Design considerations
When bonded correctly, the low tensile strengths of plastics and a large joint overlap can result in substrate failure of an assembly prior to an adhesive bond joint failure. There are five main types of forces which can be applied to adhesively bonded joints: tensile, shear, compression, peel and cleavage.
Tensile force (Figure 1) is the force applied to an adhesively bonded joint when pulling the assembly apart perpendicularly to the bond line and adjoining substrate. Many types of adhesive perform well when tested in the tension plane3.
Shear force (Figure 2) is the force applied when substrates in an adhesively bonded joint are pulled parallel from one another along a plane. In most adhesive qualification and testing, structural properties of an adhesive are evaluated in the shear plane. Many structural adhesives perform well when subjected to shear forces3.
Compression (Figure 3) is the optimal force in which to place on an adhesively bonded assembly. Compressive force is the force applied to an assembly when the adhesively bonded substrates are pushed together perpendicularly by an outside force3.
Peel (Figure 4) and cleavage forces are similar to each other and are the least desirable forces to apply to an adhesively bonded assembly. Peel and cleavage forces are applied to the leading edge of an adhesively bonded assembly. These forces apply an uneven distribution of stress to the edge of the bonded materials. Once the adhesive begins to pull apart along the leading edge, the fractures being created in the adhesive may begin propagate through bond line3.
The force, or forces, which may be applied to the bond joint are critical in the adhesive selection process. Some adhesive perform better under certain conditions or have been designed to withstand specific forces better than other adhesives.
During the adhesive selection process, manufacturers should consider the environmental conditions the assembly will be exposed to throughout its intended usage period. Temperature, UV exposure, surface contamination, surface treatments and solvent and chemical exposure are all environmental factors which should be considered during adhesive selection. Different adhesive chemistries are designed to meet specific manufacturing and end use conditions.