Deformation, yield, relaxation and recovery in partially processed leather
The aim of this research was to understand better the rheological behaviour of part processed leather in order to improve area yield. Rheological behaviour was characterised in uniaxial tests by measuring the stress-strain relationship, the immediate and recovered set and stress relaxation. The influence of moisture content, time under strain and applied strain were determined. The effect of drying under strain was also explored. Results were interpreted using rheological and structural models. It was found that for wet leather the relationship between applied strain and long term set is nonlinear. Moreover, below a critical strain no set is imparted. At this critical strain there is a change from a bending to a tensile mode of fibre deformation. Long term set only occurs in wet leather when collagen fibres are subject to tension. In wet leather, recovery occurs because collagen fibres return to their initial bent configuration. The long term set of partially processed leather at low strains increases with decreasing moisture content because collagen fibres permanently deform due to the rupture of intra-fibre adhesions. Inter-fibre adhesions formed on drying result in a yield point on the stress-strain curve. At the yield point the adhesions rupture and the mode of fibre deformation changes from stretching between adhesions to bending. A model, which views the structure of leather as adhesion nodes, joined by segments of fibres, has been used to predict increases in the initial modulus caused by drying under strain, in terms of the recruitment of straight fibre segments between nodes. When material is held under strain, there is a proportionate decrease in stress and increase in the immediate set, which is shown to be consistent with the Maxwell model. The two stage recovery of set is fitted to a multiple Voigt model. A novel way of achieving area yield without loss of tensile strength or softness is suggested in which the leather’s structure is relaxed whilst remaining wet and is then fixed by drying rapidly under strain
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