【国外标准】 Standard Test Method for Tensile Strain-Hardening Exponents (n -Values) of Metallic Sheet Materials

5.1 This test method is useful for estimating the strain at the onset of necking in a uniaxial tension test (1). Practically, it provides an empirical parameter for appraising the relative stretch formability of similar metallic systems. The strain-hardening exponent is also a measure of the increase in strength of a material due to plastic deformation.5.2 The strain-hardening exponent may be determined over the entire plastic stress-strain curve or any portion(s) of the stress-strain curve specified in a product specification.NOTE 4: The engineering strain interval 10–20% is commonly used for determining the strain-hardening exponent, n, of formable low-carbon steel products5.3 This test method is not intended to apply to any portion of the true stress versus true strain curve that exhibits discontinuous behavior; however, the method may be applied by curve-smoothing techniques as agreed upon.NOTE 5: For example, those portions of the stress-strain curves for mild steel, aluminum, or other alloys that exhibit yield point and Lüders band elongation, twinning, or Portevin–Le Chatelier effect (PLC) may be characterized as behaving discontinuously.NOTE 6: Caution should be observed in the use of curve-smoothing techniques as they may affect the n-value.5.4 This test method is suitable for determining the tensile stress-strain response of metallic sheet materials in the plastic region prior to the onset of necking.5.5 The n-value may vary with the displacement rate or strain rate used, depending on the metal and test temperature.1.1 This test method covers the determination of a strain-hardening exponent by tension testing of metallic sheet materials for which plastic-flow behavior obeys the power curve given in the Introduction.NOTE 1: A single power curve may not be a satisfactory fit to the entire stress-strain curve between yield and necking. If such is the case, more than one value of the strain-hardening exponent may be obtained (2) by agreement using this test method.1.2 This test method is specifically for metallic sheet materials with thicknesses of at least 0.005 in. (0.13 mm) but not greater than 0.25 in. (6.4 mm). The method has successfully been and may be applied to other forms and thicknesses by agreement1.3 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.NOTE 2: The value of the strain-hardening exponent, n, has no units and is independent of the units used in its determination1.4 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 and health practices and determine the applicability of regulatory limitations prior to use.