Abstract
Thin grain leathers were taken from bovine sides and composites formed by saturation. Leather and composites were characterized by mechanical tests. They deform mostly by fibre extension, decrimping, translation and rupture. The relative proportion of each is strain dependant and the restricting forces determine the tensile stress response. In unstrained leather, frictional forces or polar bonds, predominate, whilst in composites, polymer bonds prevail. Aqueous impregnants probably form additional polar bonds. The stress response of leather is largely strain rate insensitive but composites have a composition dependant, strain rate sensitivity and a higher stress relaxation rate. It is doubtful whether any bonds remain intact beyond 12% strain where leather and composites exhibit an increase in Poisson!s ratio, a change in stress relaxation behaviour and, in leathers, an ill-defined stress yield point which probably marks the end of the fibre decrimping in retan leather. Below 12%, strain, full chrome leather has a high modulus and retan a lower one. Composites also have high moduli but in those of aqueous polymers, the yield point is sharper and occurs at 5%, strain along with a maximum stress relaxation rate and, with one polymer, an increase in Poisson!s ratio. Beyond 12% strain, frictional forces or entanglements cause progressive fibre rupture, beginning at 15%, strain in full chrome leather, at 30% in retan, and increasing exponentially in both up to rupture at 40% strain. No polymer brought forward the onset of fibre rupture, most delay it. All polymers further restrict fibre movement at the highest strain rates employed but at lower rates soft polymers can act as lubricants with increasing polymer content and composite failure strain increases while secondary modulus decreases. A simple additive model describes composite stress relaxation. Tensile stress prediction requires a complex network model which does not account for the effect of polymer content.