Fracture Toughness of High-Impact Polystyrene Based on Three]=Integral Methods

Three J-integral methods and their modified versions have been used to characterize the fracture toughness of high-impact polystyrene (HIPS) with different thicknesses. The J c values obtained were highest from the E813-87 method, followed by the E813-81 method, then by the hysteresis method. The hysteresis method based on the steep rising of hysteresis energy under constant displacement-controlled loading in J c determination has many ad-vantages over the ASTM E813-81 or the E813-87 method. The requirement of crack growth length measurements is no longer necessary and the controversial issue on the crack blunting line can also be avoided. The E813-87 method resulted in significantly higher J c values for polymers, but the modified version of E813-87, by moving the offset line from the original 0.2 to 0.1 mm, resulted in comparable Jc values. Since crazes as the main failure mechanism for HIPS, well-defined crack blunting does not expect to occur and the J c obtained by the original E813-81 based on the theoretically predicted blunting line is indeed slightly higher. The modified version of E813-81 by neglecting the blunting line in J c determination is believed to be more reasonable for HIPS. The nature of polymers will determine whether the crack tip will be blunted, partially blunted, or not blunted. ASTM E813-81 is appropriate for those polymers with a well-defined blunted crack tip (such as elastomer-modified poly-carbonate), whereas the modified version of ASTM E813-81 seems better for those polymers with craze as the main failure mechanism (such as HIPS). Experimental results indicated that this hysteresis method is able to inherently adjust the crack blunting effect and therefore can be applied to any type of ductile polymer. 0 1993 John Wiley & Sons, Inc