What are the key points for the selection of composite fasteners?

MechanicalFastenersand adhesives can be used to join the composite material, which joining technique is used depending on the application and material composition. For example, composite materials used in aircraft are generally connected with mechanical fasteners and rarely with adhesives. However, in automotive applications, most composites are joined by adhesives.

In theory, all composite materials can be bonded. However, in many joints, the load is concentrated and will bear a lot of stress, so fasteners are often used in these places. In addition, when the structural components are very large, it is difficult to place tools and curing equipment for bonding, which makes the fasteners cost-effective in this case.

Titanium alloy rivets, threaded pins, blind rivets, two-piece bolts, nuts and other mechanical fasteners can be used for composite materials. Special attention should be paid to the following items when selecting:

1. Clearance fit between fastener and composite hole;

2. The effect of drilling on the structural integrity of the material (to avoid delamination);

3. Possible galvanic corrosion at joints;

4. The conductive path of the composite material in a special location or application.

In addition, the fasteners used in composite materials often have a "big head" in appearance ". The purpose is to distribute the load over a larger surface area to reduce the pressure on the surface of the composite material and avoid delamination. Fasteners and composite materials cannot be installed with a clearance fit only. A slight interference fit is permitted if necessary. However, in order to avoid delamination of composite materials caused by interference fit, sleeve fasteners should be properly selected to make the stress evenly distributed along the radial direction of the fastener.

Particular emphasis is placed here on the problem of galvanic corrosion. If aluminum fasteners are used, the carbon fiber composite material may be electrically corroded due to the potential difference between aluminum and carbon fiber, so it is necessary to apply a coating on the surface of the fastener, but this will increase the assembly cost and time. Therefore, in this case, aluminum fasteners are often replaced by more expensive titanium alloy and stainless steel fasteners. Of course, the price of fasteners still depends on the quantity.

The basic requirement of fastener selection is to meet the requirements of the structure for the load force, so first of all, it is necessary to find out whether the load at the connection is tensile stress, shear stress or both. If the tensile fatigue of the fastener is the main problem, it is necessary to choose a tensile head fastener, and there should be a certain chamfer at the connection between the head and the nail shank. However, under mild or moderate tensile stress, any type of fastener may be considered, including rivets and shear head fasteners.

Metals used in carbon fiber compositesFastenersThere is a clear tendency to corrosion. Cadmium plating is a surface treatment method for steel fasteners, but electrochemical corrosion still occurs in carbon fiber composites. Therefore, in the installation process, sealing fasteners become very common, which is commonly known as "wet" fasteners. For carbon fiber composites, titanium alloy fasteners are preferred.