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

Abstract

AbstractA 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