Fracture of Soft Foam Solids: Interplay of Visco- and Plasto-elasticity.

Fracture mechanical properties of a very soft solidified foam of polyethylene with Young's modulus about 1 MPa are studied by changing stretching velocity in a wide range, by using sheets of the material in order to suppress finite-size effects. Unexpectedly, we find that the fracture can be described well by linear elastic fracture mechanics for a given fracture rate in the wide range. This allows a direct determination of velocity-dependent fracture energy of the soft foam. As a result, we find that the fracture energy is composed of a static plastoelastic component and another dynamic viscoelastic component, elucidating a simple physical interpretation of each component and giving guiding principles useful for practical applications to reinforce industrial polymer materials. Furthermore, we introduce a finite stress criterion for fracture that is similar in spirit to the cohesive zone model and, using our data, demonstrate that this stress criterion is consistent with the Griffith's energy balance.