To characterize forming properties, the r and n values are often determined in tensile tests.
ISO 10275 specifies a method for determining the hardening exponent, often referred to as the n-value, on flat metal specimens, usually sheet or strip. To calculate the n-value, a test is carried out in accordance with ISO 6892 and a stress-strain diagram is constructed using an axial strain gauge. The n-value is calculated over the entire range of uniform plastic strain - normally from the beginning of the range of uniform hardening, after the yield point is exceeded, to just before the point at which the tensile strength Rm is calculated.
The r-value is the ratio of logarithmic width deformation to logarithmic thickness change. The same conditions apply to the sample position as for determining the n-value. An average r-value is calculated from the individual values held at angles of 0°, 45° and 90° to the rolling direction. The r-value can be used to determine whether materials (sheets) behave anisotopically. Anisotopy becomes clear when r0, r45 and r90 differ from one another.
n-values and r-values are determined exclusively on thin sheets. Thin sheets are sheets with a product thickness of max. 3 mm.
The n-value is an indicator of the hardening (increase in stress) in the ongoing tensile test (work hardening in the forming process). The determined r-value is an indicator of the vertical anisotropy. Typical values for metals (sheet metal) are 0,8 - 2,2.
The N value is determined from the dependence of tensile stress and strain values.
The R-value requires an additional width change sensor to measure the “waist formation”.
