3.4.3 Adhesive film thickness

Just like the bonding area, the adhesive film thickness must also be adapted to the adhesive and expected loads.
The following graph clearly shows the effect of the adhesive film thickness for shear loads.

In the lap shear test the strength of a bonded joint (under otherwise constant conditions) depends on the thickness of the adhesive film (in general: the highest strength is attained at a film thickness of ca. 0.1 – 0.2  mm):

Increasing adhesive film thickness:

  • Greater number of inhomogeneities with notch effect (air bubbles, regions where there is poor mixing, contaminants, …)
  • Increase in the torque due to the action of eccentric forces → peel effect

Decreasing adhesive film thickness:

  • Increase in the notch effect due to surface roughness