Differences in strength between the grain and corium layers of bovine leather
Chrome tanned bovine leather comprises two principal strata, the grain and the corium. The tensile strength and specific work of fracture of these two materials was investigated using uniaxial tensile tests and trouser tear tests respectively. Corium material was observed to be intrinsically stronger and tougher than grain material. The greater resistance of corium material to fracture is attributed to the processes of fibre debonding and pull out. The viscoelastic nature of grain and corium material was investigated by examining differences in strength and toughness over a range of deformation rates. The ultimate tensile properties and the specific work of fracture of both grain and corium material are rate dependent. Energy dissipation during a strain cycle was measured over a range of strain rates and strain levels to determine the bulk hysteresis of grain and corium specimens. The influences of specimen orientation and fatliquor (oil) on strength and toughness are also considered. The notch sensitivity of grain and corium materials has been scrutinised. Corium material is highly notch insensitive, whereas the fracture of grain material is notably sensitive to the presence of notches. The mechanism of fibre orientation and the phenomenon of fibre independence (or fibre autonomy) are responsible for the notch insensitive fracture behaviour of corium material. Strain distribution was measured in grain and corium single edge notch specimens. The radius of curvature of the notch was assessed throughout deformation / fracture and local strains at two dimensional levels were measured. Local strains ahead of the advancing crack and the radius of curvature of the crack are considerably higher with corium specimens than with grain specimens
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