Top-down design of tailored fiber-metal laminates
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Researchers at the University of Paderborn have developed a top-down approach for the design of fiber-metal laminates. The thickness-dependent characteristics profile of the multi-layered material is derived from full-vehicle simulations. The researchers are also focusing on inherent transitions between the different layers’ stiffness gradients, as well as good formability.
Drawbacks of Single Materials
Automotive lightweight design is a considerable measure to meet the worldwide need for reducing CO2 emissions. This led to an excessive portfolio development of conventional metals like steel or aluminum in the last decade. However, the lightweight potential of common materials like steel, aluminum or even fiber-reinforced plastics is limited. High-strength steels play a significant role in the design of safe and light car body structures. Nevertheless, the high density and buckling problems related to reduced sheet thickness limit the achievable mass reduction. Aluminum alloys are well known for the potential to improve the strength-to-weight-ratio of car bodies. Nonetheless, in terms of stiffness, aluminum is at a clear disadvantage due to a relatively low Young’s modulus. Even fiber-reinforced plastic (FRP) components, which display superior lightweight characteristics, show limitations for the car body design, for example catastrophic failure or high production costs. Hybrid materials combine metals and FRPs to offset the drawbacks of every single material, and thus reach an optimum balance of mechanical properties against costs. Nevertheless, the achievable lightweight potential of such materials heavily depends on the loading situation, geometry or cross section of the chosen material design.
From BIW Simulations to the New Material
The identified components, one crash- and one stiffness-relevant part, are subdivided into at least five layers.
The redesigned material leads to a weight reduction of at least 20 %.
Inherent Stiffness Gradients
Intrinsic tensions, arising from thermal curing and contact damages that build up during strain, are typical problems that are present when materials with large differences in physical properties, such as stiffness and thermal expansion coefficient, are bonded. In the production of fiber metal laminates, materials with different properties are being combined into one, which means that such problems are to be expected, but this is also the fact from which they derive many of their advantages.
Evolutionary pressure has caused biological organisms to develop a solution for the described problems, probably at an early stage. Nature uses gradient systems within the layers of tissue to bridge the wide differences in stiffness and hardness that exists between certain types of materials in the bodies of animals. This is especially important where large forces need to be transferred. Evidence for this principle can, for example, be found in the beaks of squids and in the tissue of worms that is close to their jaws.
Deep-drawing of FML Sheets
The forming technology plays an important role in the production of sheet metal components for car body applications, as it does in the project on which this report is based. Two car body components made of FML sheets are to be produced by deep-drawing. During the forming process, there are complex tension-pressure stresses and tension stresses. When using disadvantageous process and tool designs, different failure types, like cracks or wrinkles, can appear on the manufactured sheet metal parts. The forming of FML-sheets can lead to even more failure types, such as for example draping and compressing of the rovings when tangential compressive stresses are applied. This can lead to delamination, buckling and breaking of the rovings. Not only the roving itself can be damaged: Delamination can cause a loss of cohesion of the FML sheet, rendering it unfit for use in the body parts. In areas with great contact pressure, the resin can flow out of the FML. However, these undesirable effects can be successfully counteracted by implementing appropriate measures.
Material and Process Suitable for Forming
Based on the results, the deep-drawing of FML sheets requires individually adapted fiber reinforcements or patches. Therefore, the patches in the FML sheets are customized in thickness and surface direction. Central to this project is the integration of process elements from Advanced Fiber Placement. The alignment of the fibers prevents the aforementioned failures, such as buckling and breaking of fiber strands, and thus significantly improves the deep-drawability of the FML blanks.
The alignment of the fibers improves the deep-drawability of the FML blanks.
The improved forming behavior of modified FML blanks can be further enhanced through the use of adjustable multi-point blank holders and stamping systems, allowing for the manufacture of complex components. Optimized test tools and machine systems improve the intake behavior of FML sheets and lead to an enhanced interlacing of the composite material with the metal, so that car body components with optimum properties can be manufactured.
Numerical processes developed in the project allowed for the design of novel requirement-optimal hybrid materials. These materials led to a weight reduction of at least 20 % and hold further lightweight potentials. To ensure that the new materials can be used to manufacture parts in conventional production processes, as for example in deep-drawing, special tooling or process routes shall be developed too. The developed materials were hitherto investigated only in numerical simulations and coupon-based experimental tests. The experimental validation on real component geometries is still pending. The hardware tests are in preparation right now. The chosen demonstrator components will be subjected to a series of crash, stiffness and durability tests to point out the qualification for automotive applications. Further, the LHybS team focuses on series production issues and additional markets through a transfer analysis. The developed approach can find applications outside the automotive field, for example in the aeronautical or energy sector.
We would like to give thanks to the European Regional Development Fund and the State of North Rhine-Westphalia for funding the research project LHybS, and to the lead partner PTJ. Sincere thanks also to all of the industrial partners, especially to Thyssenkrupp for providing the In Car plus model.
- ATZ extra: Das Projekt ThyssenKrupp InCar plus. Lösungen für automobile Effizienz. Wiesbaden: Springer Vieweg Verlag, 2014Google Scholar