R Value Sheet Metal Forming

The r and n values are determined in the tensile test in order to characterize the forming properties.

R value sheet metal forming.

The r value of drawing quality steel is typically 1 and that of aluminum sheet 1 so the wrinkling problem is more severe in aluminum. In simple terms r values measure the effectiveness of insulation and a higher number represents more effective insulation. Less radial stretching is required with a high r value decreasing the chance of tearing failure. The r value is the ratio of the true strain in the width direction to the true strain in the thickness direction when a sheet material is pulled in uniaxial tension beyond its elastic limit see figure 1.

In practice the value is usually measured at 20 elongation in a tensile test. If a material has a higher r value it is easy to avoid plane strain deformation and the material formability is higher such as mild steel. The constant n plays a crucial role in sheet metal forming. For sheet metals the r displaystyle r values are usually determined for three different directions of loading in plane 0 45 90 displaystyle 0 circ 45 circ 90 circ to the rolling direction and the normal r value is taken to be the average.

The r value is an index that the material can resist thinning deformation in thickness direction. The larger the n value the more the material can elongate before necking. The plastic strain ratio r is considered a direct measure of sheet metal s drawability and is useful for evaluating materials intended for forming shapes by deep drawing see lead photo. R value also known as plastic strain ratio is a measurement of the drawability of a sheet metal.

It is the ability to maintain thickness as the material is drawn. In construction the r value is the measurement of a material s capacity to resist heat flowfrom one side to the other. The effect of the n value in deep drawing is ambiguous. For instance if you have a material with an r value of 12 attached to another material with an r value of 3 then both materials combined have an r value of 15.

Thus as the n value increases the material s resistance to necking increases and the material can be stretched farther before necking starts. R value is a very important material property to understand for materials that will be formed into various shapes in their end use.