Evaluation of impact fracture toughness of polymeric materials by means of the J-integral approach

The most commonly accepted method of determining impact fracture toughness of polymeric materials that exhibit small scale yielding and negligible influence of dynamic effects is given by the ISO/DIS 17281 Standard, which states that for brittle behavior, basically a linear relationship exists between the fracture energy, U, and the energy calibration factor, phi. This relationship allows calculation of the critical strain energy release rate, G(IC), from the slope of the U vs. BWphi plot. This paper describes a simpler alternative methodology capable of evaluating impact fracture toughness using the J(c) parameter. The J-integral is evaluated at the instability load point, by calculating the fracture energy required to produce cleavage behavior of a pre-cracked specimen. The methodology is limited to single edge notched three-point-bending specimens with a crack to depth ratio equal to 0.5. Tests were carried out on an instrumented falling weight impact testing machine on the following materials: PP (polypropylene), HDPE (high-density polyethylene), MDPE (mid-density polyethylene) and RT-PMMA (rubber toughened polymethylmetacrylate). Results are in excellent agreement with the critical values determined by the ISO/DIS 17281 Standard.