Fracture toughness of bulk adhesives in mode I and mode III and curing effect

Failure load predictions of adhesively bonded lap joints are either done based on a stress/strain limit criterion or using concepts of fracture mechanics. For the former case, the stress-strain curves of the adhesive must be determined accurately and for the latter case the toughness of the adhesive is needed. The present study gives for two adhesives, one brittle and one ductile, the strength and fracture properties in the bulk form. Stress-strain curves are given in tension and shear. The toughness was measured in mode I using a three point bending specimen and in mode III with a circular specimen with a notch under torsion. Mixed-mode criteria are discussed and it is shown that the experimental results are in good agreement with the strain energy theory. Finally, the effect of shrinkage stresses on the fracture toughness was studied and it is shown that they have a substantial effect on the fracture toughness.