The impact wedge-peel performance of structural adhesives

The impact wedge-peel (IWP) test is an International Standard (ISO 11343) method that is employed to measure the resistance to cleavage fracture of structural adhesives at a relatively high test-rate of 2 to 3 m/s. In the present work this test has been employed to evaluate the performance of a range of structural adhesives when used to bond either steel or aluminium-alloy substrates. Firstly, a novel test arrangement for performing these tests, using a high-speed servohydraulic machine, is described. Tests were performed at 10(-4) and 2 m/s and at test temperatures of -40 and 23 degrees C. High-speed photography was also used to investigate the failure of the IWP test specimens. Both stable and unstable types of crack growth were recorded, with the crack propagating cohesively through the adhesive layer in all cases. The methods of data analysis recommended by the International Standard are critically reviewed, and some shortcomings are highlighted. Secondly, the results from the IWP test are then directly correlated to the measured values of the adhesive fracture energies, G(c), of the adhesives, which were determined using a fracture-mechanics approach. Finally, it is demonstrated that, from knowledge of the value of G(c) of the adhesive, coupled with a finite-element analysis of the IWP test geometry, the failure behaviour of the IWP specimen may be successfully modelled and predicted. (C) 2000 Kluwer Academic Publishers.