动态断裂过程的数值分析及LY-12铝的层裂

 本文从文献[1]中用于分析柱壳动态膨胀断裂过程的损伤度函数出发，将它推广到对一维应变下层裂过程的数值模拟研究。试件材料为LY-12铝，其特性方程取为含粘性的本构方程形式。数值计算结果很好地再现了实测自由面速度u_fs随时间t的变化过程，并表现出层裂强度σ_c及层裂面上的临界损伤度α_c都分别是应变率ε_c'的单调递增函数关系。σ_c~ε_c'的这种变化规律在许多文献中已屡见报道，例如可见文献[2-3]。在10⁵ s^-1~10⁶ s^-1应变率范围内，σ_c~ε_c'关系可以表示为ε_c'exp(-11.4α_c)=2 100 s^-1，这个式子可以作为一种层裂判据使用。数值计算还给出了层裂片的损伤度剖面，其形状特征与Barbee等对回收试件的细观测量结果在定性上一致。     

Global attractor for the nonlinear strain waves in elastic waveguides

1. IntroductionIn some problems of nonlinear wave propagation in waveguides, the illteraction of waveguides and the external medium and, therefore, the possibility of energy exchange throughlateral surface of waveguide cannot be neglected. When the energy exchange between therod and the medium is considered, for one cajse, there is a dissipation of deformation wavein the viscous external medium. The general cubic double dispersion equation (CDDE) canbe derived from Hamilton principled]:where…     

Rational Manipulation of Epitaxial Strains Enabled Valence Band Convergence and High Thermoelectric Performances in Mg3Sb2 Films

Strain engineering is demonstrated to effectively regulate the functionality of materials, such as thermoelectric, ferroelectric, and photovoltaic properties. As the straightforward approach of strain engineering, epitaxial strain is usually proposed for rationally manipulating the electronic structure and performances of thermoelectric materials, but has rarely been verified experimentally. In this study, tunable and large epitaxial strains are demonstrated, as well as the resulting valence band convergence can be achieved in the Mg₃Sb₂ epi-films with the choice of substrates. The large epitaxial strains up to 8% in Mg₃Sb₂ films represent one of the most striking results in strain engineering. The angle-resolved photoemission spectroscopy measurements and the theoretical calculations reveal the vital role of epitaxial strain in tuning the crystal field splitting and the band structure of Mg₃Sb₂. Benefiting from the appropriate manipulation of the crystal field effect via in-plane compressive strain, the valence band convergence is unambiguously discovered in the strained Mg₃Sb₂ film grown on InP(111) substrate. As a result, a state-of-the-art thermoelectric power factor of 0.94 mWm⁻¹K⁻² is achieved in the strain-engineered Mg₃Sb₂ film, well exceeding that of the strain-relaxed Mg₃Sb₂. The work paves the way for effectively manipulating epitaxial strain and band convergence for Mg₃Sb₂ and other thermoelectric films.     

Impact of 2D Ligands on Lattice Strain and Energy Losses in Narrow-Bandgap Lead–Tin Perovskite Solar Cells

Mixed lead and tin (Pb/Sn) hybrid perovskites exhibit a great potential in fabricating all-perovskite tandem devices due to their easily tunable bandgaps. However, the energy deficit and instability in Pb/Sn perovskite solar cells (PSCs) constrain their practical applications, which renders defect passivation engineering indispensable to develop highly efficient and long-term stable PSCs. Herein, the mechanisms of strain tailoring and defect passivation in Pb/Sn PSCs by 2D ligands are investigated. The 2D ligands include electroneutral cations with long alkyl chain (LAC), iodates with relatively short alkyl chain (SAC) and their mixtures. This study reveals that LAC ligands facilitate the relaxation of tensile strain in perovskite films while SAC ligands cause strain buildup. By mixing LAC/SAC ligands, tensile strain in perovskite films can be balanced which improves solar cell performance. PSCs with admixed β-guanidinopropionic acid (GUA)/phenethylammonium iodide (PEAI) exhibit enhanced open circuit voltage and fill factor, which is attributed to reduced nonradiative recombination losses in the bulk and at the interfaces. Furthermore, the operational stability of PSCs is slightly improved by the mixed 2D ligands. This work reveals the mechanisms of 2D ligands in strain tailoring and defect passivation toward efficient and stable narrow-bandgap PSCs.     

Tuneable Piezoresistance of Graphene-Based 2D:2D Nanocomposite Networks

Piezoresistive nanocomposites are an important class of materials that allow the production of very sensitive strain sensors. Herein, a new class of piezoresistive nanocomposites prepared by mixing different types of 2D nanosheets is explored. In this way, three distinct types of nanocomposite are produced by mixing conducting and insulating nanosheets (graphene, Gr and boron nitride, BN), conducting and semiconducting nanosheets (graphene and tungsten diselenide, WSe₂ or tungsten disulfide, WS₂) as well as mixing two different types of conducting nanosheets (graphene and silver, Ag). For each nanocomposite type, a different dependence of composite conductivity on filler volume fraction is observed although all behaviors can be fully described by percolation theory. In addition, each composite type shows different piezoresistive properties. Interestingly, while the conductor insulator composites show the standard monotonic relationship between gauge factor and conductivity, both conductor:semi-conductor and conductor:conductor composites show very unusual behavior, in each case displaying a peak engage factor at the percolation threshold. In each case, percolation theory is used to develop simple equations for gauge factor as a function of both volume fraction and conductivity that fully describes all experimental data. This work expands the understanding of piezoresistive nanocomposites and provides a platform for the engineering of high-performance strain sensors.     

Nanoparticle-Based Strain Gauges: Anisotropic Response Characteristics,Multidirectional Strain Sensing,and Novel Approaches to Healthcare Applications

Directional strain sensing is essential for advanced sensor applications in the field of human-machine interfaces and healthcare. Here, the angle dependent anisotropic strain sensitivity caused by charge carriers percolating through cross-linked gold nanoparticle (GNP) networks is studied and these versatile materials are used for the fabrication of wearable triaxial pulse and gesture sensors. More specifically, the anisotropic response of 1,9-nonanedithiol cross-linked GNP films is separated into geometric and piezoresistive contributions by fitting the measured data with an analytic model. Hereby, piezoresistive coefficients of g₁₁ ∼ 32 and g₁₂ ∼ 21 are extracted, indicating a slightly anisotropic response behavior of the GNP-based material. To use the material for healthcare applications, arrangements of three GNP transducers are patterned lithographically and fully embedded into a highly flexible silicone polymer (Dragon Skin 30). The new encapsulation method ensures good and robust electrical contacts and enables facile handling and protection from external influences. A facile read-out with wireless data transmission using off-the-shelf electrical components underlines the great potential of these devices for applications as skin-wearable healthcare sensors.     

Highly Processable Ionogels with Mechanical Robustness

Currently, the increasing needs of conductive ionogels with intricate shapes and high processability by individual requirements of next-generation flexible electronics constitute significant challenges. Here, the design of highly processable ionogels is reported with mechanical robustness by self-assembly of a common triblock copolymer into a precursor in functional mixed ionic liquids (ILs) containing conductivity-enhancing and polymerizable strength-enhancing components. The subsequent in situ polymerization of the precursor forms physical-co-chemical cross-linked networks, in which the entanglement between physical and chemical cross-linked networks and microphase separation give rise to mechanical robustness of as-fabricated ionogel. The viscosity of the self-assembled precursor can be rationally tuned, which makes the fabrication process compatible with diverse technologies including inkjet printing, spray coating, and 3D printing. By virtue of highly processable capability of the designed ionogels, an auxetic-structured ionogel can be easily generated using 3D printing, which exhibits greatly improved sensitivity and thus is able to monitor tiny deformations. This study that relies on designing functional mixed ILs as the dispersion phase rather than focusing on synthesizing new-type polymers establishes a new route for versatile and programmable fabrication of high-performance ionogels for broader applications.     

二元超品格弹性应力的能带理论计算

根据量子能带理论,采用经验赝势方法,对(Pb)n／(Bi)n,(A1)n／(As)n的假想简立方二元超品格结构的弹性应力进行了计算．结果表明弹性应力随超晶格周期λ的增加而迅速升高至一定大小后趋于稳定,且只有在周期较小时才同过渡金属氮化物超晶格硬度随周期的变化的趋势定性一致,为超晶格硬度随周期的变化提供了一种可能的解释．     

Automatic energy–momentum conserving time integrators for hyperelastic waves

An energy–momentum conserving time integrator coupled with an automatic finite element algorithm is developed to study longitudinal wave propagation in hyperelastic layers. The Murnaghan strain energy function is used to model material nonlinearity and full geometric nonlinearity is considered. An automatic assembly algorithm using algorithmic differentiation is developed within a discrete Hamiltonian framework to directly formulate the finite element matrices without recourse to an explicit derivation of their algebraic form or the governing equations. The algorithm is illustrated with applications to longitudinal wave propagation in a thin hyperelastic layer modeled with a two-mode kinematic model. Solution obtained using a standard nonlinear finite element model with Newmark time stepping is provided for comparison.     

Weak Fiber Bragg Grating Based Sensing System for Cryogenic Static Test of Launch Vehicle Oxygen Tank

In this paper,a weak fiber Bragg grating (WFBG) based sensing system applied to the cryogenic static test of launch vehicle oxygen tanks has been developed and the results of an evaluation are reported.A customized sensor encapsulation and installation method allow for precise strain measurement at cryogenic temperatures.The reflection peaks of series-connected WFBGs with low optical loss are obtained,ensuring high system reliability in harsh environments.The experimental results show that the maximum full-scale error is less than 0.81％ full-scale error.The temperature is as low as-193 ℃ during the test.This study also demonstrates a practical method in which WFBG can be used to obtain critical parameters for structural monitoring in a cryogenic environment.