ASTM D256 is a standard test method for determining the impact resistance of plastics using a pendulum impact testing machine. This test method is used to evaluate the ability of plastics to withstand impact or shock loads and to quantify their impact resistance. The test involves preparing a standardized test specimen and positioning it on a test fixture.

Last Updated: September 18th, 2023 First Published :


What Izod Impact Testing Is ASTM D256?

In the Izod Pendulum Impact Resistance of Plastics test, a plastic specimen is broken with a hammer resembling a pendulum to determine its resistance and tensile strength. The Charpy impact test and this test are comparable.

These pivoting hammer machines must adhere to particular specifications, such as having a set hammer height. At the time of impact, a fixed velocity is produced by the height. Hammers of varied weights can provide diverse beginning energy to test samples with various levels of impact resistance. There is also room for different lengths, pendulum configurations, and rigidities.

A machined notch with a certain pattern and a defined length and depth must be used to create the specimens. The notch aids in focusing the strain and guiding the fracture caused by the swinging arm. It guarantees that the break happens behind the notch and reduces plastic distortion.

In Method A testing at Infinita Labs, the notch creates a specific stress concentration to raise the likelihood of a brittle fracture as opposed to a ductile one.

ASTM D256 Requirements

The Izod impact test requires five samples before an average can be calculated. The plastic sample needs to be cut according to these requirements:

  • It is 2.5 inches long.
  • 5 inches across.
  • The thickness is 125 inches.

The sample’s center has to have a notch cut out of it. For the notch’s angle, radius, location, and depth, accuracy is necessary. To help obtain the necessary accuracy, a broaching machine is advised.

The impact energy and notch sensitivity within the specimen is determined by the energy absorbed by the sample. The test findings are given as the amount of energy absorbed per unit of thickness. The units of measurement for the results are joules per centimeter (J/cm) or foot-pounds per inch ( As joules per meter squared (J/m2) or foot-pounds per inch squared (, they can also be expressed as the energy absorbed per unit of the cross-sectional area under the notch.

Test parameters that could have an impact on test outcomes include:

  • Fabrication process
  • Superior quality
  • Between notching and testing period
  • Test object thickness
  • Width below the notch
  • Conditioned by the environment.

Video 01:ASTM D256 Izod Impact Strength Test – QC-639R(U) Computerized Impact Tester

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