Tensile stress-strain testing is used to investigate fracture behavior of carbon black-reinforced styrene-butadiene rubber, using 50 replicate specimens. Four vulcanized rubber compounds are studied: a CB-filled SBR with standard mixing conditions (Control); the same formulation with intentional poor mixing of the CB; and materials identical to the control material but formed by adding minor amounts of 0.5 mm diameter glass microspheres (beads) – serving as large model defects/inclusions - using a two-roll mill at two levels corresponding to average values of 0.78 and 6.24 beads per gauge section region of the tensile test specimen. This paper focuses on microscopy analysis of the resulting fracture surfaces to complement our recent publication on Weibull failure statistics for distributions of tensile strength and crack precursor size (Polymers 12, 203 (2020)). All 200 fractured specimens from tensile testing at 23°C are imaged with light microscopy and exhibit fracture surfaces that are characterized by relatively smooth planes that are perpendicular to the uniaxial loading direction. The majority of tensile failures originate from the edges of the dumbbell specimens, in line with expectations from fracture mechanics. Light microscopy reveals concentric fracture ring features of high specular reflectance emanating from crack precursors, which are a universal feature of the failure process for these compounds and independent of precursor type, size, or location. Non-contact interferometric microscopy confirms that the rings are the result of variations in surface micro-roughness; proceeding outwards from the precursor as rough-smooth-rough to the edge of the fracture surface. Fracture rings are also observed for tensile tests performed at 80°C. The variation in surface roughness of the fracture surface has parallels to the stick-slip tearing behavior seen for rubbers torn at medium to high rates. To the best of the authors’ knowledge, this is the first time that such striking features have been reported.

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