Transient creep effects and the lubricating power of water in materials ranging from paper to concrete and Kevlar

A diverse class of viscous materials, which includes familiar materials such concrete, wood, and Kevlar, exhibit surprising, counterintutive properties under internal moisture content fluctuations. In test after test over the past 50 years, the viscosity of these materials is observed to decrease, often dramatically, during wetting and drying. The key characteristics of the observed viscous softening are: the decrease in viscosity is temporary, and depending on the specimen size can be greatly delayed with respect to the associated change in weight; the decrease in viscosity is absent under steady state flow.Based on recent research on the properties of water and other polar fluids confined by hydrophilic surfaces, we provide a physical explanation and propose a constitutive law. The resulting model accurately captures the interplay between the pore fluid movement and macroscopic constitutive properties in totality. The model is verified against published data for the creep of paper sheets exposed to cyclic moisture conditions. Experimental data of different materials under similar boundary conditions are compared using a new metric, the creep rate factor. The results further reinforce the idea that nanoscale movement of water enhances the internal ‘lubrication’ of the studied materials, interpreted as loosening of the hydrogen bonds.