5.2.1.1 Gap Requirements for Bonded Assemblies

5.2.1.1 Gap Requirements for Bonded Assemblies

Gap Effect in Rigid Adhesives

As a general rule, high modulus adhesives (rigid adhesives) possess high tensile strengths. Additionally, there is an inverse relationship between the bond line thickness and the tensile strength of adhesives. This is not an absolute rule; however, some adhesives require a specific gap to achieve maximum strength. Where this induced gap is critical; the adhesive will have spacer beads within the formulation and the gap will be specifically addressed in the (TDS) of the corresponding adhesive. This is most important during clamping operations as an induced gap relates to the overall clamp force applied to the assembly; care should be taken not to apply too much force, as this may cause the spacers to be crushed or displaced and the induced gap will be less than specified, resulting in a lower strength bond than desired.

If an induced gap is not specifically advised, using rigid adhesives with thin gaps is usually recommended as long as the bond-line gap is fully filled. Thicker gaps normally result in lower assembly strength because cleavage forces more readily occur due to nonaxial loading or internal stresses in the adhesive.