Значение и использование Test Methods A and B are used to establish the weight of the dart when 50 % of the specimens fail under the conditions specified.Data obtained by one test method cannot be compared directly with the other test method nor with those obtained from tests employing different conditions of missile velocity, impinging surface diameter, effective specimen diameter, and thickness. The values obtained by these test variables are highly dependent on the method of film fabrication.

The results obtained by Test Methods A and B are greatly influenced by the quality of film under test. The confidence limits of data obtained by this procedure can, therefore, vary significantly, depending on the sample quality, uniformity of film gage, die marks, contaminants, etc. (see Section 15).

Test Methods A and B have been found useful for specification purposes. Correlation between test results and field performance can usually be established.

The impact resistance of plastic film, while partly dependent on thickness, has no simple correlation with sample thickness. Hence, impact values cannot be normalized over a range of thickness without producing misleading data as to the actual impact resistance of the material. Data from these test methods are comparable only for specimens that vary by no more than ±25 % from the nominal or average thickness of the specimens tested.

Several impact test methods are used for film. It is sometimes desirable to know the relationships among test results derived by different test methods. A study was conducted in which four films made from two resins (polypropylene and linear low-density polyethylene), with two film thicknesses for each resin, were impacted using Test Methods D 1709 (Method A), D 3420 (Procedures A and B), and D 4272. The test results are shown in the Appendix. Differences in results between Test Methods D 1709 and D 4272 may be expected since Test Methods D 1709 represents failure initiated energy, while Test Method D 4272 is initiation plus completion energy. Some films may show consistency when the initiation energy is the same as the total energy. This statement and the test data also appear in the significance sections and appendixes of Test Methods D 3420 and D 4272.

Область применения

1.1 These test methods cover the determination of the energy that causes plastic film to fail under specified conditions of impact of a free-falling dart. This energy is expressed in terms of the weight (mass) of the missile falling from a specified height which would result in 50 % failure of specimens tested.

1.2 Two test methods are described:

1.2.1 Test Method A employs a dart with a 38.10 ± 0.13-mm (1.500 ± 0.005-in.) diameter hemispherical head dropped from a height of 0.66 ± 0.01 m (26.0 ± 0.4 in.). This test method may be used for films whose impact resistances require masses of about 50 g or less to about 2 kg to fracture them.

1.2.2 Test Method B employs a dart with a 50.80 ± 0.13-mm (2.000 ± 0.005-in.) diameter hemispherical head dropped from a height of 1.50 ± 0.03 m (60.0 + 0.25, −1.70 in.). Its range of applicability is from about 0.3 kg to about 2 kg.

1.3 Two testing techniques are described:

1.3.1 The standard technique is the staircase method. By this technique, a uniform missile weight increment is employed during test and the missile weight is decreased or increased by the uniform increment after test of each specimen, depending upon the result (fail or not fail) observed for the specimen.

1.3.2 The alternative technique provides for testing specimens in successive groups of ten. One missile weight is employed for each group and missile weight is varied in uniform increments from group to group.

1.3.3 The staircase technique and the alternative technique give equivalent results both as to the values of impact failure weight which are obtained and as to the precisions with which they are determined.

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

Note 1—Tests on materials that do not break, for any reason, are not considered to be valid. It has been noted that certain materials may stretch so far as to bottom out at the base of certain test instruments without actually rupturing. Subcommittee D20.19 is currently considering methods for testing these materials. Anyone interested in participating in a Task Group should contact the Chairman of Subcommittee D20.19 through ASTM International Headquarters.

Note 2—This test method is technically equivalent to ISO 7765-1: 1988, with the exception of a larger tolerance on the drop height in Test Method B. Also, the ISO method does not allow the alternative testing technique described in Section 11 of this test method.

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