Fractal analysis of microstructure-related indentation toughness of Clinocardium californiense shell

The correlation between microstructures and indentation toughness of Clinocardium californiense shell was characterized by a fractal method. It is found that the shell exhibits a hierarchical structure comprising three layers, i.e., the inner layer with a cross-lamellar structure, the middle layer with a complex cross-lamellar structure, and the outer layer with a prismatic structure. The microstructure in the middle layer with fine and complex arrangement of aragonite sheets corresponds to the largest fractal dimension for indentation-induced cracks, and consequently to the highest indentation toughness.