Study On Effects Of Cyclic Stress On The Structure And Mechanical Properties Of Bovine Upper Leather

Leathers will endure various kinds of forces in process and service,which can inevitably affect the structure and properties of leather.Cyclic stress is very common,such as cyclic stretching and bending.These kinds of forces are relatively complex and will greatly affect the tensile strength,softness and surface texture of leathers.However,few reports may be found in this respect,and little is known about the effects of forces on the structure and mechanical properties of leathers.Therefore,it is of great significance to study the mechanical performance of leathers under cyclic stress to understand the structure and properties of leathers for the optimization of leather making.The mechanical performance of bovine upper leather under cyclic stretching and its effects on leather mechanical properties was studied.In this part,bovine wet blue was retanned,fat liquored and dried to get crust leather.Two types of cyclic stretching with various stretching ratio and cycles were applied to leather.The mechanical performance of leathers after being processed under different stretching ratio was studied,and the cyclic stretching effects on leather mechanical properties and some general patterns between leather stress and strain were analyzed.Besides,optical microscope(OM)was employed to characterize collagen fiber structure on the side of thickness and cross section,and also pores on leather grain side.The results showed that cyclic stretching results in a large hysteresis loop and significant residual strain.While loading energy,hysteresis and peak stress decrease with increasing stretching cycles,unloading energy almost remains stable.Cyclic stretching will improve the softness and tensile strength of leather by unfolding the sticky collagen fibers.A higher stretching ratio will result in higher orientation of collagen fiber.In addition,there is a linear relationship between initial gradient and initial strain energy,loading energy and hysteresis,but the ratio of hysteresis and loading energy will gradually decrease with increasing the stretching cycles.In the second part,the freely recovery of leather after stretching was studied.Two methods were proposed to measure leather elasticity,according to the stress-strain of the leathers under cyclic stretching.The results showed that the recovery of leather could be divided into three stages as follows: fast recovery stage contributing the most to elasticity,medium recovery stage playing an elasticity-assisting role and slow recovery stage mainly contributing to residual strain.Linear relationships were found not only between recoverable strain and stretching ratio,but also between loading energy and unloading energy.The recovered strain as well as the ratio of unloading energy and loading energy decrease with increasing the stretching ratio.As for the method to measure the leather elasticity,the residual strain has advantages to the permanent set.The effect of bending on the structure and mechanical properties of leather was studies and the formation of folding on the grain side of leather was analyzed.Different leather samples were bended,with optical microscope and fourier transform infrared spectroscopy(FT-IR)employed to discuss the structure of folded samples and internal structure of collagen fiber.The results showed that the nature of bending is the stress concentration,leading to the formation of fold in grain side of leather and fiber fracture in flesh side by crushing and stretching.While bending can separate the sticky fiber to improve the softness of leathers to some extent,it will impair the tensile strength and fracture energy.Retanning may increase tensile strength but not anti-bending capacity.In contrast,fat liquoring can improve both the anti-bending capacity and mechanical properties as well as the softness,elongation at break and fracture energy by enhancing the relative sliding ability of collagen fiber.The tensile strength of samples parallel to backbone is higher than that perpendicular to the backbone,while the anti-bending capacity is better for the sample perpendicular to the backbone...