The mechanical properties of leather

Abstract

The many uses of leather have largely relied on the range of mechanical properties which it can provide, according to the raw material employed in its manufacture and the manufacturing processes themselves. The contrasting behaviour of a stiff sole leather and of a fine gloving leather exemplify this point. The last twenty years have seen intensive investigation of many mechanical properties of leather and the design of test methods now accepted internationally. At ambient temperatures and humidities most types of leather show mainly elastic behaviour, although delayed elastic effects may give the semblance of plasticity. The stress relaxation-time relation for constant linear strain shows the stress decaying linearly with log (time). The stress decay becomes discontinuous after sufficient time. The stress-strain relation for extension of leather strips is often markedly non-linear even at low strains (<2%). Two dimensional extension of leather has been analysed using an instrument allowing independent extension in two perpendicular directions. To a first approximation each stress component is linearly related to the two elastic strain components in the perpendicular directions. As with other materials of biological origin, the mechanical behaviour of leather varies from place to place in the skin, not only over its area, but also through its thickness. The extent of variation is briefly discussed and related to the underlying fibre structure. Leather which has been strained and then subjected to either heat alone or heat and moisture, shows much more extensive plasticity than occurs at lower temperatures. This behaviour has been used to enable leather to be given appropriate shapes, as in the heat setting of upper leathers. Quantitative studies of heat setting are reported and the influence of such variables as temperature, moisture content of the applied air stream, the air stream velocity and the duration of treatment are discussed. The plastic deformation obtained in this way is contrasted with “run” in gloving leather.