Glass-Reinforced Thermoplastic Succeeds in Car Crash Structure

Automakers face great pressure to produce passenger cars that are not only comfortable, aesthetically pleasing and handle well on the road, but fuel-stingy, safety feature-filled and recyclable as well. European "End of Life"Vehicle (EELV) legislation requires that for new cars made after January 2015, 95 percent of the vehicle weight must be reused or recovered. While composites have contributed much toward these goals in extremely expensive, few-of-a-kind supercars and sports cars, the expensive materials and slow production still present barriers to significant structural applications in the mass-produced family sedan.

ne bellwether of change, however, is the ECOLITE (Efficient COmposites – LIghtweight and ThErmoformed) program. A joint effort of Lotus Engineering, a design and manufacturing consultancy and sister company to Lotus Cars (Hethel, U.K.), and Jacob Composites (Wilhelmsdorf, Germany), one of Europe's leading manufacturers of thermoplastic parts, ECOLITE's objective is development of a passenger car with a body and chassis made primarily of composites and producible at annual volumes of 30,000 to 50,000 units. In the program's first phase, the partners are concentrating efforts on development of a mass-producible, crashworthy front-end structure.

Current State of the Art

The front end was the logical place to start, given the partners' previous experience. Lotus has been making monocoque composite cars longer than any other car maker, starting in 1957 with the Lotus Elite. The Elite epitomized the company's philosophy, still in force today, that drives the car maker to combine stiff, lightweight composite structure with small, high-powered engines to produce cars that match the performance of much more powerful yet heavier, less economical vehicles.

In 1994, Lotus launched the Elise, a lightweight sports car made in volumes of between 5,000 and 10,000 per year. The Elise features a 7.5 kg/16.5 lb glass-reinforced thermoset front structure made by resin transfer molding (RTM), which is capable of absorbing 75kJ/55,000 ft-lb of energy in an 11.5m/sec (25 mph) impact. In 2001, Aston Martin launched its carbon fiber-bodied Vanquish V12 supercar, which weighed in at more than 2 metric tonnes (4,400 lb). Lotus designed and engineered the extruded aluminum bonded body structure for the Vanquish and manufactured the structural composite assemblies - front crash structure, rear cargo deck and transmission tunnel. The Vanquish V12, made in volumes of between 500 and 1,000 per year, features an RTM'd front-end structure of carbon/epoxy composite. The assembly's weight is 22.5 kg/50 lb but it can absorb up to 140 kJ/103,000 ft-lb of energy in a 16 m/s (35 mph) impact. The much larger energy absorption required for the Vanquish was achieved by the use of carbon/epoxy side rails. Each consists of two corrugated panels joined together, one with the corrugations horizontal and the other with vertical corrugations. (See the CAD drawing on this page, bottom right.)

From Thermoset to Thermoplastic

The challenge for Lotus Engineering was how to make a similar front-end technology viable in a much higher volume production scenario, where unit cost must be low and, therefore, material costs are a more critical consideration than in niche market cars. Here, glass/thermoplastic composites promised relatively low material cost and rapid part forming plus the potential to recycle. Since Lotus' thermoplastic molding capabilities were not yet mature, the partners pooled their experience to evolve a design that would provide the energy absorption achieved by the earlier Lotus designs but could be produced in large quantities using a thermoforming technology pioneered by Jacob.

Jacob brings to the program real production experience, having made, since 2003, 20,000 thermoplastic composite bumpers per year, split between the front and rear bumper systems on the BMW M3. Its bumpers are optimized to absorb the energy of low-speed crashes - 15 kJ over a distance of 200 mm/8 inches, compared with the 140 kJ in the Vanquish over a distance of 530 mm/21 inches. The material used is glass-reinforced polyamide 6 (PA6), sourced initially from Hexcel (Dublin, Calif.), and the manufacturing method uses robotic lay up followed by press forming, a variation of thermoforming that uses a relatively inexpensive two-part mold. Preheated materials are suspended over the hard tool and then conformed to its surface under moderate pressure by a rubber upper tool. For the ECOLITE front-end structure, the glass/PA6 compound was supplied by Bond-Laminates (Brilon, Germany).

The question was, Would thermoplastics perform under crash impact? Generally, composite structures offer much better specific energy absorption than comparable metal structures. Additionally, they can crumple to a shorter final length because the method of absorbing energy is by controlled fracture and/or internal delamination. By contrast, a metal structure has to fold, which gives it a larger residual length. This "crumple distance"is a key indicator of crashworthiness, particularly in smaller cars. Crumple data, however, are often compiled from lab tests, in which round tubular structures are subjected to axial crush loads. Based on such data, composites are shown to be clearly superior to metals (see Table 1), with carbon composites performing better than glass. However, the lab data does not represent realistic crash situations. Current test regimes require simulation of real-life scenarios, such as impacts with poles or with only one side of the vehicle hitting the crash barrier, which involve off-axis loads. In these situations, automakers employ more sophisticated part shapes, in both metals and composites, to account for the off-axis impact. Lotus and Jacob have exploited lessons learned on the Elise, Vanquish and M3 to produce a front-end structure in which a thermoplastic performed better than glass/epoxy. In fact, Lotus tests showed that the glass/PA6 structure absorbs nearly 55 J/g, enough to negate the need for carbon/epoxy. Beyond the benefit of reduced cost, substitution of a glass-reinforced thermoplastic for carbon/epoxy ended concern about whether there would be enough carbon fiber available to fulfill supply requirements for production at even a moderate rate.

One concern remained: How would the thermoplastic perform at a car's high and low operating temperatures? Although there is a 20 percent performance loss at the highest operating temperatures likely to be encountered (90°C/194°F), the thermoplastic actually improves part performance at low temperature (-40°C/40°F).

The interim nonoptimized design weighed 16 kg/35 lb compared with the 22.5 kg/50 lb for the baseline steel structure. As can be seen from the force displacement plot (Table 3), there can be an initial load of nearly twice the mean load during crumpling of the structure. This is not an optimum solution, since it requires the support structure to be designed to withstand the higher load. The partners worked on various ways to initiate the failure at a lower load, including designed-in features as well as additional process steps. The latter included beveling the edge, cutting the front of the rail at 15� and cutting slots or holes in the part. As can be seen from the plots (Table 4), the 15� cut provided the most even load but the beveled-edge option absorbed a little more energy.

Initial Phase Promising

The program has shown that glass/PA6 provides for good specific energy absorption, matching that of carbon fiber/epoxy in some configurations if not in all circumstances. Table 2 shows the spread of performance for various materials. What is clear is that glass/thermoplastic solutions, as a class, perform better than glass/thermosets, which in turn, are superior to aluminum designs.

This fact makes it possible to design a much lighter structure in thermoplastic composites than aluminum or steel, to absorb the required energy - although, in reality, the extent of weight savings will be limited by the need to achieve a specific stiffness property.

The next part of the analysis was to determine the production volumes at which thermoplastic composites will be cost-competitive. Metal structures typically cost less to make per unit but have much higher investment costs. For this particular structure, the investment would be £3.2 million (about $4 million) for steel or aluminum but only £1.3 million (about $1.6 million) for composites.

Based on the design used to demonstrate the technology, Lotus determined that the glass/PA6 structure would break even with a steel design at about 80,000 units. While this is currently short of the program goal, the partners are considering various methods of optimizing the design that could improve the break-even by reducing both the investment and the unit price of a composite unit.

Lotus is now scrutinizing its all-composite structure to ensure an optimum design. In the case of the bumper beam, which has to be strong to distribute the load into the energy-absorbing composite structure, the optimum solution may prove to be an aluminum structure with selective thermoplastic reinforcements. The area where composites are most cost-effective is in the crush structures.

ECOLITE's first phase has demonstrated the economics of using composite materials to reduce vehicle weights and, hence, fuel consumption in mass-produced cars. Lotus has already begun to look at other areas of the car structure, such as side and rear impact structures