椭圆柱空腔吸声覆盖层的声学特性

采用波导有限元方法（WFEM）分析了共振腔型吸声覆盖层内轴向波的传播和损耗特性,以椭圆柱空腔吸声层为例,在保持穿孔系数不变的前提下,分别从轴向波传播和波型转换的角度分析了空腔形状改变吸声覆盖层声学特性的机理。在此基础上,结合波导有限元方法和传递矩阵（TM）建立了变截面空腔吸声覆盖层的分析模型,以圆台空腔吸声层为例,与二维解析方法对比验证了WFEM-TM方法的正确性,并讨论了椭圆台空腔吸声层的声学特性。     

Performance of functionally graded sandwich composite beams under shock wave loading

The dynamic behavior of sandwich composites made of E-Glass Vinyl-Ester (EVE) facesheets and graded Corecell™ A-series foam was studied using a shock tube apparatus. The foam core was monotonically graded based on increasing acoustic wave impedance, with the foam core layer of lowest wave impedance facing the blast. The specimen dimensions were held constant for all core configurations, while the number of core layers varied, resulting in specimens with one layer, two layer, three layer, and four layers of foam core gradation. Prior to shock tube testing, the quasi-static and dynamic constitutive behavior (compressive) of each type of foam was evaluated. During the shock tube testing, high-speed photography coupled with the optical technique of Digital Image Correlation (DIC) was utilized to capture the real-time deformation process as well as mechanisms of failure. Post-mortem analysis was also carried out to evaluate the overall blast performance of these configurations. The results indicated that increasing the number of monotonically graded foam core layers, thus reducing the acoustic wave impedance mismatch between successive layers, helped maintain structural integrity and increased the blast performance of the sandwich composite.     

The use of metal foam projectiles to simulate shock loading on a structure

Metal foam projectiles are used to generate dynamic pressure–time histories representative of shock loading in water and air. A 1D plastic shock wave analysis is performed for a foam projectile impacting a free but rigid mass. It is shown that the pressure versus time pulse exerted on the mass and the shock arrest distance within the foam depend upon the ratio of foam mass to impacted mass, and upon the ratio of quasi-static to hydrodynamic strength of the foam. The theory is supported by two sets of experiments, one where Alporas foam impacts an instrumented Kolsky pressure bar, and one where the foam is fired at a free mass. It is demonstrated that the magnitude and duration of the pressure pulse can be controlled by suitable adjustment of the velocity, length and density of the foam projectile.     

The response of clamped sandwich beams subjected to shock loading

The dynamic response of monolithic and sandwich beams made from stainless steel is determined by loading the end-clamped beams at mid-span with metal foam projectiles. The sandwich beams comprise stainless-steel pyramidal cores (with no axial stretch resistance), stainless-steel corrugated cores (with a high stretch resistance) and an aluminium alloy metal foam. High-speed photography is used to measure the transient transverse deflection of the beams. The resistance to shock loading is measured by the permanent transverse deflection at the mid-span of the beams for a fixed magnitude of projectile momentum and mass of beam. It is found that the sandwich beam with the pyramidal core was the weakest of the sandwich beams, but all sandwich beams had a higher shock resistance, then the monolithic beam. For each type of beam, the dependence of transverse deflection upon the magnitude of the projectile momentum is measured. A comparison of the measurements is made with analytical predictions for both impulsive and finite pressure loading. It is found that the impulsive loading analysis over-predicts the deflections of both the monolithic and sandwich beams. The finite pressure analysis, which considers the transient nature of the loading pressure provided by the foam projectile, can accurately predict the measured transverse deflection.     

Cavity and crater depth in hypervelocity impact

We discuss the depth of cavities and craters caused by hypervelocity impacts as a function of impact parameters such as impact velocity, projectile and target densities, and projectile diameter, in two extreme cases: the penetration of intact projectiles at low impact pressure and the hemispherical excavation at very high impact pressure. The relations between the depth and the impact parameters are obtained. Then, previous experimental results are compiled; crater depth normalized by projectile diameter and the ratio of projectile and target densities is plotted for glass, plastic, and metal projectiles and metal, rock, ice, foam, sheet-stack, and aerogel targets. The trends of the data are consistent with the relations in the extreme cases.     

岩石中爆炸冲击波能量分布规律初探

本文就冲击波能量主要消耗在爆腔扩胀,形成裂隙和引起岩石弹性变形上讨论。首先分析了冲击波作用岩石变形和破坏的特点,在此基础上探讨了炮孔柱状装药爆破时冲击波能量的分布规律以及各部分能量消耗量的求算方法。     

深孔间隔装药爆破对不同孔壁介质的影响

数值模拟研究了岩石、混凝土和土三种不同孔壁介质深孔间隔装药爆破时的扩孔特征、压力场、应力场、速度场和能量分布及传播衰减规律,还分析了间隔介质(空气和水)和起爆方式等对孔壁介质中冲击波传播规律的影响。研究表明:由于岩石、混凝土和土三种孔壁介质的波阻抗和可压缩性不同,导致爆破后分别形成"狼牙棒"型、"纺锤"型和"圆柱"型三种爆腔。与岩石和混凝土相比,在土体中的扩孔宽度分别提高约60%和约45%,土能缓解孔壁压力和等效应力、降低爆破振动效应、减缓爆炸冲击波的衰减速度和提高能量利用率,而在岩石和混凝土介质中,上述效果的差异性不太明显。与水间隔装药相比,在岩石和混凝土孔壁介质中采用空气间隔装药结构能降低约7%的孔壁压力。在岩石和混凝土孔壁介质中,采用底部起爆方式能够提高炸药的能量利用率,中部起爆方式能够减缓爆破振动效应,而在土体中并不明显。     

水中爆炸对围岩增耦的实验研究

为研究水中爆炸对围岩的增耦,在一个直径1.6 m的水泥砂浆半球体装置上,将其中心预留的直径0.3 m的腔室内注水和置入水泥砂浆芯体,对比性地进行了一系列的1.00 gTNT(2,4,6-三硝基甲苯)当量的填实和空腔爆炸实验.水泥砂浆球表运动测量数据表明,与水泥砂浆中的空腔解耦相比,水中空腔爆炸的解耦效果较差,尤其水腔中爆炸可大大增强爆炸能量的耦合.可以推断,水下爆炸会明显增强围岩中的爆炸应力波强度,近水域中的空中爆炸也可能会增强围岩中的爆炸能耦合,因而相对于围岩和堤坝中同距离、同药量的爆炸来讲,所引起的破坏危险更大.     

应力卸载过程砂岩应力波传播及衰减特性试验研究

由于岩体开挖卸载和原岩应力重新分布的复杂性,围岩体通常处于复杂的应力卸载环境中。为研究应力卸载过程砂岩应力波的传播及衰减特性,设计了长径比L/D≈65的条形试件,利用动静组合加载装置进行了不同轴向卸载应力下的小扰动撞击试验,测得了不同测点处的砂岩应力波信号,研究了不同卸载应力对应力波波形、纵波波速值、波峰的时空衰减规律、波峰及波峰衰减速率等的影响,并建立了经验规律模型。研究表明:砂岩是否具有卸载应力对应力波波形具有显著影响;随应力卸载,纵波波速呈“近似线性+非线性”衰减,波峰及其衰减率均呈指数变化;随距离和时间增加,波峰均呈指数衰减,且不同应力卸载阶段波峰的时空衰减具有不同敏感性;研究结果对爆破开挖条件下深部工程岩体稳定性分析提供了重要的理论参考。     

High velocity linear induction launchers

Electromagnetic launchers (EMLs) have received great attention in the last decades because of their potential application to a variety of energy, transportation, space, and defense systems. Particularly, they can serve as kinetic weapons, such as ground-based and naval artillery, space-based anti-missile guns, Earth-to-Orbit launcher, and mass transportation. The main advantage is that EMLs can accelerate projectiles to hyper velocities, i.e. velocities greater than those achievable with conventional cannons. The Linear Induction Launcher (LIL) is an air-cored electromagnetic coil launcher operating on the principle of the induction motor. Polyphase excitation of the coils constituting the barrel is designed to create an electromagnetic wave packet, which travels with increasing velocity from the breech to the muzzle. The projectile is a hollow conducting cylinder (sleeve) carrying the payload within it. Relative motion (slip) of the wave packet with respect to the projectile induces azimuthal currents in the sleeve that interacts with the exciting magnetic field to produce both propulsive and centering forces. This paper deals with the design of a high velocity linear induction launcher with muzzle velocity up to 6000 m/s. It addresses the design specifications of the launcher and utilizing a projectile weighing 1 kg. In the paper, the design specifications with simulation results for the phase voltages, the currents, the velocity, and the temperature rise of the sleeve are presented.     

机载焰弹发射装置弹药储运防护设计

目的为了近一步加强和提高人工影响天气行业机载焰弹发射装置弹药的安全储运和防护问题。方法依据焰弹的贮运要求和对现有机载催化作业设备的材料、技术指标、性能要求、结构原理和加工工艺等方面进行分析和研究。结果该装置由发射器、电路控制器和Ag I焰剂构成。经试验和初步应用证明,该发射装置具有手自一体2种发射模式,携弹量为400枚,可连续作业达4 h以上,与目前所使用的相关机载焰弹发射装置相比,在作业方式、作业效率和弹药安全防护等方面都有了极大的改善和提升。结论新型机载焰弹发射装置的成功研发有效地弥补了机载焰弹设备在弹药携行方面的不足和缺陷,并为其他类似的产品设计提供参考。     

弹载加速度记录仪抗冲击防护结构设计

针对弹体侵彻混凝土环境,提出一种中薄型防护外壳嵌套内壳体,两壳体间填充泡沫铝缓冲材料的记录仪抗冲击防护结构。针对应力波作用下防护外壳的屈曲和泡沫铝缓冲性能不足导致内部电路模块冲击断裂的典型失效模式,通过对记录仪抗冲击防护结构在应力波作用下的结构响应分析,提出一套以壳体厚度h、泡沫铝密度ρ、泡沫铝厚度h_b为主要设计指标的弹载加速度记录仪抗冲击防护结构设计方法。设计出防护外壳半径为29 mm,壁厚3 mm,泡沫铝密度1.1g/cm~3,泡沫铝厚度23 mm的抗冲击防护结构,经计算该结构理论抗冲击能力为63 300 g。在实弹侵彻混凝土试验中,测得冲击加速度峰值为56 300 g,在此冲击下记录仪壳体结构稳定,内部电路工作正常,验证了此抗冲击防护结构具有较高的可靠性。     

Ballistic impact experiments of metallic sandwich panels with aluminium foam core

Metallic sandwich structures with aluminium foam core are good energy absorbers for impact protection. To study their ballistic performance, quasi-static and impact perforation tests were carried out and the results are reported and analysed in this paper. In the experiments, effects of several key parameters, i.e. impact velocity, skin thickness, thickness and density of foam core and projectile shapes, on the ballistic limit and energy absorption of the panels during perforation are identified and discussed in detail.     

换流变压器阀厅抗爆门抗冲击性能实验研究

抗爆门作为换流变压器阀厅的重要安全防护设施,有着很高的质量要求,因此对阀厅抗爆门的抗冲击性能研究具有很大的实用价值.主要对300 kPa和1000 kPa冲击波超压载荷工况下,有泡沫铝填充和无填充的抗爆门抗爆性能进行实验研究,验证抗爆门的抗冲击性,并通过门扇形变量对两种不同材质抗爆门的抗爆性能进行对比评估.实验结果表明...     

Shock front analysis for axial shear wave propagation in a hyperelastic incompressible solid

Summary A wavefront analysis is employed to study the propagation of axial shear waves in an incompressible hyperelastic solid, whose strain energy function is expressible as a truncated power series in terms of the basic invariants of the left Cauchy-Green tensor. Waves are generated by the application of an axial shear stress at the surface of a cylindrical cavity in an unbounded medium. Depending on the nature of the boundary condition, an acceleration front or a shock front propagates from the boundary of the cavity. For an acceleration front, the coefficients in the wavefront expansion satisfy a sequence of transport equations which can be solved analytically. For a shock front, a wavefront analysis gives approximate formulas for the wave speed, shock front and intensity of the various field variables at the front. As well, our shock front analysis is used to devise a method of estimating the breaking distance of a shock front. In order to test the validity of the results of our wavefront analysis, numerical solutions are obtained for waves initiated by a step function or by a finite duration pulse at the boundary. Our numerical solutions are found by using a recently proposed relaxation scheme for systems of conservation laws.     

Experimental investigation of inertia effects in one-dimensional metal ring systems subjected to end impact—II. Free-ended systems

Experiments are described in which a variety of one-dimensional ring systems with free distal ends were subjected to axial impact by a rigid projectile. The elastic wave which is reflected from the free end leads to unloading and consequently to the arrest of the structural shock wave which causes the principal deformation in the system. An elastic-plastic structural shock model is used to predict the deformation distribution and change in velocity of the striker for those cases in which the rings are constrained to collapse in essentially a simple four hinge mechanism. There is good agreement between the predictions of the model and the observations from the experiments.     

Oblique and normal perforation of concrete targets by a rigid projectile

The oblique and normal perforation of a concrete target subjected to a rigid projectile impact is studied in this paper. A general three-stages model, i.e., initial cratering, tunnelling and shear plugging, is developed based on dynamic cavity expansion theory and plug formation. Analytical solutions for the ballistic performance and the perforation limit of concrete target are obtained. The proposed formulae are consistent with other empirical formulae and correlate well with the experimental data.     

泡沫电介质中高压脉冲放电的破岩机理研究

针对水介质中压力冲击波在破碎岩石方面的应用,研究高压脉冲放电在泡沫电介质中的放电机理。根据不同电介质的电离特性配比了两种阴阳离子的泡沫电介质,数值模拟分析了在不同电介质环境下高压脉冲放电时的冲击压力变化规律。运用气泡动力学方程分析了气泡在放电过程中的爆破形态,分析了不同放电电压、放电间隙等条件对气泡爆破形态的影响规律。并进行了初步室内实验,验证了泡沫作为电介质实现高压脉冲放电破碎岩石的可行性。由于泡沫可产生空泡效应,能增加爆破效果,而且泡沫在防漏、密封性能要求等方面都优于水介质,具有更广阔的应用前景。     

Dynamic response of aluminum honeycomb sandwich panels under foam projectile impact

The dynamic response of honeycomb sandwich panels under aluminum foam projectile impact was investigated. The different configurations of panels were tested, and deformation/failure modes were obtained. Corresponding numerical simulations were also presented to investigate the energy absorption and deformation mechanism of sandwich panels. Results showed that the deformation/failure modes of sandwich panels were sensitive to the impact velocity and density of aluminum foam. When the panel was impacted by the aluminum foam projectile with the back mass of nylon, the “accelerating impact” stage can be produced and may lead to further compression and damage of the sandwich structures.     

Electromagnetic induction launchers

The electromagnetic launcher consists of a system of stator coils producing a traveling field which accelerates an armature carrying currents induced by the traveling field (induction accelerator [1,2]) or persistent currents supplied from otner sources (synchronous accelerator [2,10]). The fact that their armature has no electrical contact with the stator, essentially riding on the crest of a traveling magnetic wave, makes induction accelerators very attractive for a large number of applications. This paper is devoted exclusively to the accelerator of the induction type. Efficiency considerations require that the traveling wave should accelerate at approximately the same rate as the projectile. This can be achieved either using variable (increasing) winding pitch or a continuously increasing power supply frequency or a combination of both. A new dimension was added to the induction coaxial accelerator technology with the definition at the Center for Electromechanics at The University of Texas at Austin (CEM-UT) of a new electrical machine, the Rising Frequency Generator (RFG) representing a more attractive integrated power source for induction accelerators which had previously been forced to conform to constant frequency power supplies. This paper outlines the principles of design and shows two applications of induction coaxial launchers; a half-scale aircraft launcher in which the system also acts as an electromagnetic brake, stopping the shuttle and driving it in the opposite direction, and a high performance, 18-m long launcher capable of accelerating a 1-kg aluminum projectile to a velocity of 10 km/s at an average acceleration of 250,000 G.