Ahmad Khayer Dastjerdi

The weak interfaces within tough natural composites: Experiments on three types of nacre

Mineralization is a typical strategy used in natural materials to achieve high stiffness and hardness for structural functions such as skeletal support, protection or predation. High mineral content generally leads to brittleness, yet natural materials such as bone, mollusk shells or glass sponge achieve relatively high toughness considering the weakness of their constituents through intricate microstructures. In particular, nanometers thick organic interfaces organized in micro-architectures play a key role in providing toughness by various processes including crack deflection, crack bridging or energy dissipation. While these interfaces are critical in these materials, their composition, structure and mechanics is often poorly understood. In this work we focus on nacre, one of the most impressive hard biological materials in terms of toughness. We performed interfacial fracture tests on chevron notched nacre samples from three different species: red abalone, top shell and pearl oyster. We found that the intrinsic toughness of the interfaces is indeed found to be extremely low, in the order of the toughness of the mineral inclusions themselves. Such low toughness is required for the cracks to follow the interfaces, and to deflect and circumvent the mineral tablets. This result highlights the efficacy of toughening mechanisms in natural materials, turning low-toughness inclusions and interfaces into high-performance composites. We found that top shell nacre displayed the highest interfacial toughness, because of higher surface roughness and a more resilient organic material, and also through extrinsic toughening mechanisms including crack deflection, crack bridging and process zone. In the context of biomimetics, the main implication of this finding is that the interface in nacre-like composite does not need to be tough; the extensibility or ductility of the interfaces may be more important than their strength and toughness to produce toughness at the macroscale.

An improved failure criterion for biological and engineered staggered composites

High-performance biological materials such as nacre, spider silk or bone have evolved a staggered microstructure consisting of stiff and strong elongated inclusions aligned with the direction of loading. This structure leads to useful combinations of stiffness, strength and toughness, and it is therefore increasingly mimicked in bio-inspired composites. The performance of staggered composites can be tuned; for example, their mechanical properties increase when the overlap between the inclusions is increased. However, larger overlaps may lead to excessive tensile stress and fracture of the inclusions themselves, a highly detrimental failure mode. Fracture of the inclusions has so far only been predicted using highly simplified models, which hinder our ability to properly design and optimize engineered staggered composites. In this work, we develop a new failure criterion that takes into account the complex stress field within the inclusions as well as initial defects. The model leads to an ‘optimum criterion’ for cases where the shear tractions on the inclusions is uniform, and a ‘conservative’ criterion for which the tractions are modelled as point forces at the ends of the overlap regions. The criterion can therefore be applied for a wide array of material behaviour at the interface, even if the details of the shear load transfer is not known. The new criterion is validated with experiments on staggered structures made of millimetre-thick alumina tablets, and by comparison with data on nacre. Formulated in a non-dimensional form, our new criterion can be applied on a wide variety of engineered staggered composites at any length scale. It also reveals new design guidelines, for example high aspect ratio inclusions with weak interfaces are preferable over inclusions with low aspect ratio and stronger interfaces. Together with existing models, this new criterion will lead to optimal designs that harness the full potential of bio-inspired staggered composites.

Direct Measurements of the Cohesive Behavior of Soft Biological Interfaces

The mechanical strength, reliability and toughness of biological tissues rely on high-performance bio-adhesives composed of proteins and/or polysaccharides. Without these “natural glues”, materials such as bone, teeth, seashells or eggshells would simply disintegrate. Therefore, the structure and composition of these bio-glues dictates the strength, toughness, reliability and performance of tissues at a fundamental level. Single-molecule pullout experiments using atomic force microscopes have revealed the extraordinary properties of some proteins involved in adhesion. However, how this nanoscale performance relates to macroscopic interfacial fracture toughness is currently unknown. In this work we present a new interfacial fracture experimental approach based on the standard double-cantilever beam (DCB) test for adhesives used in engineering, which was adapted to take into account the high compliance of the interface compared to the beams. Interestingly, this “rigid” DCB approach leads to the full cohesive law of the interface, which when compared to a single toughness value yields more information on the chemical, molecular and structural mechanisms operating at the interface. As an example we present a study on fibrin, a protein involved in blood clotting and used as a bio-adhesive in surgical procedures. The effects of the substrate and calcium content on adhesion are examined and discussed.

Tough and deformable glasses with bioinspired cross-ply architectures

Glasses are optically transparent, hard materials that have been in sustained demand and usage in architectural windows, optical devices, electronics and solar panels. Despite their outstanding optical qualities and durability, their brittleness and low resistance to impact still limits wider applications. Here we present new laminated glass designs that contain toughening cross-ply architectures inspired from fish scales and arthropod cuticles. This seemingly minor enrichment completely transforms the way laminated glass deforms and fractures, and it turns a traditionally brittle material into a stretchy and tough material with little impact on surface hardness and optical quality. Large ply rotation propagates over large volumes, and localization is delayed in tension, even if a strain softening interlayer is used, in a remarkable mechanism which is generated by the kinematics of the plies and geometrical hardening. Compared to traditional laminated glass which degrades significantly in performance when damaged, our cross-ply architecture glass is damage-tolerant and 50 times tougher in energy terms. STATEMENT OF SIGNIFICANCE Despite the outstanding optical qualities and durability of glass, its brittleness and low resistance to impact still limits its wider application. Here we present new laminated glass designs that contain toughening cross-ply architectures inspired from fish scales and arthropod cuticles. Enriching laminated designs with crossplies completely transforms the material deforms and fractures, and turns a traditionally brittle material into a stretchy and tough material - with little impact on surface hardness and optical quality. Large ply rotation propagates over large volumes and localization is delayed in tension because of a remarkable and unexpected geometrical hardening effect. Compared to traditional laminated glass which degrades significantly in performance when damaged, our cross-ply architecture glass is damage-tolerant and it is 50 times tougher in energy terms. Our glass-based, transparent material is highly innovative and it is the first of its kind. We believe it will have impact in broad range of applications in construction, coatings, chemical engineering, electronics, photovoltaics.

The Cohesive Law and Toughness of Engineering and Natural Adhesives

Polymeric adhesives play a critical role in engineering applications, whether it is to bond components together or to serve as matrix for composite materials. Likewise, adhesives play a critical role in natural materials where adhesion is needed (e.g. mussel byssus) or to simply preserve the integrity of natural composite materials by holding fibers together (e.g. extra-collagenous proteins in bone). In this work we use a newly developed technique to measure the cohesive law and toughness of adhesives which is similar to a standard double cantilever beam configuration, but in which the beams are replaced by two rigid blocks. We originally developed this method for extracting the cohesive law of soft and weak biological adhesives, and we here show that it can be modified to include high strength of engineering adhesives. Using this method, the cohesive law of the adhesive is directly computed from the load-deflection curve of the experiment, without making initial assumption on its shape. The cohesive law reveals the strength and extensibility of the adhesives, which is richer in information than the toughness (which is the area under the cohesive law). We also define a non-dimensional parameter which can be used to quantitatively investigate whether the assumption of rigid substrates is valid. For values of the parameter close to unity, the RDCB rigidity assumption is valid and the method directly yields the cohesive law of the adhesive. The engineering and natural adhesives we tested showed a wide range of strength, toughness and extensibility, and revealed new pathways which can be exploited in the design and fabrication of biomimetic materials.

Interfacial Fracture Toughness of Nacre

Nacre is a natural mineralized composite which is made of 95% aragonite yet is three orders of magnitude tougher than its main ingredient. Extensive research has recently been devoted to identifying the toughening mechanisms in nacre, among which the toughness of the organic component has been claimed to have the most significant impact on the overall toughness. In this study, interlaminar fracture toughness of nacre from three different species, namely red abalone, pearl oyster and top shell, is measured and reported using chevron notch fracture technique. Among the three seashells, top shell exhibits outstanding levels of interlaminar toughness which is even comparable to the values across the tablet layers. Analysis of the experimental data from top shell suggests that the intrinsic toughness of the organic glue accounts for only about 3% of the overall interface toughness, whereas the main contribution to the toughness originates from a multitude of extrinsic toughening mechanisms including ligament bridging, crack deflection, and process zone effect. While the same is true for the other two shells, the toughening mechanisms are less pronounced. This finding clearly emphasizes the role of the microstructure on the overall material properties.

Hierarchical structure, mechanical properties and fabrication of biomimetic biomaterials

Abstract: This chapter investigates the concept of hierarchy widely found in biological materials. First, natural hierarchical materials are explored in terms of their high order structures formed from universal building blocks. Hierarchical arrangement is claimed to give rise to remarkable mechanical properties of biological structures. Therefore at the next step, the significance of hierarchical structuring on mechanical properties is investigated through available analytical models. Finally, fabrication methods which could potentially lead to artificial hierarchical structures are briefly reviewed in the domain of biomimetics.