Abstract

Given the function of the bighorn sheep horn, the various parametric effects important to the microstructure and mechanical property relationships of horn keratin were quantified. These parameters included analysis of the stress-state dependence with the horn keratin tested under tension and compression, the anisotropy of the material structure and mechanical behavior, the spatial location along the horn, and the wet-dry horn behavior. The mechanical properties of interest were the elastic moduli, yield strength, ultimate strength, failure strain, and hardness. The final results showed that water dominates the ram horn mechanical behavior more than the anisotropy, location along the horn, and the type of loading state. All of these parametric effects showed that the horn microstructure and mechanical properties were similar to those of long fiber composites. In the ambient dry condition (10 wt.% water), the longitudinal elastic modulus, yield strength, and failure strain were measured to be 4.0 GPa, 62 MPa, and 4%, respectively, and the transverse elastic modulus, yield strength, and failure strain were 2.9 GPa, 37 MPa, and 2 %, respectively. In the wet condition (35 wt.% water), horn behaves more like an isotropic material; the elastic modulus, yield strength, and failure strain were determined to be 0.6 GPa, 10 MPa, and 60%, respectively. [1]

Methodology

Schematic illustration of the mechanism testing specimen locations, dimensions and orientations

Compressive and tensile testing was performed on a universal testing machine (EM Model 5869, Instron, Massachusetts, USA) equipped with a 50 kN load cell. Three sets of specimens used for tensile and compression testing were cut from the base, middle, and tip of the horn using a water-jet cutting machine. Care was taken to cut the specimens such that the fiber orientation was aligned either parallel or perpendicular to the long axis of the specimen. The dog-bone tensile specimens had a length of 37 mm, a width of 18 mm, a gage length of 12 mm, a gage width of 6 mm and a thickness of 3 mm. The cylindrical compression specimens had a diameter of 3 mm and thickness of 3 mm. A constant strain rate of 3.0 × 10^-3 s^-1 was maintained for all testing. Toughness values were calculated as the area under the average tensile stress-strain curves.

Twenty cylindrical specimens were harvested from each region of the horn (base, middle, and tip) for compression and tension testing, ten in the longitudinal direction, and ten in the transverse direction. In order to investigate the effects of moisture content of the horn keratin, five of the ten longitudinal specimens from each region, were tested in the ‘wet’ condition and five were tested in the ‘dry’ condition. Of the ten longitudinal and the ten transverse tension specimens from each region, five of each were tested in the ‘wet’ condition and five were tested in the ‘dry’ condition, i.e., each uniaxial tension and compression test was repeated five times. The resulting stress-strain curves for the duplicate tests were averaged together and the standard deviation at various strain levels was calculated. In this parametric study, sixty compression tests and sixty tension tests were performed in all. No specimens were harvested from the region of the horns where growth lines were obvious, as the growth lines could potentially affect the mechanical properties[2].

The density of the horn keratin taken from the base, middle, and tip of the horn was determined using Archimedes principle. Cylindrical samples, having a diameter of 3 mm and thickness of 3 mm, were harvested from the three horn regions. The dry weight, W1, of each sample was obtained using a digital scale. The samples were then impregnated with oil and reweighed to obtain W2. The oil-impregnated sample was then immersed in water of known density, , via a suspension wire with known mass, Ww, to obtain W3. The Archimedes density was then calculated.

A micromechanical testing machine (TI 900 Triboindentor, Hysitron Inc., Minneapolis, MN, USA) equipped with a Berkovich indentor tip was used to determine hardness and elastic modulus of the horn sheath material. An indentation profile was made across a polished cross section of the horn. Care was taken to not probe any voids within the material. Spacing between indentations was approximately 500 um to avoid any strain hardening or residual stress effects.

Fracture surfaces were examined using a field emission scanning electron microscope (SEM) equipped with EDS (JSM-6500F, JEOL Ltd., Tokyo, Japan). Specimens were mounted on aluminum sample holders and all surfaces not being examined were coated with silver paint. All specimens received a 12.5 nm platinum coating in a sputter coater (Polaron SC7640, Quorum Technologies Ltd., Connecticut, USA) prior to observation in secondary electron (SE) mode at 5 kV. SEM images were analyzed using the Image-Analyzer software package developed by the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University (MSU) to quantify the microstructural features of the ram horn keratin material.

Results

• Horn keratin behaves in an anisotropic manner similar to a long fiber composite with strengthening fibers in a matrix in terms of the elastic moduli, strengths, and failure strains and mechanisms. However, the anisotropy is lessened as water is content is increased.


• The tubules serve to longitudinally stiffen the horn in tension and absorb energy in transverse compression.


• Water dominates the horn keratin material behavior more than the anisotropy, location on the horn, and the type of loading state. This makes moisture content the most relevant parameter in regards to influence on the mechanical behavior of horn keratin.


• A clear tension-compression asymmetry exists within the horn in which the tension stress-strain behavior exhibits a greater initial modulus that is exacerbated in the wet state. This early higher modulus in the tension curve leads to higher stress-states as a function of strain and eventual fracture sooner than compression.


• Tensile failure in the longitudinal direction occurred by matrix separation followed by fracture of the reinforcing tubules and some tubule pull-out. The ambient dry horn keratin failed in a much more brittle manner, while wet horn keratin was much more ductile.


• Tensile failure in the transverse direction occurred in wet horn keratin primarily because of matrix failure, with some transverse fiber pullout. However, ambient dry horn keratin exhibited delamination and tubule fracture.


• Compressive failure in the longitudinal direction occurred by shear microbuckling followed by delamination in both the wet and ambient dry conditions.


• Compressive failure in the transverse direction, both the wet and dry specimens exhibited a shear type failure mode.

Reference

1. Trim MW, Horstemeyer MF, Rhee H, El Kadiri H, Williams LN, Liao J, Walters KB, McKittrick J, Park S-J. “The effects of water and microstructure on the mechanical properties of bighorn sheep (Ovis canadensis) horn keratin.” Acta Biomaterialia 2010;7:1228. DOI: 10.1016/j.actbio.2010.11.024.

2. Kitchener A, Vincent JFV. Composite theory and the effect of water on the stiffness of horn keratin. Journal of Materials Science 1987;22:1385