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Leather tensile strength and its auxetics properties


proprietà auxetiche nell’essiccazione pelli

Among the many properties of leather, tensile strength is still nowadays a very broad and articulated topic of study. Hides and skins have unique textures and structures, influenced by the characteristics of the animal itself and the different tanning processes.


Testing leather tensile strength can be challenging in terms of constant reproducibility: although many examples can be found in technical books, the tanning industry has probably yet to determine the best way to exploit hides and skins’ tensile strength for area yield.


For the occasion of IULTCS 2022, Fratelli Carlessi has presented an in-depth study and test on leather tensile properties, using innovative techniques and machinery for hides’ drying and conditioning. Let’s find out more about the most interesting findings.


 

Insights and trials on leather tensile strength


For centuries, the tanning industry has been studying and learning the many mechanical properties of leather, among which the theory and the practice of leather tensile strength.


Hides and skins’ tensile properties are regulated by different key characteristics. To name only a couple there are:

  • Orientation and concentration of the fibers: their disposition and density vary in relation to the different areas of the leather, once destined to certain purposes when the animal lived;

  • The level of moisture, as well as the distribution of the fat liquors in each section.


Unfortunately, the studies covering leather tensile strength are not as extensive as other materials: these properties have been evaluated in the past by cutting and testing samples out of different areas of a hide, according to its orientation to the backbone.


variations in leather tensile strength

This methodology has allowed tanneries to know more about the tensile properties of the different areas of hides and skins, and to map them accordingly in order to better predict their performances on finished productions.


In spite of this an important consideration should be raised: all the areas of the leather, no matter how different they may be, are connected and related to each other. If it were possible to better understand and take advantage of the multiple fibers linkages making up the full leather, it could be possible to exploit its real potential at the maximum.

 

What is necking and Poisson's Ratio


What we know so far about leather tensile strength is that, under traction, most testing samples show necking before the fibers start to break and, consequently, tear the sample.


By definition, necking is the localized deformation in the cross-sectional area of a material. This effect is determined by the Poisson Ratio, which measures the expansion or the contraction of the structure in the perpendicular direction to the strain’s direction.


More specifically, the Poisson Ratio is a positive or negative value that defines the proportional ratio between the transverse strain and the axial strain, meaning that:

  • a positive Poisson Ratio will show a perpendicular contraction to the applied strain;

  • a negative Poisson Ratio will show a perpendicular expansion to the applied force.


auxetic materials properties tension

While most materials have a positive Poisson Ratio, auxetic materials have a negative one: when stretched, they become thicker perpendicularly to the strain’s direction. This occurs due to a particular internal structure that allows auxetic materials to have some interesting properties, such as high energy absorption and fracture resistance.

 

Auxetic properties found in hides


During some drying trials carried out on the CRC toggle drier, an interesting phenomenon has been observed: a negative Poisson coefficient was found on some areas of the hides, that caused a surface development perpendicular to the frames’ tension.


The test was carried out by tensioning and stabilizing the leather through different steps, in order to extend the areas of the belly and backbone with the same ratio: in fact, CRC is capable of applying different forces to different degrees on both sides of the frames.


The auxetic deformations appeared linearly, starting from the backbone and spreading in the direction of the outer edge, and at the end of drying were still present in the backbone.



Based on our testings, it appears that entire hides under tension can perform an area increase in unexpected directions: not all samples have shown clear auxetics properties but this phenomena is quite frequent, as well as the hides’ necking was not so evident.


What we can confirm for sure is that leather fiber structure is absolutely unique and has a great potentiality: exploiting these auxetic tensile properties through leather drying could achieve not only a greater area yield, but also a more uniform quality of the hides.


 

The importance of leather drying


One operation which heavily influences leather tensile strength is the drying process.

Our studies on whole hides’ tensile properties could only be carried out through the most innovative tannery machinery, in order to perfectly control leather tensioning and drying.


In recent years, Fratelli Carlessi has focused on researching and designing a new drying and conditioning machine called CRC - Cell Rotary Conditioning, that works with load-based split frames in order to better focus on leather strain and moisture.


The CRC toggle dryer by Fratelli Carlessi allows to perform:

  • A perfectly controlled leather drying and conditioning through sophisticated, highly precised airflow canalizations;

  • Hides stretching in an accurate manner, by transferring the tension adequately to the different areas;

  • Constant feedback to operators in real-time on hides and skins’ tension, temperature and humidity levels.


Fratelli Carlessi’s solutions for the tanning industry can not only achieve a constant quality and reproducibility in leather processing, but also allow tanneries to lower their energy consumptions, manpower costs, production times and carbon footprints.



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