A Simplified Model for Buckling and Post-Buckling Analysis of Cu Nanobeam Under Compression

Both of Buckling and post-buckling are fundamental problems of geometric nonlinearity in solid mechanics.With the rapid development of nanotechnology in recent years,buckling behaviors in nanobeams receive more attention due to its applications in sensors,actuators,transistors,probes,and resonators in nanoelectromechanical systems(NEMS)and biotechnology.In this work,buckling and post-buckling of copper nanobeam under uniaxial compression are investigated with theoretical analysis and atomistic simulations.Different cross sections are explored for the consideration of surface effects.To avoid complicated high order buckling modes,a stressbased simplified model is proposed to analyze the critical strain for buckling,maximum deflection,and nominal failure strain for post-buckling.Surface effects should be considered regarding critical buckling strain and the maximum post-buckling deflection.The critical strain increases with increasing nanobeam cross section,while themaximumdeflection increases with increasing loading strain but stays nearly the same for different cross sections,and the underlying mechanisms are revealed by our model.The maximum deflection is also influenced by surface effects.The nominal failure strains are captured by our simulations,and they are in good agreement with the simplified model.Our results can be used for helping design strain gauge sensors and nanodevices with self-detecting ability.

Deformation and Spallation of Explosive Welded Steels under Gas Gun Shock Loading

We investigate deformation and spallation of explosive welded bi-steel plates under gas gun shock loading.Free surface histories are measured to obtain the Hugoniot elastic limit and spall strengths at different impact velocities.Pre-and post-shock microstructures are characterized with optical metallography,scanning electron microscopy,and electron backscatter diffraction.In addition,the Vickers hardness test is conducted.Explosive welding can result in a wavy steel/steel interface,an ultrafine grain region centered at the interface,and a neighboring high deformation region,accompanied by a hardness with the highest value at the interface.Additional shock compression induces a further increase in hardness,and shock-induced deformation occurs in the form of twinning and dislocation slip and depends on the local substructure.Spall damage nucleates and propagates along the ultrafine grain region,due to the initial cracks or weak interface bonding.Spall strengths of bimetal plates can be higher than its constituents.Plate impact offers a promising method for improving explosive welding.

Novel X-Ray and Optical Diagnostics for Studying Energetic Materials:A Review

含能材料在制备、加工或外刺激(如冲击压缩)过程中的热力学、物理和化学过程包含多个时空尺度,发掘其中的新现象、获取新数据以及了解内在机制都亟需时空分辨诊断技术。在本文中,我们简要回顾了一些新兴或已存在但较少应用于含能材料的新型诊断技术,包括二维(2D)和三维(3D)X射线成像、X射线衍射、相干X射线衍射成像、X射线小角度散射、太赫兹和光学吸收/发射光谱以及一维(1D)和二维激光速率/位移干涉技术。这些技术涉及的典型空间尺度主要是晶格尺度(纳米和微米),典型时间尺度包括飞秒、纳秒、微秒和毫秒;针对的科学问题和工程难题包括缺陷、强度、变形、热点、相变、反应和冲击感度。本文介绍了探测和数据分析的基本原理并辅以示例说明。同时,先进测量和实验复杂性方面还急需发展相应的数据分析和解析方法学以及多尺度模拟。

Temperature-Dependent Far-Infrared Absorption in Cyclotrimethylene Trinitramine Single Crystals Using Broadband Time-Domain Terahertz Spectroscopy

We investigate the absorption properties of cyclotrimethylene trinitramine(RDX) single crystals from ~15 to150 cm^(-1) using the terahertz time-domain spectroscopy. We observe that all the absorption modes exhibit strong anisotropic behavior in terms of the crystal orientations. We demonstrate that the anharmonic phonon model can well describe the temperature-dependent behaviors of these absorption modes. These results indicate that the intermolecular interaction plays a major role in the collective motion of large number of RDX molecules. Our findings provide important information for understanding and controlling the dynamic properties in the explosive materials.

Dynamic compression behavior of TiZrNbV refractory high-entropy alloys upon ultrahigh strain rate loading

In this study,the dynamic compressive response behavior of a body-centered cubic(BCC)single-phase TiZrNbV refractory high-entropy alloy(RHEA)was investigated under impact at speeds of 313-1584 m s^(-1)using two-stage,gas-gun-driven,high-speed plate-impact experiments;recovery sample analysis;and theoretical calculations.The strain rate and pressure were approximately 10^(7) s^(−1) and 5.07-29.37 GPa,respectively.The results showed that the TiZrNbV RHEA had a Hugoniot elastic limit of 4.12-5.86 GPa and a spall strength of 1.84-2.03 GPa.The initial yield strength of the alloy showed a strong strain-rate dependence and could be described by the modified Zerilli-Armstrong model,while the phonon-damping effect was the main reason for its high strain-rate sensitivity.Microstructural analysis showed that the dynamic deformation of the TiZrNbV RHEA was controlled by the dislocation slip,dislocation proliferation,intersection of the deformation bands,and grain refinement.The analysis also showed that the intergranular,transgranular,and mixed-type cracks dominated the spall failure of the material.The dynamic Hall-Petch effect and pinning from the lattice distortion led to high dynamic yield strength.The critical strain rate for the phonon drag effect was positively related to the relative atomic mass and local strain field of the metals.Within the experimental loading range,the RHEA showed good structural stability,and simultaneously,the theoretical calculation method for the equation of state based on a cold-energy mixture could accurately predict its shock-response behavior.The valence-electron concentration(VEC)had a direct effect on the shock-compression properties of the HEAs;higher VEC implied more difficulty in compressing the HEAs.The findings of this study provide insights into understanding the mechanical response characteristics of RHEAs under extreme conditions such as high-speed impact and ultrahigh strain-rate loading.

Dynamic mechanical properties,deformation and damage mechanisms of eutectic high-entropy alloy AlCoCrFeNi_(2.1)under plate impact

Shock compression and spallation of a eutectic high-entropy alloy(HEA)AlCoCrFeNi_(2.1)with lamellar structure are investigated via plate impact loading with free-surface velocity measurements.The as-cast and postmortem samples are characterized with transmission electron microscopy,electron back-scatter diffraction and scanning electron microscopy.An accurate Hugoniot equation of state is determined.Af-ter shock compression to∼12 GPa,both the L1_(2)and B_(2)phases retain their ordered structures.Dense dislocations in the{111}slip planes,stacking faults and deformation twins are found in the L1_(2)phase,along with fewer dislocations in the{110}slip bands in the B(2)phase.Shock-induced deformation twin-ning within the L1_(2)phase of this HEA is observed as a new deformation mechanism under various load-ing conditions.For spallation,both ductile and brittle damage modes are observed.The micro voids and cracks prefer to nucleate at the phase boundaries chiefly,then in the B(2)phase.Under similar shock stress,the spall strength of AlCoCrFeNi_(2.1)HEA is about 40%higher than those of other reported dual-phase HEAs due to the high stability of its semi-coherent phase boundaries.

Shock compression and spallation damage of high-entropy alloy Al_(0.1)CoCrFeNi

Shock compression and spallation damage of a face-center cubic phase high-entropy alloy(HEA)Al_(0.1)CoCrFeNi were investigated via plate impact experiments along with free surface velocity measurements.Postmortem samples were characterized with transmission electron microscopy and electron backscatter diffraction.The Hugoniot equation of state and spall strength at different impact strengths were determined.There exists a power-law relation between spall strength and strain rate.The spall strength of Al_(0.1)CoCrFeNi HEA is about 50%higher than those of previously studied HEAs and comparable to those widely applied structural stainless steels at the same shock stress.Dislocation glide and stacking faults are the important deformation mechanisms in the Al_(0.1)CoCrFeNi HEA.Nanotwins are only observed at high shock stress.Damage in the Al_(0.1)CoCrFeNi HEA is ductile in nature.Voids are nucleated preferentially in grain interiors,and the intragranular voids show a strong dependence on grain boundary misorientation and peak stress.

Implication of Sensitive Reactions to Ignition of Methyl Pentanoate:H-Abstraction Reactions by H and CH_(3) Radicals

Methyl pentanoate(MP)was identified as a potential candidate.To facilitate the application of MP with high efficiency in engines,a comprehensive understanding of combustion chemical kinetics of MP is necessary.In this work,the H-abstraction reactions from MP by H and CH_(3) radicals,critical in controlling the initial fuel consumption,are theoretically investigated at the DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ level of theory.The multistructural torsional(MS-T)anharmonicity is characterized using the dual-level MS-T method;the HF/3-21G and M06-2X/cc-pVTZ methods are chosen as the low-and high-level methods,respectively.The conventional transition state theory(TST)is employed to calculate the high-pressure limit rate constants at 298-2000 K with the Eckart tunneling correction.Our calculations indicate that the hydrogen atoms of the methylene functional group are easier to be abstracted by H and CH_(3) radicals.The multistructural torsional anharmonicities of H-abstraction reactions MP+H/CH_(3) are significant within the temperature range investigated.The tunneling effects are more pronounced at low temperatures,and contribute considerably to the rate constants below 500 K.The model from the work of Diévart et al.is updated with our calculations,and the simulations of the updated model are in excellent agreement with the reported ignition delay time of MP/O2/Ar and MP/Air mixtures.The sensitivity analysis indicates that the H-abstraction reactions,MP+H-CH_(3)CH_(2)CHCH_(2)C(-O)OCH_(3)/CH_(3)CHCH_(2)CH_(2)C(-O)OCH_(3)+H2,are critical in controlling the initial fuel consumption and ignition delay time of MP.

Active{1121}and{1012}extension twinning in a WE43 Mg alloy

Deformation twinning in a WE43 alloy subjected to uniaxial compression at room temperature is in-vestigated.Active{1121}and{1012}extension twinning is observed.The activation of{1121}twinning in a certain grain is influenced not only by its own Schmid factor,but also by that of the basal slip.Twin-twin interactions of the co-zone{1121}twin variants are commonly observed and systematically studied,including the crystallographic characteristics of the four typically observed twin-twin junctions(TTJs)and the associated twin-twin boundaries.Unlike the quilted-looking TTJs formed by the interaction of co-zone{1012}twin variants,crossing-like TTJs of co-zone{1121}twin variants are observed and their formation mechanisms are discussed.Crossing-like TTJs are also formed by the interaction of{1121}and{10¯12}twins.The morphology and thickening rate of{1121}and{1012}twins vary markedly due to the difference in twinning shear.The needle-like{1121}twins thicken more slowly than the lenticular{1012}twins.Twin nucleation is observed on the previously formed{1121}twin boundary due to the relatively large strain concentration on such interface,while twin nucleation on{1012}twin boundary is rarely observed.

Multiple ballistic impacts on 2024-T4 aluminum alloy by spheres:Experiments and modelling

Multiple ballistic impacts are carried out on a 2024-T4 aluminum alloy by spherical steel projectiles(5-mm diameter)at~400 m s^(-1),to investigate its dynamic deformation and damage.The ballistic impact process is captured with high-speed photography.Postmortem samples are characterized with optical imaging,three-dimensional laser scanning,microhardness testing and electron backscatter diffraction.With increasing number of impacts,crater diameter increases slightly,but crater depth and crater volume increase significantly,and strain accumulation leads to microhardness increase overall.Crater parameters all follow power-law relations with the number of impacts.Twin-like deformation bands and macroscopic deformation twins are produced by impact as a result of spontaneous dislocation self-pinning under high strain rate,large shear deformation.Under multiple impacts,shear strain accumulation in the arc-shaped region of the crater induces deformation twinning when it exceeds a critical value(~1.1-1.6).It is highly possible that the deformation twins are related to deformation bands,since they both share one set of the{111}pole with the initial matrix grain.A finite element method model is optimized to reproduce experimental observations and interpret deformation mechanisms.