Submitted to: Fiber Society Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 6/16/2004
Publication Date: 10/11/2004
Citation: Liu, C. 2004. The application of acoustic emission to characterize the lubrication of a fibrous collagen material: leather [abstract]. Fiber Society Fall Technical Conference, A Symposium on Advanced Materials and Processes. p. 13-14.
Technical Abstract: For thousands of years, collagen materials, such as leather, have been among the most dominant natural fibrous materials used by humans, especially for clothing, upholstery, and shoes. Leather is economically significant because it is the major by-product derived from the meat industry. Fatliquoring is an oil-addition process by which the leather fibers are lubricated so that after drying they will be capable of slipping over one another. The fatliquored leather therefore attains a greater softness and flexibility than is imparted by the tannages. The fatliquoring process must be done properly to ensure that the leather's fibrous structure is adequately lubricated in order to prevent its fibers from sticking together during the leather drying process. Currently the method described in this report is the only method to measure the degree of lubrication. Lubrication affects the resistance of fiber movements and deformations, and from previous studies we learned that acoustic emission measurements are very sensitive to these changes in resistance. Therefore, we recently investigated the feasibility of using acoustic emission technology to measure the degree of lubrication associated with the fatliquoring process. As a natural fibrous material, leather emits sound waves engendered by a sudden stress accompanied with any significant fiber movement or fiber deformation including breakage. From analysis of the hits, frequency and energy associated with emitted sound waves during the tensile tests of leather treated with various amounts of fatliquor, one may gain a correlation between acoustic emission quantities and the degree of lubrication. The proper lubrication of a fibrous structure such as leather is essential to its mechanical performance, and consequently to its quality. We have examined the feasibility of using the acoustic emission (AE) technique to characterize the degree of lubrication of leather produced with various fatliquor concentrations. In a tensile test, an acoustic transducer was contacted with the leather samples to collect their AE quantities and properties. The samples lubricated with a fatliquor concentration less than 10% showed twin peaks on the plot of hits rate versus time. This implied that a non-uniform fracture occurred in a leather structure that was not sufficiently lubricated. In contrast, a sufficiently lubricated leather structure showed a steady increase in hits rate with time until it fractured. This provides an insight into the reason for the increased strength of fatliquored leather. Traditional stress-strain tests did not reflect these behaviors. Observations also showed a direct correlation between the cumulative hits and fatliquor concentration. The results of this work may provide a route to identify an adequate degree of lubrication in the leather, which was previously difficult to determine. Besides using AE to measure the degree of lubrication as reported in this presentation, we also have developed AE techniques to predict tensile strength without breaking the leather, to gain insights into the reasons for tear failure and provide a new route to characterize the tear resistance of leather, to measure the degree of opening-up of the leather fibrous structure, and to quantify the bonding strength of coatings on finished leather. This research program at USDA, ARS, ERRC has provided the leather industry with insights into the structure/property relationship. These techniques drastically improve leather quality control.