For stiff materials like metals and ceramics, the material modulus is usually far in excess of the peak cohesive stress or strength of the interface. For soft solids, for example elastomers and biological tissues, a physically based value for cohesive strength is often far greater than the modulus of the material. As a consequence, cracks in such materials undergo very large deformations before fracture. The question arises: is it possible that a material be so soft that it will blunt elasti-cally? Given that the cohesive or adhesive strength of materials and interfaces are so large in comparison with the material modulus, how do soft materials fail? Blunting of a crack in a homogeneous material or at an interface has a profound influence on the fracture process, usually resulting in much greater energy dissipation. Whereas the onset of blunting is reasonably well understood in elastic-plastic materials, conditions for its occurrence in elastic solids have not been clearly established. Our premise is that a crack in a soft material will typically blunt before failure can occur. The region ahead of the blunted crack can be considered as a 'cohesive zone' since its size may still be small in comparison with typical specimen dimensions. The strength of this cohesive zone, according to our theory, is limited by the elastic modulus of the material. Failure must take place within this cohesive zone, e.g., by cavitation or by the growth of a micro-crack initiating from the original crack. Cohesive zone due to blunting must be distinguished from regions where actual separation occurs -these regions are often of molecular dimensions.
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