# Structural mechanics of paper and board

This section deals with bending stiffness, compressive strength and out-of-plane strength. Use the links below to find more detailed discussion on these properties. These mechanical properties are particularly important in the use of many board grades, but bending stiffness is important also for printing papers. High stiffness is the property that defines paperboard – a planar product with sufficient thickness. In printing papers, insufficient bending stiffness can make thumbing of printed matter difficult. Compressive strength determines the ultimate strength of boxes when crushed and insufficient out-of-plane strength can lead to delamination in offset printing or to blistering in drying.

From simple mechanics, one can understand that bending stiffness is sensitive to the distribution of the elastic modulus through sheet thickness. For example, if different furnishes are available, layered boards can be designed to maximise bending stiffness at minimum grammage.

Compressive strength refers to the failure of paper or board under compressive in-plane stress. Particularly with coated products, failure under compression in the out-of-plane direction can also be relevant but it will not be considered here. Out-of-plane strength or delamination strength, in turn, measures failure in tensile mode but in the thickness direction of the sheet. In this case, layered structures are generally weaker than uniform structures, because failure may easily localise at structural discontinuities. The interwoven structure of the fibre network is crucial for high out-of-plane strength.

The properties are dependent on each other; bending stiffness can be increased by increasing the middle-ply bulk of boxboard, but at the same time the out-of-plane strength can decrease too much. In addition, bending stiffness and compression strength are interconnected; high bending stiffness prevents the build-up of compressive stresses in a box.