AbstractConventional 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 thermoplast
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