Ribs of Pinna nobilis shell induce unexpected microstructural changes that provide unique mechanical properties

Kinga Nalepka,Katarzyna Berent,Antonio G Checa,Tomasz Machniewicz,Adrian J Harris,Paweł Nalepka,Martyna Strąg,Łukasz Maj,Aleksandra Szkudlarek,Magdalena Bieda,Krzysztof Sztwiertnia

doi:10.1016/j.msea.2021.142163

Abstract

The reinforcement function of shell ribs depends not only on their vaulted morphology but also on their microstructure. They are part of the outer layer which, in the case of the Pinna nobilis bivalve, is built from almost monocrystalline calcitic prisms, always oriented perpendicular to the growth surfaces. Originally, prisms and their c-axes follow the radii of rib curvature, becoming oblique to the shell thickness direction. Later, prisms bend to reach the nacre layer perpendicularly, but their c-axes retain the initial orientation. Calcite grains form nonrandom boundaries. Most often, three twin disorientations arise, with two of them observed for the first time. Nano-indentation and impact tests demonstrate that the oblique orientation of c-axes significantly improves the hardness and fracture toughness of prisms. Moreover, compression tests reveal that the rib area achieves a unique strength of 700 MPa. The detection of the specific microstructure formed to toughen the shell is novel.

Highlights

Bivalve shells are hierarchically complex biocomposites consisting of calcium carbonate and an organic matrix[1,2,3]
The regularities revealed in the theoretical analysis are mapped by the relation that combines the values of the mechanical property measured for representative grains in the nano-indentation test
Research conducted on biogenic calcite shows that one of the sources of improving mechanical properties is structural defects[35]

Summary

Introduction

Bivalve shells are hierarchically complex biocomposites consisting of calcium carbonate (calcite or aragonite) and an organic matrix[1,2,3]. They have been improved over millions of years of evolution to provide effective protection against predators. The shells are lightweight and exhibit outstanding mechanical properties compared to the materials from which they are built. Protective armors are an excellent source of inspiration for the formation of biomimetic engineering materials with an unique combination of high strength and fracture toughness[4,5,6,7]. The examined specimen is distinguished by a large, strongly ribbed surface

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