Scope:

ASTM D256 is a standard Izod impact test that determines the resistance of material samples by hitting and breaking a notched sample with a swinging hammer. The standard test utilizes the dynamic energy needed to instigate a fracture and continue until the sample breaks. The samples must be notched as it prevents malformation such as twisting, warping, bending of samples upon the impact. Impact toughness of industry materials is critical for quality control in product applications.

Test Procedure:

The ASTM D256 test method needs a swinging pendulum hammer. The notched side of the specimen is pivoted into the test fixture, facing the impact edge of the pendulum. The pendulum or hammer is swung and allowed to hit the sample. The sample is checked for any breakage. If no breakage occurs, the weight of the hammer is increased until failure takes place. At lower temperatures, materials like thermoplastics show a much lower impact strength. For testing of such materials, temperature simulation is achieved.

Reduced Temperature Test procedure: 

Test specimens are conditioned in a freezer at a specified temperature till they reach the state of equilibrium. Specimens are removed one by one from the freezer and tested. Although ASTM or ISO gives no specified conditioning time, typically 6 hours conditioning duration and 5 seconds freezing duration are used before impact.

Specimen size:

ASTM D256 standard for a specimen is 64 × 12.7 × 3.2 mm (2.5 × 0.5 × 0.125 inch). The thickness of the sample commonly used is 3.2 mm (0.125 inches), but the most desirable thickness, which is less likely to crush, is 6.4 mm (approx 0.25 inches). The depth under the notch of the sample is 10.2 mm (approx 0.4 inches). ISO provides a standard multipurpose sample of Type 1A with terminated end tabs. The final test sample has dimensions of 80 × 10 × 4 mm. Under the notch, the depth of the sample is 8 mm.

Data:

Impact energy as per ASTM D256 is expressed in J/m or ft-lb/in. Calculation of Impact strength is done by dividing impact energy in J (or ft-lb) by the sample’s thickness. An average of 5 samples is taken for the result.

Impact strength as per ISO is expressed in kJ/m2. The calculation involves dividing impact energy in J by the area measured under the notch. An average of 10 samples is taken for the result. 

Higher resulting numbers indicate greater toughness of the material.