  We study the dependence of the fracture surface energy on the pulling
velocity for nano-porous polypropylene (PP) sheets to find two components: the
static and dynamic ones. We show that these terms can be interpreted
respectively as plastoelastic and viscoelastic components, as has been shown
for soft polyethylene (PE) foams in a previous work. Considering significant
differences in the pore size, volume fraction and Young's modulus of the
present PP and previous PE sheets, the present results suggest a universal
physical mechanism for fracture of porous polymer sheets. The simple physical
interpretation emerging from the mechanism could be useful for developing tough
polymers. Equivalence of Griffith's energy balance in fracture mechanics to a
stress criterion is also discussed and demonstrated using the present
experimental data.
