced Composites Conference, Detroit, September.
- Rudd, C D and Johnson, C F (1999) "Current Trends in Lightweight Vehicle Manufacture: A Composite Perspective", 6th International Conference on Automated Composites, Bristol, September (ISBN 1-86125-102-5).
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