Characterizing impact behavior of thermoplastics

Conventional impact testing of thermoplastics gives a single numeric criterion of impact behavior in terms of total breaking energy. It is shown that impact behavior is characterized by an elastic phase and, after yielding, a plastic phase may occur. An autographic impact tester was built by modification of a standard pendulum hammer Izod impact unit. Force-time impulse curves show three characteristic categories in breaking; catastrophic, plastic drawing with tearing, and tearing. Photographs of impulse curves on five commercial thermoplastics clearly depict the three breaking categories. The impulse as measured by the area under the force-time curve shows a linear correlation with impact strength. The peak force at yielding exhibits over a 2 to 1 variation among the five commercial plastics. The three materials with highest impact strengths absorb approximately 70% of the total impulse after yielding. These results indicate that impact strength is not a good single measure of the mechanical behavior of the impact properties of thermoplastics.