Effects of microstructure, specimen and loading geometries on KIC of brittle honeycombs

Fracture toughness of brittle honeycombs depends on cell microstructure, specimen and geometries. A microstructure coefficient in the K_IC expression of brittle honeycombs can not be found analytically. In this paper, a finite element program is utilized to numerically determine the coefficient. In practice, fracture toughness can be measured from conducting a three-point bend or uniaxial tension test. Specimen geometry restrictions of three-point bend test for honeycombs are examined and proposed here. Meanwhile, fracture toughness of honeycombs under the two loading geometries is compared; results show that K_IC measured from uniaxial tension test is smaller than that from three-point bend test if the K_IC formulations for solid materials are employed. As a result of that, the K_IC formulation of three-point bend test is modified for honeycomb-like materials.     

In-plane elastic constants and strengths of circular cell honeycombs

The in-plane elastic modulus, Poisson’s ratio, brittle crushing strength and plastic yielding strength of honeycombs with hexagonally packed circular cells are analyzed theoretically. The resulting theoretical expressions are compared with the numerical results obtained from a series of finite element analyses for circular cell honeycombs with various relative densities, leading to a good correlation. It is also found that the in-plane mechanical properties of circular cell honeycombs are significantly affected by the ratio of cell-wall thickness to cell radius. Though the elastic constants along the two principal directions of circular cell honeycombs are the same, the brittle crushing strength and plastic yielding strength along the two principal directions are not identical. Furthermore, the in-plane mechanical properties of circular cell honeycombs are compared to those of regular hexagonal honeycombs with straight and uniform-thickness cell walls to evaluate their microstructural efficiency.     

Amplifying Strength,Toughness, and Auxeticity via Wavy Sutural Tessellation in Plant Seedcoats

Protective armors are widespread in nature and often consist of periodic arrays of tile‐like building blocks that articulate with each other through undulating interfaces. To investigate the mechanical consequences of these wavy tessellations, especially in instances where the amplitude of the undulations is near the scale of the constituent tiles as is found in the seedcoats of many plant species, an approach that integrates parametric modeling and finite element simulations with direct mechanical testing on their 3D‐printed multi‐material structural analogues is presented. Results from these studies demonstrate that these tiled arrays of largely isotropic star‐like unit cells exhibit an unusual combination of mechanical properties including auxeticity and mutually amplified strength and toughness which can be systematically tuned by varying the waviness of the sutural tessellation.     

Bacterially Produced,Nacre‐Inspired Composite Materials

The impressive mechanical properties of natural composites, such as nacre, arise from their multiscale hierarchical structures, which span from nano‐ to macroscale and lead to effective energy dissipation. While some synthetic bioinspired materials have achieved the toughness of natural nacre, current production methods are complex and typically involve toxic chemicals, extreme temperatures, and/or high pressures. Here, the exclusive use of bacteria to produce nacre‐inspired layered calcium carbonate‐polyglutamate composite materials that reach and exceed the toughness of natural nacre, while additionally exhibiting high extensibility and maintaining high stiffness, is introduced. The extensive diversity of bacterial metabolic abilities and the possibility of genetic engineering allows for the creation of a library of bacterially produced, cost‐effective, and eco‐friendly composite materials.     

Engineering and medical applications of diatoms

Biologists, and diatomists in particular, have long studied the properties of single-cell algae, and engineers are just discovering how to exploit features unique to these organisms. Their uniform nanopore structure, microchannels, chemical inertness, and silica microcrystal structure suggest many nanoscale applications. This paper proposes three potential research initiatives taking advantage of diatom morphology and mechanical and chemical properties: (1) embedding diatom frustules in a metal-film membrane; (2) magnetizing frustules for pinpoint drug delivery; and (3) producing silica nanopowders from frustules. The potential benefits of each initiative and its technical challenges are outlined.     

Experimental study on mechanical properties and fracture toughness of structural thick plate and its butt weld along thickness and at low temperatures

The thick plate induces the variation of mechanical properties and fracture toughness, especially in cold regions. At the low temperature, the brittle behaviour of steel becomes worse. A series of tests (such as uniaxial tensile test and three‐point bending test) were carried out at low temperature to investigate the mechanical properties and fracture toughness of structural steel plates of Q345B with thickness of 60 to 150 mm, as well as the fracture toughness of 150 mm thick butt welded plate. The test specimens are all manufactured from plates along thickness with small size, and the tensile test specimens included through‐thickness specimens additionally. The ductility index (percentage reduction of area) and the fracture toughness index (critical CTOD values) all decrease with the temperature decreases and the distance from plate surface increases. The results obtained in this paper provide technical basis for preventing brittle fracture of thick plate steel structures in cold regions.     

Survival probability for brittle honeycombs under in-plane biaxial loading

The failure envelopes of brittle honeycombs are affected by the cell-wall modulus of rupture. The variability in the cell-wall modulus of rupture is accounted for by assuming that it follows a Weibull distribution, giving the corresponding modulus of rupture for a prescribed survival probability. Furthermore, the existing model for the failure envelopes of honeycombs under in-plane biaxial loading is modified to take into account the effect of variability in the cell-wall modulus of rupture. Consequently, the failure envelopes of brittle honeycombs with a prescribed survival probability are developed. The effects of cell size, Weibull modulus and prescribed survival probability on the failure envelopes of brittle honeycombs are also evaluated.     

Failure surfaces for brittle honeycombs with Plateau borders under in-plane biaxial loads

Brittle honeycombs without any macrocrack under in-plane biaxial loads fail by elastic buckling and brittle rupturing of cell edges. The failure surfaces of brittle honeycombs with non-uniform thickness cell edges are affected by the cell-edge modulus of rupture and the distribution of solid between three cell edges and a vertex. In the paper, the non-uniform cross-section of cell edges is taken to be a Plateau border. Meanwhile, the variability of cell-edge modulus of rupture is taken into account by assuming that it follows a Weibull distribution. As a result of those, the failure surfaces of brittle honeycombs with Plateau borders caused by elastic buckling and brittle rupturing for a prescribed survival probability can be generated. The effects of the solid distribution in cell edges, the Weibull modulus and the prescribed survival probability on the failure surfaces of brittle honeycombs are also evaluated.     

In situ observation of nanograin rotation and deformation in nacre

Nacre is a natural nanocomposite material with superior mechanical strength and toughness. What roles do the nanoscale structures play in the inelasticity and toughening of nacre? Can we learn from this to produce nacre-like nanocomposites? Here we report in situ dynamic atomic force microscope observations of nacre with aragonite nanograins (nanoparticles) of an average grain size of 32 nm, which show that nanograin rotation and deformation are the two prominent mechanisms contributing to energy dissipation in nacre. The biopolymer spacing between the nanograins facilitates the grain rotation process. The aragonite nanograins in nacre are not brittle but deformable.     

Bioinspired engineering of honeycomb structure – Using nature to inspire human innovation

Honeycomb structures, inspired from bee honeycombs, had found widespread applications in various fields, including architecture, transportation, mechanical engineering, chemical engineering, nanofabrication and, recently, biomedicine. A major challenge in this field is to understand the unique properties of honeycomb structures, which depended on their structures, scales and the materials used. In this article, we presented a state-of-the-art review of the interdisciplinary efforts to better understand the design principles for products with honeycomb structures, including their fabrication, performance (e.g., mechanical, thermal and acoustic properties) as well as optimization design. We described how these structural perspectives have led to new insights into the design of honeycomb structures ranging from macro-, micro- to nano-scales. We presented current scientific advances in micro- and nano-technologies that hold great promise for bioinspired honeycomb structures. We also discussed the emerging applications of honeycomb structures in biomedicine such as tissue engineering and regenerative medicine. Understanding the design principles underlying the creation of honeycomb structures as well as the related scientific discovery and technology development is critical for engineering bioinspired materials and devices designed based on honeycomb structures for a wide range of practical applications.     

Crack Tip Opening Displacement in atomistic modeling of fracture of silicon

We analyze the fracture of single crystal silicon simulated by atomistic modeling with ReaxFF first principles based reactive force field. The simulations are performed at three temperatures: 500 K, 800 K and 1200 K, capturing both brittle and ductile behavior for the selected crystallographic orientation with (1 0 0) as the fracture plane. Three failure mechanisms are observed: bond breaking, amorphization and emission of dislocations. We demonstrate that the Crack Tip Opening Displacement (CTOD) gives a realistic estimate of the fracture toughness of brittle fracture, linking continuum mechanics fracture theory with the direct crack tip atomistic approach. We discuss the physics based mechanisms of failure in silicon in view of the CTOD measurements.     

Ferroelastic toughening: Can it solve the mechanics challenges of solid electrolytes?

The most promising solid electrolytes for all-solid-state Li batteries are oxide and sulfide ceramics. Current ceramic solid electrolytes are brittle and lack the toughness to withstand the mechanical stresses of repeated charge and discharge cycles. Solid electrolytes are susceptible to crack propagation due to dendrite growth from Li metal anodes and to debonding processes at the cathode/electrolyte interface due to cyclic variations in the cathode lattice parameters. In this perspective, we argue that solutions to the mechanics challenges of all-solid-state batteries can be borrowed from the aerospace industry, which successfully overcame similar hurdles in the development of thermal barrier coatings of superalloy turbine blades. Their solution was to exploit ferroelastic and transformation toughening mechanisms to develop ceramics that can withstand cyclic stresses due to large variations in temperature. This perspective describes fundamental materials design principles with which to search for solid electrolytes that are ferroelastically toughened.     

Bio‐Mimicked Silica Architectures Capture Geometry,Microstructure, and Mechanical Properties of Marine Diatoms

The authors create life‐sized synthetic replicas of marine diatom coscinodiscus sp frustules out of cyclohexyl polyhedral oligomeric silsesquioxanes (POSS). The authors demonstrate that these synthetic structures have biosilica‐like amorphous atomic‐level microstructure and mechanical attributes similar to those of a natural diatom. In situ beam bending and fracture experiments on micron‐sized excised sections of natural and synthetic diatoms reveal similarities in their mechanical properties: a Young's modulus of GPa and a fracture toughness of 0.78 ± 0.10 MPa m^?1/2 for the synthetic materials; those of natural diatoms are GPa and MPa m^?1/2, respectively. In situ single edge notched beam (SENB) bending fracture experiments reveal that fracture behavior of the natural and synthetic specimens is virtually indistinguishable and is characterized by the same brittle failure and crack‐arresting behavior enabled by the double‐wall geometry. Their fracture toughness is comparable to that of fully dense silica, which suggests that the natural diatoms’ frustule maintains its mechanical resilience even at <50% of the weight attained through multi‐scale architecture. The demonstrated ability to fabricate a synthetic hard biomaterial that is virtually indistinguishable from its natural counterpart while capturing its complex architecture, microstructure, and mechanical properties provides a powerful platform for investigating the specific role of each geometrical feature at every relevant length scale in the often sophisticated, multi‐scale hierarchical construct of hard biomaterials, and provides a robust pathway for property optimization.

     

High-Performance Polymeric Materials through Hydrogen-Bond Cross-Linking

It has always been critical to develop high-performance polymeric materials with exceptional mechanical strength and toughness, thermal stability, and even healable properties for meeting performance requirements in industry. Conventional chemical cross-linking leads to enhanced mechanical strength and thermostability at the expense of extensibility due to mutually exclusive mechanisms. Such major challenges have recently been addressed by using noncovalent cross-linking of reversible multiple hydrogen-bonds (H-bonds) that widely exist in biological materials, such as silk and muscle. Recent decades have witnessed the development of many tailor-made high-performance H-bond cross-linked polymeric materials. Here, recent advances in H-bond cross-linking strategies are reviewed for creating high-performance polymeric materials. H-bond cross-linking of polymers can be realized via i) self-association of interchain multiple H-bonding interactions or specific H-bond cross-linking motifs, such as 2-ureido-4-pyrimidone units with self-complementary quadruple H-bonds and ii) addition of external cross-linkers, including small molecules, nanoparticles, and polymer aggregates. The resultant cross-linked polymers normally exhibit tunable high strength, large extensibility, improved thermostability, and healable capability. Such performance portfolios enable these advanced polymers to find many significant cutting-edge applications. Major challenges facing existing H-bond cross-linking strategies are discussed, and some promising approaches for designing H-bond cross-linked polymeric materials in the future are also proposed.     

Graded conventional-auxetic Kirigami sandwich structures: Flatwise compression and edgewise loading

The work describes the manufacturing and testing of graded conventional/auxetic honeycomb cores. The graded honeycombs are manufactured using Kevlar woven fabric/914 epoxy prepreg using Kirigami techniques, which consist in a combination of Origami and ply-cut processes. The cores are used to manufacture sandwich panels for flatwise compression and edgewise loading. The compressive modulus and compressive strength of stabilized (sandwich) honeycombs are found to be higher than those of bare honeycombs, and with density-averaged properties enhanced compared to other sandwich panels offered in the market place. The modulus and strength of graded sandwich panel under quasi-static edgewise loading vary with different failure mode mechanisms, and offer also improvements towards available panels from open literature. Edgewise impact loading shows a strong directionality of the mechanical response. When the indenter impacts the auxetic portion of the graded core, the strong localization of the damage due to the negative Poisson’s ratio effect contains significantly the maximum dynamic displacement of the sandwich panel.     

自相似多级纳米蜂窝铝结构力学性能的分子动力学模拟

首次采用分子动力学方法预测了自相似多级纳米蜂窝铝面内和面外（轴向）的压缩力学性能（弹性模量和压缩强度）。重点研究了相对密度、层级数和长度比对自相似多级纳米蜂窝铝结构力学性能的影响。在Gibson模型中引入了表面效应因子,结果表明修正的Gibson-Ashby模型与分子动力学计算结果更加吻合。此外,通过比较一级、二级和三级纳米蜂窝铝结构的变形机制发现,二级和三级纳米蜂窝铝结构由于分别在单级蜂窝和二级蜂窝的角点处接入六边形,在压缩过程中,多级纳米蜂窝铝结构激发的位错远高于单级蜂窝铝结构。也就是说,在压缩载荷下,多级蜂窝铝结构可以更好地利用结构的承载能力,吸收更多的能量。但是,自相似纳米蜂窝铝结构的力学性能无法通过增加级数的方法来无限增强,在相对密度和长度比不变的情况下,当纳米蜂窝铝结构的级数达到二级时,其综合力学性能最佳。研究结果还表明,相对密度不变时,二级纳米蜂窝铝结构长度比分别在0.3和0.4附近时,二级蜂窝铝结构具有最佳的面内和面外力学性能。研究成果对自相似多级纳米蜂窝结构的优化设计具有重要的指导作用。      

Topologically engineered 3D printed architectures with superior mechanical strength

Materials that are lightweight yet exhibit superior mechanical properties are of compelling importance for several technological applications that range from aircrafts to household appliances. Lightweight materials allow energy saving and reduce the amount of resources required for manufacturing. Researchers have expended significant efforts in the quest for such materials, which require new concepts in both tailoring material microstructure as well as structural design. Architectured materials, which take advantage of new engineering paradigms, have recently emerged as an exciting avenue to create bespoke combinations of desired macroscopic material responses. In some instances, rather unique structures have emerged from advanced geometrical concepts (e.g. gyroids, menger cubes, or origami/kirigami-based structures), while in others innovation has emerged from mimicking nature in bio-inspired materials (e.g. honeycomb structures, nacre, fish scales etc.). Beyond design, additive manufacturing has enabled the facile fabrication of complex geometrical and bio-inspired architectures, using computer aided design models. The combination of simulations and experiments on these structures has led to an enhancement of mechanical properties, including strength, stiffness and toughness. In this review, we provide a perspective on topologically engineered architectured materials that exhibit optimal mechanical behaviour and can be readily printed using additive manufacturing.     

Bruchmechanische Prüfung von Keramiken mit R-Kurvenverhalten

Fracture Testing of Ceramics with R-Curve Behaviour Significant improvements of the fracture toughness are striven for ceramic materials using micromechanical mechanisms of crack tip shielding. The application of Fine Ceramics to structures and engines is planed increasingly in expectancy of higher strength. The aim of the article is the verification of common brittle fracture tests employed for advanced ceramics. Theoretical aspects of the R-curve behaviour are analysed to feature reliably damage of materials. Stable crack growth and failure of standard specimens are assessed using theoretical simulations of experiments. The results are compared with real data. Suggestions are made to interpret properly measurements and to select appropriate specimen geometries for materials which show a distinctive R-curve behaviour.     

Fracture mechanisms of epoxy-based ternary composites filled with rigid-soft particles

Epoxy composites filled with different amounts of aggregate-free silica nanoparticles and phase-separated submicron rubber particles were fabricated to study the synergistic effect of multi-phase particles on mechanical properties of the composites. Compared with binary composites with single-phase particles, the ternary composites with both rigid and soft particles offer a good balance in stiffness, strength and fracture toughness, showing capacities in tailoring the mechanical properties of modified epoxy resins. It was observed that debonding of silica nanoparticles from matrix in the ternary composites was less pronounced than that in the binary composites. Moreover, the rubber particles became smaller and their shape tends to be irregular, affected by the presence of rigid silica nanoparticles. The toughening mechanisms in the epoxy composites were evaluated, and the enlarged plastic deformation around the crack tip, induced by the combination of rigid and soft particles, seems to be a dominant factor in enhancing fracture toughness of the ternary composites.     

Asymptotic analysis of crack bridging by ductile fibres

The problem that is addressed relates to a brittle matrix — typically, a ceramic — reinforced by ductile fibres. If a crack develops in the matrix, its opening is restrained by the fibres and toughness is enhanced. The problem can be formulated as an integral equation which can be solved numerically, but one limiting situation, of some practical relevance, can be treated analytically. This is the case of a matrix that is sufficiently brittle to allow the crack to extend relatively far before the fibres begin to fail. Analysis so far performed has provided an analytic expression for failure load versus crack length prior to fibre failure, plus an estimate for crack length at first fibre failure, when the physical parameters allow this sequence. Current work is devoted to the analysis of the progressive failure of fibres.