【国外标准】 Standard Test Method for Plastic Strain Ratio r for Sheet Metal

4.1 The plastic strain ratio r is a parameter that indicates the ability of a sheet metal to resist thinning or thickening when subjected to either tensile or compressive forces in the plane of the sheet. It is a measure of plastic anisotropy and is related to the preferred crystallographic orientations within a polycrystalline metal. This resistance to thinning or thickening contributes to the forming of shapes, such as cylindrical flat-bottom cups, by the deep-drawing process. The value of r , therefore, is considered a measure of sheet-metal drawability. It is particularly useful for evaluating materials intended for parts where a substantial portion of the blank is drawn from beneath the blank holder into the die opening. 4.2 For many materials the plastic strain ratio remains essentially constant over a range of plastic strains up to maximum applied force in a tension test. For materials that give different values of r at various strain levels, a superscript is used to designate the percent strain at which the value of r was measured. For example, if a 20 % elongation is used, the report would show r20. 4.3 Materials usually have different values of r when tested in different orientations relative to the rolling direction. The angle of sampling of the individual test specimen is noted by a subscript. Thus, for a test specimen whose length is aligned parallel to the rolling direction, plastic strain ratio, r , is reported as r0. If, in addition, the measurement was made at 20 % elongation and it was deemed necessary to note the percent strain at which the value was measured, the value would be reported as r020. 4.4 A material that has an upper yield strength (yield point) point followed by discontinuous yielding stretches unevenly while this yielding is taking place. In steels, this is associated with the propagation of Lüders' bands on the surface. The accuracy and reproducibility of the determination of plastic strain ratio, r , will be reduced unless the test is continued beyond this yield-point elongation. Similarly, the discontinuous yielding associated with large grain size in a material decreases the accuracy and reproducibility of determinations of plastic strain ratio, r , made at low strains. 1.1 This test method covers special tension testing for the measurement of the plastic strain ratio, r, of sheet metal intended for deep-drawing applications. 1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard. 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.