Microstructure and mechanical properties of hard Acrocomia mexicana fruit shell

Abstract

Fruit and nut shells can exhibit high hardness and toughness. In the peninsula of Yucatan, Mexico, the fruit of the Cocoyol palm tree (Acrocomia mexicana) is well known to be very difficult to break. Its hardness has been documented since the 1500 s, and is even mentioned in the popular Maya legend The Dwarf of Uxmal. However, until now, no scientific studies quantifying the mechanical performance of the Cocoyol endocarp has been found in the literature to prove or disprove that this fruit shell is indeed "very hard". Here we report the mechanical properties, microstructure and hardness of this material. The mechanical measurements showed compressive strength values of up to ~150 and ~250 MPa under quasi-static and high strain rate loading conditions, respectively, and microhardness of up to ~0.36 GPa. Our findings reveal a complex hierarchical structure showing that the Cocoyol shell is a functionally graded material with distinctive layers along the radial directions. These findings demonstrate that structure-property relationships make this material hard and tough. The mechanical results and the microstructure presented herein encourage designing new types of bioinspired superior synthetic materials.

Figure 1

Cocoyol fruit and endocarp specimens. (a) Hierarchical structure of Cocoyol endocarp. (b) Specimens for microscopy analysis and mechanical testing. (c) Geometrical details of specimens.

Figure 2

Optical micrographs of Cocoyol fruit endocarp. (a) Cross section of fruit endocarp in equatorial plane. (b) Cross section of fruit endocarp in meridional plane. (c-d) Close-up views.

Figure 3

Representative 3D micro-CT and SEM images of Cocoyol endocarp. (a) 3D micro-CT scan; slices were obtained at various depths, from a central sub-volume of the scanned sample. The brighter contrast indicates distribution of cells and darker regions indicate cell lumina and space between cells. (b) SEM images of the edges of a cuboid specimen showing the 3D shape of elongated sclereids.

Figure 4

Typical uniaxial compression true stress-true strain curves. (a) Quasi-static compression tests. (b) High strain-rate (HSR) compression tests (at ∼10³ s⁻¹).

Figure 5

SEM images of crack surfaces. Identified failure mechanisms include A. Cell tearing; B. Middle lamella breakage; C. Primary cell wall breakage; D. Pull-out of elongated cells. (a) Crack surface after quasi-static compression test. (b) Crack surface after C-ring test. (c) Close-up image of surface near the outer edge after C-ring test. (d) Close-up image of surface near the inner edge after C-ring test.

Figure 6

Nanoindentation and Vickers hardness results. (a) Reduced elastic modulus E_r and hardness H of the endocarp measured from the internal edge (IE) to external edge (EE) for meridional and equatorial specimens at a maximum load of 100 mN (nanoindentation). (b) Vickers hardness results. (c-d) Typical Vickers indentation marks. (e) Zones in the ring specimen for nanoindentation and Vickers hardness tests.

Figure 7

Ashby charts for different types of materials including the Cocoyol endocarp. (a) Strength versus density (Adapted from: http://www-materials.eng.cam.ac.uk/mpsite/interactive_charts/). (b) Strength versus toughness (From data available in the literature: rigid foams, concrete, woods^,,, polymers, metals and alloys^–, composites,).