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 (c₀) 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 c₀ > and the crack growth like that under cyclic loading (dc/dn mode) at c₀ < as a result of a pronounced SIC effect near the crack tip. At c₀ < , 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 dc/dn mode at c₀ < was validated by the c₀-dependent G characterized by G = (c₀/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-c₀ 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.