LEATHER’S ELEMENTARY UNIT

Geoff Attenburrow’s PhD student David M. Wright worked on the behaviour of leather under strain (Attenburrow, 1993; Wright, 1996). They were the first to describe that leather had a “co-operating unit”, a set of structures within leather that helped the leather to resist breaking and to resist deformation at low strain.
Traditionally material scientists have tried to elucidate where the structure of this co-operating unit lies within a material. Was it a fundamental property; within the collagen hierarchy at the collagen triple helix level - or was it higher up? Leather researchers over the years have given clues, with important discernment coming through researchers such as Maeser (1960), who told the community about anisotropy (different properties of a material, depending on the orientation of the test direction) which recognises that fiber orientation in the direction of strain makes a difference. Popplewell and Ward (1963) tried to pinpoint the behaviour of this co-operating unit in linear set and discovered that unlike anisotropy the co-operating unit seemed to depend on moisture content of the leather; was affected by temperature during set; but didn’t seem to be independent of position on the hide.
Further work by Geoff’s students could show that the fracture of the leather shone a light on how the network of fibres interacted and by the 1990s energy of fracture, angle of weave, and the behaviour of how macroscopic leather fibres pulled out of the network were important.

Crack-bridging model
Soviet physicist G.I. Barenblatt (1959) first came up with the theory of cracking-bridge failure. Figure 1 shows a panel giving details and the mathematics involved with the idea. Attenburrow was hinting that the “bridges” seen in Figure 1 were being seen in leather materials.

Li et al. (2022) and Figure 1 go on to explain why these bridges are vital in the determination of tear strength in materials, particularly with regard to the materials with a slit that can focus the propagation of the tear. As a tear moves into a material three major mechanisms resist that propagation. Crack-tip blunting, crack deflection, and crack-bridging are ways in which energy is diffused; breaking is slowed; or the tensile strength of said bridges is summed to give a high tear strength.
Crack-tip blunting causes a V-shaped crack tear to deform into a rounded shape that slows and diffuses impulse energy refocussing tear energy into multiple potential tear sites. Crack deflection is seen in leather tear tests where the fracture front is deflected through the assimilation of multiple nanovoids (the spaces between bridges that can connect behind the crack-tip, see Figure 1. Of particular interest however is the third mechanism, crack-bridging which pertains to the Long et al. (2025) breakthrough, which also links back to the seminal work of Attenburrow in the 1990s and the work of leather scientists in the 1960s.

Crack-bridging
Barenblatt (1959), Li et al. (2022) and Long et al. (2025) allow a mechanism to describe the Attenburrow co-operating unit. In other words, it tries to give detail to how fatliquoring, or tanning (or retanning) gives a leather fibre strength. Previous authors to Long et al. (2025) tried to pin the properties of material strength onto a more fundamental unit in a material, such as the microfibril or fibril. The result was an inconsistent outcome of results, with some studies showing that fibril diameter increased strength; and others showing that fibril diameter decreased strength. The studies were looking at the wrong scale. The determination by researchers was to show that crack-bridging at some level was resisting tears.
Long et al. (2025) decided to look at the interfibrillary space of leather. Using a porosity measurement, the researchers were able to gain a better understanding of what the average fibril bundle diameter was. The research unlocked a better correlation with tanning type, fatliquoring, and how the collagen D-spacing was being influenced. Leather scientists knew that operations like fatliquoring and tanning influenced collagen D-spacing. Some speculated that the vegetable tanning agents targeted the D-space, which would adjust the fibril diameter and consequently the fibril bundle diameter.
The effect and results were profound. The Chinese researchers had shown that in the material that was just tanned on its own with no post tanning treatment, the material ranged from hydrophobic to hydrophilic in character as one moves from chromium, to zirconium, to an organic aldehyde. Hydrophilicity increases the fibril bundle diameter (possibly due to increased supramolecular associated water. Hydrophobic tannages decrease fibril bundle diameter and this is amplified by the association of a hydrophilic post tanning treatment. As a side note, not considered by the authors is they have described a proof for the Miles “polymer-in-a-box” theory that supports decreasing shrinkage temperature as one moves from hydrophobic to hydrophilic tannage.
The authors added three types of post tanning treatment to see the effect that it has on fibril bundle diameter. They added a hydrophobic oil, a hydrophilic polyacrylic polymer, and a combination of the oil and polymer. As expected, the hydrophobic oil resulted in a decreasing fibril bundle diameter and the polyacrylic increased fibril bundle diameter.
In conclusion, the Chinese researchers have linked tear strength and fibril bundle diameter. There is an inverse relationship. The greater the fibril bundle diameter the lower the tear strength. Maximum tear strength comes from a hydrophobic tannage, such as chromium post tanned with a hydrophobic oil that decreases fibril bundle diameter. It begs the question: can highly water-resistant leathers have even more of an impact on tear strength and do they have lower supramolecular water, shrinking the polymer-in-the-box?

African Leather: How does this compare to the tear strength of textiles and other materials that leather competes against?
H. Procter:  Leather is generally stronger, I believe.
African Leather: Is that question relevant and/or honest and/or fair?
H. Procter:  Yes, very relevant. The answer is probably underdeveloped, unconfident, and needs deeper dive. We should be able to compare materials more freely and leather should proudly parade the answer. Good question.

References
Wright, D. M. 1996. Deformation, yield, relaxation and recovery in partially processed leather (PhD thesis). University of Leicester / University of Northampton. Supervisor: Geoff E. Attenburrow.
Attenburrow, G. E. 1993. The Rheology of Leather – A Review. J. Soc. Leath. Tech. Chem., 77: 107–114.
Maeser, M. 1960. The effect of hide location and cutting direction on the tensile properties of upper leathers. J. Amer. Leath. Chem. Ass. 55: 501.
Popplewell, D. and Ward, A.G. 1963. The mechanical properties of upper leather - 1. The measurement of linear plasticity. J. Soc. Leath. Tech. Chem. 47: 502-511.
Barenblatt, G.I. 1959. The formation of equilibrium cracks during brittle fracture. General ideas and hypotheses. Axially-symmetric cracks. J. Appl. Math. Mech. 23: 622-636.
Li, X., Sheinerman, A.G., Yang, H., and Zhu, Z. (2022). Theoretical modeling of toughening mechanisms in the CrMnFeCoNi high-entropy alloy at room temperature. Int. J. Plast. 154: 103304. DOI:10.1016/j.ijplas.2022.103304
Long, W., Peng, L., Li, J., Yu, Y., and Zhang, W. 2025. A simple method for identifying the elementary units governing the mechanical properties in leather materials. Collagen and Leather 6:7 https://doi.org/10.1186/s42825-024-00184-4

In the next issue: The author of Obliquity, John Kay argues that trying to achieve a goal directly is seldom achieved. He argues that the most successful achievement system is obtained through making the achievement of a goal in an oblique fashion, indirectly. A company that is trying to achieve profitability will not do so easily, however profitability is best achieved through making the purpose of the leather business something focussed on a value or goal (that everyone involved in the enterprise buys into) is an easier way to achieve that profitability. In the next article, the principles of Obliquity will be discussed and how the leather industry should be maximising this useful way of thinking.