Patent of the month: conjoined joints

Materials World magazine
,
1 Jul 2018

Hybrid manufacturing helps to form joints between dissimilar materials. Dr Jennifer Unsworth, Patent Attorney at intellectual property law firm Withers and Rogers, reports.

As materials scientists seek to adopt the right materials for maximum effectiveness, it is increasingly found that one material does not provide all the desired properties for every part of a structure. In fact, a combination of materials is often the best solution. However, this raises the issue of how to join materials together that may be very dissimilar.

Faced with this problem, BAE Systems, defence, security, and aerospace company, UK, has secured a patent to manufacture hybrid joints between metal and composite parts.

Aircraft and defence platforms now regularly use composite materials instead of metals due to their high strength-to-weight ratio, resistance to corrosion and their potential to incur lower fabrication costs. Despite these benefits, metals are still required for high-tolerance surfaces, due to their fire-resistant properties and suitability for high-volume manufacturing applications and low-cost assembly. By combining composites and metals in a single structure, it is possible to gain advantages from each respective material, enabling increased design flexibility. Marine propulsion shafts are an example of this. Previously manufactured from metals, they may now be formed from lightweight, corrosion-resistant composite materials.

Nevertheless, metallic end fittings are still required for bearing interfaces as the composite material does not have the necessary wear resistance or dimensional tolerance. 

The main challenge with multi-material structures is how best to join dissimilar materials together. Fastened joints suffer from increased weight due to the necessity to reinforce the composite section for machining operations and the increased number of fasteners. 

Composites are also particularly sensitive to notches that sever the load-bearing fibres, which can result in the performance of the composite being degraded and additional stress concentrations being introduced to the product.

Adhesive bonding is a popular alternative, but it is sensitive to initial surface preparation, hot or wet environments, impact damage and may require a large bond area. Furthermore, high-strength, adhesively-bonded joints tend to exhibit sudden catastrophic failure. To address these problems, hybrid joining has been considered for connecting composites to metals. This process involves combining mechanical interlocking features with adhesive bonding.

Figure 11 of the patent – GB 2507128 – shows a metallic component with a series of spaced pins (22), which are hollow. A layup for a composite component is a pre-preg and forms a top layer in the figure. To assemble the joint, pressure is applied to the layup so that the pins pierce and pass through it. Once the layup is in position, with the surfaces of the parts in contact, a press forming tool (23) is lowered onto the pins to deform them, creating a constraint in the form of an enlarged head (24). A metal mesh is then layered on the upper surface of the composite layup to protect it from peeling away from the metallic component.

This process facilitates the use of low-cost pin placement methods and allows the initial use of narrow, cylindrical pins that will easily pass through the reinforced composite fabric. Once the composite fabric is in situ, the shape of the pins can be modified to increase the strength of the joint and improve its peel loading performance.

The metal mesh may also be used to provide a conductive path from the metallic component to the outside of the composite material. This can be used to detect damage in hybrid joints because the resistance between the mesh and metal components will change if the pins become damaged or broken. 

This patent shows how it is increasingly possible to combine dissimilar materials, particularly metals and composites, to deliver strong and long-lasting solutions for a variety of applications.