Shear modulus of single wood pulp fibers from torsion tests

Abstract

The shear modulus of pulp fibers is difficult to measure and only very little literature is available on this topic. In this work we are introducing a method to measure this fiber property utilizing a custom built instrument. From the geometry of the fiber cross section, the fiber twisting angle and the applied torque, the shear modulus is derived by de Saint Venant’s theory of torsion. The deformation of the fiber is applied by a moving coil mechanism. The support of the rotating part consists of taut bands, making it nearly frictionless, which allows easy control of the torque to twist the fiber. A permanent magnet moving coil meter was fitted with a sample holder for fibers and torque references. Measurements on fine metal bands were performed to validate the instrument. The irregular shape of the fibers was reconstructed from several microtome cuts and an apparent torsion constant was computed by applying de Saint Venant’s torsion theory. Fibers from two types of industrial pulp were measured: thermomechanical pulp (TMP) and Kraft pulp. The average shear modulus was determined as (2.13 $${\pm }$$ ± 0.36) GPa for TMP and (2.51 $${\pm }$$ ± 0.50) GPa for Kraft fibers, respectively. The TMP fibers showed a smaller shear modulus but, due to their less collapsed state, a higher torsional rigidity than the kraft fibers.

The effect of the strain rate on the longitudinal modulus of cellulosic fibres

“…Therefore, to eliminate the compliance of the system, the displacement was corrected. This correction factor was obtained from measurement of Platinum (Pt) bands, similar to [ 27 ]. By measuring Pt bands directly mounted into the clamping of the DMA as well as glued on the sample holder (under the identical conditions as the fibres), it was possible to obtain a correction factor which was then used to correct the response of all fibres glued onto the sample holder.…”