Fastening to composite components
Composite materials are becoming ever more common. While fibre-reinforced polymer matrix composites have made limited progress in the sphere of powertrain design and development, despite the efforts of some pioneers, they haven’t achieved the type of success they have enjoyed for almost 30 years in racing chassis. The era of the (very expensive) roadcar with carbon-reinforced polymer composites, however, is with us.
Compared to an engine block, a chassis is a relatively straightforward component – it is physically large, and the load paths are well defined and less complex than in an engine. There are some real difficulties in the manufacture of complex structural components, though, and it definitely isn't an easy task to make a successful composite engine block, for example. Part of the development bottleneck is due to restrictive regulations in some race series.
However, even if regulations were free, would we see composites supplanting conventional materials for blocks and heads? Only time will tell.
There are difficulties where we want to join one component to another, especially if we want to disassemble these parts at a later date. Bolted joints can be a real pain, and unless fastener loads are very low, it is not a good idea to try to put a thread directly into a fibre-reinforced composite component. Cut threads work badly, and moulded threads are often simply formed from weak resin.
Where we want a female thread, we need to provide a metallic ‘hard point’ in the composite, and this means bonding in or, more likely, moulding a metallic component in place. It should theoretically be possible to mould a hard point into a composite component with the thread already cut, but often this is not successful.
Experience often leads people to cut the thread after the hard point has been moulded in place. It isn’t usually prudent to rely solely on the resin to react the fastener torque and service loads, as the hard point needs to consider loading conditions and should provide some mechanical design features to prevent unwanted movement. The hard point itself can become quite bulky, especially as we need to have a certain amount of composite around these components – it can place the fastener axis far enough away from the applied load such that fastener bending stresses need to be more carefully considered.
Where a male thread is required there is a huge variety of off-the-shelf fastener components that combine a male thread with some unusual head geometry. The head of the fastener is not used to pre-load the fastener – the head is ‘buried’ within the composite and is a locking feature – and the heads of such fasteners are rarely axisymmetric. Polygon-shaped heads or discs with holes provide the opportunity for the composite to mould the head firmly in position.
The same type of fastener is also sometimes used to provide a female thread – here, a larger-diameter spigot is tapped rather than being externally threaded. However, because the fastener’s head is physically large, such fasteners don’t lend themselves to many applications where we might want a thread in a highly stressed engine component.
Written by Wayne Ward