The use of the J‐integral to investigate fracture characterization in a carbon black reinforced natural rubber is described. Three applications to crack initiation are included: two based on the use of a hypothetical zero specimen length and one on conventional testing procedures for metals. While the validity of the zero‐length methods is questionable, the conventional method yielded a consistent Jc value of 1.01 N/mm for a typical tire compound. This value was obtained from 24 combinations of varying specimen geometries and pre‐crack lengths. The J‐integral is revealed as a valid fracture parameter that is applicable not only for material evaluation but also for designing tire structures to resist premature failure. These conclusions disagree with those from an earlier investigation, so the causes for the discrepancies are examined and discussed.

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