Composites offer great potential over metals, and they’re a financial alternative to conventional materials. But the route to realising these benefits has many pitfalls. Components must be completely redesigned when made from composites, says James Anderson, Structural Group Leader at Engenuity
The need for lighter, stronger and stiffer structures has driven the use of composites in high-tech structures for many years. However, the ever reducing cost of composite materials is making the use of composites in structures a very attractive and realistic alternative for more mainstream applications. The fact that they also look fantastic is simply an added bonus!
Composites are often used as ‘black metal’ by simply taking the geometry of a metal component and replacing the material. This method attempts to achieve an increase in performance on a par with the relative specific properties of composites, which can be 10 to 15 times better than standard steel.
Such benefits don’t come without a price, and the approach to achieving a reliable and efficient composite structure must be detailed and thorough if the full benefits are to be seen.
These high strength values are unlikely to be realised because they are based on the uni-directional properties of high performance composites. In reality a multi-axial load regime often needs to be supported, requiring more layers of fibres to react to load in many directions.
Other issues arise during the design of a composite structure that can require more consideration than for an equivalent metal structure, and are consequently easily overlooked or misunderstood. These include the detailing of joints and inserts which, because they cause structural discontinuities, are often the Achilles heel of a composite design.
The Black Art
The principles behind the black art of achieving the most from composites are not straightforward. Firstly a good overview of all aspects of composites, including material types and manufacturing techniques is required. Secondly the approach used in the development of the component must be complete and open minded.
When producing a composite component there is often a temptation to jump straight into the detailed analysis. This could well result in only modestly improved structural performance, neglecting many key project stages that are required to both maximise actual improvements and ensure the component is manufacturable. An example of a more complete approach is shown in Figure 2.
Engenuity applied the project cycle above when they were approached to develop a new 8.5m camera boom for a client. The first stage was to understand the project requirements. As is often the case they were not expressed in engineering terms but as an operational requirement in terms of final film picture quality. This requirement was converted into exact engineering goals through understanding what was considered as high picture quality and how the characteristics of the boom effected this.
The concept development stage involved analysing simple models to investigate only key design drivers — in this case global stiffness and mass. The development was split into two phases; the first was to evaluate an initial concept based on the clients current shorter aluminium truss boom, and also to investigate other possible solutions.
These included a composite truss structure and box structure in both aluminium and carbon. Once the structural performance of each concept model had been assessed the second phase was started. This involved evaluating the concepts in terms of relative structural performance, manufacturability and cost. The results showed that both aluminium concepts would only be able to achieve less than 25% of the target specific stiffness. Whilst a carbon truss concept initially looked slightly more structurally efficient it was found that the connectors required to link the truss members would add sufficient mass to negate this. The carbon box structure was chosen and provided an efficient starting point for the detailed development stage.
This stage requires the building of a model that will capture sufficient detail for the analysis requirements of the project. It provides absolute results in terms of stiffness or strength, rather than the relative values that were calculated during the concept development stage. Additional features such as joint and insert details may also be included.
History of Laminate Modelling
As the use of composites has become more widespread the analysis tools used to model the characteristics and properties of composite structures have also been developed. In 1990 two IAD employees, now Engenuity directors, developed the lay-up for the Yamaha OX99 Supercar.
Whilst the composite material properties could be accurately modelled within the software available at the time, the model was created without the benefits of any automated ply definition. This meant that the properties of each individual ply region had to be calculated manually and fed into the analysis. The initial model took three months to complete, although subsequent iterations were quicker as the structure became more efficient. The results of the analysis showed that modelling and optimising the ply thickness and orientations not only gave a very efficient structure but also showed good correlation with test results. This work proved to be one of the most comprehensive composite analyses of its time.
Understanding that the time required to create such a detailed lay-up would be prohibitive in many applications they decided to develop automatic lay-up generation software This was the fore-runner to the MSC Laminate Modeller, the P-Lap software.
It’s first application was on the Peugeot 905 EV2 front wing. A wing design was already in place and during practice they found that it was deflecting too much and dragging on the ground at high speeds. Within two weeks, with the new software, a new lay-up was produced that increased the stiffness of the wing by 100% for only a 10% increase in mass.
Analysis and Lay-up
In order to produce an efficient lay-up the material should only be placed where it will be used. The potential benefits of conceptual geometry development have already been considered, allowing this stage to concentrate solely on lay-up optimisation.
The optimisation process involves starting with a minimum lay-up over the entire structure. The stresses in individual plies can then be investigated and this information is used to drive the size, position and orientation of additional layers or patches. Although this is a highly iterative process, the automated lay-up tool enables this to be undertaken very quickly. A typical race car chassis now takes 3 to 4 weeks to optimise and can include over sixty ply iterations.
It is essential that the manufacturer be consulted at the earliest possible stage in the project to understand how the manufacturing techniques might influence the design. These effects can then be assessed at the concept development stage and minimise the need for costly changes later in the project. Ongoing support throughout the manufacturing process helps to ensure the final component reflects the design intent.
Review test data
As with all components it is prudent to review any appraisal of the manufactured component that is undertaken (whether this is static or dynamic testing) or at the very least verifying the weight of the component. This closes the analysis, manufacturing and testing loop before the part is finally commissioned.
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