Page 50 - Metalorming Magazine January/February 2023

the applied stress rises above the mate- rial’s yield strength.
With increasing n-value, the sheet metal strengthens with each increment of strain. If the applied stress from deformation remains below the now- increased strength, the part continues to deform without necking or cracking. Materials with a higher n-value strengthen to a greater extent for each strain increment, which is why grades with higher n-value can form more complex stampings before failing.
Areas strained from forming increase in strength greater than adja- cent flatter regions. This locally higher strength helps resist deformation, with deformation concentrating in the lower-strained areas. Parts formed from higher n-value alloys experience more strengthening even in these low-strain regions, transferring the deformation further away and thereby minimizing strain localization over a larger portion of the stamping. More-complex stamp-
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Higher-strength steels with a lower n-value cannot accommodate as much deformation before the onset of local necking and fracture. Lower n-value also leads to a faster localization of deformation, further promoting early failure.
Some alloys, such as 3XX-series stainless steels and 5XXX-series alu- minum alloys, can achieve additional cold reduction—called tempering or strain hardening—to increase strength. Accompanying this strength increase is a decrease in formability, as the cold- rolling process uses up some of the alloys’ strain-hardening capacity.
Measurement Range Makes a Difference
ASTM E646, Standard Test Method for Tensile Strain-Hardening Exponents (n -Values) of Metallic Sheet Materials, describes the steps required to calcu-
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late n-value from tensile-test data. Although n-value derives from the slope of the log true stress-log true strain curve in strain ranges below uni- form elongation, for some alloys this slope changes based on the strain range over which calculation occurs. The standard range of 10- to 20-percent strain evolved from when low-carbon steels were the primary sheet metal used in many applications.
Advanced high-strength steels such as dual-phase (DP) grades have high n-value at lower strain ranges, then level off to values approximating that of high-strength, low-alloy steels at the same incoming yield strength. A higher n-value at lower strains minimizes strain-gradient formation and is the key reason for the improved formability seen in DP steels. Some industry and OEM standards require n-value report- ing at both low and high strain ranges to ensure the capturing of this char- acteristic. MF
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