Time-dependence of fracture behaviour of carbon black natural rubber

ABSTRACT: Fracture toughness at crack onset rate dependence was investigated performing videorecorded fracture tests on compounds having different carbon black content, under pure shear test configuration at dif- ferent displacement rates using the J-integral fracture mechanics approach. Filled rubber compounds fracture toughness slightly increases at low displacement rates while, at high displacement rates, the mechanisms which induce time-dependence are different and therefore different trends are observed depending on the car- bon black content. Results were analyzed considering the input strain energy as made of two components, one “stored” and released when fracture occurs, and the other “dissipative” related to non-catastrophic fracture events. To separate energy into these components, “loading-unloading” tensile tests were performed at in- creasing maximum strains on unnotched pure shear specimens. From these analyses, fracture toughness rate- dependence has been interpreted considering mechanisms which involve the “bound rubber”, a part of the rubber molecules bonded to the carbon black aggregates’ surface, which have restricted mobility, modifying the material orientability.