ASTM D256 Izod Impact (Notched), ISO 180

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.

The application of this test includes comparing various molding materials, monitoring tolerances as part of quality assurance and goods entry inspections, testing of completed components using machined specimens, making material cards, and measuring the effects of aging.

Test specimen dimensions as defined by ASTM D256-10

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 D256 or ISO 180 gives no specified conditioning time, typically 6 hours conditioning duration and 5 seconds freezing duration are used before impact.

Each specimen’s failure type must be noted under one of the following four categories:

• Complete Break: Specimen splits into two or more fragments.
• Hinge Break: A partially broken specimen that cannot maintain one of its parts above horizontal when the other is held vertically (less than a 90° included angle).
• Partial Break: An incomplete break that does not match the criteria for a hinge break but has cracked at least 90% of the space between the vertex of the notch and the opposing side.
• Non-Break: An incomplete break where the fracture covers less than 90% of the space between one side and the opposite vertex of the notch.

The specification of the material being tested should be consulted before using these test techniques. The materials specification shall take precedence over these test procedures with regard to any test specimen preparation, conditioning, dimensions, and testing parameters. In the absence of a material specification, the default conditions are followed.

Video 01: Digital Izod Impact Tester for ASTM D256, ISO 180

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.

The wall thickness of machined components often define the width for the specimens. The impact test according to ASTM D1822 is used to test specimens taken from thinner wall thicknesses.

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.

Conclusion:

ASTM D256 is a standard test method for determining the Izod Pendulum Impact Resistance of Plastics.