ISO 5629:2017 specifies a method for the determination of bending stiffness using the resonance method.
This method gives only a single result for each test piece tested, although in general, bending stiffness values may depend on which surface of the material forms the inner surface of the arc resulting from bending forces.
The bending stiffness of a very wide range of papers and boards can be measured using this method. However, for some materials, such as those listed below, the measured numerical values will not be strictly identical with bending stiffness as defined in 3.1:
a) multi-ply papers and boards in which the component plies can move separately during a test;
b) papers and boards with appreciable curl, especially if the axis of the curl is in the long direction of the test piece;
c) some soft papers with grammage below about 40 g/m2.
This method is not applicable to corrugated boards.