Uniform standards for measuring mechanical properties of plastics will lead to consistent test data. However, for the design engineer, the usefulness of the data in predicting part performance is also very important. If the data is dependent on test specimen geometry or the type of loading, it cannot be used to predict component behavior. Data reported for flexural strength of unfilled and fiber-filled thermoplastics is about 50% greater than the tensile strength. This is simply a miscalculation of the flexural strength since elastic beam equations are used for the nearly fully plastic behavior of thermoplastics. The proper flexural strength calculation would be 2/3 times the elastic beam equation. The flexural strength would then be about the same as the tensile strength. Also, gate location and its effect on part deflection for fiber-filled materials can be treated with an orthotropic stress analysis. Izod and Charpy impact tests are simply two different beam bending loads (cantilever and 3-point bending) yielding single-point information. To predict ductile/brittle behavior of parts, geometry can be captured with a geometric severity factor and compared to the strength ratio―the material's critical maximum principal stress at the appropriate strain rate and temperature divided by the yield stress. Creep data displayed as "effective modulus" graphs provide useful geometry-independent design information for situations where time and temperature are important. HDT is simply a geometry and loading dependent temperature that is not useful for design purposes. The plastics industry must strive to develop standard mechanical tests that are independent of specimen geometry and thus useful to the design engineer who is responsible for part performance.
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