Plane strain fracture toughness testing and the fracture morphology of polycarbonate

The critical stress intensity factor, K$\sb{\rm Ic}$, is widely used as a measure of plane strain fracture toughness of materials. Even though a standard procedure for estimating K$\sb{\rm Ic}$ for metals is outlined in ASTM standard E399-83 "Plane-strain fracture toughness of metallic materials", no such standard is available for polymers. One goal of the present research was the development of a standard fracture toughness testing technique for polymers, which required a study of the specimen geometry and an understanding of the sensitivity of the measurements to the dimensions of the specimen and also to other test parameters. Such an understanding should be aided by a study of the fracture surface morphology, which was expected to be a function of the specimen dimensions, test parameters and the measured fracture toughness.

The standard compact tension specimen (CTS), which is commonly used for the fracture toughness testing for metals, was used as a starting point for this study. The fracture tests were performed on an Instron tensile testing machine, using LEXAN$\sp\circler$ polycarbonate as a prototype for tough engineering plastics. The effects of various geometric parameters were investigated. Different precracking techniques also were evaluated. In addition, the effects of loading rate, test temperature and thermal history of the material on the fracture process were studied. The fracture morphology was interpreted using both optical and scanning electron microscopy.

The results of this investigation indicate that the initial crack length, a, and the ligament length, W, do not significantly affect the measured fracture toughness of polycarbonate as long as the a/W ratio is at least 0.3 and W satisfies the specifications outlined in ASTM standard E399-83. The fracture toughness was observed to decrease with increasing crosshead speed of the testing machine. Also, the fracture toughness increased with increasing test temperature but decreased with thermal aging.

The fracture morphology was determined to be a function of the a/W ratio, the test temperature and the sharpness of the precrack. On the basis of this study a 'critical-thickness-craze crack interaction' model (CCI) was developed for the micromechanical processes that determine the fracture morphology of brittle fracture in polycarbonate.