Influence of leather stretching to gain area yield on its stress-relaxation behavior

Abstract

Four bovine leathers subjected to five stretching procedures in water at different temperatures between 60 and 80ºC and drawing ratios between 1.20 and 1.40 were analyzed to demonstrate the suitability of the generalized Maxwell model to fit the different stress-relaxation processes related to the structural hierarchy of leather. This consists of a set of three Maxwell units connected in parallel to represent the high-rate, the medium-rate, and the low-rate stress-relaxation processes, and a Hookean spring in parallel to represent the residual stress at the equilibrium. The high-rate, the medium-rate and the low-rate relaxation times were approximately of 0.6, 10, and 200 s, respectively. Stretching of leather under different conditions to gain area yield in addition to a reduction in thickness also produces a decrease in leather density but not always induce hardening of leather. Leather stretching increases the initial stress when leather is strained 20% for stress-relaxation tests. The effect of stretching on the residual stress depends on the level of stretching. The hardening effect of stretching measured by relative softness showed a good relationship with the medium-rate relaxed stress. The harder the stretched leather the higher the decrease in the medium-rate relaxed stress. Softness also showed a good relationship with the low-rate relaxation time.