Determining crack initiation points in polymeric materials

For fracture toughness measurements, accurate identification of crack initiation is of critical importance. Maximum load point criteria can work well for brittle materials but may be inappropriate for other materials. The 5% compliance intercept criterion stipulated in the ASTM E399 standard for metals gives reproducible data but is not necessarily related to the actual fracture process in polymers. Visual observation techniques are inherently related to the fracture process but may be somewhat subjective. Crack resistance values based on the above criteria are measured for polystyrene, polypropylene, rubber modified polymethylmethacrylate, and a nylon 6/ABS blend. It is found that the method of determining characteristic points can affect the generated fracture characterizing parameters.