Abstract
Dumbbell specimens containing edge razor-cuts of various depths, c, have been prepared from sulfur-vulcanized natural rubber compositions containing 0–75 phr of N115 black. Normal tensile strengths (c=0) of the gum and all black-filled samples are similar. However, cut growth resistance (CGR) relative to the gum, depends strongly on black content and cut depth. With about 6–15 phr of black, filled specimens are weaker than the gum and fracture occurs by simple forward growth of the original cut tip, i.e., a single crack develops and propagates laterally. At about 18 phr of black, filled specimens become stronger than the gum and cracking becomes complex. Prior to catastrophic rupture, crack splitting occurs near the original cut tip - forming at least two slowly-growing, longitudinal cracks. These reduce the stress concentration caused by the cut, thereby delaying rupture, and marking the onset of “reinforcement”. All pre-cut specimens containing 18–75 phr of black are stronger than the gum and they exhibit crack splitting. Specimens containing 50 phr of black have the highest strength - being about ten times as strong as the gum when cuts are large. The decrease in CGR at low levels of black and increase at high levels is attributed to competing effects upon the addition of carbon black to NR.