Shear Displacement
Mechanical ratios describe the deformation of an adhesive layer relative to its thickness when a sliding force is applied across the bond. In structural testing, lap shear strain is calculated as the displacement of the bonded substrates divided by the original thickness of the adhesive. This value helps engineers understand how much a joint can stretch or shift before the bond fails.
It is a critical parameter for designing assemblies that experience thermal expansion or vibration.
Deformation Test
Testing involves pulling two overlapped plates in opposite directions while monitoring the movement of the interface. The resulting lap shear strain reveals the ductility of the adhesive and its ability to distribute stress across the joint. A brittle epoxy might fail at a very low strain, whereas a flexible silicone can withstand significant movement.
Metrology tools like extensometers or digital image correlation are used to measure the tiny displacements accurately during the pull test.
Joint Design
Designers use this measurement to select the appropriate bond line thickness for a specific application. If the lap shear strain exceeds the limit of the material, the joint will delaminate or the adhesive will tear. For components with large differences in thermal expansion, a thicker bond line reduces the total strain by spreading the movement over a larger volume of material.
Calculations must account for the maximum expected temperature range to ensure the strain stays within the elastic region of the adhesive. This prevents permanent deformation or creep that could lead to the misalignment of precision sensors over time.
Fracture Limit
Failure occurs when the material reaches its final capacity for deformation. Recording the peak value at the point of breakage provides a safety limit for future design iterations.
