Toughening Mechanisms of Bouligand Structure in Dactyl Club The Bouligand structure found in many natural materials is credited with imparting a very high
toughness and
fracture resistance to the overall material it is a part of. The mechanisms by which this toughening occurs are many, and no one mechanism has yet to be identified as the main source of the structure's toughness. Both computational work and physical experiments have been done to determine these pathways by which the structure resists fracture so that synthetic tough Bouligand structures can be taken advantage of. Crack deflection of one form or another is considered the main toughening mechanism in the bouligand structure. Deflection can take the form of crack tilting, and crack bridging. Another means of toughening the bouligand structure is by shear wave filtering. The periodic and hierarchical nature of the Bouligand structure, creates a shear wave filtering effect that is especially effective during high intensity dynamic loads. As the force is applied, specific frequencies that are in shear are not permitted to transmit through the layered structure, creating a
band gap in the transmitted energies and decreasing the effective energy felt by the system. The pitch angle of the layers, thickness of the layers, and number of layers present in the material all effect which frequencies are filtered out.
Adaptability Adjustment of the Bouligand structure during loading has been measured using small angle X-ray scattering (
SAXS). The two adjustment effects are the change in angle between the
collagen fibrils and tensile axis, and the stretching of collagen fibrils. There are four mechanisms through which these adjustments occur. • Fibrils rotate because of interfibrillar shear: As a tensile force is applied, fibrils rotate to align with the tensile direction. During deformation, the shear component of the applied stress causes the hydrogen bonds between fibrils to break and then reform after fibril adjustment. • Collagen fibrils stretch: Collagen fibrils can elastically stretch, resulting in fibrils re-orientating to align with the tensile direction. • Tensile opening of interfibrillar gaps: Fibrils highly misoriented with the tensile direction can separate, creating an opening. • "Sympathetic" lamella rotation: A lamella is able to rotate away from the tensile direction if it is sandwiched between two lamellae that are reorienting themselves towards the tensile direction. This can happen if the bonding between these lamellae is high. Ψ refers to the angle between the tensile axis and the collagen fibril. Mechanisms 1 and 2 both decrease Ψ. Mechanisms 3 and 4 can increase Ψ, as in, the fibril moves away from the tensile axis. Fibrils with a small Ψ stretch elastically. Fibrils with a large Ψ are compressed, since adjacent lamellae contract in accordance with
Poisson's ratio, which is a function of strain
anisotropy.
Single vs. Double Bouligand Structure The most common Bouligand structure found in nature is the twisted plywood structure where there is a constant angle of misalignment between layers. A rare variation of this structure is the so-called "double twisted" Bouligand structure seen in
Coelacanth. This structure uses stacks of two as units to be twisted with respect to each other at some constant misalignment angle. The two fibril layers in each of these units in this case lay such that their fibril orientation is perpendicular to each other. The mechanical differences between the single and double twisted bouligand structure has been observed. It was shown that the double bouligand structure is stiffer and tougher than the more common single bouligand structure. The increase in stiffness is also accompanied by a reduction of flexibility. The increased strength is attributed in part to an addition to the structure of "inter-bundle fibrils" that run up and down the stack of layers, perpendicular to the twisted fiber planes. These fiber bundles help keep the structure together by greatly increasing the energy needed for inter-fibril sliding. These bundles are coupled with the double twisted nature of the plywood arraignment, which shifts the direction a crack would like to grow drastically with each layer. It has also been observed that a structure can form mostly similar to the single twisted bouligand structure, but with a non-constant angle of misalignment. It is still unclear how this particular structural difference affects mechanical properties. == Examples in Nature ==