Strategies to fracture characterize engineering thermoplastics

FRONTINI, PATRCIA MARÍA; FASCE, LAURA ALEJANDRA; RUEDA, FEDERICO

Conferencia; 5th Bianual Polymers and Moulds Innovations conference, PMI 2012; 2012

New plastic parts with improved fracture performance and enhanced reliability can be produced in the near future only if fracture behavior of involved materials is perfectly known. Many commercially significant thermoplastics exhibit abrupt changes in fracture behavior from ductile to brittle ? or vice versa ? over relatively narrow intervals of testing or structural variables. The discontinuous transition from unstable to stable crack propagation has been described as ?brittle to ductile transition?. This pattern is characterized by a random spatial fluctuation of material toughness characteristics due to the occurrence of both, brittle and ductile failure micromechanisms simultaneously. Assuming that the fracture resistance can be measured in terms of a single critical parameter, this problem is not amenable to a conventional Linear or Non-Linear Fracture Mechanics analysis. A consistent description of the entire range of sample behavior may be done by using the statistical weakest link model. The Weibull statistical method is capable of determining the toughness threshold of polymers displaying ductile-to-brittle behavior. Typically, fracture toughness of ductile polymers is usually measured by the incremental crack growth resistance curve, J?R, where J-Integral value is plotted as a function of crack extension (the so-called Multiple Specimen approach). This procedure is hard to apply in situations such as under high-loading rates, elevated temperatures and/or aggressive environments conditions. Alternatively, Normalization Data Reduction technique has the inherent advantage of developing J?R curves directly from a single load versus load-line displacement record without using any sophisticated automated crack length monitoring system.