Transition of rupture mode of strain crystallizing elastomers in tensile edge-crack tests.

Abstract

We revisit the classical results that the fracture energy density (<i>W</i>_b) of strain crystallizing (SC) elastomers exhibits an abrupt change at a characteristic value () of initial notch length (<i>c</i>₀) in tensile edge-crack tests. We elucidate that the abrupt change of <i>W</i>_b reflects the transition in rupture mode between the catastrophic crack growth without a significant SIC effect at <i>c</i>₀ > and the crack growth like that under cyclic loading (d<i>c</i>/d<i>n</i> mode) at <i>c</i>₀ < as a result of a pronounced SIC effect near the crack tip. At <i>c</i>₀ < , the tearing energy (<i>G</i>) was considerably enhanced by hardening <i>via</i> SIC near the crack tip, preventing and postponing catastrophic crack growth. The fracture dominated by the d<i>c</i>/d<i>n</i> mode at <i>c</i>₀ < was validated by the <i>c</i>₀-dependent <i>G</i> characterized by <i>G</i> = (<i>c</i>₀/<i>B</i>)^1/2/2 and the specific striations on the fracture surface. As the theory expects, coefficient <i>B</i> quantitatively agreed with the result of a separate cyclic loading test using the same specimen. We propose the methodology to quantify the tearing energy enhanced <i>via</i> SIC (<i>G</i>_SIC) and to evaluate the dependence of <i>G</i>_SIC on ambient temperature (<i>T</i>) and strain rate (<i></i>). The disappearance of the transition feature in the <i>W</i>_b-<i>c</i>₀ relationships enables us to estimate definitely the upper limits of the SIC effects for <i>T</i> (<i>T</i>*) and <i></i> (<i></i>*). Comparisons of the <i>G</i>_SIC, <i>T</i>*, and <i></i>* values between natural rubber (NR) and its synthetic analog reveal the superior reinforcement effect <i>via</i> SIC in NR.