Molecular mechanisms involved in creep phenomena of paper

The phenomenon of mechanosorptive creep (i.e., the increasing creep occurring in some hygroscopic materials subjected to moisture cycling) was studied for paper from a molecular point of view. Paper was tested in creep at different loading levels in a constant high humidity of 90% relative humidity (RH) and in a cyclic climate between 30 and 90% RH. Throughout the creep tests, spectra from the mid‐ and near‐IR, as well as dynamic mechanical data, were recorded to determine molecular changes occurring with time. In tensile stress scans the instantaneous, dynamic elastic modulus was found to increase. It is suggested that this increase was due to orientation of the cellulose molecules, which was detected as changes in the mid‐IR spectra at 1160 cm⁻¹ assigned to the C₁ O C₄ stretching. During creep in constant and cyclic humidity, the modulus was found to increase with time, more so for the cyclic humidity. Changes in the mid‐IR spectra at 1184 and 1030 cm⁻¹, which is assigned to CH₂, CH, and C O, may indicate sliding between the cellulose chains. The near‐IR measurements mainly showed differences in the moisture content. In stress scans the moisture content increased with increasing tensile load. In creep at constant 90% RH, the moisture content was also found to increase in a manner similar to the stress scan. In the cyclic humidity with a conditioning time of 70 min at 90% RH the moisture content decreased successively with increasing numbers of cycles. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1590–1595, 2001     

Moisture‐induced effects on stacking strength of moulded‐fibre packaging in varying environmental conditions

Stacking strength of moulded‐fibre trays was investigated as both compressional creep and static compression strength at constant and varying humidity conditions. The compressional creep behaviour resembled that of other paper and board containers and was accelerated at humidity cycling between 33% and 94% relative humidity (r.h.) for 18 days compared to constant humidity conditions 91% and 94% r.h. Although the moulded‐fibre trays did not experience failure, secondary creep rate was accelerated by a factor of 10–20 and total creep strain by a factor of 1.3–1.6. Compressional creep was thus affected by mechanosorption, whereas static compression was found not to respond to cycling of environmental humidity. Static compression strength was merely determined by the moisture content of the moulded‐fibre material. The effect of varying temperature on tray moisture content was examined by transfer of the moulded‐fibre tray from preconditioning at cold storage (5°C, 59% r.h.) to ambient conditions (25°C, 54%). When a food simulant [agar gel, water activity (a_w) ～1] was sealed inside the moulded‐fibre tray, moisture condensed on the tray outer surface (moisture gain 1.4 g/100 g dry fibre) within 40 min of transfer, contrary to when empty moulded‐fibre trays were exposed to same conditions. Condensation could thus potentially induce a large initial creep deformation of the moulded‐fibre tray. Copyright © 2004 John Wiley & Sons, Ltd.     

Three Dimensional Creep Model for Wood Under Variable Humidity-Numerical Analyses at Different Material Scales

This paper presents a qualitative explanation of the creep phenomenon based on the physical and chemical mechanisms that occur at micro and ultra-structural levels of wood during moisture diffusion. This part is then completed by the formulation of a 3-dimensional non linear hydro-visco-elastic model, combined with hygro-expansion effects, and able to describe creep and recovery phenomena under variable humidity conditions. The constitutive relation is based on a generalised Maxwell model whose relaxation time functions depend on the moisture content rate, the history of accumulated moisture variations and the stress level. The model was implemented in a Finite Element (FE) program. Several applications based on external experimental tests and with creep periods ranging from 1 to 2735 days were carried out in order to prove the relevance of the approach at different structure scales. A local strain energy density criterion, associated with a flow law, allows the representation of the rupture phase initiated by the tertiary creep and is incorporated into the model in order to open up new horizons for the service life estimation of timber structures.     

Creep in Wood Under Variable Climate Conditions: Numerical Modeling and Experimental Validation

This paper deals with the modeling of linear viscoelastic behavior and strain accumulation (accelerated creep) during moisture content changes in timber. A generalized Kelvin–Voigt model is used and associated in series with a shrinkage-swelling element depending on the mechanical and moisture content states of materials. The hygrothermal aging due to climatic variations implies an evolution of rheological parameters depending upon moisture content and temperature. Two distinct viscoelastic laws, one for drying and the other for moistening, are coupled according to the thermodynamic principles when wood is subjected to nonmonotonous moisture variations. An incremental formulation of behavior is established in the finite element program CAST3M (Software developed by C.E.A. (Commissariat á l'Energi Atomique) and an experimental validation from tension creep-recovery tests is presented.