Automotive OEMs and Tier 1s are grappling with the need to reduce vehicle mass to meet fuel economy and carbon emission targets. Composite materials have the potential to contribute significantly to this light-weighting push in many areas, but cost, design issues, unfamiliar processing, and competition from other materials continue to present obstacles. To overcome these, many projects are investigating how composites can be integrated into multi-material automotive structures for maximum benefit.
One project addressing how composites can reduce automotive load-bearing structures is being conducted by the Clemson University (Clemson, SC, US) Composites Center, the Clemson University International Center for Automotive Research (CU-ICAR) and Honda R&D Americas (Raymond, OH, US), with support from the University of Delaware Center for Composite Materials (CCM, Newark, DE, US) and funding from the US Department of Energy (DOE, Washington, DC, US).
The question is whether composites can enable ultra-lightweight closure systems — doors, hoods, trunk lids — to complement concurrent advances in power-train technology and better aerodynamics: “We believe there’s potential for efficiency gains in the area of load-bearing, structural closure systems at a reasonable price point.”
“Thermoplastic door is potential and ideal”
Initial analysis included bench-marking of other OEM efforts at lightweight closures for limited market models, including the Audi aluminum door frame for its A8 model, the Porsche Panamera magnesium door frame, and BMW i8 carbon-fiber-reinforced thermoset door frame. None of these previous OEM approaches, however, met this project’s cost or weight goals. Says Pilla, “I wanted to be part of something that would benefit the future, that would contribute to a circular economy. A thermoplastic door hadn’t been attempted before, and it would be recyclable.” When stacked up against other candidate materials, including thermoset composites, aluminum, and steel, thermoplastics offered not only recyclability but very high potential for lightweight and fast processing speeds (compared to thermosets) to meet production targets.
With the original Acura MDX door as the baseline, the team broke down its material mix: 62% metal, 21% rigid neat polymer, 13% glass, and 4% elastomer. The greatest opportunity for lightweight, 60%, would come from the metallic door frame, which the team intended to reduce from a baseline weight of 15.4 kg down to the target weight of 6.2 kg. While there was no chance to reduce weight in the door’s internal components and electronics (radio speaker, a servo for raising and lowering the window, door lock, etc.), the team determined the window glass weight could be reduced by 20%, perhaps making the glass thinner but without compromising the target metrics of NVH and durability. Further, the team estimated that the weight of trim elements on the door’s inner surface could be reduced by 30%, or even eliminated.
Prospected Door Panel Data
The project’s major tasks ran concurrently for the first two years. Some team members worked on material data generation, while others tackled door design specifics. The material data group generated material testing data for a variety of thermoplastics — continuous tapes, mats, short and long fiber-reinforced polymers, and more — to determine candidate materials for the inner frame and outer panel; materials were contributed by a number of industry supplier partners. Data were evaluated via spider charts, with overall strength, shear strength, allowable cost, allowable density, stiffness, and toughness making up the chart axes.
The best-performing material options following initial data evaluation — continuous fiber tapes and long fiber-reinforced thermoplastic pellets — underwent material modeling, explains Pilla: “It was possible to construct a simple orthotropic material stiffness matrix for the continuous fiber tapes, based on Hooke’s law.” For the long fiber-reinforced polymer, however, secondary simulations were needed to predict the strength and stiffness of an injection-molded door part, because of anisotropy introduced by both the final part geometry and the mold-filling process. Adds Pilla, “Modeling of these long fiber materials is difficult because not much has been done on simulation.” To gather the data needed, the team developed a manufacturing optimization loop.
At this point, detailed CAD models were generated and FEA simulations were performed for each concept to validate static performance in compliance with Honda’s targets. Taking into account manufacturability and integration of subsystems, Concept 7 (a space frame approach) began to converge toward Concept 2 (a one-piece structural frame), so the team decided to continue with Concept 2, incorporating lessons learned from the space frame approach. That concept consists of four elements: the outer Class A panel, the door internals, an inner frame or panel, and the interior trim elements.