Source Year: 1999

The tensile strengths, Ob, and apparent fracture energies, G, of edge-cut strip specimens (28 mm wide) of black-filled vulcanizates of natural rubber (NR), cis-butadiene rubber (cis-BR), and a 50/50 blend have been determined. The BR vulcanizate exhibited a rather steady decrease in Ob with increasing cut size, c, and fractured by simple growth of the original cut tip. NR and the Ccr ~2.2 mm. High strengths are attributed, at least in part, to substantial longitudinal cracking prior to catastropic fracture, especially when c < Ccr. When strengths of edge-cut specimens were normalized by the regular (uncut) tensile strength, similar values were found for all three vulcanizates, when c > Ccr. Below Ccr, normalized strengths of the NR and NR/BR were comparable and exeeded those of BR. For all three vulcanizates, calculated fracture energies depended on cut size. Photographs of a deformed NR/BR specimen, which had developed the auxiliary cracking, revealed that longitudinal crack progresses, strain energy is released by the retraction of (sheared) material into the unstresses zone at the extremity of crack opening.