The spines of the sea urchins Heterocentrotus mamillatus and Phyllacanthus imperials are on the scale of mm to cm and are investigated as role models for architectural applications, which have the size of dm to m. The sea urchin spines are made from Mg-calcite (a carbonate), which is definitely a brittle material. Therefore, any scaling up is expected to obey the classic volume effect described by the Weibull theory. As naturally grown materials spines have a relative high variability in their structures, which are potential critical defects. Accordingly, a low Weibull modulus is expected, which should lead to a dramatic decrease in strength with increasing volume.
In order to study the effect, the strength measured by uniaxial compression and pin indentation tests of small and large spines is compared. The problem of the study is to take into account the natural heterogeneity, which effects the strength via porosity variations, which were analysed by image analyses and gravimetry and other internal structure variations analysed by SEM. Surprisingly, it seems that the strength of small and large spines is very similar, indicating that nature has found a way to mitigate the size effect. We discuss the reasons for this deviation, which is sought within the special structural features of the spines.