To use composite materials in primary structures of vehicles and mobile equipment, it is necessary to control their crash behavior. Solutions exist in sports cars to reach an energy absorption level of typically 50 to 80 kJ per kg of crushed composite material. These solutions are based on carbon/epoxy materials with complex preforming and manufacturing, and are therefore not economically applicable for common vehicles.
The main requirements in the automotive industry are low cost and high manufacturing rates of the parts. Current designs use metallic crashbox and body-in-white (BIW) to absorb the required amount of energy to protect the passengers thanks to yielding created by local buckling. Such design costs approximately €3/kg of parts to be produced, and may be made thousands of times per day thanks to process like stamping and welding.
Currently, decreasing the costs is seen as more important than reducing the weight. Compared to a penalty of €95/g of CO2 emissions above the regulatory target that will be applied by the European Commission after year 2020, a cost of €9.5/kg of lightweighting may be acceptable, as a 10-kg (22-lb) reduction in the vehicle weight results in CO2 emissions reduction of approximately 1 g/km.
Concerning the process time, production of 1000 parts/day is needed to address the mass market.
Additional requirements are given by adaptation constraint of the vehicle to its environment:
• A front-end module is fixed on the crashbox and must be kept linked with the BIW even after the crash
• Crashbox must be easily mountable and dismountable for final assembly in factory and reparability in after-sale workshops
• Dimensions are limited by the targeted vehicle compactness.
Finally, performance must be consistent on a broad range of test parameters, as crash scenarios change from one case to another. In particular, performance in off-axis crushing must be checked.
Proposed design and process solutions
Due to the high cost of the material, especially carbon fiber (CF), design-to-cost analysis shows that only very simple shapes can be used for the crashbox design. Material waste must be reduced to a minimum; therefore, preform must be produced by an in-line process. In addition, a one-step process should be chosen, including the triggering mechanisms.
The assembly solution for linking the crashbox to the car is a key aspect of the innovation. The tested solution will not be detailed here due to patent-pending matter, but the following principles are used:
• Mechanical clamping with simple interface parts, on easily produced shape, with tolerance to manufacturing scatter
• Clamp length adapted to fulfill off-axis bending strength requirement
• Regular or symmetrical shape to avoid localized effect that may change the crash behavior.
Several processes adapted to this design have been evaluated to check the achievable performance. Due to the variety of tested parameters and to the manufacturing constraint, a complete experimental plan has not been set up.
A crashbox has been made with a conical geometry of 150 mm (5.9 in) length, approximately 5 mm (0.2 in) thickness, 100 mm (3.9 in) outside diameter at larger end, and 2° of apex angle. The conical geometry has several benefits, including crash behavior that is progressive and robust in case of off-axis crush; more efficient in the proposed assembly concept; and it facilitates the demoulding which will be a plus at high production rate.
Triaxial braided preform has been used, and the following parameters have been tested:
• Carbon tow: 24K (Toray T700S) or 50K (SGL Group Sigrafil)
• Bias fiber orientation: Preforms are made with 50% of axial fiber and 50% of bias fiber with ±25°, 30°, or 45° orientation
• Matrix: Mono component epoxy (resin transfer molding [RTM] toughened aerospace grade) or bi-component epoxy (high-pressure RTM [HP-RTM] EPIKOTE resin 05475/EPIKURE curing agent 05443 and developmental grade) from Momentive Specialty Chemicals; or mono component thermoplastic (PA6).
All configurations have been produced in the same RTM mold. Epoxy RTM systems are already well established on the market, while thermoplastic RTM still need process development. 50K carbon tows were selected as the low-cost CF product option.
The RTM aerospace system is a one-component (1K) toughened system curing typically at 180°C (356°F) offering low processing viscosity and a wide processing window up to 10 h. It offers the ability to design complex parts.
The new thermo-latent fast cure EPIKOTE/EPIKURE epoxy resin systems allow the rapid and reliable processing of structural composite components using high-volume manufacturing techniques. These systems have a cure cycle time of approximately 5 min and 2 min at 120°C (248°F) and can be processed via all common RTM techniques, such as multi-component low-pressure and high-pressure RTM machines.
The new systems provide benefits such as a very low viscosity of less than 50 mPa·s at the injection temperature and a thermo-latent behavior allowing relatively long injection window up to 90 s before reacting. The resin system features excellent wetting and adhesion to carbon fibers, superior thermal and mechanical performances, and very low VOCs, making them viable solutions for a wide range of future applications.
The EPIKOTE/EPIKURE resin system with the internal mold release agent HELOXY Additive 112 has been used to build the first series of crash cones at high production rate. This system cures fully within 5 min at 120°C and allows reaching the target production rate of 1000 parts/day, and consequently enables a cost-viable solution.
A new thermo-latent fast cure and enhanced epoxy resin system is under development to offer enhanced impact performances.
The PA6 Evolite from Solvay is a polyamide-based matrix suited to continuous glass or CF composite materials. It offers enhanced impregnation capability generated by the high fluidity of its polymer compounds.
Crash test results
A high number of crash tests have been performed in the framework of this study. The results revealed that the axial crash performance increases slightly as bias angle decreases. On the contrary, for 15° off-axis, better performances are achieved with 45° bias angle. This could be linked to the enhanced hoop stiffness, which stabilizes the crush mode in off-axis crash. Crash mode is mainly fragmentation for all these tests.
Axial and off-axis crash performance decreases when low-cost heavy tow are used. The performance gap is around 25%, which is approximately the price gap between the fibers. Crash mode is mainly fragmentation for all these tests.
The fast-curing epoxy offers the same level of performance as the aeronautic 180°C grade epoxy. Higher performances were obtained when using a toughened non-commercial fast-curing epoxy grade. This grade also offers an attractive crushing mode with a good compromise of fiber and matrix fragmentation.
Fluid TP PA offers slightly lower performances than the epoxy systems tested. Obviously, the process conditions need to be adapted as the PA parts presented dry fiber domains that may explain the lower results. The crushing mode observed is different, as the thermoplastic folds and limits the fragmentation of the carbon fibers. Work is still under way to evaluate other types of thermoplastics material with this crashbox.
For process optimization matters, the influence of the fiber twist on crash performance of the composite has been examined. Fiber twisting may be useful to facilitate the textile process, but the impact to material performances needed to be reviewed. Concerning crashworthiness, it has been found that twisting does not influence the test results. Test on cylindrical weaved tubes have led to the same conclusion.
Cetim and its partner Momentive Specialty Chemicals Inc. propose to automotive designers a composite crashbox based on a conical design. A manufacturing study was performed and it demonstrated that, with only one braiding machine, one standard bi-component RTM press, an injection tool allowing simultaneous injection of nine crashbox parts, and a fast-curing epoxy system, a 1000-parts-per-day production is feasible. The cost of one part should be about €10.
Easily adaptable to current vehicle design, the conical crashbox concept assists with the lightweighting of a vehicle’s BIW—a 66% weight reduction can be achieved compared to current steel parts. In addition, obtained specific crash performances are tripled to those of a traditional steel part.
Research strategy on composites is focused on the development of a new process to allow easier use of thermoset and thermoplastic material in the automotive industry. This study obtained comparable results between fast-curing epoxy and high-performance aerospace epoxy with carbon-fiber composite. PA processed for the first time by RTM offers slightly lower performances than the epoxy systems tested. (Previous study on composite with high-performance thermoplastic matrix has shown greater SEA for this type of composite.)
Additional research can be conducted to enhance the findings in both plastic families (modified epoxy, preform quality, enhanced fiber fraction, fiber/matrix adhesion, less porosity, etc.), leaving the door open to many future developments toward tailor-made composites performances. R&D work with textile manufacturers is also needed to find better ways to produce preforms at a lower cost and with higher performances.
Damien Guillon and Matthieu Kneveller of Cetim, and Alain Leroy and Jean-Philippe Sauvaget of Momentive Specialty Chemicals Inc., wrote this article for Automotive Engineering magazine. For more information regarding the crashbox concept, email firstname.lastname@example.org or call 1-888-443-9466.