FRACTURE-TOUGHNESS AND FRACTURE MECHANISMS OF PBT PC IM BLEND .1. FRACTURE PROPERTIES

The static and impact fracture toughnesses of a polybutylene terephtha late/polycarbonate/impact modifier (PBT/PC/IM) blend were studied at d ifferent temperatures. The static fracture toughness of the blend was evaluated via the specific fracture work concept and the J-integral an alysis. A comparison of these two analytical methods showed that the s pecific essential fracture work, w(e), was equivalent to the J(IC-81) obtained by the ASTM E813-81 procedure, representing the crack initiat ion resistance of the material. The discrepancy between w(e) and J(IC- 89) of ASTM E81 3-89 was caused by the extra energy component in J(IC- 89) consumed by a 0.2 mm crack growth. Impact fracture toughness was a lso analysed using the specific essential fracture work approach. When the fracture was elastic, w(e) was equivalent to the critical potenti al energy release rate, G(IC), obtained via LEFM analysis. Temperature and strain-rate effects on the fracture toughness were also studied. The increase in impact toughness with temperature was attributed to tw o different toughening mechanisms, namely, the relaxation processes of the rubbery particles and the parent polymers in a relatively low-tem perature range and thermal blunting of the crack tip at higher tempera tures. The enhancement in static fracture toughness at temperatures be low - 60-degrees-C was thought to be caused by plastic crack-tip blunt ing, but the monotonic reduction in yield stress was largely responsib le for the toughness decreasing with higher temperatures. The temperat ure-dependent fracture toughness data obtained in static tests could b e horizontally shifted to match roughly the data for the impact tests, indicating the existence of a time temperature equivalence relationsh ip.