Fracture toughness of filled polypropylene copolymer systems

The impact strength to stiffness balance of a toughened polypropylene copolymer was modified through the addition of mineral fillers. The stiffness was improved over the base resin value for all formulations. However, the impact strength exhibited complex behavior. The fracture toughness G c calculated using the linear elastic fracture mechanics theory was indicative of the materials resistance to crack propagation. The G c values were modified significantly with the addition of fillers and for some formulations was greater than the base resin value. The LEFM analysis indicates that this is due to an increase in the damage zone size r p where the energy absorbing mechanisms are concentrated. However, the specific energy absorbed per unit volume decreased with the addition of fillers. The total energy to fracture measured using unnotched samples was indicative of crack initiation and crack propagation energies. This upper bound value decreased for all formulations indicating a reduction in the crack initiation resistance, in the presence of stress concentrating heterogeneities in the filled systems.