5

Structural Materials

The challenge for materials technology is to support a dramatic reduction in vehicle weight to achieve the PNGV goal of up to three times today 's fuel economy. Lighter weight PNGV vehicles should maintain the performance, size, utility, and cost of ownership of today's comparable vehicles, and their operation should meet or exceed federal safety and emissions requirements. The sensitivity of vehicle weight to the efficiency needs of the powertrain is depicted in figure 5-1. An ability to achieve in excess of 40 percent vehicle mass reduction could reduce the demands for power source thermal efficiency from over 50 percent to approximately 40 percent. At a vehicle mass reduction of 40 percent, advanced conventional power sources have the potential to attain the necessary thermal efficiency to reach the objective of three times today's fuel economy. Thermal efficiencies of approximately 40 percent are associated with more mature power sources, which are the most likely to meet the necessary performance requirements within the PNGV timeframe. At the other end of the scale, power sources with much higher efficiencies, such as fuel cells1, may be less demanding in terms of needed vehicle-mass-reduction requirements but are relatively immature in their development and, hence, have a lower probability of meeting PNGV schedules.

THE PNGV APPROACH

The objectives for vehicle mass reduction in support of the PNGV Goal 3 vehicle have now been established, as shown in table 5-1. In pursuit of these objectives, the PNGV vehicle engineering team is directing manufacturing, materials, and system analysis teams in support of subsystem

1  

Assumes use of hydrogen fuel stored onboard the vehicle.



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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT 5 Structural Materials The challenge for materials technology is to support a dramatic reduction in vehicle weight to achieve the PNGV goal of up to three times today 's fuel economy. Lighter weight PNGV vehicles should maintain the performance, size, utility, and cost of ownership of today's comparable vehicles, and their operation should meet or exceed federal safety and emissions requirements. The sensitivity of vehicle weight to the efficiency needs of the powertrain is depicted in figure 5-1. An ability to achieve in excess of 40 percent vehicle mass reduction could reduce the demands for power source thermal efficiency from over 50 percent to approximately 40 percent. At a vehicle mass reduction of 40 percent, advanced conventional power sources have the potential to attain the necessary thermal efficiency to reach the objective of three times today's fuel economy. Thermal efficiencies of approximately 40 percent are associated with more mature power sources, which are the most likely to meet the necessary performance requirements within the PNGV timeframe. At the other end of the scale, power sources with much higher efficiencies, such as fuel cells1, may be less demanding in terms of needed vehicle-mass-reduction requirements but are relatively immature in their development and, hence, have a lower probability of meeting PNGV schedules. THE PNGV APPROACH The objectives for vehicle mass reduction in support of the PNGV Goal 3 vehicle have now been established, as shown in table 5-1. In pursuit of these objectives, the PNGV vehicle engineering team is directing manufacturing, materials, and system analysis teams in support of subsystem 1   Assumes use of hydrogen fuel stored onboard the vehicle.

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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT FIGURE 5-1 PNGV design space. SOURCE: PNGV, 1995. development2. Directives are established through workshops where the PNGV technical teams and vehicle engineers identify potential materials and applications. Challenge tables have been created to prioritize developments needed to support technology selection in 1997. These tables compare present state-of-the-art technologies with PNGV requirements. A series of workshops has been held with materials suppliers to identify materials research projects in response to the developments identified in the challenge tables. Workshop participants are encouraged to submit white papers outlining their proposed research. Opportunities may exist to leverage materials developments in non-automotive areas —notably in the aerospace sector—in pursuit of PNGV goals. Advanced alloys, ceramics, and composite materials developed for a range of high-performance applications may offer potential for automotive use. MATERIALS OPTIONS Three candidate materials have now been identified for body structure; namely, steel, aluminum, and graphite fiber-reinforced, polymer-matrix composites (GFRP). Many opportunities exist to develop improved 2   The PNGV technical organization structure is addressed in chapter 9.

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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT TABLE 5-1 Vehicle Mass Reduction Targets for PNGV Goal 3 Vehicle System Current Vehicle (lb) PNGV Vehicle Target (lb) % Mass Reduction Body 1,134 566 50 Chassis 1,101 550 50 Powertrain 868 781 10 Fuel/Other 137 63 55 Curb Weight 3,240 1,960 40 SOURCE: PNGV, 1995. materials for candidate powertrain components, such as fuel cells, flywheels, and ultracapacitors. However, no major studies in this area have yet been initiated by the PNGV Materials and Structures Team. Therefore, the present discussion is limited to materials for vehicle body structure. Steel Steel is the dominant material in use today for automotive body structures. Efforts sponsored by the American Iron and Steel Institute (AISI) are underway to reduce the weight of steel-intensive vehicles by 20 percent or more through improved design techniques, advances in manufacturing technology, and wider use of high-strength steels. An analytical study by the Ultralight Steel Auto Body international consortium forecasts a weight of 205 kg (450 lb) for a body-in-white 3, based on incorporation in the design of off-the-shelf, near-term approaches that relate primarily to Goals 1 and 2 (Martin, 1995). Although increased use of high-strength steel was included in the study, weight reductions were achieved primarily through design approaches rather than new materials. Such considerations are applicable to other metallic materials. Current activities form a basis for exploring and using more advanced technologies directed toward Goal 3 needs. Ongoing studies are investigating steel-based materials with improved strength, ductility, and stiffness-to-weight ratio; methods for strengthening after forming are also being considered. Manufacturing studies are addressing increases in production rates for new processes for stamping and assembly, including challenges in joining dissimilar materials, such as the welding of steel to aluminum. A major focus is the development of a systems-based design approach that can be implemented on a desktop computer-aided design system rather than requiring a mainframe supercomputer, with associated accessibility problems. An important issue, yet to be addressed by the PNGV, relates to the 3   Defined as the vehicle assembly, comprising the assembled but unpainted body, excluding glass, trim, and chassis items.

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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT design process and how systems analysis will be used to evaluate tradeoffs between various objectives for materials and components. The specification of low-cost conventional materials in some areas can be balanced with high-cost advanced materials in other areas to achieve targeted vehicle weight reductions optimized with respect to cost (both piece and life cycle). Available data and information on planned programs indicate that 20 percent overall vehicle mass reduction would be a very challenging objective for steel (Sherman, 1995). However, the advantages of this material are that it is the lowest cost material (per pound) of the three candidates; it has the largest technical experience base for automotive structures; and it would not need either the added investments in new manufacturing processes required for aluminum or the major investments necessary to support GFRP. Aluminum The intensive use of aluminum is forecast to yield weights close to target values for the overall vehicle. However, based on current manufacturing practices, it is unlikely the target weight can be achieved within the PNGV timeframe with no cost increase. While the forecast weight savings for aluminum usage are not controversial, the outstanding tasks listed by the PNGV suggest that more data are needed on the strength of aluminum weld bonds and crashworthiness of the aluminum intensive vehicles (Sherman, 1995). Weight reductions of up to 45 to 50 percent for body/chassis are considered attainable, based in part on supporting data from Ford's Aluminum Intensive Vehicle (AIV) design and testing program, which included crash vehicles (Sherman, 1995). Analysis by the PNGV Materials and Structures Team suggests that, taking into account potential weight savings, a cost of $1/lb for aluminum sheet would be competitive with steel for vehicle structural applications. Data from recent experimental manufacturing studies for sheet aluminum should be helpful in assessing the feasibility of meeting the $1/lb cost target. Recent workshops with aluminum suppliers will likely yield many white papers on technical issues, together with suggestions for materials solutions that can be investigated in the coming year (Sherman, 1995). Broader questions to be addressed (through systems analysis) relate to the life-cycle cost of aluminum, that is, the overall cost from mining of bauxite ore through use, recycle, and reuse; and to the availability of bauxite ore, smelters, and material formers to meet the requirements of a potential annual market in the United States for 15 million aluminum-intensive vehicles.

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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT Composites The PNGV has developed a challenging vision statement for composite materials as follows: “To develop and demonstrate reinforced polymer composite manufacturing processes capable of producing complex, high-volume vehicle structures at low cost, which meet all manufacturing and vehicle performance requirements.”4 There are major design and manufacturing process changes involved in transitioning from a steel-based to a composite-based vehicle. The design of the body structure has to be approached quite differently to take advantage of the unique properties of GFRP and the relationship of specific manufacturing details, to final in-service performance of one component, as well as the ability to integrate two or more existing components into one. For composites, manufacturing details, such as the method of fiber preform fabrication, fiber architecture, fiber angle, braid material properties, and molding processes, as well as the choice of fiber and resin systems, can be manipulated to alter substantially the in-service performance of a component. Thus, unlike most automotive structural metallic materials, for which the manufacturing processes generally provide simply a means for attaining a specific geometrical shape, the manufacturing process for composites is an essential determinant of both the properties and geometry of a finished part. Thus, an integrated product design approach to composite applications in automotive structures is required to evaluate adequately the cost effectiveness of composites in meeting the cost and performance requirements for Goal 3. Investigations of such product design approaches are underway in the USCAR Automotive Materials Partnership (appendix D). Intensive use of GFRP could readily meet the 50 percent body and chassis weight-reduction target for a Goal 3 vehicle, but today' s price of $8 to $15/lb for graphite fibers would result in a major cost penalty of approximately $6,000 per vehicle. Current projections suggest that an annual demand for one billion pounds of graphite fiber could drive its cost down to between $3/lb and $5/lb, within reach of the PNGV cost targets (Michno, 1995). Testing of glass-fiber-reinforced composites using a Ford Aerostar vehicle underbody crossmember has shown high-volume potential for lower cost, recyclable, structural resin-injection-molded composites. Limited progress has been reported by PNGV to date in establishing the crashworthiness of composite structures in front, side, and rear collisions. Crash energy management needs to be resolved with extremely high confidence. Computer simulation of crash events for composite materials 4   High volume is defined to be greater than 250,000 units. The low-cost requirement refers to the cost in an assembled vehicle.

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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT cannot currently be made with confidence. Consequently a large number of prototypes must be built and crashed under a variety of conditions that can occur in the field to develop the databases, computer models, and analytical methods needed to provide a reliable predictive capability. However, such tests cannot be performed until the matrix resin for the composite has been selected. The choice of resin also greatly influences recycling methods, as well as joining and repairability techniques. ISSUES The PNGV has planned its next steps towards achieving the desired materials objectives as follows: Based on white papers developed following the supplier workshops, help initiate research projects with materials suppliers and other organizations aimed at meeting the challenges for aluminum and composite vehicles. Work with the AISI to develop structural concepts for steel vehicles and define materials challenges and research needs. Pursue activities defined in the Technical Roadmap for chassis and powertrain materials. The powertrain technical teams, materials suppliers, universities, and government laboratories will be involved in these activities. The committee concluded that progress towards the PNGV materials objectives has clearly been made through the process (challenge tables, supplier workshops, white papers, etc.) established by the Materials and Structures Team, and through the work of the USCAR Automotive Materials Partnership. Materials suppliers are major supporters of the program and are investing heavily in technology development for both metals and composites, including innovative processing techniques. In particular, the PNGV materials program has made significant progress in developing steel and aluminum for structural automotive applications. Despite some progress, the composites program is lagging, with no clearly scheduled activities that would enable this class of materials to be viable alternatives to steel and aluminum by 1997. To assess adequately the potential of composites to meet program objectives, vis-à-vis high-strength steel or aluminum, the PNGV team needs to place high priority on an integrated composites evaluation program involving feature-based

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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT computerized design, analysis of manufacturing costs and tradeoffs, modeling of composite automotive structure performance under impact, and the evaluation of newly developed manufacturing technology oriented towards low-cost, high-volume production. RECOMMENDATIONS Recommendation. The systems analysis effort recently initiated by the PNGV should be used to drive the optimization of materials usage for the various vehicle components based on part configuration tradeoffs and on incorporation of data on manufacturing costs, structural effectiveness, recyclability, and other properties. Recommendation. The USCAR should continue to use the process it has developed—incorporating its substantial leverage through integrated industry programs—to pursue the very promising developments in steel and aluminum materials made by materials suppliers and trade associations. The development of innovative manufacturing processes for aluminum and steel should be encouraged and accelerated. Recommendation. The PNGV should establish an integrated product design program to provide better evidence for the advantages and viability of using polymer-based composites for automotive body structures. The program should: Develop computerized feature-based design and decision support tools to enable an integrated product design evaluation of the cost effectiveness of composites for vehicle structural applications. Validate the projected cost of $3 to $5/lb at high-volume through production process pilots. Address the development of a database and models to establish the crashworthiness of composite structures. Take into account relevant experience with composites in the aerospace industry. The approach adopted to date by the PNGV (materials workshops, white papers, etc.) should be pursued on an accelerated schedule as a basis for establishing the above program.

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REVIEW OF THE RESEARCH PROGRAM OF THE PARTNERSHIP FOR A NEW GENERATION OF VEHICLES: SECOND REPORT REFERENCES Martin, D. 1995. Materials Technology—Steel. Presented to the Standing Committee to Review the Research Program of the PNGV at the National Academy of Sciences, Washington, D.C., October 30, 1995. Michno, D. 1995. Why Carbon Fibers? Presented to the Standing Committee to Review the Research Program of the PNGV at the National Academy of Sciences, Washington, D.C., October 30, 1995. PNGV (Partnership for a New Generation of Vehicles). 1995. Technical Roadmap (draft). Dearborn, Michigan: PNGV. Sherman, A. 1995. Review of PNGV Materials Roadmap. Presented to the Standing Committee to Review the Research Program of the PNGV at the National Academy of Sciences, Washington, D.C., October 30, 1995.