ASTM D256 covers an impact test which is used to measure the impact resistance and strength of plastic material. The sample is held by the specimen holder in a vertical position which differs from Charpy impact testing where the sample is held horizontally.
The plastic sample must be cut according to exact specifications. There are several different specimen sizes, however the most common uses a sample of:
Length: 2.5 inches
Width: 0.5 inches
Thickness: .125 inches
The plastic specimen must also be notched with a broaching machine. The notch is cut out of the middle of the sample. The ASTM standard is very specific about the angle of the notch, radius of the notch, and the placement and depth of the notch. Variations in the cut out are the number one cause of variations in the testing data. Therefore, using a broaching machine is recommended instead of using any sort of hand tools. UGC carries a line of broaching machines which can be found in the sample preparation tools section of the website.
Universal Grip sells and supports a complete line of impact testers. The impact hammer that is used is specific to Izod testing and is different than the Charpy hammer. Since plastics have relatively low impact strength, a benchtop impact tester can be used. Our table top impact testing machines are capable of testing with between 1-25 Joules.
Test data calculations are relatively simple for impact testing. The impact resistance or impact strength is given by Joules per Meter (J/m) or foot pounds per inch (ft.lb./in.). An average is taken of at least 5 specimen tests. Generally, multiple lots of 5 are tested.