In-plane tensile properties

In-plane mechanical properties in tension are important in papers for printing or other uses in the form of a web, and in packaging papers and boards. The fundamental quantities are tensile stiffness, or elastic modulus and tensile strength. The in-plane stiffness of sheets is roughly given by the typical fibre axial modulus times a small numerical constant accounting for the fact that not all the fibres carry load equally. The tensile strength measures how much load a paper specimen can hold. If expressed in terms of its own length, the tensile strength of a paper web typically corresponds to the weight of 10 km of the same paper.

The tensile strength of paper is not the only important strength property. For example, fracture toughness may control the mechanical runnability of a paper web. Fracture toughness relates to the tensile strength, elastic modulus and load-elongation behaviour of paper. The elastic modulus of paper is particularly relevant for two reasons. The tension of a running web is small and controlled by the elastic modulus. The elastic modulus also controls the bending stiffness and structural rigidity of paper and board sheets or box templates (see Chapter 6).

All the mechanical properties of paper directly relate to the bonding degree of the fibre network. For the elastic modulus, this relation is easy to understand. Models also exist for the tensile strength and fracture toughness of paper that relate them to the strength of fibres and inter-fibre bonds. However, in contrast to the traditional view, we promote the use of macroscopic properties, such as the elastic modulus and fracture energy, as the characteristics that help us to understand what happens when paper breaks. In the theoretical treatments of paper strength, continuum mechanics has accordingly replaced microscopic models.