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

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