CUT GROWTH IN VULCANIZATES OF NATURAL-RUBBER, CIS-POLYBUTADIENE, AND A 50 50 BLEND DURING SINGLE AND REPEATED EXTENSION/

A typical passenger sidewall composition was employed using three elas tomer systems: NR, BR, and a 50/50 blend of NR and BR. Tensile strips with edge cuts of various depths were strained in two ways: continuous ly increasing extension up to rupture, or repeatedly stretching them t o progressively higher strains until failure occurred. For BR, the ten sile strength sigma(b) decreased steadily with increasing cut depth, c , while NR and NR/BR showed an abrupt decrease in sigma(b) at a critic al cut size, c(cr). This is attributed to crystallization of NR throug hout the specimen at high strains, when c<c(cr), while crystallization is confined to the crack tip region when c>c(cr). Furthermore, in sam ples of NR and NR/BR the crack tip split, forming two secondary cracks which grew a considerable distance parallel to the direction of strai ning, and even turned backwards, before stopping. Rupture of the speci men did not occur until a new (third) crack was initiated from the smo oth torn surface of the secondary crack. Under repeated straining NR s pecimens developed secondary cracks repeatedly in a characteristic alt ernating pattern. These are instances of severe ''knotty'' tearing. In contrast, in BR samples the initial cut always grew in a forward dire ction and secondary, longitudinal cracks did not develop.