Volume 3, Issue 8 (September 2006)
An Investigation of Specimen Geometry Effects on the Fracture Behavior of a Polytetrafluoroethylene Polymer
Recent interest by the U.S. Navy in metal filled polymers for weapons applications has resulted in a desire to characterize interactions between polymer properties associated with strain rate sensitivity, pressure sensitivity, failure mechanisms, damage mechanisms, spall strength, crystalline response to shock loading, and dynamic fracture mechanics as well as the effects of loading the polymer with various metal fillers . Polytetrafluoroethylene (PTFE) is an important structural polymer that in the “7C” derivative is used for gaskets, bearing pads, piston rings, and diaphragms and when 5μm aluminum spheres are added, has important ballistic properties. Prior work [2–4] evaluated the fracture toughness of PTFE and aluminum filled PTFE over a range of temperatures and loading rates using compact tension (C(T)) specimens and the normalization method [5–7] described in Annex A15 of ASTM E 1820. Moreover, prior work  has shown that filling the PTFE with 10 to 25 % aluminum 5μm spheres can increase stiffness and strength while the effect on toughness can be either positive or negative depending on the loading rate and test temperature. The overall effect on fracture toughness properties derived from adding the aluminum spheres to the PTFE is small and for that reason the unfilled PTFE material was used here to study the effects of geometry on fracture toughness in this material. The normalization method was initially chosen for this project because of the scarcity and cost of specimen material, especially the filled, reactive material, the many different test conditions to be investigated, and the inadequacy of alternative methods. It was quite fortunate that the normalization method was used initially to characterize this polymer since the fracture toughness resistance curves of this material often contains ductile instabilities or “pop-ins” which would not have been well characterized by multispecimen methods. The objective of this work is to compare the fracture toughness measurements obtained from C(T) and precracked Charpy (SE(B)) specimens tested under similar test conditions. A comparison is also made between J-R curves obtained using the single specimen normalization method and the “Basic” or multispecimen procedure. A limited study of the effects of side grooving on the crack extension and on the resulting J-R curves is included here.