Exploring causal relationships in elastomeric fracture

RESUMO

The conventional interpretation of elastomeric fracture treats the tearing energy G as an output determined by tear speed, taken to equal crack velocity vc, concluding that higher vc produced greater viscoelastic dissipation to increase G. Based on spatially and temporally resolved polarized optical microscopic (str-POM) measurements of tip stress tip stress, we use prenotched pure shear experiments to clarify the causality: Local crack-tip stress determines the network lifetime, making vc the passive kinetic output. In the elastic limit that can be readily achieved for a well crosslinked elastomer in a wide range of temperatures, Rivlin-Thomas scaling holds, and vc depends only on the applied strain, independent of stretch rate, indicating one-to-one correspondence between vc and tip stress. In a highly stretchable elastomer made with reduced crosslink density, vc no longer correlates well with the far-field load as rate-dependent viscoelastic processes emerge to affect network lifetime. Because of the rheological effects on how chain tension builds in the elastomeric network, vc is higher at a common nominal strain when it is imposed with higher stretch rate. confirm that different stretch rates produce different levels of tip stress. The same tip stress still produces the same vc. With str-POM measurements we are also able to elucidate the nature of temperature dependence: at a given tip stress,, crack growth is slower at lower temperatures; conversely it requires higher tip stress to produce the same vc at lower temperatures.