Hybrid Composite and Steel - the Best of Both Worlds

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Conventional automotive sheet steel averages 1.1-mm thick, ranging from 0.65 mm to 2.5 mm, depending on the automotive component it represents. The thicker the sheet steel, the heavier the automobile. And that’s not good news for automotive fuel efficiency. Reducing the total body panel thickness from 0.75 mm to 0.55 mm could reduce the automobile’s body mass by about 27 percent. But, thinner gauge steel panels alone have been found to be subject to inherent sheet-forming defects such as denting, vibration at higher speeds, and waviness in the steel sheets.

To address this challenge, researchers from Oak Ridge National Laboratory and Idaho National Laboratory are collaborating with Detroit-based Diversitak to test a novel carbon fiber reinforced epoxy (CFRE) developed by the automotive technology company. The research involves coating thinner gauge steels on one side with the CFRE to “stiffen” them. Specifically, the team is developing processes for applying the coating to the steel, determine its performance, and evaluate long-term durability of materials manufactured using this technology - with the ultimate goal of maturing the technology for manufacturing readiness. Each partner has undertaken specific roles in the project. ORNL is leading optimization and selection of the best CFRE materials, including optimal fiber length, concentration, and coating thickness for best vehicle function and performance at the least cost. INL is heading up characterization of corrosion properties of steel panels coated with CFRE. The panels are being investigated for suitability as replacement materials for automotive body parts to reduce mass. And Diversitak is producing different epoxy formulations and determining the physical and chemical properties that are critical to mixing with carbon fibers and producing a stable coating that can be cured in the time range to support the automotive assembly processing time of 10 to 15 seconds. A second industry partner, steel and mining company ArcelorMittal, is characterizing the advanced high-strength steel used with the CFRE.

In the first year of this project, ORNL took the reins to investigate the optimal fiber length and concentration in the epoxy resin system, using recycled carbon fiber 100, 200, and 400 microns long and 100, 200, and 400 microns in concentration. The team found that the shorter fibers in lower concentrations resulted in a more homogenous CFRE, although length did not show a significant difference in overall flexural strength. They also determined that there is no improvement in flexural strength of the panel with the absence of carbon fiber in the epoxy. And they learned there are minimal strength enhancement differences among different ratios of fiber in the epoxy. Thus, the team determined that based on viscosity and flexural strength, 15 percent loading by weight and 100 micron fiber is optimum in the formulation. The team discovered that when covering the entire door with the CFRE, 450 g of mass was added, while applying isolated patches added 115 g for a 0.5-mm-thick patch and 230 g for a 1.0-mm-thick patch. The team also noted a 50- to 70-percent improvement in strength of a 0.5- mm CFRE-coated panel over a non-coated panel. Finally, they determined an increase in flexural strength linear with an increase in applied thickness.

By investigating CFRE properties on thinner gauge steel, ORNL and its collaborators will provide the automotive manufacturing industry with highly durable and strong steel panels for lighter-weight vehicles.