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Fracture toughness of polymers in the ductile-to-brittle transition region: Statistical approach and lower bound determination



A method available in literature was adapted and proposed for treating scatter and nonlinearity effects in fracture toughness of polymers in the ductile-to-brittle transition regime. The materials used were polypropylene homopolymer (PPH) and a polypropylene-elastomeric polyolefin blend (PPH/POes 20 wt %), at room temperature and at 20-mm/min test rate. Under such conditions, the fracture toughness presents a large scatter and a mean value can not be used as a design parameter because it leads to toughness overestimation. Then, there is a need to find a threshold of toughness, as a safe characteristic value for design. The toughness was evaluated by using the J-integral approach. Large sets of specimens, 53 samples per each material, were tested with the purpose to reveal a reliable tendency in fracture behavior. As the toughness was considered nonuniform throughout the material, a weakest link model was assumed, and then results were analyzed statistically by means of a three-parameter Weibull model (3P-W). The PPH responded well to this 3P-W model, whereas some deviations from the original model were observed in the PPH/POes blend. However, lower-bound toughness values could be determined for both materials by censoring nonvalid data (Δa > 0.1b0). From an engineering point of view, the results are very encouraging, since this methodology allows to obtain a threshold of fracture toughness from a given population, that is suitable to characterize the material fracture toughness at a given temperature and strain rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3674–3684, 2005