Micromechanics of Fracture in Structural Adhesive Bonds

The high mode-I fracture surface energies, G_IC , of structural adhesives can be attributed to their ability to form large crack-tip deformation zones prior to failure. It has been suggested that this feature also controls the dependence of the adhesive bond G_IC on bond thickness. The proposed explanation asserted that the physical constraint of the adherents and the nature of the crack-tip stress field in an adhesive joint alter the size and shape of the deformation zone, and this in turn changes the fracture behaviour. To examine this hypothesis, motion pictures were taken of fracture specimens during loading, and the stress whitening that occurred at the crack tip was used to judge the relative dimensions of the deformation zone. The results generally support the hypothesis. Moreover, the pictures furnish a clear image of the deformation zone's growth patterns during loading, and this provides a critical test for future modelling efforts.