Fatigue Crack Growth Behavior and Mechanistic Insights of Carbon Black- and Silica-Filled Styrene-Butadiene Rubbers

Fatigue crack growth (FCG) tests were performed on eleven styrene-butadiene rubber (SBR) vulcanizates using pure-shear specimens at a frequency of 5 Hz and a strain ratio of 0.1, in air at room temperature. The vulcanizates were either unfilled, carbon black (CB)-filled, or silica (SC)-filled, with filler contents ranging from 5 to 25 vol.%. While the FCG data for all vulcanizates could be fitted using power-law equations, deviations from the fitted curves were observed in the high tearing energy regime when the filler content exceeded 15 vol.%. This deviation is attributed to a transition in the crack growth mechanism from cycle-dependent to time-dependent behavior. For both CB- and SC filled vulcanizates, the threshold tearing energy increased and the power-law exponent decreased with increasing filler content; however, both values tended to plateau beyond 15 vol.%. Notably, at equivalent filler contents, SC-filled vulcanizates exhibited threshold tearing energy values approximately three times higher than those of CB-filled compounds, despite comparable hysteresis ratios. This unique threshold behavior was interpreted in terms of crack tip deformation, as evaluated by crack opening displacement, and the underlying fracture mechanisms, namely chain scission and interfacial delamination.