ASTM D256-10(2018)

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:

5.4 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.

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:

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.

5.9 The value of the impact methods lies mainly in the areas of quality control and materials specification. If two groups of specimens of supposedly the same material show significantly different energy absorptions, types of breaks, critical widths, or critical temperatures, it may be assumed that they were made of different materials or were exposed to different processing or conditioning environments. The fact that a material shows twice the energy absorption of another under these conditions of test does not indicate that this same relationship will exist under another set of test conditions. The order of toughness may even be reversed under different testing conditions.

1.1 These test methods cover the determination of the resistance of plastics to “standardized” (see Note 1) pendulum-type hammers, mounted in “standardized” machines, in breaking standard specimens with one pendulum swing (see Note 2). The standard tests for these test methods require specimens made with a milled notch (see Note 3). In Test Methods A, C, and D, the notch produces a stress concentration that increases the probability of a brittle, rather than a ductile, fracture. In Test Method E, the impact resistance is obtained by reversing the notched specimen 180° in the clamping vise. The results of all test methods are reported in terms of energy absorbed per unit of specimen width or per unit of cross-sectional area under the notch. (See Note 4.)

1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses are for information only.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.