Dynamic photoelastic study of the transient stress field in solids during shock wave lithotripsy

Photoelastic and shadowgraph imaging techniques were used to visualize the propagation and evolution of stress waves, and the resultant transient stress fields in solids during shock wave lithotripsy. In parallel, theoretical analysis of the wavefront evolution inside the solids was performed using a ray-tracing method. Excellent agreement between the theoretical prediction and experimental results was observed. Both the sample size and geometry were found to have a significant influence on the wave evolution and associated stress field produced inside the solid. In particular, characteristic patterns of spalling damage (i.e., transverse and longitudinal crack formation) were observed using plaster-of-Paris cylindrical phantoms of rectangular and circular cross sections. It was found that the leading tensile pulse of the reflected longitudinal wave is responsible for the initiation of microcracks in regions inside the phantom where high tensile stresses are produced. In addition, the transmitted shear wave was found to play a critical role in facilitating the extension and propagation of the microcrack.     

Dynamics of laser-induced cavitation bubble during expansion over sharp-edge geometry submerged in liquid – an inside view by diffuse illumination

Laser ablation in liquids is growing in popularity for various applications including nanoparticle production, breakdown spectroscopy, and surface functionalization. When laser pulse ablates the solid target submerged in liquid, a cavitation bubble develops. In case of “finite” geometries of ablated solids, liquid dynamical phenomena can occur inside the bubble when the bubble overflows the surface edge. To observe this dynamics, we use diffuse illumination of a flashlamp in combination with a high-speed videography by exposure times down to 250 ns. The developed theoretical modelling and its comparison with the experimental observations clearly prove that this approach widens the observable area inside the bubble. We thereby use it to study the dynamics of laser-induced cavitation bubble during its expansion over a sharp-edge (“cliff-like” 90°) geometry submerged in water, ethanol, and polyethylene glycol 300. The samples are 17 mm wide stainless steel plates with thickness in the range of 0.025–2 mm. Bubbles are induced on the samples by 1064-nm laser pulses with pulse durations of 7–60 ns and pulse energies of 10–55 mJ. We observe formation of a fixed-type secondary cavity behind the edge where low-pressure area develops due to bubble-driven flow of the liquid. This occurs when the velocity of liquid overflow exceeds ~20 m s⁻¹. A re-entrant liquid injection with up to ~40 m s⁻¹ velocity may occur inside the bubble when the bubble overflows the edge of the sample. Formation and characteristics of the jet evidently depend on the relation between the breakdown-edge offset and the bubble energy, as well as the properties of the surrounding liquid. Higher viscosity of the liquid prevents the generation of the jet.     

Comparative visualized investigation of impact-driven high-speed liquid jets injected in submerged water and in ambient air

This paper is a comparative study on the characteristics of high-speed liquid jets injected in surrounding water and air using shadowgraph technique. One of the main objectives is to investigate the effects of liquid’s physical properties, used to generate the high-speed liquid jets, on jet generation’s characteristics. Moreover, comparative investigations on effects of those liquid jets after injected in water and air are reported. The high-speed liquid jets were generated by the impact of a projectile launched by a horizontal single-stage power gun. The impact-driven high-speed liquid jets were visualized by shadowgraph technique and images were recorded by a high-speed digital video camera. The process of impact-driven high-speed liquid jet injection in air and water, oblique shock waves, jet-induced shock waves, shock waves propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud regeneration were clearly observed. It was found that different properties of liquid (surface tension and kinematic viscosity) affect the jet maximum velocity and shape of the jet. Bubble behaviors were only found for the jet injected in water. From the shadowgraph images, it is found that the maximum average jet velocity, expansion and contraction velocities of bubble in axial direction increase when the value of the multiplied result of surface tension by kinematic viscosity increases. Therefore, surface tension and kinematic viscosity are the significant physical properties that affect characteristics of high-speed liquid jets.     

Fracture of diamond coatings by high velocity sand erosion

This paper describes a study of the behaviour of diamond coatings when subjected to solid particle erosion from sand particles. The coatings were deposited by chemical vapour deposition (CVD) onto tungsten substrates and tested using a high velocity air–sand erosion test facility. The erosion tests were conducted using particle impact velocities of between 33 and 268 m/s. Examination of the eroded test specimens showed that the principal damage features were circumferential cracks and pin-holes. Comparison with Hertz impact theory revealed that the measured circumferential crack diameters were more than double the predicted Hertzian contact diameter. Moreover, a trend of increasing circumferential crack diameter with coating thickness, which is not predicted by Hertz, was found. Instead, the crack diameters showed good agreement with those predicted by the theory of stress wave reinforcement, which is more commonly associated with liquid impact damage of brittle materials. During impact, the bulk compression and shear waves are reflected at the rear surface of debonded regions of the coating to return to the front surface and reinforce the Rayleigh surface wave, which generates a tensile stress. Where this stress exceeds the local tensile strength of the coating, a ring of cracks surrounding the area of impact is created. The results from the present study therefore suggest that stress wave reflection is responsible for the formation of the cracks at locally debonded regions of the coating. This hypothesis was supported by images acquired using scanning acoustic microscopy, which showed that circumferential cracks and pin-holes were only found on areas of the coating that had become delaminated by multiple particle impacts during the erosion tests.     

Mode selection of Lamb waves for the evaluation of solid plates with liquid loading

This paper studies the mode selection of Lamb waves for evaluating solid plates with liquid loading. For this purpose, the Lamb wave selected should have the features such as zero normal displacement components at the plate surface in contact with liquid, small dispersion, and maximum group velocity. It is found that when the phase velocity of Lamb wave is equal to the longitudinal wave velocity of the plate material, its normal displacement at the plate surface is always zero. Through the numerical analyses, the specific S₂ Lamb wave that has zero normal displacement component at the plate surface, small dispersion and maximum group velocity compared with the other Lamb waves has been found. With respect to the specific S₂ Lamb wave, some experimental examinations have been carried out. It is found that the liquid loading on the plate surface has less influence on the specific S₂ Lamb wave signal but it can effectively eliminate the other signals. Moreover, the specific S₂ Lamb wave selected exhibits the capability of detecting multiple defects in the solid plate with the liquid loading. It can be concluded that the specific S₂ Lamb wave selected is suitable for the evaluation of solid plates with liquid loading.     

Two-dimensional numerical simulation of acoustic wave phase conjugation in magnetostrictive elastic media

The effect of parametric wave phase conjugation (WPC) in application to ultrasound or acoustic waves in magnetostrictive solids has been addressed numerically by Ben Khelil et al. [J. Acoust. Soc. Am. 109, 75-83 (2001)] using 1-D unsteady formulation. Here the numerical method presented by Voinovich et al. [Shock waves 13(3), 221-230 (2003)] extends the analysis to the 2-D effects. The employed model describes universally elastic solids and liquids. A source term similar to Ben Khelil et al.'s accounts for the coupling between deformation and magnetostriction due to external periodic magnetic field. The compatibility between the isotropic constitutive law of the medium and the model of magnetostriction has been considered. Supplementary to the 1-D simulations, the present model involves longitudinal/transversal mode conversion at the sample boundaries and separate magnetic field coupling with dilatation and shear stress. The influence of those factors in a 2-D geometry on the potential output of a magneto-elastic wave phase conjugator is analyzed in this paper. The process under study includes propagation of a wave burst of a given frequency from a point source in a liquid into the active solid, amplification of the waves due to parametric resonance, and formation of time-reversed waves, their radiation into liquid, and focusing. The considered subject is particularly important for ultrasonic applications in acoustic imaging, nondestructive testing, or medical diagnostics and therapy.     

A study of electron beams extracted from a plasma produced by laser interaction with a solid target

In this work, wave formation in laser-produced plasma is investigated by an analysis of time-of-flight signal of the electron pulse. Electrons are extracted from a non-equilibrium plasma, generated by pulsed laser ablation on a solid Ge target. The process is represented by ion-acoustic waves, which are generated from an external perturbation, given by the positive bias voltage of a Faraday cup. The characteristics of the waves depend substantially on the geometry of the plasma expansion chamber and on laser fluence, but are independent on bias potential. A KrF excimer UV laser was employed for plasma generation. Measurements were performed at two different laser fluences, 4 and 7 J/cm². The plasma created propagates with a mean velocity of about 1.1?×?10⁴ m/s. A movable Faraday cup was employed in order to collect electrons at different bias voltage values.     

A comparative study of droplet impact dynamics on a dual-scaled superhydrophobic surface and lotus leaf

The impact dynamics of water droplets on an artificial dual-scaled superhydrophobic surface was studied and compared with that of a lotus leaf with impact velocity V up to 3 m/s. The lower critical impact velocity for the bouncing of droplets was about 0.08 m/s on both surfaces. At relatively low impact velocities, regular rebound of droplets and air bubble trapping and flow jetting on both surfaces were observed as V was increased. For intermediate V, partial pinning and rebound of droplets were found on the artificial dual-scaled surface due to the penetration of the droplets into the micro- and nano-scale roughness. On the lotus leaf, however, the droplets bounced off with intensive vibrations instead of being partially pinned on the surface because of the irregular distribution of microbumps on the leaf. As the impact velocity was sufficiently high, droplet splashing occurred on both surfaces. The contact time and restitution coefficient of the impinging droplets were also measured and discussed.     

Wave velocities in shock-compressed cubic and hexagonal single crystals above the elastic limit

When solids are subjected to high-pressure shock-wave loading, multiple stress waves propagate with velocities dependent upon the elastic and inelastic compressibilities of the solid. The present paper shows that the inelastic or plastic waves in cubic and hexagonal single crystals do not necessarily propagate with the bulk sound speed as they do in isotropic elastic-plastic solids. This result is a consequence of anisotropy in the plastic deformation which depends on the slip plane orientation in the crystal and has important consequences with regard to the determination of compressibilities from shock-wave data. In particular, for wave propagation in the <110> directions of cubic crystals the departure from the bulk velocity can be significant (5–25 per cent). For wave propagation normal to the c-axis in hexagonal crystals, the plastic wave velocity also differs from the bulk sound speed (10–25 per cent). Plastic wave velocities are tabulated for a number of cubic crystals on the basis of the various slip systems common to these materials. The calculated velocities are then compared with experimental data on shock-loaded single-crystal aluminum and sodium chloride.     

Ion irradiation induced spontaneous hypersonic long-range stimulation of silicon nitride synthesis in silicon

We studied the nature of the effect of medium-energy ion implantation on the defect system of a crystal target over distances exceeding by three to four orders of magnitude the average projected range of ions in the target material. Recently, we discovered an especially strong manifestation of this long-range effect in crystal targets: argon ion bombardment stimulated the formation of a Si₃N₄ phase in nitrogen-saturated layers of a silicon wafer, the effect being observed at a distance of up to 600 μm away from the ion stopping zone. An analysis of changes in the electrical and optical properties of the nitrogen-saturated layer depending on the argon ion dose, in comparison to the morphology development on the ion-irradiated silicon surface, suggests that sufficiently effective pulsed sources of hypersonic (in the initial propagation stage) shock waves appear in the Ar⁺ ion stopping zone. These shock waves arise as a result of the jumplike formation and evolution of a network of dislocation loops and argon blisters, accompanied by explosions of the blisters. These processes probably proceed in a self-synchronized or spontaneous manner. Argon in the blisters occurs at T = 773 K in a solid state under a pressure of 4.5×10⁹ Pa, the blister energy reaching up to 5×10⁸ eV. Estimates show that the synchronized explosions of blisters in the region of a nitrogen-saturated layer at the rear side of a 600-μm-thick silicon wafer may produce a peak pressure at the wave front exceeding 10⁸ Pa, which is sufficient to cause the experimentally observed changes.     

强脉冲激光在AZ31B镁合金中诱导冲击波的实验研究

为了研究强脉冲激光在镁合金中诱导冲击波的衰减,采用Nd:Glass脉冲激光(1054 nm,23 ns)对AZ31B变形镁合金试样表面进行冲击,并利用响应快、测量范围大的PVDF压电膜传感器以及示波器实时测量了强脉冲激光在镁合金靶中诱导激光冲击波的相对压力.根据冲击波每次在靶材背面反射时,所经过距离的不同得到激光冲击波在镁合金中的衰减规律.结果表明,在激光能量为5J的强脉冲激光作用下,镁合金中冲击波衰减的平均速度为5.83×10³ m/s,与镁合金中应力波纵波的传播速度相符;强脉冲激光诱导冲击波在镁合金中是以指数规律衰减的.试验所得分析结果对激光冲击强化镁合金的应用具有重要意义. 关键词： 激光 镁合金 压电膜传感器 衰减规律     

Numerical simulation of ultrasonic enhancement on mass transfer in liquid–solid reaction by a new computational model

Mass transfer coefficient is an important parameter in the process of mass transfer. It can reflect the degree of enhancement of mass transfer process in liquid–solid reaction and in non-reactive systems like dissolution and leaching, and further verify the issues by experiments in the reaction process. In the present paper, a new computational model quantitatively solving ultrasonic enhancement on mass transfer coefficient in liquid–solid reaction is established, and the mass transfer coefficient on silicon surface with a transducer at frequencies of 40 kHz, 60 kHz, 80 kHz and 100 kHz has been numerically simulated. The simulation results indicate that mass transfer coefficient increases with the increasing of ultrasound power, and the maximum value of mass transfer coefficient is 1.467 × 10⁻⁴ m/s at 60 kHz and the minimum is 1.310 × 10⁻⁴ m/s at 80 kHz in the condition when ultrasound power is 50 W (the mass transfer coefficient is 2.384 × 10⁻⁵ m/s without ultrasound). The extrinsic factors such as temperature and transducer diameter and distance between reactor and ultrasound source also influence the mass transfer coefficient on silicon surface. Mass transfer coefficient increases with the increasing temperature, with the decreasing distance between silicon and central position, with the decreasing of transducer diameter, and with the decreasing of distance between reactor and ultrasound source at the same ultrasonic power and frequency. The simulation results indicate that the computational model can quantitatively solve the ultrasonic enhancement on mass transfer coefficient.     

Effects of an absorptive coating on the dynamics of underwater laser-induced shock process

The effects of an absorptive coating on the dynamics of underwater laser-induced shock process have been observed from the end of laser pulse to hundreds of microseconds after irradiation by time-resolved imaging techniques. A laser pulse of 13 ns at 1,064 nm was focused by a 40-mm focal length lens onto the surface of epoxy-resin blocks immersed in water to induce the shock process in the confining regime. A custom-designed time-resolved photoelasticity imaging technique and a high-speed laser stroboscopic videography technique in photoelasticity mode were used to analyze the evolution of shock waves in the water phase, the strength of stress waves in the solid phase, the oscillation of cavitation bubbles, and the generation of bubble-collapse-induced shock waves. We showed that black paint coating enhances the strength of laser-induced stress wave inside the solid, drives faster shock waves traveling in the water phase, and produces higher-energy cavitation bubbles. We propose that even at power densities of 1 GW/cm² and above, an absorptive coating can intensify the shock process by enhancing the absorption of laser energy by plasma.     

Effect of water deposition on the surface dynamics of mesopores in MCM-41

The surface acoustic waves in empty cylindrical pores in the amorphous silica MCM-41 as well as in the same pores partially filled with water are studied with the use of a continuum model. The model is shown to be adequate to predict dispersion relations, cut-off wave vectors and the Airy phases for the secular surface waves of the lowest azimuthal indices n. Quantitative predictions are presented both in the liquid and in the polycrystalline solid phase of water. Two sagittal surface waves exist when water is in the liquid phase. The phase transition to the solid phase (ice) results in the disappearance of the high-frequency mode. All the effects occur in the Terahertz frequency region.     

Propagation of Alfvén surface waves along the ionospheric plasma slab with the effect of gravity

Ionospheric regions connecting the neutral gas atmosphere have been considered to be an incompressible plasma slab surrounded by incompressible plasma on one side and neutral gas on the other side. The effect of gravity on Alfvén surface waves in the slab geometry is studied. As a special case, the propagation of ASW along the plasma-neutral gas interface is also discussed. The existence of two modes of surface waves has been identified and their characteristic behaviour affected by the gravity has been discussed.     

Fracture characteristics of bulk metallic glass under high speed impact

High speed impact experiments of rectangular plate-shaped Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (BMG) were performed using a two-stage light gas gun. Under spherical shock waves with impact velocities ranging from 0.503 km/s to 4.917 km/s, obvious traces of laminated spallation at the back (free) surface and melting (liquid droplets) at the impact point were observed. The angles about 0?, 17?, 36?, and 90? to the shocking direction were shown in the internal samples because of the interaction between the compressive shock waves and the rarefaction waves. The compressive normal stress was found to induce the consequent temperature rise in the core of the shear band.     

The lattice Boltzmann phononic lattice solid

I present a Boltzmann lattice gas-like approach for modeling compressional waves in an inhomogeneous medium as a first step toward developing a method to simulate seismic waves in complex solids. The method is based on modeling particles in a discrete lattice with wavelike characteristics of partial reflection and transmission when passing between links with different properties as well as phononlike interactions (i.e., collisions), with particle speed dependent on link properties. In the macroscopic limit, this approach theoretically yields compressional waves in an inhomogeneous acoustic medium. Numerical experiments verify the method and demonstrate its convergence properties. The lattice Boltzmann phononic lattice solid could be used to study how seismic wave anisotropy and attenuation are related to microfractures, the complex geometry of rock matrices, and their couplings to pore fluids. However, additional particles related to the two transverse phonons must be incorporated to correctly simulate wave phenomena in solids.     

空气中激光支持爆轰波实验及理论分析

为了研究激光击穿空气产生的等离子体爆轰波形成机制和传播规律,利用高能量CO2激光器产生强激光,进行了空气中产生激光支持等离子体爆轰波实验。实验中:设置了诱导靶板,用于诱发和定位空气中的激光支持爆轰波;以激光器升压过程球隙放电产生的光信号作为触发源,触发高时间分辨率(纳秒级)的高速相机,记录了激光支持爆轰波的成长和传播全过程。分析了激光支持爆轰波的形成机理和传播规律。采用C-J爆轰理论,计算了激光支持爆轰波的压力和温度。研究结果表明:激光支持等离子体爆轰波形成初期,等离子体爆轰波发光体为球形;随着时间增加,等离子体爆轰波发光体的形状类似流星,且头部为等离子体前沿吸收层,亮度较高,而尾部等离子体温度较低,亮度较弱。等离子体爆轰波高速向激光源的方向移动,爆轰波速度高达18 km/s,温度约为107K。随着激光强度的减弱,爆轰波速度迅速按指数规律衰减,当爆轰波吸收的激光能量不能有效支持爆轰波传播时,爆轰波转变为冲击波。     

Performance analysis of a new biolistic gun using high power laser irradiation

Impingement of a high power laser pulse (above 10⁹ W/cm²) on a metal foil causes ablation, which is characterized by a rapid expulsion of matter and initiation of a strong shock wave inside the solid metal. The shock propagates through the foil and reverberates on the rear side causing instant deformation of the foil, whose surface is treated with micro particles prior to ablation. Based on this principle of micro particle ejection, we develop a new biolistic gun with improved controllability, stability, efficiency of our previous system, and perform characterization of the penetration shapes at varying confinements and energy levels. The confinement media include BK7 glass, water, and succulent jelly (ultrasound gel). Biological tissue was replicated by a gelatin-water solution at a 3% weight ratio. Present data show that confinement effect results in a conspicuous enhancement of penetration reached by 5 μm cobalt micro particles. Also, there exists an optimal thickness at each energy level when using liquid confinement for enhanced particle delivery.