Written by Jody Muelaner
Lap shear strength is the standard way of measuring the strength of an adhesive joint and is the joint’s ability to resist in-plane forces. The maximum shear stress that an adhesive can support, the lap shear strength, is the force per unit area.
It’s good practice to design bonded structural joints so they’re loaded to ensure the adhesive predominantly endures the shear stresses. Surface contact can transfer compressive forces without any adhesive, so it’s only tensile forces that must be avoided.
Heat can affect the lap shear strength in two ways:
1. Temperature differences and thermal distortion of parts can impede the joint formation during adhesive application or curing.
2. Temperature changes during service life may affect the strength of an adhesive joint that’s already formed.
When a joint is being formed, thermal expansion of the parts can cause excessive gaps or movements, which can result in variation in the bond line adhesive thickness. It can also cause separation of surfaces before the adhesive has fully cured.
All materials exhibit different properties at different temperatures. At very low temperatures, the materials become brittle and more susceptible to cracking. This reduces their fatigue life and also makes them more susceptible to sudden failure under impact loadings.
At higher temperatures materials become more plastic, reducing the chance of cracking but also reducing their stiffness and ultimate strength. Adhesives behave similarly, and some may debond or delaminate at low temperatures.
Under optimal conditions, the lap shear strength of epoxy joints can be up to 25 MPa for composite components. For aluminum components, it’s typically 6 MPa. Heat-cured epoxies perform much better at elevated temperatures, experiencing reduced strength at temperatures of more than 120° C — or even higher for some epoxies.
Epoxy cured at room temperatures starts to lose strength at just 50° C. Epoxies typically perform very well at low temperatures and some can even be used in cryogenic applicates at close to absolute zero. They do, however, become brittle at lower temperatures.
For very tightly conforming interfaces between metallic components, cyanoacrylate (super glue) can a lap shear strength of up to 17 MPa. Good performance is typically maintained between -50° to 120° C.
The range of temperatures over which nominal strength can be maintained varies greatly between different types of adhesives.
It’s important to check whether a specific adhesive is suitable for a particular application by referring to the manufacturer’s specifications.