Written by Jody Muelaner

Adhesives (including glues and cements) bind surfaces together, a process known as bonding. A load-carrying adhesive, also known as a structural adhesive, is a material that has sufficient bond strength to support structural loads. 

The exact definition depends on the application. However, structural adhesives can give improved stress distribution when compared to mechanical fasteners or welding.

Ideally, joints should be designed so that an adhesive transfers shear forces. In other words, the adhesive prevents the surfaces from sliding past each other. The strength of an adhesive loaded in this way is known as its lap-shear strength. 

If loading causes the surfaces being joined to pull apart, this means the adhesive is loaded in tension, which is a condition it is unlikely to withstand. Generally, joints should be designed to minimize tension between mating surfaces.

Some structural adhesives, together with typical lap-shear and tensile strength, include:

• Epoxy resin: Typically, an ideal structural adhesive for many applications, including metals and polymers. Epoxy resin achieves a high strength even when filling gaps between components. Lap-shear strength can be as high as 25 MPa when bonding composite parts with an epoxy matrix although for aluminum parts this is typically just 6 MPa.
• PVA bonding of wood: PVA adhesive is commonly used for joining wood. Joints typically have a lap-shear strength of about 7 MPa and a tensile strength of 3 MPa. Depending on the type of wood and direction of loading, relative to the grain, this can be stronger than the wood itself.
• Cyanoacrylate (super glue): Superglue can achieve high strength when a very thin glue line is achieved. For aluminum parts lap-shear strength can be as high as 17 MPa.