In many technological applications of cork, this biomaterial is under strongly localized contact stresses, which largely differ from the homogeneous distribution of stresses of the typical uniaxial compression tests. Indentation tests constitute an excellent form of determining the behavior of the materials under localized stresses. In the present study, the applicability of Hertzian and Brinell indentation tests to the evaluation of the mechanical properties of cork is tested. One of the main conclusions of the study is that the elastic anisotropy of the material is related to the anisotropic structure of the different sections cut from a cork sample, a clear difference between the back tangential section and the other sections being observed.
Cellular solids are materials possessing cellular microstructures,which are seen in natural materials such as wood, cork, sponge, cancellous bone, and coral [
Cork is a widely used material due to its excellent mechanical properties, low density, impermeability, thermal and acoustic insulation, and so forth. Examples of its use are as the stopper in bottles of quality wine, floorings, and wall coverings, and so forth. Despite its qualities for such a diversity of applications, there has been a certain unjustifiable lethargy in scientific attention to this material. There have however been some recent studies published on the structure and mechanical properties of cellular solids in general [
In many technological applications, cork is subjected to strong contact stresses localized in small zones of the material, not to the homogeneous distribution of stresses that are involved in uniaxial compressive strength tests. The mechanical behavior of other materials such as foams of different types has been tested [
The Hertzian test involves the application of a load,
Cork is a natural product of the cork oak (
Diagram showing the different sections and directions of a piece of cork relative to its position on the tree. Also indicated above is the morphology of the cells of each section.
Samples of cork were supplied by the Institute for the Promotion of Cork, Wood, and Coal (ICMC, Spain). An automatic cutoff machine (Struers Accutom-50) with a diamond wheel was used to cut regular parallelepiped specimens (see Figure
The tests were performed on a universal testing machine (Instron Model 1122), applying loads in the range from 0 to 300 N using spherical tungsten carbide (WC) indenters of radii 5.5 and 12.7 mm. The speed of application of the load was 0.05 mm/min. After several trials (gold sputtering onto the sample, spraying with ink, etc.), it was decided that the best approach to defining the region of contact during indentation was to cover the WC ball with ink. The contact radius was measured under optical microscopy using a digital comparator clock coupled to the mobile turntable.
In Hertzian indentation test, the stress field scales with the contact pressure,
Diagram of a Hertzian indentation test (left) and typical indentation stress-strain curve (right).
In Hertzian elastic contact theory, the shear stress is maximum on the load axis at a depth of
Thus, from (
In a Brinell test, hardened steel spheres are used to produce an imprint on the surface of the tested material. The diameter of the imprint is measured, and the hardness (HBN) of the material is calculated from the relationship between the applied load and the area of the imprint. The area can be substituted by the diameter of the imprint, with the final expression being
The coefficient
Figure
Load-unload curves corresponding to the different sections of a cork specimen.
In general, a large deformation of the specimen at relatively low loads is observed. In the unloading curve, one observes an initial slow recovery which increases as the load is released, although the energy stored by the sample is still relatively high at total unloading. Above a certain value (between 6 and 10 N), the load curve deviates from a more or less linear behaviour, showing increasing resistance to deformation. This change is probably due to the progressive collapse of cells with increasing contact pressure.
The same effect can be seen in Figure
SEM micrographs of the TE and the TR or RD sections (a) before and (b) after applying a contact load of 100 N.
Figure
Young's modulus
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M2 |
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Indentation stress-strain curves for the TE, TB, TR, or RD sections of a cork sample.
The yield stress of the cork samples was obtained from the critical indentation stress for the onset of plastic deformation (
To determine the Brinell hardness index, one must first fix the testing constant,
In this study, Hertzian and Brinell indentation tests were used to evaluate the mechanical properties of cork. The main conclusions and implications to be drawn from the study are as follows. The methodological approach is well suited to the characterization of the elastic and plastic behaviour of soft materials of diverse nature (cellular materials). The observed elastic anisotropy is closely related to the anisotropic structure of the different sections of cork. The “back” tangential section is weaker than the other sections of the material. The specimens presented large deformations at relatively low stresses, consistent with the low value of the material’s elastic modulus. The material’s elastic zone was relatively small (low values of the yield stress), but well defined. Finally, we wish to note that Hertzian and Brinell indentation testing would seem to be well suited not only to characterize mechanically such materials as cork, but also to evaluate the intrinsic quality of these materials.
The authors declare that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria, educational grants, participation in speakers bureaus, membership, employment, consultancies, stock ownership, or other equity interests, and expert testimony or patent-licensing arrangements), or nonfinancial interest (such as personal or professional relationships, affiliations, knowledge, or beliefs) in the subject of matter or materials discussed in this paper. Consequently, the authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank Dr. A. Pajares and Dr. A. L. Ortiz for the fruitful discussion and the Institute for the Promotion of Cork, Wood, and Coal (ICMC), Junta de Extremadura (Spain), for supplying the cork samples.