Effects of thermal treatments on the fracture behaviour of a silica-filled NBR: A study on the recovery of the Mullins effect and on the stretch-induced cavity formation

This research work is focused on the study of the fracture behaviour of a silica-filled NBR. The Mullins effect and its thermally-induced recovery are initially studied performing tensile tests: comparing the material's response in uniaxial tension and pure shear deformation conditions, a lower recovery is observed in the pure shear configuration. A similar study is then performed considering the fracture behaviour of the material. Tests are performed in quasi-static loading conditions and by applying a fracture mechanics approach. The fracture toughness is evaluated as J-integral at the crack onset and at the unstable fracture initiation; further, the crack propagation is analysed and the resistance curve is obtained. The dependence of the overall fracture behaviour of the material on the applied thermal history is evaluated. Even though the crosslinking degree seems not to be affected by the thermal treatment, the fracture behaviour is modified. A correlation of these results with a change in the material's propensity to form cavities under stretching is proposed.