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 elastomer 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: continuously increasing extension up to rupture, or repeatedly stretching them to progressively higher strains until failure occurred. For BR, the tensile strength sigma(b) decreased steadily with increasing cut depth, c, while NR and NR/BR showed an abrupt decrease in sigma(b) at a critical cut size, c(cr). This is attributed to crystallization of NR throughout the specimen at high strains, when c<c(cr), while crystallization is confined to the crack tip region when c>c(cr). Furthermore, in samples of NR and NR/BR the crack tip split, forming two secondary cracks which grew a considerable distance parallel to the direction of straining, and even turned backwards, before stopping. Rupture of the specimen did not occur until a new (third) crack was initiated from the smooth torn surface of the secondary crack. Under repeated straining NR specimens developed secondary cracks repeatedly in a characteristic alternating pattern. These are instances of severe ''knotty'' tearing. In contrast, in BR samples the initial cut always grew in a forward direction and secondary, longitudinal cracks did not develop.