Shear fracture

The material aspects of mode II and III fracture in adhesively bonded joints were elucidated for a range of adhesives using DCB type test specimens and scanning electron microscopy. The adhesive thickness was varied from as little as a small fraction of the natural crack tip damage zone to a value large enough to expose the bulk fracture behavior. The fracture process in either mode started with the development of tensile microcracks ahead of the crack tip and continued with the spread of this damage through interfacial microcrack linkage and intralaminar adhesive yielding, of which the severity increased with adhesive ductility. Crack growth occurred when the damage zone became fully matured, at which point stable cracking took place with increasing load. Perhaps the most interesting finding was that irrespective of adhesive mechanical properties, energetically and, in essence, morphologically, the forward and anti-plane shear fracture components coincided. The shear fracture work, G_sc, exhibited a plateau followed by a phase of monotonic decline as the adhesive thickness, t, decreased. In the limit t → 0 the shearing and opening fracture works converged to a common, "intrinsic" value, G_c, that was only a small fraction of the bulk material toughness as identified by the plateau value of G_sc.