Granular crystals as strong and fully dense architectured materials.

Dense topologically interlocked panels are made of well-ordered, stiff building blocks interacting mainly by frictional contact. Under mechanical loads, the deformation of the individual blocks is small, but they can slide and rotate collectively, generating high strength, toughness, impact resistance, and damage tolerance. Here, we expand this construction strategy to fully dense, 3D architectured materials made of space filling building blocks or "grains." We used mechanical vibrations to assemble 3D printed rhombic dodecahedral and truncated octahedral grains into fully dense face-centered cubic and body-centered cubic "granular crystals." Triaxial compression tests revealed that these granular crystals are up to 25 times stronger than randomly packed spheres and that after testing, the grains can be recycled into new samples with no loss of strength. They also displayed a rich set of mechanisms: nonlinear deformations, crystal plasticity reminiscent of atomistic mechanisms, geometrical hardening, cross-slip, shear-induced dilatancy, and microbuckling. A most intriguing mechanism involved a pressure-dependent "granular crystal plasticity" with interlocked slip planes that completely forbid slip along certain loading directions. We captured these phenomena using a three-length scale theoretical model which agreed well with the experiments. Once fully understood and harnessed, we envision that these mechanisms will lead to 3D architectured materials with unusual and attractive combinations of mechanical performances as well as capabilities for repair, reshaping, on-site alterations, and recycling of the building blocks. In addition, these granular crystals could serve as "model materials" to explore unusual atomic scale deformation mechanisms, for example, non-Schmid plasticity.

Solidification of Polyurethane Model Foams via Catalyst Drainage from a Secondary Foam.

Due to their unique mechanical and thermal properties, polyurethane foams are widely used in multiple fields of applications, including cushioning, thermal insulation or biomedical engineering. However, the way polyurethane foams are usually manufactured - via chemical foaming - produces samples where blowing and gelling occur at the same time, resulting in a morphology control achieved by trial and error processes. Here, a novel strategy is introduced to build model homogeneous polyurethane foams of controlled density with millimetric bubbles from liquid templates. By producing a polyurethane foam via physical bubbling without a catalyst and gently depositing a secondary foam containing catalyst on the top of this first foam, it is possible to take advantage of drainage mechanisms to trigger the solidification of the bottom foam. The characterization of the samples performed by X-ray microtomography allows to study quantitatively the structure of the final solid foam, at the global and at the local scale. Using the tomographic 3D images of the foam architectures, the superimposed foam technique introduced in this article is shown to be promising to produce foams with a good homogeneity along the vertical direction, with a density controlled by varying the concentration of catalyst in the secondary foam.

Simultaneous improvements of strength and toughness in topologically interlocked ceramics.

Topologically interlocked materials (TIMs) are an emerging class of architectured materials based on stiff building blocks of well-controlled geometries which can slide, rotate, or interlock collectively providing a wealth of tunable mechanisms, precise structural properties, and functionalities. TIMs are typically 10 times more impact resistant than their monolithic form, but this improvement usually comes at the expense of strength. Here we used 3D printing and replica casting to explore 15 designs of architectured ceramic panels based on platonic shapes and their truncated versions. We tested the panels in quasi-static and impact conditions with stereoimaging, image correlation, and 3D reconstruction to monitor the displacements and rotations of individual blocks. We report a design based on octahedral blocks which is not only tougher (50×) but also stronger (1.2×) than monolithic plates of the same material. This result suggests that there is no upper bound for strength and toughness in TIMs, unveiling their tremendous potential as structural and multifunctional materials. Based on our experiments, we propose a nondimensional "interlocking parameter" which could guide the exploration of future architectured systems.

Active Fabrics With Controllable Stiffness for Robotic Assistive Interfaces.

Assistive interfaces enable collaborative interactions between humans and robots. In contrast to traditional rigid devices, conformable fabrics with tunable mechanical properties have emerged as compelling alternatives. However, existing assistive fabrics actuated by fluidic or thermal stimuli struggle to adapt to complex body contours and are hindered by challenges such as large volumes after actuation and slow response rates. To overcome these limitations, inspiration is drawn from biological protective organisms combining hard and soft phases, and active assistive fabrics consisting of architectured rigid tiles interconnected with flexible actuated fibers are proposed. Through programmable tessellation of target body shapes into architectured tiles and controlling their interactions by the actuated fibers, the active fabrics can rapidly transition between soft compliant configurations and rigid states conformable to the body (>350 times stiffness change) while minimizing the device volume after actuation. The versatility of these active fabrics is demonstrated as exosuits for tremor suppression and lifting assistance, as body armors for impact mitigation, and integration with electrothermal actuators for smart actuation with convenient folding capabilities. This work offers a practical framework for designing customizable active fabrics with shape adaptivity and controllable stiffness, suitable for applications in wearable exosuits, haptic devices, and medical rehabilitation systems.

Calcitic Prisms of The Giant Seashell Pinna Nobilis Form Light Guide Arrays.

The shells of the Pinnidae family are based on a double layer of single-crystal-like calcitic prisms and inner aragonitic nacre, a structure known for its outstanding mechanical performance. However, on the posterior side, shells are missing the nacreous layer, which raises the question of whether there can be any functional role in giving up this mechanical performance. Here, it is demonstrated that the prismatic part of the Pinna nobilis shell exhibits unusual optical properties, whereby each prism acts as an individual optical fiber guiding the ambient light to the inner shell cavity by total internal reflection. This pixelated light channeling enhances both spatial resolution and contrast while reducing angular blurring, an apt combination for acute tracking of a moving object. These findings offer insights into the evolutionary aspects of light-sensing and imaging and demonstrate how an architectured optical system for efficient light-tracking can be based on birefringent ceramics.

A bioinspired 3D-printable flexure joint with cellular mechanical metamaterial architecture for soft robotic hands.

Compliant flexure joints have been widely used for cable-driven soft robotic hands and grippers due to their safe interaction with humans and objects. This paper presents a soft and compliant revolute flexure joint based on the auxetic cellular mechanical metamaterials with a heterogeneous structure. The heterogeneous architecture of the proposed metamaterial flexure joint (MFJ), which is inspired by the human finger joints, provides mechanically tunable multi-stiffness bending motion and large range of bending angle in comparison to conventional flexure joints. The multi-level variation of the joint stiffness over the range of bending motion can be tuned through the geometrical parameters of the cellular mechanical metamaterial unit cells. The proposed flexure joints are 3D printed with single flexible material in monolithic fashion using a standard benchtop 3D printer. The application of the MFJ is demonstrated in robotic in-hand manipulation and grasping thin and deformable objects such as wires and cables. The results show the capability and advantages of the proposed MFJ in soft robotic grippers and highly functional bionic hands.

Layered-Fabric Materiality Fibre Reinforced Polymers (L-FMFRP): Hysteretic Behavior in Architectured FRP Material.

L_FMFRP is an architectural fiber composite surface element with an airy internal structure and variable section. This architectured material is the product of an alternative design and fabrication process that integrates fabric materiality, suggesting moldless shaping of the material through pleating and layering. Initial study of the mechanical properties of the element showed a structural behavior that would satisfy the requirement for schematic architectural cladding configurations, indicating a unique hysteretic behavior of the material. This paper further investigates the hysteretic capacities of L-FMFRP, examining the behavior under repeated loading and the effect of its internal material architecture. Parallels to entangled materials are suggested for a deeper understanding of the phenomenon, and the potential future application as an energy-absorbent material for façade cladding is outlined.

Characterization of Ultralow Density Cellular Solids: Lessons from 30 years of Bone Biomechanics Research.

Advances in additive manufacturing techniques have enabled the development of micro-architectured materials displaying a combination of low-density and lightweight structures with high specific strength and toughness. The mechanical performance of micro-architectured materials can be assessed using standard techniques; however, when studying low- and ultralow density micro-architectured materials, standard characterization techniques can be subject to experimental artifacts. Additionally, quantitative assessment and comparisons of microarchitectures with distinct lattice patterns is not always straightforward. Cancellous bone is a natural, ultralow density (porosity often exceeding 90%), irregular, cellular solid that has been thoroughly characterized in terms of micro-architecture and mechanical performance over the past 30 years. However, most the literature on cancellous bone mechanical properties and micro-structure-function relationships is in the medical literature and is not immediately accessible to materials designers. Here we provide a brief review of state-of-the-art approaches for characterizing the micro-architecture and mechanical performance of ultralow density cancellous bone, including methods of addressing experimental artifacts during mechanical characterization of ultralow density cellular solids, methods of quantifying microarchitecture, and currently understood structure-function relationships.

The Earth's Lithosphere Inspires Materials Design.

Structural patterns found in living organisms have long been inspiring biomimetic materials design. Here, it is suggested that a rich palette of patterns occurring in inanimate Nature, and especially in the Earth's lithosphere, could be not less inspirational for design of novel architectured materials. This materials design paradigm is referred to as lithomimetics and it is demonstrated that some of the patterns found in the lithosphere can be emulated by established processes of severe plastic deformation. This opens up interesting avenues for materials design in which potentially promising structural patterns are borrowed from the lithosphere's repository. The key aim here is to promulgate the "lithomimetics" paradigm as a promising approach to developing novel architectured materials.

Stiff, strong and tough laminated glasses with bio-inspired designs.

Glass is an attractive material with outstanding transparency, hardness, durability and chemical stability. However, the inherent brittleness and low toughness of glass limit its applications. Overcoming the brittleness of glass will help satisfy the rapidly increasing demands of glass in building materials, optical devices, electronics and photovoltaic systems, but it has been a challenge to create glass that is stiff, strong and tough while maintaining its transparency. In this study we explore how the basic design of laminated glass can be enriched with bio-inspired architectures generated with laser engraving. We assess the performance of designs based on continuous plies (90° cross plies, Bouligand), finite glass blocks (segmented Bouligand, nacre-like brick-and-mortar) and hybrid designs. It shows that simultaneous improvements of stiffness, strength and energy absorption upon continuous ply designs can be achieved by promoting delocalized shearing of the polymeric interlayer over brittle fracture of the glass building blocks, and by only placing enriched architectures under tensile deformation so that interlayer shearing can be realized. This principle can be realized simply by adjusting size and arrangement of the building blocks, and by combining continuous plain layers with architectured layers.

Sawtooth patterns in flexural force curves of structural biological materials are not signatures of toughness enhancement: Part II.

Stiff biological materials (SBMs), such as nacre and bone, are composites that display remarkable toughness enhancements over their primary constituents, which are brittle minerals. These enhancements are thought to be a consequence of different mechanisms made possible by the SBMs' internal lamellar architecture. One such mechanism is the Cook-Gordon (crack-arrest-and-reinitiation) mechanism, whose operation manifests in flexural tests as a sawtooth pattern in the force-displacement curves. The curves from flexural tests carried out on marine sponge spicules, which also possess a lamellar architecture, also display a sawtooth-pattern, suggesting the presence of the Cook-Gordon mechanism. Intriguingly, the spicules were recently found not to display any significant toughness enhancement. To resolve this apparent contradiction, in the preceding paper (Kochiyama et al., 2021), we put forward the hypothesis that the sawtooth pattern was due to the spicules slipping at the tests' supports. In this paper, we present a model for the spicule's flexural tests in which we allow for the possibility for the specimen to slip at the test's supports. We model contact between the specimen and the test's supports using the Coulomb's friction law. By choosing experimentally reasonable values for the friction coefficient, we were able to get the model's predictions to match experimental measurements remarkably well. Additionally, on incorporating the spicules' surface roughness into the model, which we did by varying the friction coefficient along the spicule's length, its predictions can also be made to match the measured sawtooth patterns. We find that the sawtooth patterns in the model are due to slip type instabilities, which further reinforces the hypothesis put forward in our preceding paper.

Sawtooth patterns in flexural force curves of structural biological materials are not signatures of toughness enhancement: Part I.

Layered architectures are prevalent in tough biological composites, such as nacre and bone. Another example of a biological composite with layered architecture is the skeletal elements-called spicules-from the sponge Euplectella aspergillum. Based on the similarities between the architectures, it has been speculated that the spicules are also tough. Such speculation is in part supported by a sequence of sudden force drops (sawtooth patterns) that are observed in the spicules' force-displacement curves from flexural tests, which are thought to reflect the operation of fracture toughness enhancing mechanisms. In this study, we performed three-point bending tests on the spicules, which also yielded the aforementioned sawtooth patterns. However, based on the analysis of the micrographs obtained during the tests, we found that the sawtooth patterns were in fact a consequence of slip events in the flexural tests. This is put into perspective by our recent study, in which we showed that the spicules' layered architecture contributes minimally to their toughness, and that the toughness enhancement in them is meager in comparison to what is observed in bone and nacre [Monn MA, Vijaykumar K, Kochiyama S, Kesari H (2020): Nat Commun 11:373]. Our past and current results underline the importance of inferring a material's fracture toughness through direct measurements, rather than relying on visual similarities in architectures or force-displacement curve patterns. Our results also suggest that since the spicules do not possess remarkable toughness, re-examining the mechanical function of the spicule's intricate architecture could lead to the discovery of new engineering design principles.

Increasing Necking Strain through Corrugation: Identifying Composite Systems That Can Benefit from Corrugated Geometry.

Under some circumstances, composites with a corrugated reinforcement geometry show larger necking strains compared to traditional straight reinforced composites. In this work, finite element modeling studies were performed for linearly hardening materials, examining the effect of material parameters on the stress-strain response of both corrugation and straight-reinforced composites. These studies showed that improvements in necking strain depend on the ability of the corrugation to unbend and to provide a boost in work hardening at the right time. It was found that there is a range of matrix yield strengths and hardening rates for which a corrugated geometry will improve the necking strain and also a lower threshold of reinforcement yield strength below which no improvement in necking strain is possible. In addition, benefit maps and surfaces were generated that show which regions of property space benefit through corrugation and the corresponding improvement in necking strain that can be achieved.

Additive Manufacturing and Performance of Architectured Cement-Based Materials.

There is an increasing interest in hierarchical design and additive manufacturing (AM) of cement-based materials. However, the brittle behavior of these materials and the presence of interfaces from the AM process currently present a major challenge. Contrary to the commonly adopted approach in AM of cement-based materials to eliminate the interfaces in 3D-printed hardened cement paste (hcp) elements, this work focuses on harnessing the heterogeneous interfaces by employing novel architectures (based on bioinspired Bouligand structures). These architectures are found to generate unique damage mechanisms, which allow inherently brittle hcp materials to attain flaw-tolerant properties and novel performance characteristics. It is hypothesized that combining heterogeneous interfaces with carefully designed architectures promotes such damage mechanisms as, among others, interfacial microcracking and crack twisting. This, in turn, leads to damage delocalization in brittle 3D-printed architectured hcp and therefore results in quasi-brittle behavior, enhanced fracture and damage tolerance, and unique load-displacement response, all without sacrificing strength. It is further found that in addition to delocalization of the cracks, the Bouligand architectures can also enhance work of failure and inelastic deflection of the architectured hcp elements by over 50% when compared to traditionally cast elements from the same materials.

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.