Elastic crack growth in finite elements with minimal remeshing

A minimal remeshing finite element method for crack growth is presented. Discontinuous enrichment functions are added to the finite element approximation to account for the presence of the crack. This method allows the crack to be arbitrarily aligned within the mesh. For severely curved cracks, remeshing may be needed but only away from the crack tip where remeshing is much easier. Results are presented for a wide range of two‐dimensional crack problems showing excellent accuracy. Copyright © 1999 John Wiley & Sons, Ltd.     

Intrinsically Polar Elastic Metamaterials

In many applications, one needs to combine materials with varying properties to achieve certain functionalities. For example, the inner layer of a helmet should be soft for cushioning while the outer shell should be rigid to provide protection. Over time, these combined materials either separate or wear and tear, risking the exposure of an undesired material property. This work presents a design principle for a material that gains unique properties from its 3D microstructure, consisting of repeating basic building blocks, rather than its material composition. The 3D printed specimens show, at two of its opposing faces along the same axis, different stiffness (i.e., soft on one face and hard on the other). The realized material is protected by design (i.e., topology) against cuts and tears: No matter how material is removed, either layer by layer, or in arbitrary cuts through the repeating building blocks, two opposing faces remain largely different in their mechanical response.     

Elastic T stress estimates for circumferential surface‐cracked cylinders*

On the basis of detailed three‐dimensional (3D) elastic finite element (FE) analyses, this paper provides tractable approximations for elastic T stress solutions for circumferential inner‐surface cracks in cylinders. Internal pressure and global bending moment were considered. The FE model and analysis procedure employed in the analysis were verified using existing solutions for both elastic stress intensity factor and T stress. To cover a practical range, three different values of the ratio of the mean radius of cylinder to the thickness, R_m/t, were selected; furthermore, four different values of the ratio of the crack depth to the thickness, a/t, ranging from 0.1 to 0.75 and three different values of θ/π ranging from 0.1 to 0.4 were selected. On the basis of FE analyses results, polynomial approximations were proposed at three different locations: surface point, middle point and deepest point. On the basis of the detailed 3D elastic FE analysis, the solutions presented are believed to be the most accurate, and thus provide valuable information for structural integrity assessment considering a crack‐tip constraint.     

Colloidal Particles that Rapidly Change Shape via Elastic Instabilities

The fabrication and properties of pH‐responsive colloidal particles are reported, which change shape rapidly (less than 200 ms), nearly independent of the diffusion of the pH altering species that trigger their actuation, and far more rapid than their Brownian motion. These particles are mechanically bistable, as revealed by their hysteretic shape response. Finite element analysis (FEA) shows that mechanical hysteresis and bistability derives from the colloids' spherical curvature. Mechanical characterization of the bilayered polymers comprising the colloidal particles shows that viscoelastic relaxation plays a non‐negligible role in limiting the shape switching rate; however, energy landscapes obtained from FEA simulations suggest that by tuning the elastic moduli and thicknesses of the constituent polymer layers, microparticles of the size shown here may be fabricated to actuate on timescales as fast as 1 μs.     

Plasmonic Elastic Capsules as Colorimetric Reversible pH‐Microsensors

There is a crucial need for effective and easily dispersible colloidal microsensors able to detect local pH changes before irreversible damages caused by demineralization, corrosion, or biofilms occur. One class of such microsensors is based on molecular dyes encapsulated or dispersed either in polymer matrices or in liquid systems exhibiting different colors upon pH variations. They are efficient but often rely on sophisticated and costly syntheses, and present significant risks of leakage and photobleaching damages, which is detrimental for mainstream applications. Another approach consists of exploiting the distance‐dependent plasmonic properties of metallic nanoparticles. Still, assembling nanoparticles into dispersible colloidal pH‐sensitive sensors remains a challenge. Here, it is shown how to combine optically active plasmonic gold nanoparticles and pH‐responsive thin shells into “plasmocapsules.” Upon pH change, plasmocapsules swell or shrink. Concomitantly, the distance between the gold nanoparticles embedded in the polymeric matrix varies, resulting in an unambiguous color change. Billions of micron‐size sensors can thus be easily fabricated. They are nonintrusive, reusable, and sense local pH changes. Each plasmocapsule is an independent reversible microsensor over a large pH range. Finally, their potential use for the detection of bacterial growth is demonstrated, thus proving that plasmocapsules are a new class of sensing materials.     

Highly Stretchable Metallic Nanowire Networks Reinforced by the Underlying Randomly Distributed Elastic Polymer Nanofibers via Interfacial Adhesion Improvement

On‐skin electronics require conductive, porous, and stretchable materials for a stable operation with minimal invasiveness to the human body. However, porous elastic conductors that simultaneously achieve high conductivity, good stretchability, and durability are rare owing to the lack of proper design for good adhesion between porous elastic polymer and conductive metallic networks. Here, a simple fabrication approach for porous nanomesh‐type elastic conductors is shown by designing a layer‐by‐layer structure of nanofibers/nanowires (NFs/NWs) via interfacial hydrogen bonding. The as‐prepared conductors, consisting of Ag NWs and polyurethane (PU) NFs, simultaneously achieve high conductivity (9190 S cm^?1), high stretchability (310%), and good durability (82% resistance increase after 1000 cycles of deformation at 70% tensile strain). The direct contact between the Ag NWs enables the high conductivity. The synergistic effect of the layer‐by‐layer structure and good adhesion between the Ag NWs and the PU NFs enables good mechanical properties. Furthermore, without any adhesive gel/tape, the conductors can be utilized as breathable strain sensors for precise joint motion monitoring, and as breathable sensing electrodes for continuous electrophysiological signal recording.     

Strong Graphene 3D Assemblies with High Elastic Recovery and Hardness

The rational design and construction of 3D graphene assemblies is a crucial step to extend the graphene properties for practical applications. Here, a novel interfacially reactive self‐assembling process is reported to prepare well‐organized 3D honeycomb‐like graphene assemblies with unique polygonal nanopores interconnected by silicon‐oxygen chemical bonds. The newly developed silicate‐bridged graphene assembly (SGA) exhibits an exceptionally high hardness of 13.09 GPa, outperforming all existing 3D graphene materials, while maintains high Young's modulus (162.96 GPa), elastic recovery (75.27%), and superb thermal stability (600 °C in air). The observed unusual merits are resulted from unique pore structure combining the mechanical stability of the trihedral‐nanopore structure and the deformability of the other polygonal nanopores. As a filling material, a merely 0.05% (w/w) addition of SGA could double the impact resistance of unsaturated resins (e.g., polyester). While SGA is attractive for various applications, including body armors, wearable electronics, space elevators, and multifunctional reinforcement fibers for automobiles, and aerospace vehicles, the novel liquid sodium–water interfacial reactive self‐assembling developed in this study could open avenues for further development of various well‐defined 3D assemblies from graphene and many other materials.     

On the Evaluation of the Elastic Modulus of Soft Materials Using Beams with Unknown Initial Curvature

A non‐linear optimization procedure is established to determine the elastic modulus of slender, soft materials using beams with unknown initial curvature in the presence of large rotations. Specifically, the deflection of clamped‐free beams under self‐weight – measured at different orientations with respect to gravity – is used to determine the modulus of elasticity and the intrinsic curvature in the unloaded state. The approach is validated with experiments on a number of different materials – steel, polyetherimide, rubber and pig skin. Because the loading is limited to self‐weight, the strain levels attained in these tests are small enough to assume a linear elastic material behaviour. This non‐destructive methodology is also applicable to engineered tissues and extremely delicate materials in order to obtain a quick estimate of the material's elastic modulus.     

Highly Conductive Ti3C2Tx MXene Hybrid Fibers for Flexible and Elastic Fiber‐Shaped Supercapacitors

Fiber‐shaped supercapacitors (FSCs) are promising energy storage solutions for powering miniaturized or wearable electronics. However, the scalable fabrication of fiber electrodes with high electrical conductivity and excellent energy storage performance for use in FSCs remains a challenge. Here, an easily scalable one‐step wet‐spinning approach is reported to fabricate highly conductive fibers using hybrid formulations of Ti₃C₂T_x MXene nanosheets and poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate. This approach produces fibers with a record conductivity of ≈1489 S cm^?1, which is about five times higher than other reported Ti₃C₂T_x MXene‐based fibers (up to ≈290 S cm^?1). The hybrid fiber at ≈70 wt% MXene shows a high volumetric capacitance (≈614.5 F cm^?3 at 5 mV s^?1) and an excellent rate performance (≈375.2 F cm^?3 at 1000 mV s^?1). When assembled into a free‐standing FSC, the energy and power densities of the device reach ≈7.13 Wh cm^?3 and ≈8249 mW cm^?3, respectively. The excellent strength and flexibility of the hybrid fibers allow them to be wrapped on a silicone elastomer fiber to achieve an elastic FSC with 96% capacitance retention when cyclically stretched to 100% strain. This work demonstrates the potential of MXene‐based fiber electrodes and their scalable production for fiber‐based energy storage applications.     

Elastic wave propagation in unsaturated porous media using hybrid‐Trefftz stress elements

The stress model of the hybrid‐Trefftz finite element is formulated for the analysis of elastodynamic problems defined on unsaturated porous media. The supporting mathematical model is the theory of mixtures with interfaces and considers the full coupling between the solid, fluid and gas phases, including the effect of seepage acceleration. Hybrid‐Trefftz stress elements use the free‐field regular solutions of the homogeneous Navier (or Beltrami) equation to construct the approximation of the generalized stresses in the domain of the element. The influence of non‐homogeneous terms in the Navier equation is modelled using solutions of the corresponding static problem. The resulting elements are highly convergent under p‐refinement and robust to both low and high excitation frequencies, as the trial functions embody relevant physical information on the modelled phenomenon. Copyright © 2013 John Wiley & Sons, Ltd.     

Elastic Energy and Elastic Interaction Ordering of Binary Solid Solutions

A theory in the framework of continuum elasticity has been developed to calculate the totalcontribution of "atomic size effect" or "strain energy effect" to free energy of binary solidsolutions. It is found that elastic free energy consists of two parts f elastic self energy (ESE),and elastic interaction energy(EIE). The former is a function of composition alone, the latter isalso a function of atomic configuration. Minimization of total elastic free energy with respect toatomic arrangement resuIts in an ordered arrangement of atoms, which is caIIed elastic interactionordering (EIO), as it originates from elastic interaction among atoms. EIO is a kind of localordering within a "characteristic range", and it is found to be important in determining the Stateof solid solutions and structures of superlattices. The present theory also gives good explanationto the coexistence of ordering and decomposition which can not be understood in conventionaltheories.     

天然的弹性地面

橡胶已经成为现代工业生产的一种重要材料，而工业橡胶材料的生产和使用也为环境保护带来了一系列的新问题。于是天然橡胶的生产和使用逐渐得到了人们的重视。