Reduced Volume Fracture Toughness Characterization of Polycarbonate

Justin Pogacnik
Special Instructions: 
Lunch will be provided.
Friday, February 6, 2009 - 12:00pm
Teer Room 203
Seminar Contact(s): 
Firas Khasawneh
Semester & Year: 
Spring 2009
A combination of experimental testing and finite element modeling is used to characterize fast, unstable fracture in the smallest possible volumes of polycarbonate specimens. Polycarbonate is a transparent engineering polymer noted for its toughness. The U.S. Army is particularly interested in transparent engineering polymers, and new polymers are being synthesized almost daily. This project aims to reliably determine catastrophic failure properties (i.e. fracture toughness) of engineering polymers (such as polycarbonate) on specimens that are smaller than ASTM designated samples. The purpose of this type of work, in general, is to bridge the gap between polymer synthesis and mechanical characterization. It is most desirable that a rapid, simple tool be developed for the prediction or determination of material behavior and failure. In the fracture of glassy polymers, the craze is a region that immediately precedes the crack. The craze consists of polymer fibrils aligned in the direction of applied stress. Voids form in the craze as it grows and the voids eventually coalesce into the crack, which propagates through the specimen. Microscopic analysis is being used with transmitted light, stereo-zoom, and scanning electron (SEM) microscopes with a wide range of magnifications. Microscopic analysis is being used to study the fracture surfaces of failed specimens to find microscopic features, which provide insight into the failure characteristics of the material. The Finite Element Method (FEM) is used and developed to model fracture in this type of problem. We consider methods to model the formation, expansion, and failure of the craze region to model fracture in glassy polymers in a new way. Previously developed nodal release techniques for modeling fracture and allowing crack growth in models that include momentum effects (dynamic simulations) have been used in this work. Further, transparent engineering polymers lend themselves to photoelastic stress analysis. This couples perfectly with finite element analysis as a visual means to analyze the stress-state in specimens and also offers a means to validate finite element results.