Cover Story Materials
Opportunities for Composite Sheet Hybrid Technology in Lightweight Design Thermosets reinforced with continuous carbon fiber, so-called CFRP, are already widespread in aircraft construction. They are now increasingly being used to produce cars’ body parts with a high load-bearing capacity. In this application area, however, hybrid structures as glass-fiber reinforced plastics (GRP) based on polyamide composite sheets and injection molded polyamide ribs, are also becoming increasingly popular. Lanxess can now simulate all the process steps in this hybrid technology with a high level of precision. Lightweight design components can be reliably configured for the appropriate load on the computer. 22
a uth o r s
Dipl.-Ing. Ralf Zimnol
heads the Application Development at the Semi-Crystalline Products Business Unit of Lanxess D eutschland GmbH in Leverkusen (Germany).
General Requirements
Thermoset plastics reinforced with continuous carbon fiber exhibit exceptionally high stiffness and strength. In the field of body design, they are seen as one of the lightweight design materials of the future. These materials are particularly high in demand in the areas of aircraft and wind turbine construction. It will therefore be a major challenge to provide the automotive industry with the huge quantities of carbon fibers it needs while achieving a price level that is cost-effective for manufacturers in this sector. Multi-track Lightweight Design Strategy
Dipl.-Ing. Frank Lutter
is CAE Manager at the Semi- Crystalline Products Business Unit of Lanxess Deutschland GmbH in Leverkusen (Germany).
Dipl.-Ing. Thomas Malek
is Manager for Structural C omponents and Hybrid Technology at the Semi-Crystalline Products Business Unit of Lanxess D eutschland GmbH in Leverkusen (Germany).
Dr.-Ing. Tim Arping
is Manager for Innovation Projects at the Semi-Crystalline Products B usiness Unit of Lanxess D eutschland GmbH in Leverkusen (Germany).
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Volume 12
One of the pioneers in automotive lightweight design for over 20 years with its plastic/metal composite technology (hybrid technology), Lanxess is therefore employing a multi-track strategy. The company plans to use this approach to expand the current lightweight design technologies – such as the hybrid technology with steel or aluminum inserts, bonded compound materials or continuous glass fiber reinforced polyamide composite sheets – in the long term by tapping into the excellent opportunities afforded by the continuous carbon fiber reinforced plastic (CFRP) composite systems. The development work is focusing on thermoplastic rather than thermoset matrices. This makes it possible, using the injection molding process, to produce high quantities of costeffective components with great styling freedom and without reworking while also integrating numerous functions. In the short term, Lanxess sees great prospects in automotive lightweight design for polyamide composite sheet hybrid technology with polyamide and glass fibers. This is a further development of the plastic/metal composite technology that has proved effective in volume production. In many applications, it presents a costeffective alternative to CFRP. This is be cause its mechanical performance potential is often sufficient to meet the requirements placed on such things as body components. All the materials needed for polyamide composite sheet hybrid technology – continuous or short glass fibers and polyamide 6 and 66 – are established construction materials and available in practically unlimited quantities, so that
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Cover Story Materials
Heating > TM IR lamps Shaping (and trimming if necessary) in the injection molding tool …
Demolding
… and injection of plastic for example for ribs
❶ Integration: single-step manufacturing process for polyamide composite sheet hybrid components in the injection molding tool
nothing stands in the way of the costeffective mass production of structural automotive components. Potential applications for polyamide composite sheet hybrid technology, which Lanxess believes can be implemented quickly and easily, include front ends, car seats, seat crossmembers, airbag housings and pedals. Great Weight-saving Potential
Lanxess estimates indicate that polyamide composite sheet hybrid components are much lighter than comparable all-steel components without compromising on strength. It is important to note that key properties, such as stiffness and strength, are temperature-dependent. Therefore, any potential uses must be considered and tested carefully during the concept phase. Polyamide composite sheet hybrid tech-
nology has been tested in volume production. For example, the front end of the Audi A8 features a U-section in the lower beam just 1.0 mm thick and made of polyamide composite sheet, reinforced by injection-molded ribs. It weighs 20 % less than its aluminum equivalent. At present, Lanxess is working with partners on numerous other projects for volume-produced body and safety components in polyamide composite sheet hybrid style. What is Hybrid Technology with Polyamide Composite Sheets?
Polyamide composite sheets are light and rigid semi-finished products made of a thermoplastic matrix embedded in a continuous fiber fabric of materials such as glass, aramid or carbon fibers. The fibers of a fabric layer are set perpendicular to
each other (orthotropic). They can be distributed evenly (balanced) in warp and weft directions or lie mostly lengthwise. The fibers are stretched in the matrix and therefore have a high degree of alignment. Composite sheets also boast a high glass fiber content (for example 60 % by weight). The flow of force between the force transmission points ideally takes place via the fibers, which greatly increases the mechanical strength of the component. To manufacture a hybrid component, a polyamide composite sheet is shaped (draped), placed in an injection mold and back-injected at selected points with ribbing made of polyamide reinforced with short glass fibers. Experience has shown that polyamide composite sheets do not have to be shaped in a separate step, but that the shaping can be integrated into the injection molding process, ❶. To this end, the polyamide composite sheet is heated to above the melting temperature of the polyamide matrix and positioned in a holding frame between the open halves of the injection mold. When the mold is closed, the polyamide composite sheet is shaped and back-injected with polyamide. This way, shaping and injection molding take place in one shot (single-step manufacturing process). And only one injection mold is required. Unlike metal shaping for plastic/metal hybrid technology, for example, no molds are needed for subsequent steps. Both cuts investment costs and has a positive impact on the component price.
❷ Polyamide composite sheet shaping: simulation (left) and reality (right) are a good match; the calculated shear strain in the glass fiber fabric is shown in color, folds are created if it exceeds a maximum value
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All-plastic Solution with Great Styling Freedom
Polyamide is used as both the back-injection material and matrix of the polyamide composite sheet. Unlike hybrid technology with steel sheets, suitable process management results in a material bond between the composite sheet and backinjected areas at all points of contact. This bonded connection significantly increases the mechanical strength of the component as a whole. The back injection process has a decisive impact on the component’s mechanical performance. It can be used to add ribs and other reinforcements to component areas that are subject to particularly high loads. These significantly increase stiffness, for example. The back injection process is also responsible for the exceptional design freedom that sets this method of style apart. It facilitates the integration of numerous functions, such as mountings, guides and snap-fit hooks, in a single step. Precise CAE Simulation of all Processing Steps
In order to take a polyamide composite sheet hybrid component into series production within a short development time, precise CAE simulation of all processing steps is essential. Lanxess is now in a position to do so. Reliable, load-compatible configuration of hybrid components can now be performed virtually. This development and computational expertise is part of HiAnt, a competence brand that pools the full range of expertise that the company has developed in materials, design, simulation, method and process technology to deliver tailored customer service. The main challenge with CAE simulation for polyamide composite sheet is that its mechanical properties are anisotropic (direction-dependent), non-linear and a function of strain rates. In addition, the tensile and compression properties are not symmetrical, and the non-homogeneous laminar structure gives rise to tensile and flexural strengths that are intrinsically different. Lanxess has therefore developed a dedicated new material model for polyamide composite sheets that considers the anisotropic material behavior and implemented it in the commercial finite element (FE) code Abaqus [1] by Dassault Systèmes. 01I2012
Volume 12
INTERNATIONAL VDI CONFERENCE 2012
Simulation in Automotive Lightweight Engineering Focus: Material Behavior
Internationally renowned technology leaders will present their latest results on the following topics: • how to define material properties requirements with respect to virtual development in the automotive industry • how to simulate fiber-reinforced plastics and create realistic FRP models for integration in the CAE process • how to predict strengths and failures of glued and welded hybrid material structures • how to integrate manufacturing process data in crash behavior simulations of high-strength steels and cast components
Hear from experts including: BASF • BMW Group • Daimler • DLR • Ford • Lamborghini • Lanxess • Magma • Magna Steyr • Opel • Politecnico di Torino • Polytec UK • Suisse Technology Partners • University of Leicester • University of Padova
Place and Date:
Chairman:
09th and 10th May, 2012 Dorint Hotel Pallas, Wiesbaden
DR.-ING. RALPH STENGER Director GME vehicle simulation, Adam Opel AG, Rüsselsheim
www.vdi.de/simulationlightweight Organized by VDI Wissensforum Phone +49 211 6214-201 | Fax +49 211 6214-154
Cover Story Materials
❸ Three-point flexural test on the prototype of a front-end upper beam produced using the polyamide composite sheet hybrid design to validate the simulation of the entire manufacturing process
Calculation of Local Fiber Alignment
To be able to use the new material model for computations, users need to acquire the information about the local alignment of the continuous glass fibers in the shaped polyamide composite sheet, usually through shaping simulation. This simulation for polyamide composite sheets is extremely complex and is based on different deformation mechanisms than used in the deep-drawing process for metal sheets. Polyamide composite sheets can only be
deformed by shearing the continuous glass fiber fabric (Trellis effect). This reduces the surface area of the polyamide composite sheet and increases its thickness. In areas with pronounced curvature, this causes the fibers to lose their orthogonal arrangement. At a critical angular deformation, the fiber strands lock and create folds. To accurately compute the draping of a polyamide composite sheet over a three-dimensional mold geometry, the shear stiffness as a function of the temperature must be known. This has been achieved by means of picture frame tests. ❷ uses a pan-shaped
Test result
Simulation
Force [N]
Durethan BKV60EF
DOI: 10.1365/s35595-012-0091-2
Durethan BKV30
Displacement [mm]
❹ Three-point flexural test: simulated and measured force/distance curve for a front end upper beam made of polyamide composite sheet and ribbing made of polyamide 6 Durethan BKV 30 H2.0 and Durethan DP BKV 60 H2.0 EF (with 30 and 60 % glass fiber percentage, respectively)
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geometry to show how accurately the process of shaping polyamide composite sheets can be simulated. It is now possible to calculate how fibers will be aligned locally after shaping, where folds are created and whether the critical shear angle is reached. This makes it possible to determine the optimal insertion position for the semi-finished product. This also applies to polyamide composite sheets with multi-layer continuous glass fiber fabrics. Model for Non-linear, Anisotropic Material Behavior
The results of the shaping simulation are incorporated into Lanxess’s material model and are linked with the mechanical structural analysis for the purpose of integrative simulation. The model also factors in different local fiber alignments in shaped polyamide composite sheets and their anisotropic, non-linear and strain-rate dependent behavior. This determines all the key properties of a polyamide composite sheet hybrid component – such as the fracture behavior that is essential for crash simulation. Validation of the Entire Manufacturing Process
The validation process for the prototype of a front end upper beam shows how well the entire manufacturing process for a poly
amide composite sheet hybrid component can be simulated today, ❸. For its production a polyamide composite sheet (Tepex [2]) plus two different polyamide 6 grades as back injection materials were used. ❹ illustrates the force/deflection curve in the three-point flexural test. The curves show a very good match between the simulation and the measurement for both injection molding compounds. This makes it possible, for example, to accurately predict the load under which the component will fail. It is also possible to calculate the precise point of failure, ❺. Therefore, all the requirements are in place to design a polyamide composite sheet hybrid component on the computer – quickly, costeffectively and accounting for the individual load situation.
❺ Comparison of test (top) to calculation (bottom) – the “location” of component failure can also be determined by means of simulation; once these design “weak points” have been computed, they can be counteracted, for example by increasing the wall thickness or injecting reinforcing ribs
CAE Tailored Service
Lanxess has incorporated the simulation of the various process steps, the material parameters needed for mechanical structural analysis and the new material model into a dedicated, complex development process. This simulation expertise is being made available to partners as part of the service HiAnt [3] for the development of polyamide composite sheet hybrid components. The customer service provided with HiAnt includes everything from support with material selection, estimation of the production costs and CAE calculations
to support for mold construction, component testing and help with the start of volume production. Outlook
It is difficult to predict how the availability and price level of continuous carbon fibers will develop in the future. One thing, however, is certain – they represent an extremely attractive alternative to continuous glass fibers. They significantly improve the stiffness and strength of
600 500
Strength
References
[1] Abaqus: http://www.simulia.com/products/ abaqus_fea.html. Access: 13 December 2011 [2] Tepex. http://www.tepex.com. Access: 13 D ecember 2011 [3] HiAnt. http://techcenter.lanxess.com, I nnovations, HiAnt. Access: 13 December 2011
Stiffness 400 Property [%]
polyamide composite sheets, which in turn improves their performance and potential for use in lightweight design, ❻. That is why Lanxess is working on transferring the expertise acquired with continuous glass fiber reinforced polyamide composite sheet hybrid technology to continuous carbon fiber reinforced polyamide systems. This step should be relatively straightforward, because many challenges have already been overcome with the continuous glass fiber systems. There are also plans to develop short carbon fiber reinforced polyamide variants as, for example, a back injection material for polyamide composite sheet hybrid technology.
300 200 100 0 PA6-GF30, dry
PA6-GF60, dry
PA6-GF polyamide composite sheet, dry
PA66-CF polyamide composite sheet, dry
❻ Reinforcement of the polyamide PA 6/66 with long carbon fibers (CF, right) compared to glass fibers (GF, three times left) dramatically improves the mechanical properties of polyamide composite sheets
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Volume 12
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