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Material follows function

Due to their low weight and adaptability to various industries, especially in the automotive sector, composites are big news. In spite of this, they have hardly been used in large-scale production so far. This is due to the fact that the fibre-reinforced material only develops its properties during the manufacturing of the component. The approach normally taken for metals, namely starting with the structural design, cannot be transferred to the product development stage easily. The composite experts at development service provider ARRK|P+Z Engineering therefore adopt a simulation-driven strategy instead: a combination of modern tools and practical experience thus ensures that not only form follows function, but also the material. This new approach to development thus allows an ideal geometrical design without excess weight. At the same time, this concept ensures transparency throughout the whole process of generating solutions.    

Today, simulations are mostly used for the validation of construction proposals, such that the geometry has already been set out, checked and, if necessary, improved for a particular material. This happens to such a degree that manufacturers do not use technology that they cannot simulate, which is a real obstacle to the successful establishment of composite materials. The actual properties of fibre composites depend on numerous variables such as fibre volume content, layer structure, the nature of the semi-finished products, fibre draping and the processing parameters, and only reveal themselves during manufacture. Of course, there are specific values for the original materials, and these are often used in the same way as the material data for classic materials, but this only denies the manufacturer the biggest advantage that fibre-reinforced composites have to offer: instead of optimising the geometry of prototypes, which is time-consuming and cost-intensive, a customised development process allows the material properties to be adapted with ease. “Following a conventional, construction-based approach involving composites results in a component that is lighter than steel, but is not as light as it could be,” says Dr Thomas Burkart, Group Leader for CAE & Simulation at ARRK|P+Z Engineering, summing up the problem.       

First research load paths, then choose geometry and material accordingly

As an alternative, the company’s experts have devised an approach that is tailored to the properties of composites and based on technical calculations. The starting points for this are the predefined installation space besides the development objectives and requirements for application. A topology analysis is first carried out to establish the main load path on this basis. “This means that we are able to assess at which point single axial or multi-axial loads occur at a very early stage of the concept phase, and adjust our planning accordingly,” explains Monika Kreutzmann, Head of Centre of Competence (CoC) for composites at ARRK|P+Z Engineering. “The use of standard methods means that it takes longer to establish where the particular loads fall, which leads to additional and expensive development loops.”

Using the calculated load paths, the next step sees the creation of an initial design for the geometry and thus directly selects the appropriate configurations of materials and fibre orientation. In this way, the forces being exerted in composite components can, in an ideal situation, be balanced simply by using a suitable fibre orientation, rather than stronger walls or supporting ribs. At this early stage, expertise from other disciplines is therefore called upon in order to prepare a target-oriented concept. Manufacturing expertise is vital in this collaboration as decisions must be taken as to the method of construction and the production technique, amongst other aspects. These elements are largely determined by the client’s requirements – generally either high quantities or top mechanical performance – which in turn involve other processes.

Moreover, the connections within the components and with the neighbouring components need to be considered separately when creating the design. Instead of using modular solutions as in metal structures, here it is necessary to consider the flux of forces. The topology-based concept and the flexible material also offer specific solutions, whereby individual parts are combined or the geometries are selected in such a way that the separation ranges all lie within less loaded zones. Robustness is thus also increased as a whole, while the effort required to achieve bonding is reduced. “If metal inserts are integrated into the composites themselves in order to provide reinforcement and load dissipation, it is also necessary to watch out for factors such as different thermal expansion coefficients or risk of corrosion, and to set out the material structure accordingly.”    

Select the level of detail for the simulation as required

A CAD model is created using these two parameters for the design – material and geometry – and is used in pre-processing as the basis for further development and simulation. With modelling, simplifications can be used both in relation to the actual form and in terms of the size and number of the elements calculated. “It is important not to forget these simplifications at later stages, as they can affect the accuracy of the results,” says Dr Burkart. He recommends working largely with shell elements in a size of mesh that ensures a uniform stress distribution. In addition, the structure of the model must be considered, along with the correct alignment of the elements and the layer structure. Screw, bolt and adhesive joints are adequately replicated in the rough model through rigid body, beam or solid elements. For detailed analyses, however, points that are at risk of failure should be refined separately.

Numerous specialised solvers are now available for making the calculations themselves. For the post-processing of fibre composite materials it is even more important to ensure that the correct settings are used, as this has a significant bearing on the results and the quality. For composites, the evaluation should always be carried out in a material coordinate system, or in an element coordinate system designed on this basis. In addition, for shell elements it is necessary to consider which points are being evaluated. “In general, error indices should be used when analysing the results of stress as they make a linearly scalable statement about the load,” says the simulation expert. Furthermore, some post-processors now offer their own tools for composites to represent critical points of failure, as well as strain and stress curves. 

Detailing in several stages

Based on the findings from the simulation, the geometry and, in particular, the layer structure of the composite material and the draping of the fibres are optimised with the aid of suitable tools in several iteration loops and checked with an eye to efficiency. However, in order to find a structure that both corresponds to the manufacturing requirements and also satisfies technical boundary conditions such as the specific stiffness targets with minimal material input, the expertise of engineers is also required. Detailing is carried out in several stages at ARRK|P+Z Engineering: first, a design concept is generated for each load zone, with the necessary thicknesses and fibre alignments. These specifications are then implemented through the configuration of the laminate and the individual layer bundles, and the concept thus optimised with feasibility in mind. Finally, the ideal stacking order of the layers must be determined, whereby a manual examination of the laminate structure is advised as current solvers cannot provide adequate support in this area at present. 

Furthermore, it must be decided whether the connection technology has been represented with sufficient precision in order to provide reliable failure values. This is all the more important given that all of the requirements are now to be considered in the detailing, alongside the main development objective. As conflicting objectives often also emerge between the different areas, the interplay between design, choice of material and the use of the installation space must sometimes be adapted once again at this point. At the end, all of the load cases to be observed are simulated and recorded once again. “In addition, however, because the properties of the composite materials only take shape in their final form when they are manufactured, the component should always undergo practical tests,” advises Dr Burkart.

Summary and outlook

In general, the simulation-based approach used today allows composite components to be developed very quickly whilst, most importantly, making the most of all the advantages that composite materials offer. A range of modern-day software tools cover a variety of tasks. Another advantage of the process, and one that is of at least equal importance, is that the development processes are made traceable and transparent, which should, in the long term, facilitate their uptake by the industry. 

Further steps are still needed, however, especially in terms of a comprehensive standardisation of composite technology. ARRK|P+Z Engineering is already working to self-imposed standards covering all matters relating to composite materials in order to deliver a consistent level of quality across the whole company. “Yet that is not enough for composite materials to become truly established in large-scale series production. We need fixed standards and consistent ranges of tools in order to convince companies used to the predictability of steel of the reliability of fibre-reinforced synthetic materials,” explains Head of CoC Composites, Monika Kreutzmann. Together with her colleagues, she is therefore also involved in the international MAI Carbon professional network and Carbon Composites e.V., which work on promoting standardisation. “The aim is to get to a point where development using composite materials is no longer seen as a ‘mysterious art’, but rather as a robust and, above all, valid engineering craft.”