【国外标准】 Standard Test Method for Tear Resistance (Graves Tear) of Plastic Film and Sheeting

5.1 Tear resistance of plastic film or sheeting is a complex function of its ultimate resistance to rupture. The specimen geometry and speed of testing in this test method are controlled to produce tearing in a small area of stress concentration at rates far below those usually encountered in service. Experience has shown the test to have its best reliability for materials which do not have brittle failure or do not elongate greater than two hundred percent during testing.5.2 The data from this test method furnish comparative information for ranking the tearing resistance of plastic specimens of similar composition. Actual use performance in tearing of some plastics may not necessarily correlate with data from this test method.5.3 The resistance to tear of plastic film and sheeting, while partly dependent upon thickness, has no simple correlation with specimen thickness. Hence, tearing forces measured in newtons (or pounds-force) cannot be normalized over a wide range of specimen thickness without producing misleading data as to the actual tearing resistance of the material. Data from this test method are comparable only from specimens, which vary by no more than ±10 % from the nominal or average thickness of all specimens tested. Therefore, the tearing resistance is expressed in maximum newtons (or pounds-force) of force to tear the specimen.5.4 The tear resistance of plastic film may be a specification that requires the use of this test method, but with some procedural modifications that take precedence when adhering to the specification. Therefore, it is advisable to refer to that material specification before using this test method. Table 1 of Classification System D4000 lists the ASTM materials standards that currently exist.1.1 This test method2 covers the determination of the tear resistance of flexible plastic film and sheeting at very low rates of loading, 51 mm (2 in.)/min. and is designed to measure the force to initiate tearing. The specimen geometry of this test method produces a stress concentration in a small area of the specimen. The maximum stress, usually found near the onset of tearing, is recorded as the tear resistance in newtons (or pounds-force). The method is not applicable for film or sheeting material where brittle failures occur during testing or where maximum extension is greater than 101.6 mm (4 in.).1.1.1 Although resistance to tear can be expressed in newtons per microns, (pounds-force per mil) of specimen thickness, this is only advisable where correlation for the particular material being tested has been established. In most cases, comparison between films of dissimilar thickness is not valid.NOTE 1: Film has been arbitrarily defined as sheeting having nominal thickness not greater than 0.25 mm (0.010 in.).1.2 Constant-Rate-of-Grip Separation Test—This test method employs a constant rate of separation of the grips holding the test specimen.1.2.1 Specimen extension shall be measured in this test method by grip separation.1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are provided for information only.1.4 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.NOTE 2: There is no known ISO equivalent to this standard.1.5 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.