INDUSTRY FOCUS MILITARY, AEROSPACE & DEFENCE Aerospace manufacturers have achieved weight reductions of almost 50% in the


latest generation of commercial aircraft, but has lightweighting with composite materials now reached its limit? Not according to Phil Burge, marketing and communications


manager at SKF, who explains how new design approaches are extending the potential for composites to replace metal, even in structural interface components


COMPOSITE


INNOVATIONS ADD STRENGTH TO


THE AEROSPACE INDUSTRY


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anufacturers are increasingly using light composites in their designs,


driven largely by demand for more efficient performance and lower emissions. However, while these can provide plenty of strength when subjected to in-plane loading (loads applied along the direction of the component’s mounting interface), their capacity for out-of-plane loading (perpendicular to the mounting interface) tends to be much lower. In addition, expense also becomes an


issue if composite structural interface components are assembled from several separately manufactured subcomponents. Integration of design, manufacture and function is needed to combat this. With its Black Design technology,


SKF has developed a way of creating high-performance CFRP (carbon-fibre- reinforced polymer) parts suitable for out-of-plane loads. Innovative techniques are used which radically alter part shape and geometry to gain maximum strength from the material. The opportunities this opens up are


further enhanced by integrated designs which incorporate bearings – and a variety of different functions – into a


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is around 50 times weaker than the fibres. Good examples of structural interface


With its Black Design technology, SKF has developed a way of creating high- performance CFRP (carbon-fibre-reinforced polymer) parts suitable for out-of-plane loads


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parts facing out-of-plane loading include T-shaped fittings and cleats. When made of metal, failures in these components often involve folding or unfolding. Composite parts built to the same shape may also suffer delamination in 90˚ corners, where interlaminar stresses in the resin separate the CFRP plies. Resin is at its strongest when


compressed, so Black Design’s first principle is to change the design, so that compression is maintained when the part is loaded. A key to this has been SKF’s development of hemispherical washers with carbon fibre soles. Whether the load on the component is


WITHSTAND TAND WITHSTANDTAN AND WITHSTANDTAND


tensile or compressive, the washer’s shape results in compression of the resin. Meanwhile, the sole holds the CFRP material firmly in shape to maintain local compression. Black Design’s second principle is to


3 t 32 5t FEBRUARY 2019 | DESIGN SOLUTIONS 20 t20


alter the shape of structural interface parts to improve their mechanical stability and stiffness. Rounded shapes, avoiding 90° corners, are highly effective in this respect, enabling optimal alignment of fibres to resist folding, unfolding and delamination.


single consolidated structure.


HOW BLACK DESIGN WORKS To understand Black Design, we need to look at the typical make-up of CFRP materials. They consist essentially of stacked layers, or plies, of carbon fibre impregnated with resin. Resin holds the plies together and enables transmission of loads between the fibres. These fibres give high strength and stiffness in the orientation of the layers. Any load applied perpendicular to this plane, however, must be carried mainly by the resin, which


AN INTEGRATED APPROACH SKF can now efficiently embed bearings directly into composite parts. For heavy loads, high speeds and wide temperature ranges, a high-strength interface to the outer ring of a rolling bearing can be provided. For low friction and wear, and great reliability, the inner ring of a plain (non-rolling) or plain spherical bearing can be brought into direct sliding contact with a composite surface. By removing assembly steps and


combining manufacturing times, this kind of consolidation cuts costs, but SKF believes integration can do much more. Instead of just looking at the individual requirements and properties of each component in an assembly, a holistic view should be taken on what will work best together. For instance, if components are


specified individually for stiffness, it might be found that carbon fibre is needed for some but glass fibre is good enough for others. Such a mixture of materials could be expensive, wasteful and impractical. SKF suggests an alternative strategy


in which the system’s overall stiffness function is distributed across the assembly. A specification can be chosen that makes best use of composites and gives the required performance, at a lower cost. Taking this further, the design can be optimised for other functions such as mass distribution and damping.


THE BENEFITS Whether the priority is to cut fuel consumption, limit emissions or enhance performance, weight reduction through replacement of metal with composites has much to offer. Black Design makes this possible for a much wider range of components. It has already demonstrated weight savings in excess of 40%, while continued development of component consolidation promises more. As well as solving fatigue problems,


these composite technologies avoid corrosion, enable easier integration of features and functions, and allow noise and vibration to be designed out of assemblies. Better still, their competitive advantages are cost-effective to achieve.


SKF www.skf.com


/ DESIGNSOLUTIONS


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