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A Bouligand structure is a layered and rotated microstructure resembling plywood, which is frequently found in naturally designed materials. It consists of multiple lamellae, or layers, each one composed of aligned fibers. Adjacent lamellae are progressively rotated with respect to their neighbors. This structure enhances the mechanical properties of materials, especially its fracture resistance, and enables strength and in plane isotropy. It is found in various natural structures including the cosmoid scale of the coelacanth, and the dactyl club of the mantis shrimp and many other stomatopods.

Due to its desirable mechanical properties, there are ongoing attempts to replicate Bouligand arrangements in the creation of failure resistant bioinspired materials. For example, it has been shown that layered composites (such as CFRP) utilizing this structure have enhanced impact properties. However, replicating the structure on small length scales is challenging, and the development and advancement of manufacturing techniques continually improves the ability to replicate this desirable structure.

Proposed Changes: Influence of Bouligand Structure on Additive Manufacturing of Cement

The most current and notable research relating to Bouligand structure deals with additive manufacturing of cement. Many researchers have created tool paths that deposit cement in a Bouligand structure in the hopes to increase the mechanical properties of the 3D structures, which is one of the most critical issues surrounding AM. This topic may be a subsection under “Biomimicry” and will explore development of specific Bouligand-inspired tool paths and how this impacts the mechanical properties of the AM parts (as compared to cast parts). An image will be included in this section showing how the concrete is deposited in a Bouligand type pattern.

Crab and Lobster Exoskeletons
Arthropods have exoskeletons that provide protection from the environment, mechanical load support, and body structure. The outer layer, called the epicuticle, is thin and waxy and is the main waterproofing barrier. Below is the procuticle, which is designed as the main structural element to the body. The procuticle is made of two sections, the exocuticle on the outer part, and the endocuticle on the inner part. The exocuticle is denser than the endocuticle; the endocuticle makes up about 90 volume % of the exoskeleton. Both the exocuticle and endocuticle are made with a Bouligand structure.

The arthropod exoskeleton is highly hierarchical. Polysaccharide chitin fibrils arrange with proteins to form fibers, the fibers coalesce into bundles, and then the bundles arrange into horizontal planes which are stacked helicoidally, forming the twisted plywood Bouligand structure. This results in a highly mineralized structure. Repeating Bouligand structures form the exocuticle and endocuticle. Differences in the Bouligand structure of the exocuticle and endocuticle have been found to be critical for analyzing the mechanical properties of both regions.

Crab
In crab exoskeletons, calcite and amorphous calcium carbonate are the minerals deposited in the chitin-protein heirarchical matrix. The sheep crab (Loxorhynchun grandis), like other crabs, has a highly anisotropic exoskeleton. The spacing between the (x-y) plane Bouligand lamellae in the crab exocuticle is ~3-5μm, whereas the interlamellar spacing in the endocuticle is much greater, about 10-15μm. The smaller spacing of the exocuticle results in a higher lamellae density in the exocuticle. There is a higher hardness measurement in the exocuticle than the endocuticle, which is attributed to a higher mineral content in the exocuticle. This gives a higher wear resistance and hardness on the surface of the exoskeleton, thus giving the crab a greater degree of protection. Under stress, the Bouligand planes fail via normal bundle fracture or bundle separation mechanisms. The exocuticle-endocuticle interface is the most critical region and typically where failure first occurs, due to the anisotropic structure and Bouligand discontinuity at this interface.

In the z-direction, porous tubules exist normal to the Bouligand planes that penetrate the exoskeleton. The function of these tubules is to transport ions and nutrients to the new exoskeleton during the molting process. The presence of these tubules, which have a helical structure, results in a ductile necking region during tension. An increased degree of ductility increases the toughness of the crab exoskeleton.

Lobster
The Homarus americanus (American lobster) is an anthropod with an exoskeleton structure similar to the crabs above, and with similar trends comparing the endo- and exo- cuticles. An important note for the lobster exoskeleton structural/mechanical properties is the impact of the honeycomb structure formed by the Bouligand planes. The stiffness values for the exocuticle in lobster range from 8.5-9.5 GPa, while the endocuticle ranges from 3-4.5 GPa. Gradients in the honeycomb network, especially at the interface between the endo- and exo- cuticle are believed to be the reason for this discrepancy between the structures.

Influence of Bouligand Structure on Additive Manufacturing
Additive manufacturing is a popular upcoming form of industry which allows for complex geometries and unique performance characteristics for AM parts. The main issue with mechanical properties of AM parts is the introduction of microstructural heterogeneities within layers of deposited material. These defects, including porosity and unique interfaces, result in anisotropy of the mechanical response of the workpiece, which is undesirable. To combat this anisotropic mechanical response, a Bouligand-inspired tool path is used to deposit the material in a twisted Bouligand structure. This results in a stress transfer mechanism which uses interlayer heterogeneities as stress deflection points, thus strengthening the workpiece at these points. Bouligand tool paths are used specifically in cement/ceramic deposition AM. Bouligand-inspired AM parts have been observed to behave better than cast elements under mechanical stress.

Pitch Angle
A critical parameter in the development of the Bouligand-inspired tool path is the pitch angle. The pitch angle γ is the angle at which the helicoidal structure is formed. The relative size of the pitch angle is critical for the mechanical response of a Bouligand-inspired AM toolpiece. For γ < 45° (small angle), interfacial crack growth and interfacial microcracking is observed. For 45° < γ < 90° (large pitch angle), dominant crack growth through the solid is observed.

Additive Manufacturing and Performance of Architechtured Cement-Based Materials
* Note* all content in this section comes from this article, this section is for notes

AM of materials introduces microstructural heterogeneities (porosity and interfaces) which result in anisotropic mechanical properties.

Properties depend on printing directions.

A current approach to remedy these heterogeneities is "incorporation of multi-scale hierarchical and bioinspired design principles over a broad range of architectures of fabricated materials, from nano to micro, in order to engineer the mechanical properties to significantly enhance thestrength and tensile performance, load bearing capacity, compliancy, and impact resistance and to overcome the brittleness and flaw sensitivity limitations of these materials."

Current research efforts focus on improving stress transfer across interfaces in 3D printed elements

The focus of this work is on the 3D printing of brittle cement-based materials, in chich the ability to control the internal architecture of the structure at the macroscopic level (mm scale) may play a significant role by enabling novel performance characteristics, such as quasi brittle mechanical behavior, fracture and damage tolerance, unique load displacement response and enhanced flexural strength.

Cast cement paste behaves as a brittle material and does not show nonlinear post-peak load displacement behavior, however controlling the internal architecture of elements can spread the damage and improve the overall inelastic response of composite materials (brittle ceramics).

Modulus of Rupture (MOR) and Work of Failure (WOF) are critical parameters that indicate successful material properties.

The implication of engineering of the architecture of the 3D-printed elements enables mitigation of catastrophic failure that is not attainable in cast elements. These architectures offer enhanced fracture properties by enabling crack propagation in a stepwise pattern, crack redirection, branching, and prevention of catastrophic failure in various biological organisms. Cracks grow in twisted patterns following the direction of fibers. These twisting patterns have been found to be responsible for increasing toughness and promote the spread of damage.

Pitch Angle

Small pitch angles allow for crack growth at the interface and for control of crack path whereas large pitch angles facilitate crack advancement through the solid material.

Microcrack advancement is observed.

In Bouligand structures, characteristic heterogeneous interfaces exist and are not necessarily detrimental ot the overall performance but can facilitate mechanisms that lead to novel deformation response (increased deflection, enhanced WOF, etc). Depends on the ability of material to spread damage over larger volumes.

Strength and toughness enhancement in 3d printing via bioinspiredtool path

-another article that supports the findings in [4] regarding mechanical properties and tool path

In the News
Researchers at Purdue and in China have been developing biomimetic Bouligand-based tool paths for 3D-printing of cement structures. These methods increase the mechanical properties by promoting crack propagation across whole interfaces, therefore minimizing local stress distribution and high impact damage.

Many companies are using 3D-printing of cement to build houses and other structures.

Resources
Mohamadreza Moini, Jan Olek, Jeffery P. Youngblood, Bryan Magee, Pablo D. Zavattieri. "Additive Manufacturing and Performance of Architectured Cement-Based Materials." Advanced Materials Vol 30, Issue 43. 2018. 201802123.