LONGITUDINAL CRACKING IN A CARBON-BLACK FILLED NATURAL-RUBBER VULCANIZATE DURING CHEMICAL STRESS-RELAXATION

Thin specimens of a black-filled, natural rubber vulcanizate have been held in uniaxial tension at 72 degrees C and 200% elongation in a for ced air oven. After substantial oxidative attack (inferred from stress relaxation), small edge cracks formed. Initially, these cracks grew p erpendicular to the loading direction, but, upon reaching about 0.1 mm in depth, longitudinal crack growth commenced and fracture progressed by a kind of 0 degrees-peel process with ''splitting-off'' of success ive strands of rubber. This phenomenon is attributed to anisotropy in strength caused both by straining and by oxidative attack.