ISO 5628:2012(en)–Paper and board — Determination of bending stiffness — General principles for two-point, three-point and four-point methods
Bending stiffness is regarded as an important property of paper and board, and a large number of test methods have been used for its determination. This is a result, in part at least, of the wide range in the bending stiffness of paper and board. For paper and board in the grammage range 50 g/m2 to 500 g/m2, bending stiffness might vary by a factor of over 1 000. This wide variation is reflected in the design of instruments intended for the measurement of this property.
A second factor to be taken into account is that, in general terms, bending stiffness (as defined here) can only be determined with accuracy within certain limits with regard to the degree of deformation imposed upon the test piece. These limits depend on the dimensions of the test piece and on the test method used.
This International Standard is intended to enable the bending stiffness (as defined here) to be measured and described in a consistent way, despite the variations in material type and instrument design.
It will be found that many commercially available instruments can be regarded as giving results in accordance with this International Standard for only part of the range of bending stiffness, or for only some of the materials for which they were originally designed.
It is intended, therefore, that this International Standard will be used as the basis for preparing detailed methods for determining bending stiffness, using particular instruments.
This International Standard 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, see Reference .