The thermodynamic behaviour (that is, the simultaneous mechanical and thermal behaviour) of paper and other sheet-like materials will be reported. It will be shown that the thermodynamic analysis will reveal much more of the deformation mechanism than the mechanical analysis alone.
Results obtained show that the initial straining of paper is controlled by energy elastic forces and in accordance with Kelvin’s thermoelastic equation. The plastic region straining of paper is controlled by irreversible intrafibre deformation. Simultaneously, some interfibre bond breakage does occur, but this breakage is partial and it is not a prerequisite of plastic deformation of paper.