The ASTM D256 standard describes the flexural impact tests using the Izod test for determination of the notched impact strength of plastics. Impact tests to Izod are also described in standards ISO 180 and ASTM D4508.
Within the scope of ASTM, the notched impact strength of plastics is normally measured using the Izod test to ASTM D256.The flexural impact stress is applied to a notched specimen that is gripped on one side. The result is presented as the thickness-related energy absorption of the specimen.
The Izod notched flexural impact test to ASTM D256 generates characteristic values for the impact strength and notch sensitivity at high strain rates in the form of a thickness-related energy value. The tests are normally carried out in a normal climate of 23° / 50% relative humidity to ASTM D618.
- The comparison of different molding materials
- Tolerance monitoring within the scope of goods inwards checks and quality assurance
- Testing of finished parts based on machined specimens
- Creation of material cards
- Measurement of aging effects
Izod notched impact tests are also offered in the form of instrumented tests, that is, with a fast force measurement. However, there is no standard for this yet.
The notched flexural impact test to ASTM D256 is used on all plastics to characterize the behavior resulting from impact stresses. The flexural impact stress is applied with edgewise impact to a notched specimen that is gripped on one side. The result is presented as the thickness-related energy absorption of the specimen.
This standard supports various procedures, notch sizes and notch arrangements to test the notch sensitivity of a polymer material
- Method A is used for plastics with an Izod notched impact strength greater than or equal to 27 J/m. In this case, a notch radius of 0.25 mm is used. The result is calculated directly from the height of rise of the pendulum hammer after the impact.
- Method C is used for very brittle plastic materials with an Izod notched impact strength of less than 27 J/m. It corresponds to Method A, however the measured impact energy is corrected by the amount of the calculated centrifugal work of the specimen.
- Method D is used to characterize the notch sensitivity of a polymer material. Here, the notched impact strength is measured on specimens with different notch radii, and the notch sensitivity is calculated as a linear gradient over the notch radius.
- Method E is used to estimate the impact strength on unnotched specimens. For this method, the specimen is gripped rotated by 180° so that the notch is positioned opposite the direction on impact. The result is only conditionally comparable with the test of an unnotched specimen.
For Izod notched impact tests to ASTM D256, pendulum impact testers are used with a pendulum hammer that consists of a pendulum rod and an impactor and the other end of which is attached to a low-friction bearing.
The measurement principle is based on a pendulum hammer with specified energy capacity and drop height, which releases part of its kinetic energy when it penetrates the specimen. As a result, the pendulum hammer does not return to the original drop height after the impact. The measured height difference between drop height and height of rise therefore becomes a measure of the energy absorbed. By determining the drop height, the impact velocity is also defined so that the tests are performed with reproducible strain rates.
The standard pendulum hammer to ASTM D256 has an initial potential energy of 2.7 J at a specified drop height of 610 ±2 mm. Additional hammer sizes result in each case by doubling the initial potential energy at the same drop height. This results in an impact velocity of approx. 3.46 m/s for all pendulum hammers.
Every pendulum hammer can be used for measurements of impact energies up to 85% of its initial potential energy. If several hammer sizes can be used here, the lightest pendulum hammer should be selected in each case.
The specimen is gripped standing on edge in a specified orientation so that the notch is located exactly at the corner of the gripping point, that is, in the area of the highest bending moment. Since the gripping force can affect the result, pneumatic gripping or control of the gripping force is useful.
The type of measurement implies that all energy losses are attributable to the specimen. It is therefore important to minimize, correct, or completely eliminate all external sources for error. There are strict specifications, as well as checks that are part of regular calibrations, regarding friction losses that inevitably occur due to air friction and friction at the bearing points of the pendulum hammer. The correction values are measured and assigned to the respective pendulum hammer. Sufficient instrument mass and a vibration-free installation of the pendulum impact tester on a very stable laboratory table, on a worktop bolted into a solid wall, or a masonry platform are essential for the quality of the measurement. Internal vibrations in the instrument are minimized by design. For this reason, ZwickRoell uses pendulum hammers with double rods made of unidirectional carbon materials, which are very low in mass and at the same time offer optimum stiffness of the pendulum rods, ensuring minimal oscillation losses.