The mechanical response of paper to external tension is described by the load-elongation curve, often called a stress-strain curve. Formation-like variations in local grammage, fibre orientation, and other factors induce non-uniform stresses and strains in paper. At the fibre level, the stress field of the random network is quite complex.
The load-elongation curve characterises the mechanical properties of paper in situations where paper is not loaded all the way to ultimate failure. The test result is a curve rather than a single number and the shape of the curve can be compared with finite element analyses for various loading conditions of paper and board products. The curve can also reveal microscopic fibre properties, for instance based on such comparisons with computational models.
We concentrate on the tensile behaviour before failure. We first explain general properties of the load-elongation curve of paper and relate them to microscopic yield mechanisms and theoretical models. Then we discuss practical furnish and papermaking factors that control the shape of the curve.- In addition, triaxial deformations are discussed.