Is J-integral at crack onset an intrinsic property of filled elastomers? Experimental testing and finite element modelling for fracture toughness evaluation

The fracture behaviour of two filled NBR based compounds, differing in filler type (carbon black or silica) and content (75 and 60 phr respectively) has been evaluated performing fracture tests under different loading conditions. Single edge notched tension (SENT) tests, tests in pure shear (PS) deformation conditions on single edge notched specimens and equibiaxial (EB) tensile tests on centre-notched square shaped specimens have been carried out at room temperature at a nominal deformation rate of 1 min-1. Different approaches were adopted for the material’s fracture toughness evaluation. From both SENT and PS tests, for which analytical expressions for the tearing energy are available, the material’s tearing energy was evaluated. From PS tests, J-integral at fracture onset was also evaluated, since also this energy-based parameter can be determined analytically in this case. Further, once defined the materials’ constitutive equations, J-integral has been numerically evaluated at fracture onset in uniaxial tensile, PS and EB tests. A comparison between the different parameters have been performed for the two materials in relation to the evaluation method adopted.