Cohesive law and notch sensitivity of adhesive joints

A large non-linear zone develops during fracture of a polyurethane adhesive bonded to steel adherends. The fracture process zone was characterised by a traction-separation law, a so-called cohesive law. The cohesive law was determined experimentally by the use of a J integral based approach on double cantilever beam sandwich specimens loaded with pure bending moments. The cohesive law shape was found to be highly non-linear. The cohesive stress increased with increasing separation, reached a peak and then decreased with increasing opening. The effects of loading rate and thickness of the adhesive layer on the cohesive law were investigated. An excellent agreement was found between measured strengths of bonded panels having a central notch and strength predictions based on cohesive law parameters.