Determination of JIc for Polymers Using the Single Specimen Method

Abstract

The fracture toughness of several polymes is characterized using the J-integral method. The single specimen method has been applied, and the tests were conducted on three-point bend specimens of polyvinylidine difluoride, medium and high density polyethylene. The results are compared with the J-R curves obtained from the multiple specimen method, which is often used for polymers and has proved to be successful. This has enabled a good degree of confidence be attached to the single specimen method. For each type of polymer, the J-R curves obtained from both methods are in good agreement. Hysteresis loops are observed during unloading and reloading because of the viscoelastic nature of the polymers, and the shape of the loops was affected by the friction at the support rollers. The distinction of JIc values determined according to the ASTM Standards E 813-81 and E 813-87 is shown. The E 813-87 procedure predicts a more consistent JIc value than the E 813-81, but the adoption of the 0.2 mm offset blunting line does not reflect the actual initiation point and the advantages and drawbacks of each method are discussed. The blunting line suggested in the E 813 protocols is found to be inappropriate for describing the blunting mechanisms of polymers. Some tests were performed at different loading rates to determine the rate sensitivity of both the JIc values and J-R curves. It is suggested that the maximum allowable crack extension recommended for the J controlled growth condition is too conservative for polymers, and it is noted that the crack growth is difficult to measure accurately. A maximum allowable crack extension of 10% of the uncracked ligament is considered to be appropriate.