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Lightweight Design worldwide

, Volume 11, Issue 2, pp 24–29 | Cite as

Top-down design of tailored fiber-metal laminates

  • Alan A. Camberg
  • Katja Engelkemeier
  • Jan Dietrich
  • Thomas Heggemann
Materials Fiber-Metal Laminate
  • 204 Downloads

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.

The Approach

To account for these limitations, a novel process is necessary. The LHybS (Leichtbau durch neuartige Hybridwerkstoffe) project aims to design a material, which — different than usual — is not developed in a bottom- up way, but rather in a top-down manner, Figure 1. Within the scope of the project, a new numerical approach is developed, which for the first time provides a methodology that allows for a design of sophisticated requirement-optimal layered materials. The through thickness property profile of the material takes into account not only loading-dependent requirement, but also demands that are related to the direct application of the material in the vehicle. The goal of the project is to develop a lightweight hybrid material with processing characteristics similar to materials conventionally used in body-in-white (BIW) production plants. This task requires versatile know-how provided by the six researchers of the chair of Automotive Lightweight Design and by ten industrial partners.
Figure 1

The approach of the project: developing a hybrid material with tailored through thickness properties (© Automotive Lightweight Design, Paderborn University)

From BIW Simulations to the New Material

The Thyssenkrupp In Car plus model is used as a basis for the material development. The steel-intensive structure of the In Car plus BIW represents the current state of the art by utilizing high-strength and hot formed steels [1]. Starting with crash and noise vibration harshness (NVH) simulations, the BIW properties are investigated and critical components are identified through an internal energy-based method, Figure 2. The assessment of potential components is supplemented by a sensitivity analysis to identify structural parts with a high potential for hybridization. In that manner, it can be ensured that the material improvement will be realized for those parts that show a great impact on the overall BIW properties, and additional costs can be justified through better overall vehicle characteristics. The identified components — one crash- and one stiffness-relevant part — are subdivided into at least five layers, Figure 3. In the first optimization loop, the algorithm can freely parameterize the material parameters for each single layer. Once an optimum is found, the idealized material properties of each layer are compared with a material database to select an appropriate equivalent. The optimization led in both cases to an initially unexpected material profile. In spite of different loading conditions and demands, as well as free material parameters, the algorithm proposed a quite similar profile design in both cases consisting of two top layers of steel and an FRP core. However, due to different component requirements, the material profiles are distinct, therefore different steel grades and different FRP polymer components and bonding agents are used. Finally, the developed materials undergo a series of full vehicle and BIW simulations. The results are compared with the In Car plus reference structure to highlight the potentials of the tailored material design. In both cases, a redesign of the through thickness properties with a tailored hybrid stack leads to a weight reduction of at least 20 % for each part while maintaining or even improving the overall vehicle properties.
Figure 2

Component selection process (© Automotive Lightweight Design, Paderborn University)

Figure 3

Tailored fiber-metal laminates (© Automotive Lightweight Design, Paderborn University)

The identified components, one crash- and one stiffness-relevant part, are subdivided into at least five layers.

Surface Structuring

A major challenge in the production of fiber-metal laminates (FMLs) is the achievement of good adhesion between the different materials of the individual layers. An improved bond strength can be achieved on the one hand by increasing the metallic surface (increasing the number of mechanical clamping points) and on the other hand through the chemical functionalization of the metal surface. For the applicability of a surface method, the surface finish (pure, refined, coated) of the material itself is often crucial. The project investigated galvanized and hot-dip galvanized steels whose coating improves the corrosion properties of the FML. One method that allows for a fast and reproducible adjustment of nano- and micro-roughness on the steel is short-pulse laser structuring. An advantage of the method is that the laser treatment can be applied independently of the surface. It is also applicable when the surface has a galvanic coating a few micrometers thick and the laser beams cannot cut into the base metal. For the structuring of the steel surfaces, an yttrium-vanadate (YVO4) fiber laser by Keyence is used in the project, which produces an ordered micro-scale structure of laser spots and a porous fractal nanoscale substructure, Figure 4. Results from wetting studies on cleaned and laser-structured steel surfaces show that a homogeneous, area-wide structuring significantly increases the wettability of the surface. A drop contour analysis shows that the contact angle on cleaned steel decreases by up to 43 % when the steel surface is laser treated additionally. The wetting studies also show a significant increase in free surface energy of up to 53 %, compared to only cleaned steel. Recent studies show that the significantly increased wettability of the steel material, due to the short-pulse laser treatment, leads to an increase in the strength of the developed FML.
Figure 4

SEM photographs of a cleaned (a) and a laser-structured steel surface (b and c), as well as results of wetting studies on these surfaces (© Department of Material Science, Paderborn University)

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.

In the project, a bionic approach is used to develop a solution to the problem that similarly arises during the production of the semi-finished hybrid material. Between each sheet of the fiber-metal laminate, a bonding layer consisting of two to five individual adhesive layers is applied. These sheets are created from structural adhesives with different levels of additive filling, Figure 5. They are applied to yield a higher stiffness close to the metal interface of the adhesive joint. Simulations and static experiments have shown that consequently, strains can be dispersed within the adhesive layer and an improved crash performance can be observed. The stiffness gradient principle is presently under further investigation.
Figure 5

Modular adhesive interlayer setup (© Coatings, Materials and Polymers, Paderborn University)

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

In order to develop a tool and process design adapted for FML forming, complementary numerical and experimental methods were developed in the project, Figure 6. In the previously mentioned forming simulations, the data of the In Car plus model formed the basis for the modeling of tool elements in the forming process. The simulations were initially used for basic investigations, for example for investigating the effects and interactions of variations of individual design and process parameters on the forming process. In addition, experimental work with a gradual increase in complexity was performed. Typical load situations during the forming of real components were investigated in the production of model geometries (cup and hemisphere geometry and U-profile). These provided important insights into the development of guidelines for the process design and the product design for deep-drawing of FML components.
Figures 6

Material and process design for improved formability (© Chair of Forming and Machining Technology, Paderborn University)

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.

Outlook

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.

Notes

Thanks

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.

References

  1. [1]
    ATZ extra: Das Projekt ThyssenKrupp InCar plus. Lösungen für automobile Effizienz. Wiesbaden: Springer Vieweg Verlag, 2014Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Alan A. Camberg
    • 1
  • Katja Engelkemeier
    • 1
  • Jan Dietrich
    • 1
  • Thomas Heggemann
    • 1
  1. 1.Paderborn UniversityGermany

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