强脉冲激光束引起圆柱壳动屈曲破坏的理论估算

1 引言研究强激光对靶材破坏的机理,主要目的在于确定破坏阈值.高强度短脉冲激光束辐照靶材时,靶面物质由于汽化、电离向外喷射对靶材产生反冲冲量,高速冲击可能引起材料层裂等早期材料破坏效应以及由于大变形和大应变而产生的后期结构破坏效应,结构破坏阈值一般比材料破坏阈值低.本文先假设一均匀的强脉冲激光束辐照到铝材料的圆柱壳体时,壳体将受到一余弦分布的脉冲载荷,然后根据Andcrson 的假设认为壳体的变形达到某一临界值时,就会     

激光辐照诱导的热与力学效应

对激光辐照诱导的热与力学问题研究进展进行了综述,包括材料在高温、高升温速率下的本构关系,典型薄板和柱壳等结构在激光辐照下的热力破坏效应,多层材料体系的激光破坏行为等几个方面,并着重介绍了包含相变与烧蚀过程的激光破坏分析模型与机制研究,激光辐照效应的流–热–固耦合数值模拟方法,以及短脉冲激光引起的冲击与破坏机理等方面的研究新进展。     

Progress in thermal-mechantical effects induced by laser

对激光辐照诱导的热与力学问题研究进展进行了综述,包括材料在高温、高升温速率下的本构关系,典型薄板和柱壳等结构在激光辐照下的热力破坏效应,多层材料体系的激光破坏行为等几个方面,并着重介绍了包含相变与烧蚀过程的激光破坏分析模型与机制研究,激光辐照效应的流-热-固耦合数值模拟方法,以及短脉冲激光引起的冲击与破坏机理等方面的研究新进展.     

激光辐照诱导的热与力学效应

对激光辐照诱导的热与力学问题研究进展进行了综述,包括材料在高温、高升温速率下的本构关系,典型薄板和柱壳等结构在激光辐照下的热力破坏效应,多层材料体系的激光破坏行为等几个方面,并着重介绍了包含相变与烧蚀过程的激光破坏分析模型与机制研究,激光辐照效应的流–热–固耦合数值模拟方法,以及短脉冲激光引起的冲击与破坏机理等方面的研究新进展.     

热-力联合作用下柱壳结构变形的计算分析

本文研究了激光连续加热和恒内压作用下柱壳结构的变形规律。以4340钢材料作为研究对象，其本构方程选用热粘塑性本构模型（Johnson-Cook模型）；选取某一应变率为临界值（本文中临界应变率取为1s^-1)。考察了激光功率密度，预载荷大小，激光作用时间等对结构变形规律的影响。主要结论有：结构失稳是导致激光辐照下充内压柱壳破坏的重要原因；激光功率密度和内压越高，结构破坏时间越短；根据辐照时间的长短，预测到了结构急速破坏，延迟破坏和不破坏三种模式。本工作对于深入认识激光作用下预载结构的热－力联合破坏有一定意义。     

低功率密度激光辐照结构的热动力失效

研究了低功率密度激光辐照结构的热动力失效。考虑材料的高温软化特性，研讨了温升速率对材料和结构的影响、结构热软化后的宏观受力和变形的重新分布以及可能的热运动。给出了几种热动力失效的结构破坏实验结果。     

激光辐照下预内压低碳钢圆柱壳爆裂断口分析

通过实验揭示了预内压低碳钢薄柱壳在激光辐照下裂纹萌生、扩展和止裂过程的较完整物理图象。对断口特性进行了深入分析，发现辐照区上裂纹的形成和扩展机制以韧性为主，断口在柱壳壁厚度方向有明显分层，如外层为塑性变形层，内层是韧窝或层状撕裂，部分试件的最内层出现脆性石头状断口。给出了激光辐照区应力分布与裂纹萌生之间的关系，同时探讨了激光辐照区材料金相组织变化对材料细观损伤和初始裂纹萌生的影响，有助于进一步对激光辐照下柱壳的断裂问题进行更深入的理论研究。     

激光辐照与拉伸预应力作用下复合材料试件的破坏研究

对复合材料T300/FAG80试件在室温及激光辐照下的力学性能进行了测试,得到材料的纵横向有效弹性模量以及拉压破坏强度,并分析激光参数对于材料力学性能的影响.试验采用波长为1064nm的Nd:YAG连续输出激光器,在不同拉伸预应力下,以不同功率密度激光辐照复合材料,并使用高速摄像机进行同步观测,对烧蚀破坏机理进行了分析并预测激光强度对于材料性能影响的趋势走向.使用有限元软件对激光辐照复合材料试件破坏时间进行数值建模,将计算结果与试验结果相对比,数据基本吻合.     

强激光破坏机制研究进展

本文对强激光造成靶材破坏机理进行了评述，将激光破坏机理在Ｉ－ｒ平面内分为冲击破坏、热烧蚀破坏和非熔化条件下的热应力破坏三个区域，并分别对各区激光－靶材相互作用过程及破坏方式进行了评述，给出了国内外的最新研究成果及研究展望。     

箱型结构内部爆炸破坏研究进展

结构内部爆炸破坏机理和规律是常规武器毁伤效能预测与评估、建筑物和舰船抗爆防护设计的重要支撑。基于结构内爆炸载荷、内部爆炸作用下结构塑性响应、内部爆炸作用下箱壁结构破坏模式、内部爆炸作用下多箱型结构破坏模式和分布四个方面详细论述了箱型结构内部爆炸破坏的研究现状及存在的问题,并对内部爆炸后续研究给出了建议。建议研究并建立更加复杂的结构内部爆炸载荷和破坏效应描述模型、内部爆炸作用下箱壁的动力响应机理、多箱型结构与内部爆炸波产生的耦合效应、内部爆炸作用下结构的破坏模式和破坏范围的快速准确预测方法等。     

Experimental study of underwater rock drilling using a pulsed Ho:YAG laser-induced jets

This paper is primarily an assessment of laser-induced water jets for boring rock surfaces. It also reports the result of preliminary experiments of pulsed Ho:YAG laser-induced jets applied to drill a submerged rock specimen. The irradiation of pulsed Ho:YAG laser beams at 3 Hz inside a thin metal tube produces intermittent water vapor bubbles which result in liquid jet discharge from the exit of the metal tube. The laser-induced water jets are visualized by shadowgraphs and images are recorded by a high-speed digital video camera. High stagnation pressures were eventually generated by the jet impingements. Simultaneously shock waves of about 22.7 MPa were generated at bubble collapse, which effectively cracked the surface of the rock specimens. Repeated exposures of these laser-induced jets against submerged rock specimens have a potential to practically bore holes on rock surfaces.     

iLIF: illumination by Laser-Induced Fluorescence for single flash imaging on a nanoseconds timescale

The challenge in visualizing fast microscale fluid motion phenomena is to record high-quality images free of motion-blur. Here, we present an illumination technique based on laser-induced fluorescence which delivers high-intensity light pulses of 7 ns. The light source consists of a Q-switched Nd:YAG laser and a laser dye solution incorporated into a total internal reflection lens, resulting in a uni-directional light beam with a millimeter-sized circular aperture and 3° divergence. The laser coherence, considered undesirable for imaging purposes, is reduced while maintaining a nanoseconds pulse duration. The properties of the illumination by laser-induced fluorescence (iLIF) are quantified, and a comparison is made with other high-intensity pulsed and continuous light sources.     

Multirail electromagnetic launcher powered from a pulsed magnetohydrodynamic generator

The operation of an electromagnetic multirail launcher of solids powered from a pulsed magnetohydrodynamic (MHD) generator is studied. The plasma flow in the channel of the pulsed MHD generator and the possibility of launching solids in a rapid-fire mode of launcher operation are considered. It is shown that this mode of launcher operation can be implemented by matching the plasma flow dynamics in the channel of the pulsed MHD generator and the launching conditions. It is also shown that powerful pulsed MHD generators can be used as a source of electrical energy for rapid-fire electromagnetic rail launchers operating in a burst mode.     

铝板中激光Lamb波信号的模态分析与缺陷检测研究

激光激励的Lamb波信号具有较宽的频带,且包含多个模态信息。本文采用二维傅里叶变换和时频分析等信号分析技术用于检测信号中的模态成分及缺陷信息识别。首先,对200组激光Lamb波信号进行二维傅里叶变换,得到信号的频率-波数图,可识别出激光Lamb波信号中的低阶A0、S0和高阶模态,并且A0模态能量高,可用于缺陷检测。随后对有、无缺陷状态下Lamb波信号进行连续小波变换,从时频图中识别出缺陷信号的频率成分,进一步提取特定频率下的小波系数幅值信号,实现了缺陷信息的识别。结果表明,二维傅里叶变换能较好地识别激光Lamb波的模态成分,而提取出的连续小波变换系数图,能准确实现缺陷定位。     

Hybrid analytical/numerical modeling of nanosecond laser-induced micro-jets generated by liquid confining devices

The generation of micro-jets with pulsed laser irradiation is a key enabling technique for microfluidic devices, printers and needle-free drug injectors. Modeling approaches for such devices are essential to optimize their design and performance. Here we present a hybrid analytical/numerical model to simulate nanosecond laser-induced micro-jets generated by a dual-chamber liquid confining device. The simulated device consists of two chambers; the first one is closed and filled with a propellant liquid and the second is filled with the liquid to be ejected and equipped with a nozzle. Laser-induced cavitation is generated in the first chamber, which is separated by an elastic membrane from the second one, to reduce the thermo-mechanical impact of the absorbed laser energy on the liquid to be ejected. By modifying the generalized form of the Rayleigh–Plesset equation to account for the pressure variation inside the chamber, we show that the geometry of the liquid confining device affects drastically laser-induced bubble dynamics and the resulting jet ejection dynamics. We also demonstrate the effect of the membrane size, laser energy and nozzle size variation on the micro-jet dynamics. We found that such devices can generate micro-jets (velocity: 0.93 m/s to 48.39 m/s) suitable for micro-drop printing (volume: 0.097 nL to 7.68 nL). Although we focused on printing applications, the modeling approach presented here can be widely adapted for designing and optimizing needle-free drug injectors and microfluidic devices.     

Laser-induced break-up of water jet waveguide

In this article, an optical method to control the break-up of high-speed liquid jets is proposed. The method consists of focusing the light of a pulsed laser source into the jet behaving as a waveguide. Experiments were performed with the help of a Q-switched frequency doubled Nd:Yag laser (=532 nm). The jet diameter was 48 µm and jet velocities from 100 to 200 m/s. To study the laser-induced water jet break-up, observations of the jet coupled with the high power laser were performed for variable coupling and jet velocity conditions. Experimentally determined wavelength and growth rate of the laser-generated disturbance were also compared with the ones predicted by linear stability theory of free jets.     

Observation of Piezoelectrically Induced Lamb Wave Propagation in Thin Plates by Use of Speckle Interferometry

Lamb waves have shown to be favorably applicable in the development of health monitoring systems in structural mechanics since they show reflections, refractions, as well as mode conversions at fault locations. For the observation of the wave propagation, the double pulsed electronic speckle pattern interferometry (ESPI) as a high precision, full-field, and non-contact measurement technique is under investigation in this work. First, it is shown that symmetric and antisymmetric Lamb waves can be captured in a faultless plate. Moreover, mode conversions are observed at stiffness discontinuities in a specially prepared plate. Finally, a riveted lightweight structure is under investigation. In these experiments, mode conversions indicate the location of the rivets.     

Pressure Improvement on a Supercritical High-Lift Wing Using Simple and Modulated Pulse Jet Vortex Generator

In this paper, an experimental study, aimed at delaying flow separation on a high-lift device using a pulsed blowing excitation method, is reported. The main objective of this investigation was to evaluate a new pulsed jet generation strategy to enhance flow control performance. In these experiments, two types of signal waveform were implemented to produce the unsteady blowing; a simple square-wave excitation signal for the first case, and a burst modulated excitation signal for the second case. The signal modulation was the first time to be used for a fast-switching solenoid valve actuator. Another objective of this study was to evaluate a new arrangement of the jet exit slots, in the form of a vortex generator which was employed for the first time on the high-lift device. For this purpose, a NASA SC(2)-0714 airfoil with a single slotted flap was employed. The vortex generator jets emanated from the shoulder of the trailing-edge flap with excitation frequencies from 40 to 1000 Hz. Pressure distribution around the model and wake total pressure deficit were measured. The results indicated that ejection from vortex generator slot pairs was able to prevent flow separation completely in most conditions. These measurements revealed that the burst modulated excitation signal was accompanied by more aerodynamic improvements and less air consumption relative to the simple pulsed jet excitation signal. In the best flow control mode, the results showed about a 53% increase in the value of the suction pressure peak on the flap and a 38% decrease in drag with a reduction in total pressure loss.

     

New insight into two-color LIF thermometry applied to temperature measurements of droplets

When laser-induced fluorescence of droplets is used for measurements such as droplet temperature, a new dependence of the droplet size on the spectral distribution of fluorescence has been highlighted. The two-color laser-induced fluorescence technique applied to droplet temperature measurement requires a single fluorescent tracer and two spectral bands of detection for which the temperature sensitivity is different. Generally, the ratio of the intensities measured on each of the spectral bands of detection is assumed to be only temperature dependent. However, droplet dependence on diameter is also likely to influence the intensities ratio. This study provides some illustrations of the phenomenon, first on sprays with different mean statistical diameters and secondly on single droplets, for two temperature-sensitive fluorescent tracers in their solvents: sulforhodamine B dissolved in water and pyrromethene 597-8C9 dissolved in n-decane.     

Experimental transition delay using glow-discharge plasma actuators

In the present work plasma actuators were applied in a flat-plate boundary layer with an adverse pressure gradient to influence the transition of the boundary layer. The first actuator downstream of the leading edge is operated in pulsed mode to introduce perturbations into the boundary layer to promote transition. Two steady operating actuators further downstream damp the perturbations significantly, which results in transition delay.