The effects of carbon black and stress-induced crystallization on the crack growth resistance of natural rubber

J-integral and tearing energy concepts were used to measure the fracture resistance of carbon black filled natural rubber. The J line integral around the crack tip of the non-elastic material was path independent; the fracture resistance for crack initiation, rupture and fatigue crack growth were increased by carbon black fillers and decreased by biaxial stretching; increase of specimen thickness decreased the crack growth resistance at crack initiation, but increased the resistance at rupture. To understand these phenomena, carbon black and stress state effects on stress induced crystallization were studied by DSC and stress relaxation tests. Carbon black enhanced the stress induced crystallization in natural rubber while biaxial stretching impeded it. To connect the carbon black effects on microstructural change with the effects on fracture resistance change, the strain or stress distribution in the crack tip region was analyzed, and the crystallization as function of carbon black and stress state were determined in the crack tip region. The results showed that higher crystallinity and larger crystalline zone and anisotropic mechanical properties in the crack tip region were responsible for the increased toughness.