This theory has been developed to explain the curious behavior of the failure of bonded materials. Upon failure, many adhesive bonds break not at the adhesion interface, but slightly within the adherend or the adhesive, adjacent to the interface. This suggests that a boundary layer of weak material is formed around the interface between the two media. Impurities in the bond and adverse chemical reactions are common causes of weak boundary layers. The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure.
Adhesive failure is the interfacial failure between the adhesive and one of the adherends.
It indicates a weak-boundary layer often from improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. The difference is illustrated in Figure 3.2. Ideally, the bond will fail within one of the adherends or the adhesive. This indicates that the maximum strength of the bonded materials is less than the strength of the adhesive strength between them.