The effects of bond thickness, rate and temperature on the deformation and fracture of structural adhesives under shear loading

The deformation and fracture in shear of a structural adhesive undergoing large-scale yielding is studied as a function of bond thickness, h, temperature, T , and strain rate using the Napkin Ring specimen. The lack of edges in this test, and the fact that the strain rate can be locally controlled, allow for a meaningful evaluation of the mechanical response throughout the deformation process. In accord with Airing’s molecular activation model, the yield stress linearly decreases with T while logarithmically increasing with the strain rate. The ultimate shear strain, γF, is little sensitive to rate while decreasing with h and increasing with T . Some complementary fracture tests are carried out using the ENF bond specimen in order to explore the relation between the mechanical properties of the nominally unflawed adhesive and the mode II fracture energy, GIIC. For sufficiently thin bonds, GIIC/h correlates well with the ultimate energy density (i.e., the area under the stress-strain curve in the Napkin Ring test), given, to a first approximation, by τYγF, where τY is the yield stress in shear. Accordingly, the fracture energy of the bond would be greatly affected by temperature, tending to a small value at the absolute as well as the glass transition temperatures while attaining a maximum in between these two extremes. Because the yield stress does not vary much with h, the variation of GIIC with the bond thickness reflects that of γF. A large-deformation fracture analysis, based on a cohesive zone like model, is developed to account for the observed variations of γF with h. The analysis assumes that a crack preexist in the bond, either at its center or at the interface. The results suggest that the observed increase of γF with decreasing h is due mainly to two geometric effects. The first is due to the interaction of the bonding surfaces with the stress field generated by the crack and the second has to do with the probability of finding large flaws in the bond to trigger the fracture.