The width-related bending stiffness S describes the resistance of a specimen to bending. In the elastic deformation range, it is the product of the modulus of elasticity E valid for bending and the area moment of inertia I of the sample.
The bending stiffness is an important parameter for the further processing of paper and board. The stiffness or flexibility of the paper can have a detrimental effect on processing operations in packaging machines and lead to malfunctions.
The determination of the width-related bending stiffness S is carried out according to DIN 53121, the four-point method. For this purpose, the specimen, which is supported on both sides, is stressed by two individual forces acting symmetrically between these supports. The measurements are only carried out in the elastic range. No plastic deformations may be recorded. 10 measured values each are determined in the longitudinal and transverse directions, whereby these are subdivided again into 5 with the upper side facing upwards and 5 with the lower side facing upwards. The measurement results are determined in Nm and indicated separately for longitudinal and transverse direction. An average value is calculated from each of the 10 individual measurements in each direction.