Volume 31, Issue 2 (March 2003)
Fracture Toughness Testing of Polymers Using Small Compact Specimens and Normalization
Fracture toughness tests were conducted on a polymeric material used in medical implants. The fracture behavior was ductile so that the toughness was measured using a J-R curve and JIc. The object of the test program was to compare the fracture toughness properties for different material treatments. The focus of this paper is to describe the procedure used to test the materials. Small specimens of the compact geometry are tested. Many parts of the procedure were similar to the standard test method used for metallic materials, but three issues that required a different approach were the focus of this work: the introduction of an initial defect, the testing rate, and the measurement of crack extension. The crack was introduced with a pressed razor blade cut, the loading rate was maintained constant at a rate of 0.06 in./s (0.15 mm/s) and the method of normalization was used to measure crack extension.
The results showed that the razor cut procedure for introducing the initial defect gave comparable toughness results to a specimen that was precracked in fatigue. The loading rate must be maintained over a closer range than the one used for metal testing. Metal testing allows a loading rate range of two orders of magnitude, whereas rate-influenced differences in toughness were observed for these polymer materials over this range. The methods of crack extension measurement used in the standard for metallic materials do not work for polymers. The method of normalization can give comparable results to that of the multiple specimen method used as the basic method for measuring JIc. The procedures studied in this work could form the basis for developing a standard fracture toughness test for polymeric materials.