ISO 5628:2012 specifies three test methods for determining the bending stiffness of paper and paperboard. The test methods differ in the type of loading mode, thus giving rise to the two-point, three-point and four-point bending test methods.
For paper and paperboard in a low thickness range, the two-point bending method and the three-point bending method are suitable.
For corrugated fibreboard and board with a higher thickness, the four-point bending method is recommended.
The measurement conditions are defined in such a way that the test piece is not subjected to any significant permanent deformation during the test, nor is the range of validity of the equations for calculating the bending stiffness exceeded.
In these bending tests, the test pieces of paper and board are regarded as "beams" as defined by the science of the strength of materials.