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 thicken-ing contributes to the forming of shapes, such as cylindrical flat-bottom cups, by the deep-drawing process. The r value, 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 must be drawn from beneath the blank holder into the die opening.

For many materials this ratio remains essentially constant over a range of plastic strains up to maximum applied force in a tension test. For materials that give different r values at various strain levels, a superscript is used to designate the percent strain at which the r value was measured. For example, if a 20 % elongation is used, the report would show r²⁰.

Materials usually have different r values when tested in different orientations relative to the rolling direction. The angle of sampling of the individual test coupon is noted by a subscript. Thus, for a test specimen whose length is aligned parallel to the rolling direction, the r value would be 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.

A material that has a yield 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 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 r made at low strains.

1. Scope

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 and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

ASTM Standards

E6 Terminology Relating to Methods of Mechanical Testing

E8 Test Methods for Tension Testing of Metallic Materials

E83 Practice for Verification and Classification of Extensometer Systems

E92 Test Method for Vickers Hardness of Metallic Materials

E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods

E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

Keywords

drawability; earing tendency; plastic strain ratio; r ; r value; Ductility--metals/alloys; Metals and metallic materials; Plastic strains; Sheet metal; Steel sheet;

ICS Code

ICS Number Code 77.040.10 (Mechanical testing of metals)

Citing ASTM Standards

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