Characteristics such as humidity, chemical, and electrical resistance are intrinsic properties of the adhesive material. The manner of processing is dictated by the choice of adhesive and its performance requirements in the application. The goal of any proper bond is to be strong enough to achieve substrate failure, in which the bond is stronger than the materials themselves. The next best expectation is cohesive failure, wherein the adhesive splits in failure but remains firmly attached to both substrates. Adhesive failure, in which the adhesive releases from one surface or the other, is generally considered unacceptable.
Adhesives work best in compression, and anything that promotes compressive loading is useful.
Pure tension and shear are good loading modes for structural adhesives. Adhesives that load well in tension may fail easily in peel. The thicker the bond line, the more likely the adhesive will fail cohesively. Because adhesive failure must be avoided, proper surface preparation is essential. For an adhesive to function, it must wet the surface.