1060Al/SS304板材铝箔气化焊接界面形成过程数值模拟

目的 对1060Al/SS304板材铝箔气化焊接过程进行数值模拟,揭示焊接界面的形成与演变规律.方法 基于非线性有限元软件LS-DYNA,建立1060Al/SS304板材铝箔气化焊接过程的光滑粒子流体动力学(SPH)模型,分析冲击速度对焊接界面形貌的影响,对界面形貌进行实验对比分析.结果 研究结果显示1060Al/SS304板材铝箔气化焊接界面包含了3个典型区段,即初始平直区、后继紊乱波形区和终了稳定波形区.当冲击速度达700 m/s、碰撞角度为16°时,波长和振幅分别稳定在260μm和45μm,界面波峰处呈现出尾部旋涡状,旋涡区内温度高于1060Al母材熔点.结论 在保持碰撞角度不变的条件下,稳定区界面的波长与振幅均随着冲击速度的增大而增大.平直状结合界面附近形成了一高温层,当冲击速度足够大时,该薄层发生熔化,数值模拟结果与结合界面实验观察结果一致.     

不同边界扰动形式对充气天线反射面形面精度的影响

充气天线的性能主要取决于其形面几何精度。该文以几何非线性有限元法为基础, 对不同焦径比和口径的天线反射面进行分析, 探讨了三种具有代表性的边界扰动分布形式(均匀扰动、局部扰动及波形扰动)对天线反射面几何形状及应力的影响特性及原因, 并分别获得了形面精度对这三种扰动的敏感度。研究从影响区域、影响值大小、影响敏感度及褶皱分布等几个方面对三种扰动形式进行了定性和定量的比较, 得到如下主要结论:均匀扰动及波形扰动对整个曲面形状均有影响, 在边界影响区曲面会同时产生平移和曲率变化, 中间区域则主要发生曲面平移。局部扰动则只对局部区域有影响;三种扰动形式中, 在扰动值相同的情况下, 均匀扰动引起的误差值最大, 但形面误差值对局部扰动和波形扰动更敏感;除施加向外均匀扰动时无褶皱以外, 其他扰动形式均会导致在影响区域内出现褶皱。此外, 研究还探讨了天线焦径比和口径的影响。最后, 在上述研究成果的基础上给出了反射面形面精度被动控制和主动控制的合理建议。     

不同界面约束对爆炸作用下液体抛撒影响分析

采用高速摄影仪拍摄了不同界面约束条件液体抛撒的高速撮影图片,通过对这些图片的分析,得出了不同界面约束条件液体抛撒的最大速度、作用时间、半径、速度衰减系数以及液体抛撒的不对称性。依据不可压缩柱面流体界面运动、变化规律,在分析了爆炸作用下液体抛撒初期界面初始扰动运动、变化的基础上,探讨了不同界面约束对扰动放大因子及界面稳定的影响。根据液体二次破碎理论,分析了不同界面约束条件对液体二次破碎的影响;结合实验结果讨论了不同界面约束对液体抛撒的作用时间、半径以及速度衰减系数的影响。     

转矩扰动激励下盘式制动器非线性振动分析

基于汽车盘式制动器单自由度模型研究转矩扰动对其非线性振动的影响。依据Stribeck模型分析盘式制动器的振动模式和不同模式下的平衡点及稳定性。在未受转矩扰动激励下,制动器可能表现为滑动、滑-擦边、粘-滑-擦边等多种振动形式,且粘滞擦边振动会引起小幅振荡行为。周期性的转矩扰动可将原本的振动分为两部分,且在最值处出现转换。转矩扰动会影响粘滞运动和滑动运动的过渡边界,导致振动行为更加复杂。转矩扰动频率的改变同样可导致制动器出现滑动和粘-滑运动等。无论是否受到激励,随着制动压力的增加,制动器的振幅都将逐渐增加;在相同制动压力下,当制动器受到转矩激励时,振动幅值大于未受激励情况。研究可为揭示转矩扰动对制动器振动的影响机理提供参考。     

单相凝固亚结构的特征尺度与形态转变

从定向凝固界面形的非稳态分析出发，对单相凝固亚结构的几个特征尺度包括平界面失稳初始扰动波长、胞晶间距、枝晶一次间距、枝晶尖端半径及二次间距与两有态转变即胞枝转变和高梯度绝对稳定性进行了实验研究和理论分析。     

火箭弹起始扰动的仿真与试验研究

目的 研究火箭弹起始扰动的形成与减小火箭弹起始扰动的技术途径。方法 通过计算机数值仿真和射击试验,对影响火箭弹起始扰动的因素进行分析。结果 得到了发射过程中各种扰动因素对火箭起始扰动影响的量值。结论 推力偏心、动不平衡、弹管间隙、俯仰体振动及车体振动对火箭弹起始扰动的影响较大,应该在火箭武器系统设计中对此加以考虑。     

爆炸复合钢板疲劳裂纹的增长行为

通过在不变的应力强度因子范围内进行疲劳裂纹增长试验及通过对疲劳裂纹增长进行有限元弹-塑性分析,研究了爆炸复合钢板疲劳裂纹的增长行为。探讨了初始残余应力、硬化层和每一原始材料的屈服点对裂纹传播的影响。由有复合界面、接近界面的硬化层和残余应力的爆炸复合材料的检验得出如下结论:疲劳裂纹增长速率与应力强度因子范围无关,但与裂纹尖端应变范围的最大值紧密相关。     

大振幅平面声波在多层介质中的传播

本文用逐级近似法就一维情况下分析了大振幅平面声波在多层介质的界面上发生反射和透射的传播特性。在二级近似下计算了精确到二阶小量的二至三个界面各层的反射波和透射波的二次谐波，并推广至几个界面，得到一些规律性的结果。从拉格朗日坐标下的一级近似和二级近似的波动方程式出发，假定声波在多层介质中传播在各个界面仍是连续的，即各层界面的边界条件由界面两侧的压力平衡和质点速度相等来决定，由此求得各层界面在二级近似下的二次谐波的反射波和透射波。计算结果表明，二级近似下的反射波和透射波均由两部分组成，一部分是由一级反…     

低频振动对塑性成形表面微观形貌的影响

目的 通过对塑性成形界面施加纵向和法向低频振动,探究不同振动参数对成形过程中表面质量的影响规律。方法 采用自主设计的低频振动发生器完成纵向振动作用下的摩擦实验和法向振动作用下的压缩实验,统计材料表面变形区域的平均粗糙度,评价成形质量。结果 在干摩擦条件下,不同频率(0~50 Hz)的纵向小振幅(0.1 mm)振动有利于提高成形界面的表面质量,变形区域的表面粗糙度与频率成反比,但是较大振幅(0.4 mm)的纵向振动会增加接触面的材料磨损,降低表面质量;不同频率(0~50 Hz)和不同振幅(0~0.4 mm)的法向振动均有利于提高干摩擦成形界面的表面质量,在0.2 mm振幅条件下表面粗糙度下降最为明显,振幅超过0.2 mm后,增加振幅对提升表面质量的影响不明显。结论 低频振动对成形界面微观形貌影响较大,合理优化低频振动工艺参数可显著提高成形界面的表面质量。     

入口扰动数据周期对湍流预混射流火焰直接模拟的影响

该文对湍流平面射流预混火焰进行了直接数值模拟。射流入口的扰动速度数据依据各向同性衰变湍流的能谱生成,研究了扰动数据周期的选取对模拟结果的影响。表明当入口扰动数据的周期大于总的计算时间时,得到的不同时刻的瞬态模拟结果有较明显的差异,并得到了时均定常的气体平均温度、速度和组分浓度分布。而当扰动数据周期小于总的计算时间时,入口扰动数据的周期性会对模拟结果产生影响,导致瞬态结果出现周期性。     

Effect of stretching on interfacial stability

Summary. The effect of stretching on the stability of a horizontal interface between two fluids with different viscosities and densities is discussed. A local elongational flow acts to reduce the amplitude of perturbations and increase the wavelength of periodic waves, and thereby alter the instantaneous growth rate of disturbances. Linear stability analysis for Stokes flow reveals that, in the case of a horizontal interface between two semi-infinite fluids subjected to orthogonal stagnation-point flow, interfacial stretching is not able to suppress the Rayleigh-Taylor instability of unstably stratified fluids. In contrast, stretching is able to suppress the growth of periodic waves on the surface of a flat film resting on a horizontal surface. Numerical simulations based on the boundary-integral method for Stokes flow confirm that localized perturbations on the film surface are suppressed when the elongational flow is sufficiently strong.     

Propagation of Rayleigh, Scholte and Stoneley waves along random boundaries

The effect of boundary roughness on the propagation of Rayleigh, Scholte and Stoneley waves is analysed under the assumption of the wavelength being much larger than the roughness scale of the interface. The scattering of the fundamental interface waves is represented by systems of plane waves in both neighbouring half-spaces; the formulas for amplitudes of these waves are determined using the perturbation method. The phase velocity of the interface waves is analysed by an approximate technique and is found to increase with the growing boundary roughness for all three types of interface waves.     

On the complete extinction of selected imperfection wavelengths in dynamically expanded ductile rings

In this work the inception and development of multiple necks in dynamically expanded ductile rings with ab initio geometric imperfections has been addressed. Finite element simulations and linear perturbation analysis have been applied for that task. In the numerical calculations a selected wavelength is included into the model defining along the circumference of the ring an array of periodic geometric imperfections of predefined amplitude. In the stability analysis a perturbation of a given mode is added to the background solution and the growth rate of the perturbation is evaluated. The attention has been focused on the extinction of both long and short wavelength imperfections and the appearance of a dominant necking pattern which emerges when the geometric imperfections are vanished. The role played by the loading rate on the extinction of imperfections is also addressed. Moreover, the necking strain is found to be dependent on the imperfection pattern and the loading rate. Its maximum value is registered for the loading cases in which the initial imperfections distribution is completely extinguished.     

Space-time modulation of an X-ray beam by ultrasonic superlattice

Space-time modulation of an X-ray beam is investigated by total external reflection on a YZ-cut of a LiNbO₃ crystal modulated by surface acoustic waves. It is shown that the X-ray diffraction is determined by the amplitude and the wavelength of the surface acoustic waves. Both experimental and theoretical studies have revealed the possibility of fabricating X-ray optical elements based on space-time modulation by controlling the wavelength and the pulse modulation of the surface acoustic waves. These experiments were carried out at CNRS (Grenoble, France) using a rotating copper anode     

Optical detection of evanescent ultrasound waves in water

Acoustical perturbation by targets smaller than the wavelength can generate evanescent waves, which decay with the distance of propagation. By putting such targets immediately under the free surface of water, the diffracted acoustical field can excite the surface membrane before evanescence, and it produces a static ripple because of the radiation pressure. A collimated beam of light reflects at the perturbed surface, and it becomes modulated in phase. In this work we show experimental results where we achieve an optical image of the acoustical evanescent waves produced by an array-like target. Arising from the theory, we present a numerical calculus of the optical image produced by the ultrasonic field diffracted from the target in order to verify the experimental results. With this method, we look for a spatial resolution smaller than acoustical wavelength, for normal incidence and plane waves. In our experimental device, we use a sound wavelength in water greater than 1.5 mm, generated by a PZT transducer. We can resolve an array of 1.0 mm of periodicity.     

Effects of sinusoidal crack-face perturbations on scattering of elastic waves

The scattering of a plane longitudinal wave from a two-dimensional crack, with a sinusoidal surface perturbation whose amplitude and wavelength are much smaller than the length of the crack, is investigated. The amplitude of the cylindrical body waves in the far field are calculated from a Kirchhoff approximation that utilizes the solution to the reflection from the sinusoidal surface profile of a semiinfinite solid. The results are compared to those for a flat crack, and conditions for significant differences of the amplitude as a function of the angle of observation are discussed. Characteristic changes in the scattered field produced by profiles with different amplitudes and periods are explained.     

Shear band spacing in gradient-dependent thermoviscoplastic materials

We study thermomechanical deformations of a viscoplastic body deformed in simple shear. The strain gradients are taken as independent kinematic variables and the corresponding higher order stresses are included in the balance laws, and the equation for the yield surface. Three different functional relationships, the power law, and those proposed by Wright and Batra, and Johnson and Cook are used to relate the effective strain rate to the effective stress and temperature. Effects of strain hardening of the material and elastic deformations are neglected. The homogeneous solution of the problem is perturbed and the stability of the problem linear in the perturbation variables is studied. Following Wright and Ockendon's postulate that the wavelength whose initial growth rate is maximum determines the minimum spacing between adjacent shear bands, the shear band spacing is computed. It is found that the minimum shear band spacing is very sensitive to the thermal softening coefficient/exponent, the material characteristic length and the nominal strain-rate. Approximate analytical expressions for the critical wave length for heat conducting nonpolar materials and locally adiabatic deformations of gradient dependent materials are also derived.     

Stability of thin solid films

A small amplitude perturbation analysis is used to determine the conditions under which a solid film several hundred ångströms thick on a substrate will rupture. If the perturbation grows with time the film is unstable and rupture may occur, whereas if the perturbation decays the film is stable. Film rupture is caused essentially by diffusion of atoms along the free interface of the film which can, under certain conditions, amplify a perturbation applied to the film-gas interface. This surface diffusion is generated by a gradient of the chemical potential along the free interface. The chemical potential is affected by the curvature of the interface, by the pre-existing internal stresses normally found in thin films (they generate a strain energy term in the chemical potential) and by interaction forces between the atoms at the gas-solid interface with those of the film and substrate. The thin film is assumed to behave like an elastic body. The difference in the forces which act on a volume element in a film thinner than the range of interaction forces between the atoms of the film and substrate and the forces in a bulk solid is accounted for by introducing a body force into the equations of displacement of an elastic solid. Because of the difficulties in writing boundary conditions at the film-substrate interface, two limiting situations are considered: (1) a thin film on a rigid substrate and (2) a thin free film. A critical internal stress necessary for rupture is identified. The time of rupture is estimated from the inverse of the maximum growth coefficient of the perturbation. The dominant wavelength corresponding to the maximum growth coefficient gives an idea as to the size of the islands formed through rupture.     

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs.

The kinetics of formation of Al₂O₃ in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol⁻¹. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.     

Investigation on the fatigue crack behavior of Zr702/TA2/Q345R composite plate with a crack normal to interface

In this paper, the effects of maximum load, load ratio, and average load on fatigue crack propagation of Zr702/TA2/Q345R composite plate with a crack normal to the interface are studied by experiment and finite element method. When crack propagates to the interface from the compliant material side, the crack growth rate decreases to the minimum at first. After crack penetrates through the interface, the fatigue crack growth rate accelerates continuously. When crack propagates to the interface from the stiff material side, the fatigue crack growth rate generally increases with the crack length. Regardless of the direction of crack growth, the increase of load ratio will weaken the difference of crack growth rate near the interface caused by material property mismatch. Finite element results show that elastic modulus mismatch significantly changes the variation of the stress intensity factor amplitude. All results demonstrate that crack growth rate is dependent on the competition of the stress intensity factor amplitude, the fatigue crack growth rate in the corresponding material, and the interface strength.