Izod Impact Strength Testing


Izod Impact Testing (Notched Izod) is a test to determine notch sensitivity in plastics.

ASTM D256 – 10(2018) Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics

ISO 180:1993 Plastics Determination of Izod Impact Strength of Rigid Materials



Izod impact testing is an ASTM standard method of determining the impact resistance of materials. A pivoting arm is raised to a specific height (constant potential energy) and then released. The arm swings down hitting a notched sample, breaking the specimen. The energy absorbed by the sample is calculated from the height the arm swings to after hitting the sample. A notched sample is generally used to determine impact energy and notch sensitivity.

The test is similar to the Charpy impact test but uses a different arrangement of the specimen under test. The Izod impact test differs from the Charpy impact test in that the sample is held in a cantilevered beam configuration as opposed to a three-point bending configuration.

Sample Submission Instructions

The standard specimen for ASTM is 64 x 12.7 x 3.2 mm (2½ x ½ x 1/8 inch). The most common specimen thickness is 3.2 mm (0.125 inch), but the preferred thickness is 6.4 mm (0.25 inch) because it is not as likely to bend or crush. The depth under the notch of the specimen is 10.2 mm (0.4 inches).

The standard specimen for ISO is a Type 1A multipurpose specimen with the end tabs cut off. The resulting test sample measures 80 x 10 x 4 mm. The depth under the notch of the specimen is 8mm.

Significance and Use

5.1 Before proceeding with these test methods, reference should be made to the specification of the material being tested. Any test specimen preparation, conditioning, dimensions, and testing parameters covered in the materials specification shall take precedence over those mentioned in these test methods. If there is no material specification, then the default conditions apply.

5.2 The pendulum impact test indicates the energy to break standard test specimens of specified size under stipulated parameters of specimen mounting, notching, and pendulum velocity-at-impact.

5.3 The energy lost by the pendulum during the breakage of the specimen is the sum of the following:

5.3.1 Energy to initiate fracture of the specimen;

5.3.2 Energy to propagate the fracture across the specimen;

5.3.3 Energy to throw the free end (or ends) of the broken specimen (“toss correction”);

5.3.4 Energy to bend the specimen;

5.3.5 Energy to produce vibration in the pendulum arm;

5.3.6 Energy to produce vibration or horizontal movement of the machine frame or base;

5.3.7 Energy to overcome friction in the pendulum bearing and in the indicating mechanism, and to overcome windage (pendulum air drag);

5.3.8 Energy to indent or deform plastically the specimen at the line of impact; and

5.3.9 Energy to overcome the friction caused by the rubbing of the striker (or other part of the pendulum) over the face of the bent specimen.

5.4 For relatively brittle materials, for which fracture propagation energy is small in comparison with the fracture initiation energy, the indicated impact energy absorbed is, for all practical purposes, the sum of factors 5.3.1 and 5.3.3. The toss correction (see 5.3.3) may represent a very large fraction of the total energy absorbed when testing relatively dense and brittle materials. Test Method C shall be used for materials that have an Izod impact resistance of less than 27 J/m (0.5 ft·lbf/in.). (See Appendix X4 for optional units.) The toss correction obtained in Test Method C is only an approximation of the toss error, since the rotational and rectilinear velocities may not be the same during the re-toss of the specimen as for the original toss, and because stored stresses in the specimen may have been released as kinetic energy during the specimen fracture.

5.5 For tough, ductile, fiber filled, or cloth-laminated materials, the fracture propagation energy (see 5.3.2) may be large compared to the fracture initiation energy (see 5.3.1). When testing these materials, factors (see 5.3.2, 5.3.5, and 5.3.9) can become quite significant, even when the specimen is accurately machined and positioned and the machine is in good condition with adequate capacity. (See Note 7.) Bending (see 5.3.4) and indentation losses (see 5.3.8) may be appreciable when testing soft materials.

NOTE 7: Although the frame and base of the machine should be sufficiently rigid and massive to handle the energies of tough specimens without motion or excessive vibration, the design must ensure that the center of percussion be at the center of strike. Locating the striker precisely at the center of percussion reduces vibration of the pendulum arm when used with brittle specimens. However, some losses due to pendulum arm vibration, the amount varying with the design of the pendulum, will occur with tough specimens, even when the striker is properly positioned.

5.6 In a well-designed machine of sufficient rigidity and mass, the losses due to factors 5.3.6 and 5.3.7 should be very small. Vibrational losses (see 5.3.6) can be quite large when wide specimens of tough materials are tested in machines of insufficient mass, not securely fastened to a heavy base.

5.7 With some materials, a critical width of specimen may be found below which specimens will appear ductile, as evidenced by considerable drawing or necking down in the region behind the notch and by a relatively high-energy absorption, and above which they will appear brittle as evidenced by little or no drawing down or necking and by a relatively low-energy absorption. Since these methods permit a variation in the width of the specimens, and since the width dictates, for many materials, whether a brittle, low-energy break or a ductile, high energy break will occur, it is necessary that the width be stated in the specification covering that material and that the width be reported along with the impact resistance. In view of the preceding, one should not make comparisons between data from specimens having widths that differ by more than a few mils.

5.8 The type of failure for each specimen shall be recorded as one of the four categories listed as follows:

C = Complete Break—A break where the specimen separates into two or more pieces.
H = Hinge Break—An incomplete break, such that one part of the specimen cannot support itself above the horizontal when the other part is held vertically (less than 90° included angle).
P = Partial Break—An incomplete break that does not meet the definition for a hinge break but has fractured at least 90 % of the distance between the vertex of the notch and the opposite side.
NB = Non-Break—An incomplete break where the fracture extends less than 90 % of the distance between the vertex of the notch and the opposite side.

For tough materials, the pendulum may not have the energy necessary to complete the breaking of the extreme fibers and toss the broken piece or pieces. Results obtained from “non-break” specimens shall be considered a departure from standard and shall not be reported as a standard result. Impact resistance cannot be directly compared for any two materials that experience different types of failure as defined in the test method by this code. Averages reported must likewise be derived from specimens contained within a single failure category. This letter code shall suffix the reported impact identifying the types of failure associated with the reported value. If more than one type of failure is observed for a sample material, then the report will indicate the average impact resistance for each type of failure, followed by the percent of the specimens failing in that manner and suffixed by the letter code.

ASTM Standards

  • D618 Practice for Conditioning Plastics for Testing
  • D883 Terminology Relating to Plastics
  • D3641 Practice for Injection Molding Test Specimens of Thermoplastic Molding and Extrusion Materials
  • D4066 Classification System for Nylon Injection and Extrusion Materials (PA)
  • D5947 Test Methods for Physical Dimensions of Solid Plastics Specimens
  • D6110 Test Method for Determining the Charpy Impact Resistance of Notched Specimens of Plastics
  • E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

ISO Standard

ISO 180:1993 Plastics Determination of Izod Impact Strength of Rigid Materials

Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.

ICS Number Code 29.035.20 (Plastic and rubber insulating materials)

UNSPSC Code 13102000(Thermoplastic plastics)

DOI: 10.1520/D0256-10R18



ASTM D256-10(2018), Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics, ASTM International, West Conshohocken, PA, 2018, www.astm.org