Shock waves in molecular solids: ultrafast vibrational spectroscopy of the first nanosecond

A novel technique which uses a microfabricated shock target array assembly is described, where the passage of a shock front through a thin (0.5μm) polycrystalline layer and the subsequent unloading process is monitored in real time with ultrafast coherent Raman spectroscopy. Using a high repetition rate laser shock generation technique, high resolution, coherent Raman spectra are obtained in shocked anthracene and in a high explosive material, NTO, with time resolution of ∼ 50 ps. Spectroscopic measurements are presented which yield the shock pressure (up to 5 GPa), the shock velocity (∼ 4 km/s), the shock front risetime (t _r < 25 ps), and the temperature (∼ 400°C). A brief discussion is presented, how this new technique can be used to determine the Hugoniot, the equation of state, the entropy increase across the shock front, and monitor shock induced chemical reactions in real time. Received 28 October 1996 / Accepted 12 November 1996     

Ultrafast vibrational spectroscopy imaging of nanoshock planar propagation

Variations of the velocity and pressure of repetitive 4 GPa laser-driven shock fronts in a polymer thin film are studied using spatially resolved ultrafast coherent anti-Stokes Raman Spectroscopy (CARS) measurements of shock compression of an anthracene optical gauge. Even though the radial profile of the laser pulse that generates the shock is a Gaussian that falls off steeply at the edges, optical saturation effects in the shock generation layer flatten out the shock front. Detailed measurements show the shock pressure, as indicated by the blueshift of an anthracene vibrational transition, and the shock velocity, as indicated by the arrival time at the anthracene gauge layer, remain constant within better than 5% over the central region probed by CARS, over a run distance of at least m. Received 30 July 2001 / Accepted 13 March 2002 Published online 17 June 2002     

Time- and space-resolved studies of shock compression molecular dynamics

Two kinds of experiments are described, that measure the dynamical effects of shock compression on molecules with high time and space resolution. Time resolution is obtained using ultrafast nonlinear coherent vibrational spectroscopy. Space resolution greater than the diffraction limit is obtained by building in nanostructures of known dimensions. In the first experiments, 100 ps laser flash heating is used to suddenly vaporize Al nanoparticles embedded in reactive oxidizing polymers nitrocellulose (NC) and Teflon. The hot nanoparticles react with a surrounding shell of oxidizer and generate spherical shock waves > 10 GPa. The propagation of shock-induced chemistry in time and over distances ranging from 50 to 1,000 nm is measured and discussed. In the second experiment, femtosecond laser-driven planar shock waves run through a molecular monolayer of linear hydrocarbon chains. The methyl –CH₃ groups that terminate the chains form a plane ～1.5 Å thick. The C–H stretching vibrations of these groups are monitored as the shock front passes over. A combination of experiment and molecular simulations shows that chains with odd (15) numbers of carbon atoms become shorter by bending behind the shock front, whereas chains with even numbers (18) of carbon atoms undergo mechanical failure and shorten by forming gauche defects.     

薄膜式锰铜压阻计的动态标定

介绍了以飞片平面撞击标定某新型锰铜压阻计的过程及方法. 标定在Phi 130 mm轻气炮上进行, 采用有机玻璃, Cu和W 3种材料为飞片和靶板材料, 实现不同速度下的 同种材料平面正撞击, 获得0.66$\sim$25.4\,GPa范围的入射压力, 并利用几何修 正和$D$-$u$关系修正两种方法对结果进行了修正, 给出了标定结果以及拟合曲线.     

一个改进的电磁波传输激波管实验

在中国科学院力学研究所$\varPhi $ 800 mm高温低密度激波管上进行电磁波在等离子体中传输机理研究时,低密度和强激波条件下,由于气体解离和电离等非平衡过程,使得激波后2区宽度显著减小;同时由于边界层效应造成激波衰减和接触面加速,使得激波后2区长度进一步减小.这两个效应导致激波管2区实验观测 时间减小,2区气体处于非平衡状态,增加了观察数据的不稳定性和数据分析的难度.本文提出在$\varPhi 800 $ mm高温低密度激波 管中采用氩气(Ar)和空气(Air)混合气替代纯空气作为激波管实验介质气体.利用Ar不解离和难电离的特性,减小激波前后压缩比,从而 增加激波后2区实验时间和气体长度. 采用Langmuir 静电探针和微波透射诊断技术测量激波后电子密度,同时利用探针测量激波后2区实验时间.结果显示,在Ar+Air混合气实验中,激波波后电子密度可达与纯Air同样的10$^{13}$cm$^{ - 3}$量级.在与纯Air相同的电子密度和碰撞频率条件下,采用95%Ar+5%Air和90%Ar+10%Air两种混合气,激波后2区实验时间和气体长度约为纯Air条件下的5$\sim $10倍,其中2区实验时间为300$\sim $800 $\mu$s,2区气体长度1$\sim $1.5 m.在$\varPhi $800 mm激波管中采用Ar+Air介质气体进行电磁波传输实验,获得了比在纯Air介质中与理论预测更一致的结果.      

Attenuation of shock waves by granular filters

Attenuation of shock waves in granular filters has been studied. Both pressurized air and solid explosives have been used for generating shock waves in a shock tube. The shock tube had a total length of m, and an internal diameter of 355 mm. Two large scale experiments have also been carried out in a tunnel with a cross-sectional area of 6.5 m². The results are compared with results found in the literature (Zloch, 1976; Medvedev et al., 1990; Britan et al., 2001) and previous experiments in a smaller scale by Slungaard (2002). A simple correlation based on the work of Zloch (1976) for shock wave attenuation in tube bundles and an extensive amount of experiments, is proposed. The correlation can be used to estimate the attenuation of the shock wave through a granular filter with filter characteristic . Setting B=6 will give a conservative estimate of the attenuation, while setting B=3 will give the best fit to all the results from this study and the results found in the literature. The correlation is independent of the type of driver (pressurised air or solid explosives) and upstream shock strength.Received: 30 September 2002, Accepted: 19 December 2002, Published online: 6 March 2003     

Dense particle cloud dispersion by a shock wave

A dense particle flow is generated by the interaction of a shock wave with an initially stationary packed granular bed. High-speed particle dispersion research is motivated by the energy release enhancement of explosives containing solid particles. The initial packed granular bed is produced by compressing loose powder into a wafer with a particle volume fraction of $\phi _\mathrm{p} = 0.48$ . The wafer is positioned inside the shock tube, uniformly filling the entire cross-section. This results in a clean experiment where no flow obstructing support structures are present. Through high-speed shadowgraph imaging and pressure measurements along the length of the channel, detailed information about the particle shock interaction was obtained. Due to the limited strength of the incident shock wave, no transmitted shock wave is produced. The initial solid-like response of the particle wafer acceleration forms a series of compression waves that eventually coalesce to form a shock wave. Breakup is initiated along the periphery of the wafer as the result of shear that forms due to the fixed boundary condition. Particle breakup is initiated by local failure sites that result in the formation of particle jets that extend ahead of the accelerating, largely intact, wafer core. In a circular tube, the failure sites are uniformly distributed along the wafer circumference. In a square channel, the failure sites, and the subsequent particle jets, initially form at the corners due to the enhanced shear. The wafer breakup subsequently spreads to the edges forming a highly non-uniform particle cloud.     

钨球对高硬度钢斜侵彻效应

为研究高硬度钢板抗不同着角钨球的侵彻性能及破坏模式,通过弹道枪进行了?8 mm、?11 mm钨合金球形破片以0°、20°、40°着角撞击厚度为6 mm、8 mm的高硬度钢板试验,得到了极限贯穿速度v₅₀;分析了钨球轴向径向变形及靶板失效模式与撞击速度的关系,发现高硬度钢板失效模式主要为压缩开坑破坏和沿厚度方向剪切破坏。采用有限元方法对试验进行了模拟,验证了数值模型及参数的合理性,并运用数值模拟方法研究了撞击着角对靶板吸能模式影响,结合试验数据,修正已有极限贯穿速度计算公式。结果表明:随侵彻着角增大,极限贯穿速度提高,且着角越大,极限贯穿速度增长越快;随着角增大,靶板吸能模式逐渐由压缩开坑向剪切冲塞过渡,且着角大于50°时,剪切冲塞耗能将超过压缩开坑耗能;修正后极限贯穿速度计算公式适用范围更广、精度更高,具有较好工程应用价值。     

完整岩石拉压应力状态下的张裂破坏准则

一点的应力状态可由3个主应力$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$来表示, 当规定主应力以压为正时, 沿最大主应力$\sigma_{1}$方向将产生收缩变形, 若中间主应力$\sigma_{2}$和最小主应力$\sigma_{3}$都远小于$\sigma_{1}$, 则沿$\sigma_{2}$和$\sigma_{3}$方向会产生横向扩张变形, 当横向扩张变形达到一定极限时, 将会在平行于$\sigma _{1}$的方向产生张裂破坏. 如何建立这种张裂破坏的强度准则目前尚缺乏研究, 最大拉应变理论(第二强度理论)有时被用来解释张裂破坏, 但最大拉应变理论难以应用于三向受力状态. 本文分别用$\varepsilon_{1}$, $\varepsilon_{2}$表示最大张应变和次大张应变, 则最大拉应变理论认为当$\varepsilon_{1}$达到单向拉伸屈服应变时, 材料将产生破坏. 而本文将根据$\varepsilon_{1}+\varepsilon_{2}$之和达到极限值$\varepsilon_u$来建立张裂破坏准则. 可以证明$\varepsilon_{1} +\varepsilon_{2}$所表示的是$\sigma_{1}$主平面的面积增长率. 当$\sigma_{3}<\sigma_{2} \ll \sigma_{1}$时, 大部分岩石都具有脆性破坏的特点, 所以可将破坏前的岩石视为满足广义胡克定律的线弹性材料, 这样用$\varepsilon_{1}$, $\varepsilon_{2}$表示的强度准则可通过$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$来表示. 在这个过程中还可考虑岩石在拉伸和压缩时具有不同弹性参数和强度的特点, 并可通过单向拉伸和单向压缩的破坏状态来确定$\varepsilon_u$. 不管$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$是压应力, 还是拉应力, 或者$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$中有拉有压的情形, 基于$\varepsilon_{1} +\varepsilon_{2} =\varepsilon_u$都可建立相应的强度准则. 所建立的准则可以反映中间应力$\sigma_{2}$对强度的影响规律, 通过建立的强度准则还可以证明: 静水拉力能引起屈服, 而静水压力不能产生屈服; 压缩破坏能使塑性体积增大, 其结果比Mohr-Coulomb准则更能反映实际情形. 并通过拉压应力状态下的试验数据验证了所建立的强度准则, 所得理论计算结果和已有的试验数据吻合得很好. 通过提出的强度准则和圆盘劈裂的试验结果, 可获得更为可靠的岩石单轴抗拉强度.      

Shock response of a heavy tungsten alloy

This article describes the results of shock wave experiments performed on a heavy tungsten alloy containing W, Ni, and Fe in the ratio of 92.85:4.9:2.25 by weight. These experiments provide information about the shear strength under compression and tensile strength, as measured by the spall threshold, of this alloy to 24 GPa. The results of these experiments show that: (i) the magnitude of its Hugoniot elastic limit (HEL) is 2.76±0.26 GPa; (ii) this alloy deforms plastically above its HEL and thus retains its shear strength to 24 GPa; (iii) the spall strength of the alloy is found to be 1.9±0.4 GPa and is independent of the impact stress and duration of the shock compression pulse; and (iv) the tensile impedance of the alloy, determined from a new experiment designed to measure this impedance, is 68±10 Gg/m² s.     

[100]LiF单晶在60 GPa以内冲击加载下的高压屈服强度特性

获取光学窗口自身的高压强度特性是开展材料高压高应变率冲击响应行为精密测量和数据反演的重要基础。利用平板撞击和双屈服面法,通过冲击-卸载、冲击-再加载原位粒子速度剖面精细测量和数据反演,获得了约60 GPa范围内[100]LiF屈服强度特性随冲击压力的变化规律。结果表明:在实验压力范围内,[100]LiF的屈服强度随加载压力的提高而显著提高,压力硬化效应显著;同时,LiF在冲击加载下的屈服强度高于磁驱准等熵加载结果,应变率硬化效应强于热软化效应。采用Huang-Asay模型确定了可描述冲击加载[100]LiF强度特性的本构模型参数,为LiF在强度、相变、层断裂等加窗测量实验中的深入应用和数据准确解读提供了重要支撑。     

One-dimensional numerical model for degradation and combustion of polymethyl methacrylate

A transient one-dimensional model is applied to study degradation and combustion of a poly methyl methacrylate (PMMA) sample. Ignition of PMMA is a complex interaction among different mechanisms, including solid fuel degradation, heat transfer, in-depth absorption of radiation, surface regression, gas-phase advective heat/mass transfer, and combustion. The present task has the significant feature of coupling the solid and gas phases. Besides the mathematical model has been solved numerically by using a fast iterative method and has yielded realistic results.     

Size Effect on The Bending And Tensile Strength of Micromachined Polysilicon Films for MEMS

I. INTRODUCTION Microelectromechanical systems (MEMS) have achieved impressive progress and become a very area of research. But long-term durability of various MEMS devices requires a fundamental understaof the fatigue and fracture characteristics of su…     

Direct laser-driven ramp compression studies of iron: A first step toward the reproduction of planetary core conditions

The study of iron under quasi-isentropic compression using high energy lasers, might allow to understand its thermodynamical properties, in particular its melting line in conditions of pressure and temperature relevant to Earth-like planetary cores (330–1500 GPa, 5000–8000 K). However, the iron alpha-epsilon solid–solid phase transition at 13 GPa favors shock formation during the quasi-isentropic compression process which can depart from the appropriate thermodynamical path. Understanding this shock formation mechanism is a key issue for being able to reproduce Earth-like planetary core conditions in the laboratory by ramp compression. In this article, we will present recent results of direct laser-driven quasi-isentropic compression experiments on iron samples obtained on the LULI 2000 and LIL laser facilities.     

16MnR钢缺口试样解理断裂行为研究

研究了低合金热轧钢16MnR缺口试样在$-196\,{^\circ}$C和$-130\,{^\circ}$C的解理断裂机 理. 拉伸试验、单、双缺口四点弯曲实验、断口形貌观察以及有限元分析结果表明, 缺口试 样发生解理断裂时均起裂于夹杂物粒子, 一种位于缺口根部前端(IC型), 另一种位于距缺口 根部较远的条形裂纹前端(SIC型); 且随温度升高, 起裂源的类型从$-196\,{^\circ}$C下的IC 型转变为$-130\,{^\circ}$C下的SIC型. 微裂纹均形核于夹杂物, 最终的断裂由铁素体晶粒尺 寸的微裂纹扩展控制. 缺口试样IC型解理断裂遵循裂纹形核条 件$\varepsilon_{\rm p} \ge \varepsilon_{\rm pc}$和裂纹扩展条件$\sigma_{yy} \ge \sigma_{f}$, 而SIC型解理断裂条件则演化为$\varepsilon_{\rm p}+\varepsilon_{\rm ps} \ge \varepsilon_{\rm pc}$和$\sigma_{yy} +\sigma_{yy{\rm s}} \ge \sigma_{f}$.     

A study of shock wave interaction with a rotating cylinder

Shock wave reflection over a rotating circular cylinder is numerically and experimentally investigated. It is shown that the transition from the regular reflection to the Mach reflection is promoted on the cylinder surface which rotates in the same direction of the incident shock motion, whereas it is retarded on the surface that rotates to the reverse direction. Numerical calculations solving the Navier-Stokes equations using extremely fine grids also reveal that the reflected shock transition from RR MR is either advanced or retarded depending on whether or not the surface motion favors the incident shock wave. The interpretation of viscous effects on the reflected shock transition is given by the dimensional analysis and from the viewpoint of signal propagation.Received: 24 April 2002, Accepted: 16 August 2002, Published online: 25 March 2003     

On the relation between particle velocity and shock wave speed for thermoelastic materials

Results of shock-wave experiments in solids often suggest a nearly-linear relation between the particle velocity behind the shock and the shock wave speed. The present note reconsiders the question of whether thermoelastic material models may be consistent with such observations. Emphasis is placed on the role played by the response of the material in severe compression, as distinguished from its response for small or moderate deformations. The details are illustrated for materials of Mie-Grüneisen type. Received 21 December 2001 / Accepted 22 April 2002 Published online 8 July 2002     

Non-uniqueness of solutions in asymptotically self-similar shock reflections

The present study addresses the self-similar problem of unsteady shock reflection on an inclined wedge. The start-up conditions are studied by modifying the wedge corner and allowing for a finite radius of curvature. It is found that the type of shock reflection observed far from the corner, namely regular or Mach reflection, depends intimately on the start-up condition, as the flow “remembers” how it was started. Substantial differences were found. For example, the type of shock reflection for an incident shock Mach number $M=6.6$ and an isentropic exponent $\gamma =1.2$ changes from regular to Mach reflection between $44^\circ $ and $45^\circ $ when a straight wedge tip is used, while the transition for an initially curved wedge occurs between $57^\circ $ and $58^\circ $ .     

Phase transition and dynamics of iron under ramp wave compression

The ramp wave compression experiments of iron with different thicknesses were performed on the magnetically driven ramp loading device CQ-4. Numerical simulations of this process were done with Hayes multi-phase equation of state (H-MEOS) and dynamic equations of phase transition. The calculated results of H-MEOS are in good agreement with those of shock phase transition, but are different from those under ramp wave compression. The reason for this is that the bulk modulus of the material in the Hayes model and the wave velocity are considered constant. Shock compression is a jump from the initial state to the final state, and the sound speed is related to the slope of the Rayleigh line. However, ramp compression is a continuous process, and the bulk modulus is no longer a constant but a function of pressure and temperature. Based on Murnaghan equation of state, the first-order correction of the bulk modulus on pressure in the Hayes model was carried out. The numerical results of the corrected H-MEOS agree well with those of pure iron in both ramp and shock compression phase transition experiments. The calculated results show that the relaxation time of iron is about 30 ns and the phase transition pressure is about 13 GPa. There are obvious differences between the isentropic and adiabatic process in terms of pressure–specific volume and temperature–pressure. The fluctuation of the sound speed after 13 GPa is caused by the phase transition.