• Pili nutshell: high strength and toughness. • Scale-dependent behavior: mechanical properties vary across hierarchical levels. • Bipyramidal shape and triangular cross-section: enhancing stability. • Sandwich-structure: crack deflection and energy dissipation. • Brittle base material with high strength. Cracking the endocarp of pili nuts is no easy task – it requires pre-notching or the expertise of skilled workers to open the shell. This difficulty arises from its exceptional strength and toughness, which are due to its hierarchical structure and form. We combine various imaging techniques, mechanical testing, and computational modelling to elucidate the relationship between micro- and macrostructure and mechanical properties. Compression tests with digital image correlation on whole and half nuts, along with finite element simulations, show how shell geometry influences the strain distribution and, thus, crack initiation and propagation. The three-sided bipyramidal shape provides high stability of the endocarp under compression, due to optimized load distribution. Simulations predict regions of maximum principal stresses. In brittle materials, cracks are likely to initiate here as the tensile strength is exceeded first. This hypothesis is corroborated by our lab tests on half-nuts. Toughening mechanisms operate at the macro-scale, deflecting cracks along weak interfaces in the sandwich structure. Thus, the hierarchical design – combining a strong though brittle base material with weaker interfaces deflecting cracks, and a geometry that evenly distributes loads – offers inspiration for lightweight, robust composite containers made from materials otherwise inherently prone to sudden, catastrophic fracture.
Ali et al. (Sun,) studied this question.