CUT GROWTH IN VULCANIZATES OF NATURAL RUBBER, cis-POLYBUTADIENE, AND A 50/50 BLEND: PART II. CRACKING PATTERNS IN THE STRAINED STATE

The tensile strengths. (deita(b), and apparent fracture energies, G, of edge-cut strip specimens (28 mm wide) of carbon black-filled vulcanizates of natural rubber (NR). cli-butadiene rubber (cis--BR), and a 50/50 blend have been determined. The BR vulcanizate exhibited a rather steady decrease in (deita(b) with increasing cut size, c, and fractured by simple growth of the original cut lip. NR and the NR/BR blend exhibited much higher strengths as well as a drop in (deita(b) at a critical cut size, c(c^r)~~2.2 mm. High strengths are attributed, at least in part, to substantial longitudinal cracking prior to catastrophic fracture, especially when c < c(c^r). When strengths of edge-cut specimens were normalized by the regular (uncut) tensile slrength, similar values were fouind for all three vulcamzates, when c > c(c^r).Below c(c^r) normalized streugths of the NR and NR/BR were comparable and exceeded 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 tips appear as "corners" propagating along the loading direction. As a longitudinal crack progresses, strain energy is released by the retraction of (sheared) material into the unstressed zone at the extremity of crack opening.