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Fibre Reinforced Polymers - From Aerospace to Infrastructure |
By Dr Robert Ian Mair FTSE
INTRODUCTION
Fibre Reinforced Polymers (FRP) can no longer be considered as exotic materials suitable only for niche applications where the performance demands justify very high prices, such as in aerospace or premium sporting goods markets. Today high performance FRP materials are starting to challenge that most ubiquitous of engineering materials, steel, in everyday applications as diverse as automobile bodies and civil infrastructure. It would be naive to suggest that FRP will displace steel from its dominant role; however, continuous advances in the manufacturing technologies and performance of FRP have intensified the competition in a growing range of applications leading to significant growth in market acceptance (Figure 1).
Figure 1: US Growth in polymer composites shipments compared with aluminium and steel (1)
Fibre reinforced polymers represent a class of materials that fall into a category referred to as composite materials, or more colloquially, composites. Composite materials have a long history. Consisting of two or more materials that retain their respective chemical and physical characteristics when combined together, they allow the properties of each to be utilised to their greatest effect. Amongst the earliest composites must surely be the simple straw and mud brick that continues to be used even today! The higher tensile strength of the straw binder overcomes the limitations of the friable clay matrix to give a durable building block.
Modern composite materials constitute a significant proportion of the engineering materials market ranging from everyday products, such as steel reinforced concrete for buildings, through to sophisticated niche applications like high performance ceramic armour designed to resist explosive impacts. Each type of composite brings its own performance characteristics that give it favour in selected applications. Most notably for fibre reinforced polymers they can be tailored to give high strength coupled with relatively low weight, corrosion resistance to most chemicals, and offer long term durability under most conditions of environmental exposure (Table 1).
Table 1: Advantages of Polymer Composites
- High specific strength properties (20-40% weight savings)
- Ability to fabricate directional mechanical properties
- Outstanding corrosion resistance
- Excellent fatigue and fracture resistance
- Lower tooling cost alternatives
- Lower thermal expansion properties
- Simplification of manufacturing by parts integration
- Potential for rapid process cycles
- Ability to meet stringent dimensional stability requirements
Increasingly enabled by the introduction of newer polymer resin matrix materials and high performance reinforcement fibres of glass, carbon and aramid, the penetration of these advanced materials forms has witnessed a steady expansion in uses and volume. With increased volume has come an expected reduction in costs. High performance FRP can now be found in such diverse applications as the fuel cylinders of natural gas vehicles, the blades of wind-powered turbines, industrial drive shafts, the support beams of highway bridges and even paper making rollers. An examination of the diversity of some of these newer applications and the social and commercial pressures that underpin their introduction gives an instructive insight into the future place of high performance FRP.
HIGH PERFORMANCE FRP APPLICATIONS IN PERSPECTIVE
Whereas high specific strength and low mass were often the dominant criteria to be achieved, particularly for aerospace applications, there is today an increasing emphasis on other criteria such as environmental durability, energy absorption or fire resistance, in addition to strength and mass benefits. Sometimes referred to as "advanced composites" where high performance demands set the design and manufacturing criteria, the designation is now less sharply delineated. Even the materials previously regarded as being synonymous with high performance FRP, such as carbon fibre, are today much more affordable and hence not always used to the limit of their capabilities.
The working definition for "high performance FRP" used in this paper will refer to composite systems developed for and applied to situations that demand sophisticated design and manufacturing control to achieve cost-effective solutions under the operating conditions to which they are exposed. In general the presentation will also confine itself to those applications which fall within the broad ambit of structural applications, that is those required to have a sustained load carrying capacity or to meet extreme loading events.
Whilst it is a valid argument that many sporting applications fall within these criteria they too will be excluded from further detailed consideration. Indeed, water sport applications, and in particular racing yachts, present very demanding service performance requirements. Equally, it has been stated that up to half of the world's current production of carbon fibre is consumed in the manufacture of golf club shafts!
Figure 2: Relative importance of cost and performance
At a basic level the choice of material solution in a given situation is a trade off between cost and performance. Figure 2 indicates the general hierarchy of emphasis for the key industry sectors noted for their evaluation of high performance FRP applications. For both cost and performance, however, their relative determination is dependent on the factors and timescales to be taken into consideration during any analysis. Equally, the outcomes can be dictated by the quality of the information available and the degree of sophistication in the methodology adopted by the system design team. Limited experience with the application of composite materials can lead to their exclusion as a result of over-conservative evaluations.
Acceptable costs for the introduction of high performance FRP are not absolute. For any given application and industry sector the final choice is often a competitive outcome of alternative solutions, including advances in alternative materials such as aluminium alloys and metal -composite hybrids. New developments in materials and manufacturing systems, changing balances between supply and demand and reassessments of community values can all impact on the result. The latter in particular has played a growing part through legislation and shifts in consumer preferences, most notable in recent years in attitudes towards such matters as industry deregulation, recycling, safety, energy usage and CO2 emissions.
RECENT DEVELOPMENTS AND THE DRIVERS OF CHANGE
(a) Aerospace Industry
The drivers for continued development differ between the military and commercial sectors (2). Historically an emphasis on performance over cost has been dominant in military applications in the face of national security imperatives. Today more complex criteria are applied; nevertheless with national security still the goal. These have been summarised as Affordability, Performance (predominantly low mass), In-service cost, and Low Observability. The challenge of Affordability in particular has given impetus to efforts to dramatically reduce the manufacturing costs for all materials, including polymer composites. Specifically for polymer composites the Manufacturing Science and Technology Program of the US Department of Defence, as an example, has set key goals to achieve a 75% reduction in design time; 25% reduction in material costs; and 50% reduction in fabrication time (3).
Figure 3:Airbus Industries carbon fibre composite wing section under development containing Australian components developed within the CRC for Advanced Composite Structures.
Figure 4: Vehicle Manufacturing and Life Costs as a Function of Annual Production Volume (9)
The commercial aircraft sector on the other hand is characterised in recent years by increased consolidation in the number of major airframe producers and greater competition through deregulation in the number of aircraft operators. Notwithstanding positive projections for growth in aircraft numbers internationally to match continued growth in passenger numbers, the industry sector is prone to significant swings in demand that impact upon aircraft deliveries and heighten the competition between airframe producers to secure new orders. Consequently, interest continues to be shown in the potential for high performance FRP where they can demonstrate cost-effectiveness in initial and through-life costs. It has been stated that the key competitiveness factors for prime and sub-system manufacturers now resides in proprietary product technologies and competency in utilising and adapting non-proprietary process technologies (4). The CRC for Advanced Composite Structures is working collaboratively with Australia's principal sub-system manufacturers to secure the technologies that will contribute to long-term success (Figure 3).
(b) Automotive Industry
The automotive industry is a significant contributor to the manufacturing sector of all industrialised nations. Within Australia the automotive industry accounts for about seven per cent of total manufacturing value added and about one per cent of Australia's GDP. The automobile, however, is also a significant user of petroleum products and a major contributor to the national output of greenhouse gasses. In 1990, transportation accounted for around 16 per cent of Australia's carbon dioxide emissions (5). For the USA and Canada the contributions are even more significant, with lower values for Germany and Japan. International community concerns and uncertainties about the long term effects of CO2 emissions, coupled with decreasing self sufficiency in petroleum products production, particularly in the USA, are driving efforts to develop more fuel efficient vehicle technologies. Whilst adoption to date of polymer composites has not reached early expectations for chassis and suspension components, the adoption for body construction continues to grow (6). In the USA, more than 25 production-model cars and trucks in 1998 were using sheet moulding compound (SMC), a special form of FRP, for bonnet and body panels (7).
A major initiative being undertaken by the USA government and major automotive companies is the Partnership for New Generation Vehicles (PNGV) which aims to achieve fuel efficiency improvements of up to three times (8). Lightweight materials are critical to the development of highly fuel-efficient vehicles. Since 1991 the USA Office of Advanced Automotive Technologies has been working with the Automotive Composites Consortium to develop composite material technologies for vehicle body applications. Under the PNGV program weight-reduction potential has been determined for selected body structures. Aluminium is a leading candidate for the vehicle structure. Glass-reinforced thermoplastic polymer is also a promising material for weight reduction because of the relatively low cost of the fibre, its fast cycle time and its ability to facilitate parts integration. Carbon fibre reinforced polymer is another candidate but will require breakthroughs in cost and manufacturing techniques to be cost effective for high volume production. Research toward this end is starting to focus on some promising options and may lead to a re-evaluation in later designs. The economic choice of material is, however, also very dependent on production volume.
As shown in Figure 4 the cost benefits arising from the use of polymer composites can be significant at lower volumes up to around 50,000 vehicles per year. This is well above the production volumes being considered for regional manufacture within emerging Asian markets, and also for the more specialist alternative-energy vehicles now under development worldwide. The Composite Concept Vehicle from Chrysler Corp., for instance, was designed for new buyers in the emerging economies of China, India and Southeast Asia. The only use of steel is in the chassis / frame and significantly reduces tooling and manufacturing plant space requirements (10). Nearly every country in the Asian region uses local-content quotas, import duties and local regulations to make the sale of imported cars relatively expensive and local manufacture at lower volumes is a serious consideration.
Figure 5: Composite concept car tubular frame skeleton from aXcess Australia (11)
Interest in the use of polymer composites was highlighted by its use in the aXcess Australia concept car built by Millard Design using a carbon fibre and epoxy resin tubular frame skeleton (Figure 5). Whilst not designed as a production vehicle it was fully functional and attracted worldwide attention (11).
Pressure for reductions in energy use and lower emissions levels, along with emerging market regions, will continue to drive interest in advanced composite options for the automotive sector.
(c) Offshore Oil and Gas Industry
Steel and concrete are the materials of choice for offshore oil and gas production platforms, with steel dominant in the topside applications. Polymer composites have found their way into limited applications, particularly where corrosion and the need to reduce high maintenance costs has been an issue. As the industry moves to greater water depths, however, the significance of weight saving has become increasingly important in conjunction with the application of buoyant tension leg structures. For instance the US Department of Commerce has estimated that US$0.25m could be saved per meter of water depth if metal riser pipes could be replaced by FRP!
Research is advancing options for the use of FRP in primary structural areas, although few underestimate the challenges involved. Nevertheless, the commercial incentive to extend the application of polymer composites is compelling and has driven an industry-supported effort since 1986 to explore the boundaries for the applications of polymer composites in topside structures. Coordinated through Marintech Research of the UK, a consortium of offshore oil producers and researchers has recently announced its key findings from a study commissioned through Odebrecht SLP (an offshore design and fabrication contractor) and MSP (formerly Maunsell Structural Plastics).
Starting from an existing steel topside structural design the team considered two alternative polymer composite approaches; (a) a "conforming architecture solution" which replaced steel with polymer composites wherever possible, and (b) a "radical architecture solution" which utilised the fullest capabilities of polymer composites. In the former the weight saving amounted to 38% with the corresponding cost of the fabricated structure reduced by 10%. For the latter the more dramatic savings amounted to 55% reduction in weight and 25% reduction on fabricated costs!
Coupled with growing confidence based on in-service experience, and motivated by constant pressure to reduce capital and operating costs, the interest in advanced composites for offshore applications will continue to receive active attention, particularly as the industry moves into deeper offshore exploration areas.
(d) Construction Industry
Polymer composites have long been used in the construction industry. Applications range from non-structural gratings and claddings to full structural systems such as framing for industrial supports, buildings, long span roof structures, tanks, bridge components, and complete bridge systems. Their benefits of corrosion resistance and low weight have proven attractive in many low stress applications. An extension to the use of high performance FRP in primary structural applications, however, has been slower to gain acceptance although there is much development activity (12).
Bridges represent only one aspect of construction industry activity, but one that has attracted strong interest for the utilisation of high performance FRP (13). Not only is their use for the repair, seismic retrofitting and upgrading of concrete bridges finding increased adoption as a way to extend the service life of existing structures, they are also being considered as an economic solution for new bridge structures. The commercial viability for repairs has been proven with hundreds of field applications in Europe, Japan and North America. The incentives to look to polymer composite solutions are readily found in the statistics for the cost of replacement of deteriorating bridge structures already in place. The annual worldwide potential for FRP composite repair systems is estimated as at least US$10 billion/yr! Polymer composites are seen to offer advantages that are not to be found in the more traditional materials, particularly their resistance to corrosive attack in those areas that rely on the application of de-icing salts to maintain road access. Design approaches and manufacturing efficiencies developed for road bridge applications will benefit their introduction into a broader range of civil construction fields.
The impediments to the rapid introduction of polymer composites into infrastructure share much in common with all other fields of application, albeit perhaps 20 years behind the aerospace industry in developing viable approaches. In particular, the in-service experience appropriate to the lifetime expectations of many infrastructure applications is not generally available. Amongst the issues that the polymer composites industry must address to achieve penetration of the construction market are the following:
- Long-term durability and fatigue characteristics when under load or exposed to the operating environment;
- Creep behaviour under loaded conditions and the need to inspect the quality of repair or upgrade installations;
- Failure mechanisms of laminates and composite structural systems;
- Behaviour characterisation of the material system over time;
- Developing material performance specifications that define and predict minimum behaviour;
- Establishing approved industry design codes;
- Developing reliable analysis and design methodologies
- Introducing cost-effective manufacturing processes with standard processing parameters;
- Providing common repair systems; and
- Increasing awareness and confidence in the applications of advanced composites.
Whilst they pose some challenges in developing the necessary information, none of the issues is insurmountable.
CLOSURE
It is evident from this brief overview of the applications and drivers for the introduction of high performance polymer composites into a broad range of industry sectors that effort will continue on their development. The economic incentives are substantial and lasting. It is equally evident, however, that the diversity of applications and operating environments pose significant challenges in distilling out those aspect of composites design and performance that are appropriate to a given application. Australian research and development activity is highly regarded internationally and contributing to the wider acceptance of polymer composites.
REFERENCES
- Composites Fabrication, January 1999.
- Long, G., "Future Directions in Aeronautical Composites", International Council of the Aeronautical Sciences, 21st Congress, Melbourne, September 1998
- USA Department of Defence, "Technology Area Plan Summary (1995-2001)", Manufacturing Science and Technology Program, 1994.
- Industry Canada, "Aircraft and Aircraft Parts", Sector Competitiveness Framework Series, December 1995.
- ______, Climate Change Science: Current Understanding and Uncertainties, Australian Academy of Technological Sciences and Engineering, February 1995.
- Harrison, A.R., "Opportunities for Cost-Effective Materials in the Automotive Industry", Paper No. C536/016/98, IMechE, 1998.
- Anon, "More SMC on new model vehicles", Composites Technology, July / August 1998, p13.
- Patil, P.G., "Technology Paths to Triple Automotive Fuel Economy", US Department of Energy, Office of Transportation Technologies;
- Mascarin, A.E. et al, "Costing the Ultralite in Volume Production: Can Advanced-Composite Bodies-in-White be Affordable?" Rocky Mountain Institute, December 1996;
- Ashley, S., "Plastic Cars for Developing Nations", American Society of Mechanical Engineers, 1997
- Smith, G., "Composite frame forms backbone of concept car", Reinforced Plastics, October 1998, 68-70.
- Nanni, A. et al, "International Research on Advanced Composites in Construction (IRACC-96)", Final Report to US National Science Foundation, August 1996. (Updates October 1997, June 1998)
- _____, "A Look at the World's Composite Bridges: Charting the Evolution of Bridge Engineering Using FRP Composites", Market Development Alliance, SPI Composites Institute, New York, 1998.
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Dr Ian Mair is CEO of the Cooperative Research Centre for Advanced Composite Structures, a position he assumed in October 1998. He is a graduate of the University of Melbourne with qualifications in Civil Engineering. After graduation he was employed by BHP, starting in 1970 until 1997, where he held positions in BHP Research, Corporate Planning, Corporate Business Development and BHP Steel Technology and Planning. From 1989 to 1992 he spent four years in Japan as senior technical representative and Vice President Development. In 1997 Dr Mair left BHP to start his own consultancy in technology planning and management prior to joining the CRC-ACS. Dr Mair is a past-President of the Institution of Engineers, Australia, and a former member of the Prime Minister's Science and Engineering Council. He is currently a Professorial Associate in Civil Engineering at the University of Melbourne, Chairman of Engineering Education Australia, a Director of the Advanced Engineering Centre for Manufacturing and a Member of the Board of the Australia - Japan Foundation.
ATSE Focus is a non-refereed publication. The views expressed in the above article are those of the author(s) and do not necessarily represent the views of the Academy.
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