Problems in fracture mechanics characterization of rubber-modified glassy polymers, using double torsion

Abstract

The double torsion fracture test was tentatively applied to some rubber-modified glassy polymers. By varying the specimen thickness, a substantial variation of the fracture toughness values was obtained, which cannot be attributed to variations of material's properties. Two sorts of problems were encountered, due to the large deformations attained during the test. Firstly, large deformations imply variations in the load moment arm since radii of load points are finite; secondly, system and/or material non-linearities, that are outside the scope of the classic theory of double torsion, may become significant. The application of a purely geometrical correction factor, to account for the large deformations, somewhat reduces, but does not eliminate, the observed thickness dependence of fracture toughness.