Bottom-up Design of Three-Dimensional Carbon-Honeycomb with Superb Specific Strength and High Thermal Conductivity

Low-dimensional carbon allotropes, from fullerenes, carbon nanotubes, to graphene, have been broadly explored due to their outstanding and special properties. However, there exist significant challenges in retaining such properties of basic building blocks when scaling them up to three-dimensional materials and structures for many technological applications. Here we show theoretically the atomistic structure of a stable three-dimensional carbon honeycomb (C-honeycomb) structure with superb mechanical and thermal properties. A combination of sp(2) bonding in the wall and sp(3) bonding in the triple junction of C-honeycomb is the key to retain stability of C-honeycomb. The specific strength could be the best in structural carbon materials, and this strength remains at,a high level but tunable with different cell sizes. C-honeycomb is also found to have a very high thermal conductivity, for example, &gt;100 W/mK along the axis of the hexagonal cell with a density only similar to 0.4 g/cm(3). Because of the low density and high thermal conductivity, the specific thermal conductivity of C-honeycoMbs is larger than most engineering materials, including metals and high thermal conductivity semiconductors, as well as lightweight CNT arrays and graphene-based nanocomposites. Such high specific strength, high thermal conductivity, and anomalous Poisson&#39;s effect in C-honeycomb render it appealing for the use in various engineering practices.</p

INVESTIGATION OF MECHANICAL BEHAVIOR OF INTERFACES IN NANOSTRUCTURED METALS

将常规多晶材料的粗晶粒尺寸缩小到纳米尺度时,这些纳米晶体材料会呈现出与其对应的粗晶材料迥异的物理现象.与材料力学行为最相关的是强度及塑形变形机理这两个方面.考虑到晶界的变形与破坏可能是纳米晶体材料低塑性的根源,克服纳米晶体材料中强度与韧性之间存在的&ldquo;熊掌和鱼不可兼得&rdquo;的问题,也通常称为晶界工程.在众多的晶界中,孪晶界面被发现可同时保持材料的强度和韧性.本文主要就纳米金属材料中界面的力学行为做一个简要述,包含晶界的强化力学机理以及新型孪晶界面的力学行为与揭示内在尺度效应的模型研究。</p

A stochastic description on the traction-separation law of an interface with non-covalent bonding

We formulate a stochastic description about the mechanical response of an interface composed of non-covalent bonds. In such interfaces, the evolution of bonding probability in response to deformation plays the central role in determining their traction-separation behavior. The model connects atomistic and molecular level bonding properties to meso-scale traction-separation relationship in an interface. In response to quasi-static loading, the traction-separation of a stochastic interface is the resultant of varying bonding probability as a function of separation, and the bonding probability follows the Boltzmann distribution. The quasi-static stochastic interface model is applied to understand the critical force while detaching a sphere from an infinite half space. We further show the kinetics of interfacial debonding in the context of the Bell model (1978) and two of its derivatives - the Evans-Richie model (1997) and the Freund model (2009). While subjected to constant force, an interface creeps and its separation time curve shows typical characteristics seen during the creep of crystalline materials at high temperature. When we exert constant separation rate to an interface, interfacial traction shows strong rate-sensitivity with higher traction at faster separation rate. The model presented here may supply a guidance to bring the stochastic nature of interfacial debonding into theories on cracking initiation and growth during fatigue fracture. (C) 2014 The Author. Published by Elsevier Ltd

Crack deflection in brittle media with heterogeneous interfaces and its application in shale fracking

Driven by the rapid progress in exploiting unconventional energy resources such as shale gas there is growing interest in hydraulic fracture of brittle yet heterogeneous shales. In particular how hydraulic cracks interact with natural weak zones in sedimentary rocks to form permeable cracking networks is of significance in engineering practice. Such a process is typically influenced by crack deflection material anisotropy crack-surface friction crustal stresses and so on. In this work we extend the He-Hutchinson theory (He and Hutchinson 1989) to give the closed-form formulae of the strain energy release rate of a hydraulic crack with arbitrary angles with respect to the crustal stress. The critical conditions in which the hydraulic crack deflects into weak interfaces and exhibits a dependence on crack-surface friction and crustal stress anisotropy are given in explicit formulae. We reveal analytically that with increasing pressure hydraulic fracture in shales may sequentially undergo friction locking mode II fracture and mixed mode fracture. Mode II fracture dominates the hydraulic fracturing process and the impinging angle between the hydraulic crack and the weak interface is the determining factor that accounts for crack deflection; the lower friction coefficient between cracked planes and the greater crustal stress difference favor hydraulic fracturing. In addition to shale fracking the analytical solution of crack deflection could be used in failure analysis of other brittle media. (C) 2017 Elsevier Ltd. All rights reserved.</p

On the influence of interfacial properties to the bending rigidity of layered structures

Layered structures are ubiquitous, from one-atom thick layers in two-dimensional materials, to nanoscale lipid bi-layers, and to micro and millimeter thick layers in composites. The mechanical behavior of layered structures heavily depends on the interfacial properties and is of great interest in engineering practice. In this work, we give an analytical solution of the bending rigidity of bilayered structures as a function of the interfacial shear strength. Our results show that while the critical bending stiffness when the interface starts to slide plastically is proportional to the interfacial shear strength, there is a strong nonlinearity between the rigidity and the applied bending after interfacial plastic shearing. We further give semi-analytical solutions to the bending of bilayers when both interfacial shearing and pre-existing crack are present in the interface of rectangular and circular bilayers. The analytical solutions are validated by using finite element simulations. Our analysis suggests that interfacial shearing resistance, interfacial stiffness and preexisting cracks dramatically influence the bending rigidity of bilayers. The results can be utilized to understand the significant stiffness difference in typical biostructures and novel materials, and may also be used for non-destructive detection of interfacial crack in composites when stiffness can be probed through vibration techniques. (C) 2016 Elsevier Ltd. All rights reserved

The influence of crack-orientation distribution on the mechanical properties of pre-cracked brittle media

Cracks in kerogen-rich shales and other brittle rock-like materials have a tremendous impact on their elastic properties and strength. In this paper, we investigate the effective mechanical properties of shale plates with pre-existing cracks. We employ the extended finite element method (XFEM) to investigate a pre-cracked medium with an elastic, isotropic and brittle shale matrix. We show how the mechanical properties of the orthotropic shale plates are dependent on the crack density and the standard deviation of crack angles. Both the Young&#39;s modulus and the Poisson&#39;s ratio of the cracked media exhibit a linear dependence on the standard deviation of crack angles, in contrast to the nonlinear dependence of the strength on the angle deviation. Finally, we propose mechanical models to capture the relationship between the mechanical properties and the distribution characteristics of pre-existing cracks in shales. These phenomenological models could be applied to estimate the fracking behavior of shales in engineering practice. (C) 2016 Elsevier Ltd. All rights reserved.</p

Theory on Bending in Cantilever Beams With Adsorbed Islands

Traction between adsorbed islands and the substrate is commonly seen in both living and material systems: deposited material gathers into islands at the early stage of polycrystalline film deposition and generates stress due to lattice mismatch, cells exert cellular traction to extracellular matrix to probe their surrounding microenvironment in vivo, and so on. The traction between these islands and the substrate can result in perceivable macroscopic deformation in the substrate and may be measurable if the substrate is a cantilever beam. However, currently broadly used Stoney equation is incapable of handling such boundary condition. In this paper, we give the closed-form expression on the resulted curvature in substrate beams by distributed tractions. Such a relationship could be employed to monitor the stress evolution during thin film deposition, to quantify the stress level of cell traction as cells adhere to cantilever beams, and other related mechanical systems like charging-discharging induced stress in island-patterned electrode films. Moreover, we found that follower traction induced by an array of islands could lead to negative curvature. It shields light on the early stage compressive stress during polycrystalline film deposition.</p

A polycrystal based numerical investigation on the temperature dependence of slip resistance and texture evolution in magnesium alloy AZ31B

Low thermoplastic formability is a key factor limiting the usage of magnesium alloys, which otherwise can have broad application in automotive industry for their competitive strength to density ratio. Combining with experimental calibration and validation, we report a systematic numerical investigation about the plastic deformation of magnesium alloy AZ31B at different temperatures and subjected to different boundary conditions. By employing 3D Voronoi grains based microstructure and the crystal plasticity constitutive model developed by Staroselsky and Anand (2003), which accounts for both dislocation slip and deformation twinning in polycrystalline magnesium, we estimate the dependence of critical resolved shear stresses (CRSS) of different slip/twinning systems on temperature. We further obtain the fractional plastic strains contributed by individual slip/twinning systems at different loading conditions. Grain level deformation analysis indicates that there exists significant stress and plasticity inhomogeneity among grains. (C) 2013 Elsevier Ltd. All rights reserved