Influence of viscoelastic behavior on curl of paper

The influence of viscoelastic stress relaxation on the curl response of pap er subjected to transient humidity changes was investigated numerically and experimentally. We considered asymmetric two-ply paper sheets consisting o f low (L) and high (H) anisotropy plies bonded together. Experiments were c onducted at room temperature at various constant relative humidities (RH) t o obtain the mechanical properties, moisture content, moisture expansion co efficients, and stress relation curves of the constituent plies. The time-d ependent curl response of an unconstrained sheet was analyzed using finite- element analysis. Geometrically linear analysis predicted a double curvatur e shape of the sheets while geometrically nonlinear analysis predicted a cy lindrical (single-curvature) shape with the axis in the direction of the ma jor stiffness (MD). The analysis revealed that viscoelasticity has little i nfluence on curling in dry conditions. With increased humidity, the moistur e absorbed softens the material, reduces the relaxation time, and the curva ture response becomes highly viscoelastic. Predicted time-dependent curvatu re response was in reasonable agreement with experimental measurements, alt hough it was observed that the analysis underpredicted the curvature change during the increasing phase of the moisture loading. Such a deviation is a ttributed to the assumption of uniform through-thickness distribution of mo isture in the analysis.