7.1.3 Substrate Failure

Substrate failure occurs when external loads exceed the strength of one or the other substrates. When this happens, the substrate reaches the point of deformation and eventually its yield point, resulting in failure. This type of failure does not indicate failure of the adhesive; rather, if the external loads to which the assembly was subjected are reasonable proxies for real-world conditions, a stronger substrate is needed. Put simply, cohesion and adhesion are stronger than the substrate (in the specific dimensions and assembly).

Delamination is another form of substrate failure that may occur in non-homogenous substrates, such as fibre-reinforced plastics, or laminated materials, such as plywood. Alternatively, coated substrates may suffer substrate failure in the form of the coating tearing off the base material; this is most often seen with paints and other surface coatings (see Section 4).