The forces acting on the adhesive layer of a bonded joint under external loads can be categorized into four types: shear, tension, uneven separation, and peeling.
A. Shear: External forces are equal in magnitude, opposite in direction, basically parallel to the bonded surface, and evenly distributed across the entire bonded area.
B. Tension: Also known as uniform separation, the joint is subjected to opposite tensile forces that are perpendicular to the bonded surface and evenly distributed over the entire bonded area.
C. Uneven separation: Also called cleavage, the direction of the external force is also perpendicular to the bonded surface, but the force distribution is uneven.
D. Peeling: The direction of the external force forms a certain angle with the bonded surface, and the force is mainly distributed along a straight line on the bonded surface.
In the same adhesive system, it is highly likely that several of these four forces exist simultaneously; the key difference lies in which type of force dominates.