Modeling of stress relaxation and elastic recovery of wet paper

Understanding and control of the viscoelastic behavior of paper web is important for the runnability of modern paper machines and for the success of converting processes that contain finite deformations at a high strain rate. It is well known that tension in a wet and dry paper web is non-linearly proportional to strain, and depends on the strain rate. Strain in paper is not completely recovered after the tensile load is removed, part of the recovery is timedependent and part is non-recoverable [1]. A relaxation phase in the stressstrain cycle makes the behavior more complex and challenging. The objective of this study was to model the rheological, time-dependent stress-strain cycle that includes a relaxation, de-loading and an elastic recovery phase. The modeling was based on experimental data that was produced for softwood and hardwood kraft pulps at different strain rates.