Mechanical fasteners

Mechanical joining is perhaps best for situations where the components to be joined are thicker and the loading paths are complex. Mechanical joining is widely used for both thermosetting and thermoplastic matrix composites. Key points for mechanical joining (bolting and riveting) include:

• Mechanical joints can be disassembled without imparting damage.
• Improved inspection / replacement possibilities compared to bonded joints.
• Holes must be drilled in the composite parts before these methods can be applied and these give rise to stress concentrations and are potential damage initiation sites if poorly machined.
• Bolts / rivets specifically designed for joining composites are available.

The number, form and size of bolts or rivets are design factors that will greatly influence the strength and stiffness of a joint. Mechanical joints are designed to avoid premature failure of the fasteners through shear or bending and also to prevent premature failure of the composite through tension, bearing, shear out or cleavage. The form of the composite (short fibre or continuous fibre) and orientation of the reinforcement are also factors that will influence how a joint carries load and how it will fail.