Unusual elastic behaviour of a wood-like material during bending, milling and nanoindentation

Abstract

Pistachio hardshells exhibit isotropic composite sclerenchyma at the length-scale of nanoindentation. The cells are composed of the common wood constituents and their macroscopic elasticity is unusual: upon reversible bending to less than 50% of the clear-length the height to length ratio increases up to 10-fold. Such behaviour is not reflected by the reduced elastic modulus value from nanoindentation compared with very close values for brittle walnut and macadamia shell. This may indicate outstanding values of Poisson's ratio for pistachio, but it covers only the sclerenchymatous cell wall. Three-dimensional light microscopy on fresh fracture surfaces revealed highly structured pistachio cells with micrometre-sized protrusions in all directions, in contrast to the smoothness of walnut cells in the outer shell. Neither of the hardshells exhibited a fibrous microstructure. Elasticity and isotropy of pistachio hardshells were also evident in their unusual breaking into uniform brick-type fragments upon milling. The cell interlocking in pistachio shells (three varieties and also a salted roasted brand) may be interpreted as a bionics model for elasticity with potential use for the construction of macroscopic structures, and pistachio shells may be used as elastic feedstock rather than being wasted.