Effects of constraint on crack tip stress fields and fracture toughness in adhesive joints

Experimental investigation and finite element analysis have been carried out to study the fracture behaviour of adhesive joints under mode-I loading. Fractography and fracture toughness showed apparent variation with bond thickness. Numerical results indicated that at the same loading level, a higher opening stress was obtained in the joint with a smaller bond thickness (h) under both elastic and elastic-plastic conditions. The region dominated by crack singularity is much smaller than the bond thickness. Small-scale yielding conditions for homogeneous material cannot be directly applied to adhesive joints. Beyond the crack tip region, a self-similar stress field can be described by the normalised loading parameter J/hδ_o. The relationship between J and crack tip opening displacement is dependent on the bond thickness. The strong dependence of toughness upon bond thickness is likely to be a result of the competition between two different fracture mechanisms. A simple model has been proposed to predict the variation of toughness with bond thickness.