Adhesive Layer Shrinkage in Bonds Subjected to Thermal Cycling

Residual stress states within certain classes of polymeric adhesives andcoatings subjected to cyclic temperature conditions can becomeincreasingly tensile with exposure to cyclic temperature conditions.Under some conditions, these tensile stresses are believed to beresponsible for initiating debonds which may eventually propagatethroughout the bond. In certain potting-type applications in which thepolymer is constrained in a particular fashion, these debonded layerscan shrink extensively. In an industrial application, this shrinkage hasreached as much as 15%, nearly two orders of magnitude larger thanexpected based on simple thermal contraction. This paper provides athermal racheting explanation for this dramatic shrinkage. Numericalstudies have confirmed the explanation, and provide additional insightsinto the effects of temperature range, coefficient of friction betweenthe adhesive and substrates, and the viscoelastic nature of theadhesive. Based on the hysteretic nature of the normal forces whichaffect the frictional forces, the proposed mechanism usesfriction hysteresis under cyclic thermal conditions to explain observedshrinkage.