Figure 1

Schematic illustration of a pulp fiber. Most of the fiber volume is in the S2 layer, while the S1 layer forms an outer shell. During the papermaking process, the S1 layer is damaged, partially exposing the underlying S2 layer. The fiber is characterized by the base \(\{{{\varvec{x}}}_{T1}, {{\varvec{x}}}_{T2},{{\varvec{x}}}_{L}\}\), while the material base is represented by \(\{{{\varvec{a}}}_{1},{{\varvec{a}}}_{2},{{\varvec{a}}}_{3}\}\). Relative to the fiber axis, the microfibrils are aligned at an angle, typically between 0° and 40°¹, illustrated here by the angle between the axis \({{\varvec{x}}}_{L}\) and \({{\varvec{a}}}_{3}\). The S2 layer is transversely isotropic, with the axis of symmetry (the strong direction) oriented along \({{\varvec{a}}}_{3}\). The S1 layer has a less well-defined fibril orientation, which is, here, represented by two different orientations of the fibrils in the plane of the fiber surface.