Effect of drying under strain on the mechanical properties of leather

The influence of drying leather at 30% strain on the tensile strength and crack resistance of the grain layer of chromium tanned bovine leather was examined. The effect of drying under different strains was also examined. Bovine wet blue splits were stretched (by 30%) either along one axis (uniaxial) or two perpendicular axes (biaxial) and dried in this stretched condition. The strength and the fracture toughness of the resulting crust leather, was then evaluated. Drying leather under 30% uniaxial strain resulted in an increase in the tensile strength for samples cut along the stretch axis. However the variation of tensile strength with angle of cutting the specimen with respect to the stretch axis can be described by the Tsai-Hill theory that is applicable to fibre composite materials. The essential work of fracture approach was applied for the first time to leather specimens cut both longitudinally and transversally to the stretch axis. For all the samples examined, a strong anisotropy in the fracture behaviour was observed. The fracture resistance for cracks propagating along the stretch direction in uniaxial samples was lower than in the transverse direction, this fracture anisotropy being enhanced at higher applied strains. Drying under uniaxial strains above 15%, resulted in an increase in the fracture toughness for cracks running transversally to the strain direction, but at the same time, it seriously reduced the fracture resistance for cracks propagating along the strain direction resulting in global yielding, a phenomenon that rendered the evaluation of fracture resistance by use of essential work of fracture approach unattainable. Drying under biaxial strains above 15% seriously reduced the fracture resistance of leather both perpendicular to and parallel to the backbone. The application of optical microscopy to observe the propagation of cracks through the grain layer led to the conclusion that drying under biaxial strain can cause a reduction in crack resistance due to changes in the size and nature of a highly orientated fibrous zone close to the crack tip. X-ray diffraction studies were made in order to relate the fracture behaviour to leather structure. The X-ray data was correlated with experimental data in order to investigate the influence of pre-stretching on the strength and hence quality of leather during manufacture and their relationship to the fibrous collagen microstructure.