The high undercooling of binary Co-4.54%Sn alloy has a significant influence on its microstructural characteristics and physical properties. Here, we report that the dendritic growth and physical properties of broad-temperature-range Co-4.54%Sn alloy remarkably depends on the undercooling during the rapid solidification. The maximum undercooling attains 208 K at molten state, and the dendritic growth velocity is quite sluggish in highly undercooled liquid Co-4.54%Sn alloy because it has a broad solidification range of 375 K (0.21 TL); the maximum value is only 0.95 m/s at the undercooling of 175 K, which then decreases with undercooling. The microstructure refines visibly and the volume fraction of the interdendritic βCo3Sn2 phase obviously decreases with undercooling. The microhardness and electrical resistivity increase with undercooling owing to the enhancement of solute content of the primary αCo phase and refinement of the microstructure where the increased crystal boundary hinders the electronic transmission. Meanwhile, the saturation magnetization also reduces with undercooling due to the crystal particle and boundary increasing significantly, and the dendritic growth velocity and solute content increase in the primary αCo phase under rapid solidification.

为了提高列车的耐撞性能,采用基于模型的多目标遗传算法的优化方法,研究城轨列车方锥式防爬吸能结构碰撞力学参数的最优配置.设计一种方锥式防爬吸能结构,通过建立8编组列车碰撞纵向多体动力学模型,分析吸能结构不同力学参数配置对列车耐撞性能的影响;提出头车吸能量(EA)和列车总体减速度(TMA)的车体端部耐撞性评价指标,基于实验设计采用径向基函数法建立F,K和Fmax关于EA和TMA的响应面模型,并基于模型采用多目标遗传算法对防爬吸能结构的力学参数进行多目标优化.研究结果表明:在最优参数配置下,EA提高了1.933％,TMA降低了14.810％;优化后的力学参数配置方案降低了列车碰撞减速度,提高了头车吸能量占比和整车的耐撞性能.

Modifying epoxy resin molecules using phosphorus-containing functional groups can enhance the liquid oxygen compatibility (LOC) of polymeric matrix composites, but will significantly change the mechanical and fracture properties of the functionalized thermosets due to the increased complexity of the molecular architecture. The underlying mechanisms responsible for these property changes are not well understood. In this work, we unveiled the molecular-scale fracture mechanisms of epoxy resins modified by 10-(2,5-dihydroxyphenyl) -10-hydrogen-9-oxal -phenanthroline-10-oxide (ODOPB) using molecular dynamics (MD) simulations. We adopted a two-step reaction scheme to prepare the modified cross-linked networks of the epoxy thermosets. Then, a fracture simulation approach was developed based on hybrid use of non-reactive and reactive force field parameters, which enables accurate and efficient bond scission representation during tensile deformation. Efforts were made to explore the impact of varying the P content (proportional to ODOPB amount) on the molecular architectures and mechanical performance. It was found that the chain length distribution was a crucial determinant of the mechanical and fracture properties. More intriguingly, the simulation results showed that at a fixed P content, properties such as the fracture energy could be enhanced by regulating the chain length. This study offers valuable insights into the design and fabrication of high-performance aerospace composites with remarkable tolerance to harsh engineering environments.

Manmade high-performance polymers are typically non-biodegradable and derived from petroleum feedstock through energy intensive processes involving toxic solvents and byproducts. While engineered microbes have been used for renewable production of many small molecules, direct microbial synthesis of high-performance polymeric materials remains a major challenge. Here we engineer microbial production of megadalton muscle titin polymers yielding high-performance fibers that not only recapture highly desirable properties of natural titin (i.e., high damping capacity and mechanical recovery) but also exhibit high strength, toughness, and damping energy - outperforming many synthetic and natural polymers. Structural analyses and molecular modeling suggest these properties derive from unique inter-chain crystallization of folded immunoglobulin-like domains that resists interchain slippage while permitting intra-chain unfolding. These fibers have potential applications in areas from biomedicine to textiles, and the developed approach, coupled with the structure-function insights, promises to accelerate further innovation in microbial production of high-performance materials.

To understand the effect of chemical composition, cross-link density, and microstructure on the linear and nonlinear viscoelasticity of ethylene propylene diene monomer (EPDM) rubber, we carried out high-frequency oscillatory shear molecular dynamics simulations at varying shear strain rates. Sweeping through different EPDM compositions with varying ethylene, propylene, and diene ratios, a positive correlation was observed between the ratio of the propylene monomer and the complex shear modulus of EPDM in the high-frequency glassy regime. For small deformations in this regime, we found that the simplest measure of local molecular stiffness, namely, the Debye-Waller factor, is predictive of the complex shear modulus and loss modulus of 20 unique systems with distinct compositions and cross-link densities. Polymer design parameters that reduce the Debye-Waller factor, including cross-linking or increased propylene content generally, result in higher moduli. Remarkably, large-amplitude oscillatory shear simulations revealed that dissipation becomes strongly influenced by polymer entanglements, which results in divergent optimal compositions for small-strain vs large-strain applications of EPDM. Utilizing time-temperature superposition and varying strain rates in simulations, we were able to capture rheological properties over 6 orders of magnitude in frequency. The data was captured well using a Rouse model superposed with a stretched exponential function, which was used to predict key constants that determine the mechanical behavior in these regimes. Our findings establish a chemistry-specific molecular simulation approach for capturing the constitutive behavior of elastomers and pave the way for multiscale analyses linking composition and microstructure to performance.

数字岩芯可提供无差别化仿真计算模型,是研究岩石物理力学性质的理想模型,精准高效建模一直制约着数字岩芯重构技术的推广.传统方法处理CT切片扫描数据费时费力,主要受限于2个方面,一是扫描层数有限;二是孔裂隙识别依赖于传统阈值分割算法.以煤岩为例,引入人工智能识别实现4种微观相态:孔隙、裂隙、高密度矿物和基质的智能识别,并开展分形重构.基于微米CT扫描建立4种微观相态数据集并进行了数据增强,开发了专用标注软件可实现跨尺度孔裂隙的准确标注.算法上优化了全卷级神经网络智能识别架构,建立Crack-FCN网络结构,网络层次少且错误率低.同时引入矢量化算法实现了裂隙面积、长度和宽度的定量计算;进而引入中心线细化算法实现了复杂裂隙拓扑结构的有效提取.最后开发局部自相似分形重构算法,并基于优化策略解决了快速插值问题,解决了相邻CT层扫描信息缺失的问题.结果 表明分形插值与直线插值和三阶样条插值相比局部粗糙特性明显,且保证了裂隙断面的粗糙性和连续性.工作引入全卷级神经网络智能识别技术用于构造数字岩芯,为高效精准建立数字岩芯提供了新的技术支撑.

Development of schemes to form complex three-dimensional (3D) mesostructures in functional materials is a topic of broad interest, thanks to the ubiquitous applications across a diversity of technologies. Recently established schemes in the mechanically guided 3D assembly allow deterministic transformation of two-dimensional structures into sophisticated 3D architectures by controlled compressive buckling resulted from strain release of prestretched elastomer substrates. Existing studies mostly exploited supporting substrates made of homogeneous elastomeric material with uniform thickness, which produces relatively uniform strain field to drive the 3D assembly, thus posing limitations to the geometric diversity of resultant 3D mesostructures. To offer nonuniform strains with desired spatial distributions in the 3D assembly, this paper introduces a versatile set of concepts in the design of engineered substrates with heterogeneous integration of materials of different moduli. Such heterogeneous, deformable substrates can achieve large strain gradients and efficient strain isolation/magnification, which are difficult to realize using the previously reported strategies. Theoretical and experimental studies on the underlying mechanics offer a viable route to the design of heterogeneous, deformable substrates to yield favorable strain fields. A broad collection of 3D mesostructures and associated heterogeneous substrates is fabricated and demonstrated, including examples that resemble windmills, scorpions, and manta rays and those that have application potentials in tunable inductors and vibrational microsystems.

The ribbons selectively bonded to a prestrained elastomeric substrate may buckle into three-dimensional (3D) microstructures after the prestrain release, leading to three possible deformation modes, global, local, and no buckling, depending on the adhesion between the ribbons and substrate. This note establishes analytically the critical length-to-thickness ratio of ribbons, above which the global buckling mode (preferred for mechanically guided 3D deterministic assembly) occurs without material failure.

Approaches capable of creating three-dimensional (3D) mesostructures in advanced materials (device-grade semiconductors, electroactive polymers etc.) are of increasing interest in modern materials research. A versatile set of approaches exploits transformation of planar precursors into 3D architectures through the action of compressive forces associated with release of prestrain in a supporting elastomer substrate. Although a diverse set of 3D structures can be realized in nearly any class of material in this way, all previously reported demonstrations lack the ability to vary the degree of compression imparted to different regions of the 2D precursor, thus constraining the diversity of 3D geometries. This paper presents a set of ideas in materials and mechanics in which elastomeric substrates with engineered distributions of thickness yield desired strain distributions for targeted control over resultant 3D mesostructures geometries. This approach is compatible with a broad range of advanced functional materials from device-grade semiconductors to commercially available thin films, over length scales from tens of microns to several millimeters. A wide range of 3D structures can be produced in this way, some of which have direct relevance to applications in tunable optics and stretchable electronics.

Low modulus, compliant systems of sensors, circuits and radios designed to intimately interface with the soft tissues of the human body are of growing interest, due to their emerging applications in continuous, clinical-quality health monitors and advanced, bioelectronic therapeutics. Although recent research establishes various materials and mechanics concepts for such technologies, all existing approaches involve simple, two-dimensional (2D) layouts in the constituent micro-components and interconnects. Here we introduce concepts in three-dimensional (3D) architectures that bypass important engineering constraints and performance limitations set by traditional, 2D designs. Specifically, open-mesh, 3D interconnect networks of helical microcoils formed by deterministic compressive buckling establish the basis for systems that can offer exceptional low modulus, elastic mechanics, in compact geometries, with active components and sophisticated levels of functionality. Coupled mechanical and electrical design approaches enable layout optimization, assembly processes and encapsulation schemes to yield 3D configurations that satisfy requirements in demanding, complex systems, such as wireless, skin-compatible electronic sensors.

Bubbles blown up from a water surface can only last for seconds before bursting due to gravity, surface tension and evaporation. Although adding certain surfactants and depressing evaporation can significantly extend the bubbles' lifetime, there is still no method to prevent the bubble film from getting thinner and avoid the effects of evaporation. Here we report our experimental observation that centimeter length scale water bubbles can last for over a month at room temperature in the open natural environment with evaporation if they are covered with densely distributed microparticles on the bubble top surface. The underlying stability mechanism to balance out evaporation water loss is revealed to be the existence of negative pressure in the water between the two water-air interfaces of the film of the bubbles. This negative pressure is generated by surface tension of the locally curved water-air interfaces spanned over the particles and acts against gravity to suck water up from the water bulk and self-adaptively compensate the water loss due to evaporation. A theoretical model of the above water supplementary mechanism is built and computed numerically using Surface Evolver. A three-dimensional fluorescence experiment is also designed to verify the above water transfer process. This mechanism is generally valid for making ultra-long lifetime bubbles not only with water, but also for other liquids and suitable particles that satisfy certain contact angle requirements.

Precise, quantitative in vivo monitoring of hydration levels in the near surface regions of the skin can be useful in preventing skin-based pathologies, and regulating external appearance. Here we introduce multimodal sensors with important capabilities in this context, rendered in soft, ultrathin, ‘skin-like’ formats with numerous advantages over alternative technologies, including the ability to establish intimate, conformal contact without applied pressure, and to provide spatiotemporally resolved data on both electrical and thermal transport properties from sensitive regions of the skin. Systematic in vitro studies and computational models establish the underlying measurement principles and associated approaches for determination of temperature, thermal conductivity, thermal diffusivity, volumetric heat capacity, and electrical impedance using simple analysis algorithms. Clinical studies on 20 patients subjected to a variety of external stimuli validate the device operation and allow quantitative comparisons of measurement capabilities to those of existing state-of-the-art tools.

The functional amyloid curli fiber, a major proteinaceous component of biofilm extracellular matrices, plays an important role in biofilm formation and enterobacteriaceae adhesion. Curli nanofibers exhibit exceptional underwater adhesion to various surfaces, have high rigidity and strong tensile mechanical properties, and thus hold great promise in biomaterials. The mechanisms of how curli fibers strongly attach to surfaces and detach under force remain elusive. To investigate curli fiber adhesion to surfaces, we developed a coarse-grained curli fiber model, in which the protein subunit CsgA (curli specific gene A) self-assembles into the fiber. The coarse-grained model yields physiologically relevant and tunable bending rigidity and persistence length. The force-induced desorption of a single curli fiber is examined using coarse-grained modeling and theoretical analysis. We find that the bending energy penalty arising from high persistence length enhances the resistance of the curli fiber against desorption and thus strengthens the adhesion of the curli fiber to surfaces. The CsgA-surface adhesion energy and the curli fiber bending rigidity both play crucial roles in the resistance of curli fiber against desorption from surfaces. To enable the desorption process, the applied peeling force must overcome both the interfacial adhesion energy and the energy barrier for bending the curli fiber at the peeling front. We show that the energy barrier to desorption increases with the interfacial adhesion energy, however, the bending induced failure of a single curli fiber limits the work of adhesion if the proportion of the CsgA-surface adhesion energy to the CsgA-CsgA cohesive energy becomes large. These results illustrate that the optimal adhesion performance of nanofibers is dictated by the interplay between bending, surface energy and cohesive energy. Our model provides timely insight into enterobacteriaceae adhesion mechanisms as well as future designs of engineered curli fiber based adhesives.

Liquid Ag-5% Bi alloy has been rapidly solidified in the form of 84-1140 mu m droplets under free fall condition. The results show that there are dendritic (Ag)(1) phase, interdendritic (Ag)(2) phase and (Bi) solid solution phase in this alloy respectively. The microstructure of (Ag)(1) phase displays a conspicuous "dendritic-equiaxed" morphology transition with the decrease of droplet diameter, which is attributed to the fragmentation of dendrites induced by the recalescence of highly undercooled liquid alloy. The theoretical analysis with the LKT/BCT dendritic growth model predicts that the growth velocity of (Ag) dendrite increases with undercooling, and the kinetics transition from solute diffusion-controlled growth to thermal diffusion-controlled growth occurs at 98 K during solidification. Meanwhile, the remarkable solute trapping effect takes place according to theoretical calculated calculation.

针对地下水对隔水煤柱和矿井地下水库边界的反复浸水损伤过程,研究反复浸水对煤样的力学性质损伤作用,对矿井地下储水和含水层保护等问题有借鉴意义.文中选择15个不同浸水次数的典型煤样进行单轴压缩声发射实验.研究发现,煤样的峰值应力、弹性模量、应变软化模量、后峰值模量均随着煤样浸水次数的增加而呈现不同程度的减小,而峰值应变随着浸水次数的增加而上升.综上反映了水对煤样的明显弱化作用.声发射累计计数、能率均能很好地表示出裂隙发育的各个阶段,对求取弹性模量研究煤样力学性质有重要作用.实验结果对反复浸水作用下煤样的力学性质和裂隙损伤的研究具有借鉴作用.