The forces exerted on the adhesive layer of a bonded joint under external loads can be categorized into four types: shear, tension, uneven peeling, and cleavage.
A. Shear: External forces are equal in magnitude, opposite in direction, basically parallel to the bonding surface, and evenly distributed across the entire bonding area.
B. Tension: Also known as uniform peeling, it is subjected to pulling forces in opposite directions that are perpendicular to the bonding surface and evenly distributed over the whole bonding area.
C. Uneven peeling: Also referred to as splitting, the direction of the applied external force is perpendicular to the bonding surface as well, but the force distribution is uneven.
D. Cleavage: The direction of the external force forms a certain angle with the bonding surface, and the force is distributed essentially along a straight line on the bonding surface.
In the same adhesive system, it is highly likely that several of these four types of forces act simultaneously, with only the difference in which one dominates.