The energy dissipation of the sea urchin spine as biomimetic concept generator for cellular ceramics and load-bearing systems

Abstract

The sea urchin spine of Phyllacanthus imperialis is characterized by a lightweight construction undergoing energy dissipation by quasi-plastic deformation based on multiple fracturing, even though the constituting material is basically brittle magnesium calcite (CaxMg1-xCO3) with a very low content of organic matter and yet is organized in a hierarchical construction. The correlation between the structural and mechanical characteristics of the sea urchin spine of Phyllacanthus imperialis is analyzed in depth to discover the operating principle in order to transfer them into ceramic- and concrete-based materials. High-resolved micro-computed tomography (μCT) proved to be ideally suited to analyze the internal structures of the adult spine of Phyllacanthus imperialis non-invasively in regard of the strut arrangement, configuration and connectivity. Based on μCT reconstruction of the microstructural elements of the spine, the relationship of the varying structural elements was defined and summarized accurately in a two-dimensional map as abstracted, structural key elements. A comparative analysis of the microstructure of the adult and of a not fully grown spine shows that the growth process of the spine is characterized by an interplay of calcite resorption and precipitation caused by phagocytes and sclerocytes, respectively. The type of mechanical behavior of adult spine segments of Phyllacanthus imperialis is dependent on the volumetric proportion of the outer layer, the cortex. Two main failure modes are identified: a) a high volumetric quantity of the cortex in the spine segment results in high initial strength and stiffness and b) when the volumetric quantity of the cortex is low, a comparable large energy dissipation capacity combined with a low initial strength and stiffness is given. A comparison between the spine interior (radiating layer and medulla) and the radiating layer in terms of the mechanical properties has revealed that the wedge-like enclosing of the radiating layer with the lateral side branches of the medulla provides a larger mechanical stability and is responsible for the controlled detachment of segments at advanced stages of compression. A transfer of the highly interconnected and directional structures of the sea urchin spine in ceramic-based materials is feasible utilizing the freeze-casting method. Both agar and gelatin as additives in a water-based ceramic suspension are suitable to manufacture directional and cellular cell geometries in the ceramics. The concentration of gelatin in the ceramic suspension determines the degree of the connectivity of the cell walls, which dictates, in turn, the type of mechanical behavior. Two categories of failure modes are determined: a) a progressive interplay of flaking and crumbling and b) a segmental division of the ceramic into several lath-like segments. The Zhang and Ashby (1992) honeycomb model prediction is not applicable for freeze-cast ceramics with a directional pore system, because it models an elastic/ plastic honeycomb foam behavior rather than brittle fracturing. Brittle fracturing can be covered by the pore model by Pabst and Gregorová (2014) and is a reasonable approach to cover the physical constraints for freeze-cast ceramics. Freeze-cast alumina ceramics characterized by prolate pore shape are associated with low Eshelby (1957)-Wu (1966) exponents and exhibit a high resilience in the light of strength and stiffness. Ceramics, which are comprising isolated cell walls have the lowest resilience. Their high values of the Eshelby (1957)-Wu (1966) exponents indicate an oblate pore shape. The relation of the elastic moduli and the pore shape by Pabst and Gregorová (2014) is applicable to freeze-cast alumina ceramics. The operating principles of the sea urchin spine of Phyllacanthus imperialis have been revealed and are suitable as concept generator for biomimetic materials as well as for concrete-based constructions, which might be fabricated in pursuing research activities. III