ABSTRACT: The intrinsic strengths for carbon black filled (50 phr) rubber materials based on Natural Rubber (NR), Butadiene Rubber (BR) and NR/BR blends in the volume ratio 50/50 have been evaluated for unaged material, and material aged at 50°C for 720 hours. The measurement was based on quasi-static tension loading on an edge-cracked pure shear specimen, combined with frictionless cutting via a sharp blade. The cutting force and pre-stress parameters were varied automatically using the Intrinsic Strength Analyzer (ISA©) instrument manufactured by Coesfeld. It was observed that both stiffness and the intrinsic cutting energy increase with aging. The cutting measurement provides a convenient approach for estimating fatigue threshold, a measurement with otherwise might require much longer testing periods.