John W. Hutchinson (School of Engineering and Applied Sciences, Harvard University, USA). Mechanics of the fracture of tough gels and elastomers.

Recent research has produced a class of hydrogels with exceptional stretchability and toughness enabling recoverable uniaxial tensile stretches as large as 20 and fracture toughness values as large as 10 KJ/m². The gels are hybrids formed from an interpenetrating double network of two gel components such as the long chained polyacrylamide and the shorter chained alginate.  The hybrid toughness can be more than 25 times that of either component.  Fracture experiments on the tough hybrid gels will be discussed which reveal that the high stresses and strains induced by a crack or notch produce dissipation extending well away from the crack tip.  This dissipative mechanism in the hybrid gel has a number of aspects in common with the manner in which plasticity contributes to the effective toughness of ductile metals.  Other features in common with the fracture of ductile metals revealed by our experiments are substantial crack growth resistance following crack initiation and subsequent attainment of steady-state tearing conditions after sufficient crack advance.  We will outline how nonlinear fracture mechanics can be brought to bear on these exceptional new materials.