Most tests of adhesive strength examine linear or peel stress1, which induce cracking and drive adhesives to the point of failure. When cracks develop either adhesive or cohesive failure can result and over the years extensive mathematical representations have been developed to model this. For those who are interested, some of the references at the end of this article discuss mathematical methods in detail. Here though, I'm hoping to highlight some of the basics of that underlie common adhesive breakdown behaviour and the maths that's used to describe it.
Mode I is the symmetrical opening of an adhesive bond in the vertical plane, from pulling the joined materials apart. Mode II is a sliding shear stress, in which the substrates are pushed in opposite directions in the plane of the bond, maintaining a linear arrangement. By contrast mode III, known as antiplane shear, is caused by horizontal forces perpendicular to the stresses experience in mode II.Cracking caused by solvent absorption also follows a similar power law.