In specific adhesion, its the molecular attraction between contracting surfaces. Mechanical adhesion occures when applies to rough or porous surfaces. Effective adhesion combines specific and mechanical to form optimum strength vs. stress. Stress is the force applied to the materials through four basic types. Tensile is exerted equally over the entire surface in a pull direction bonding is for optimum strength.
Cleavage is force concentrated at one edge and exerts a prying force.
The other edge is now under zero stress. Peel is concentrated along a thin line at the edge. Bonding has a flexible surface to stay out in front of the mating surface. Adhesives allow uniform distrubition of stress over the entire bonded area. This eliminates stress concentration caused by rivets and holes needed for mechinical fasteners, screwing, and drilling.