Simpleware Case Study: Elastic Properties of Bivalve Shell Nanocomposites

Material Measurements

Experimental testing was carried out to obtain material properties of the bivalve samples. Five bivalve species were sourced and scanned using scanning electron microcopy (JEOL JSM-820 SEM – JEOL Ltd., Tokyo, Japan) to compare microstructures. Other testing included novel 4-point bending measurements using a Tinius Olsen 5 Kn Universal Test Machine (Tinius Olsen Ltd., Redhill, UK). Nanoindentation measurements were also taken to determine the Young’s modulus and hardness of different microstructure, as well as the anisotropy of properties.

4-point bending jig: The sample is placed across the bottom rods and stress is applied downwards through the upper two bending points. The upper bending points span a third way along the bottom points, permitting the use of the third span equation to calculate the Young’s modulus

There are limitations to this approach in terms of capturing the true geometry of the bivalve shell microstructure. FEA-based homogenization analysis provides a more robust solution as it is based on actual microstructures taken from X-ray tomography. This method takes into account the multiple microstructures that are typical of bivalves, and how they come together to create an adaptive advantage for different environments.

X-Ray Image-Based Analysis using Simpleware Software

To capture these results, the bivalve shells were scanned using a Bruker SKYSCAN 1172 micro-CT machine to generate 3D images of the microstructure. The data was then imported to Simpleware ScanIP to carry out image processing techniques, including thresholding and segmentation, to identify different phases of the shell. Once segmented, the image data was meshed using the Simpleware FE module, which ensures that meshes are ready-to-use in a simulation tool.

Segmentation and mesh generation of the prismatic structure of a Pinna nobilis from CT scan: (a) Organic material (dark gray) can be distinguished from calcite prisms (lighter gray). The cuboid had dimensions 0.4 x 0.4 x 0.27 mm³; (b-d) FEA model of the different structures; (e) the two phases defined by a finite number of tetrahedral elements

In this case, the Simpleware SOLID module, part of a set of Physics modules available with the software, was used due to its ability to calculate the effective stiffness tensor of the composite structure through the use of (simulated) external forces. The finite element-based homogenization tools in Simpleware SOLID were particularly valuable, in this study, for obtaining results on the unique microstructure of the shells from the high-quality X-ray tomography data. This data was then used to model a much larger section of the shell, reducing workflow run-times and cutting down on hardware requirements.

Conclusions

The workflow developed by the University of Cambridge was able to accurately model compressional and tensional stresses within the shells, and to understand how strains are managed by the distribution of phases in the material. Analysis also revealed how intracrystalline organic material affects stiffness in the whole microstructure, and the extreme anisotropy of properties arising from the prismatic microstructure. Specific insights were gained into how elastic properties of mollusk shells are adapted in species with swimming capabilities, as compared to resistance to damage from impact by predators.

Stress distribution using the Simpleware SOLID module in the prismatic part of the shell of a Pinna nobilis when tensile forces are imposed in the Z-direction (parallel to prisms). Calcite prisms are visible in green in XY plane and orange in ZX plane. The organic matrix is light blue (XY plane) and dark blue (ZX plane)

The use of Simpleware software was essential for incorporating X-ray tomography-based FEA to studies of complex biocomposites. By using this workflow, researchers can better understand how biocomposites function, and how their adaptive properties might carry over improving the design of other products.