Real time NDE of laser shock processing with time-of-flight of laser induced plasma shock wave in air by acoustic emission sensor

Acoustic emission sensor is used to research the time-of-flight of the shock wave induced by laser-plasma in air for real time nondestructive evaluation (NDE) of laser shock processing. The time-of-flight of the shock wave propagating from the source to the sensor declines nonlinearly and similarly at the different distances for different laser energies. The velocity of the shock wave at the distance of 30 mm increases faster than that of the distance of 35 mm. The relationship between the laser energy and the distance is almost linearly when the signal with distortion is measured by acoustic emission sensor. Finally, Taylor solution is used to analyze the experimental results, and the empirical formula between the energy of the shock wave and the laser energy is established, which will provide a theoretical basis for real time NDE of laser shock processing.     

Influence of air pressure on mechanical effect of laser plasma shock wave

The influence of air pressure on mechanical effect of laser plasma shock wave in a vacuum chamber produced by a Nd:YAG laser has been studied. The laser pulses with pulse width of 10ns and pulse energy of about 320mJ at 1.06$\mu $m wavelength is focused on the aluminium target mounted on a ballistic pendulum, and the air pressure in the chamber changes from $2.8\times 10^{3}$ to 1.01$\times $10$^{5 }$Pa. The experimental results show that the impulse coupling coefficient changes as the air pressure and the distance of the target from focus change. The mechanical effects of the plasma shock wave on the target are analysed at different distances from focus and the air pressure.     

激光烧蚀水下金属产生冲击波和空泡效应的研究

采用自行研制的高灵敏度光束偏转测试系统,对脉冲激光烧蚀水下金属产生的等离子体冲击波和空泡效应进行了实验研究。实验得到了激光等离子体冲击波的传播规律、冲击波与激光空泡的分离过程、空泡的脉动特性以及空泡溃灭冲击波的形成机制。结果表明,空泡最大和最小泡径、振荡周期均随着脉动次数的增加呈减小趋势,且减小幅度较大。最小收缩泡径由泡能和腔内含气量共同决定。在同一次脉动过程中,空泡膨胀所需的时间明显大于收缩所需的时间。     

Compton散射下强激光等离子体空气冲击波波前的传输特性

 根据球对称激光等离子体空气冲击波在自由空间中传输的约束条件，对多光子非线性Compton散射的强激光等离子体空气冲击波波前的传输特性进行了研究，给出了散射下空气冲击波波前的运动方程，并进行了数值模拟。结果表明：该冲击波的衰减过程不仅与爆炸源和爆炸过程的特性、释放总能量、空气的弹性有关，而且还与散射有关，散射效应使冲击波的初始半径增大，衰减过程加快，能量转移率提高；数值模拟与实验结果符合得很好。     

强脉冲CO2激光对红外材料的破坏现象

实验表明强激光对红外材料的破坏分为两种类型:烧蚀和冲击破坏。在空气中,光功率密度低于大气的光学击穿阈值时,以烧蚀为主。超过击穿阈值时,冲击破坏是主要的。在我们的实验条件下,用强激光辐照红外脆性材料的靶,靶极易产生微裂纹或粉碎性破坏,其破坏效果与激光的脉冲能量、功率密度、脉冲宽度,以及着靶面积有关。为了直观,文中给出了不少辐照损伤的照片。 关键词：     

Spatial and temporal study of shock waves generated by laser ablation for Ti target

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Computer simulation of perturbation of a shock wave in nitrogen by optical radiation

The possibility of a displacement of the front of a shock wave formed in a 1D nitrogen flow to the low gas pressure region (in front of the shock wave) upon absorption of the laser pulse energy in the region of the shock wave front is demonstrated using computer simulation based on the finite difference technique. The low-pressure region formed in the region of the initially high pressure under the action of a light pulse moves in the direction opposite to the direction of propagation of the shock wave front. Analysis is carried out for a typical experimental situation corresponding to the growth of carbon-nitride nanofilms.     

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     

基于神光III原型的整形激光直接驱动准等熵压缩实验研究

整形激光直接驱动准等熵压缩实验技术对新材料科学、冲击波物理、地球物理学研究等领域有重要意义. 设计特定的实验条件, 可以将材料压缩到冲击压缩和等熵压缩都无法达到的状态, 从而为材料研究提供新的技术途径. 介绍了在神光III原型装置上开展的直接驱动准等熵压缩实验, 对理论基础、实验靶型、实验结果、关键技术、实验特点和数据都进行了较详细的分析. 通过实验和相应的数据处理程序, 获得了加载面上压力超过400 GPa的数据, 这是目前国际上用激光直接驱动的方式获得的Al材料中最高的压力. 同时, 对经过LiF窗口获得的表观粒子速度与真实粒子速度的关系进行了分析, 获得了一条对表观粒子速度进行动态修正的曲线, 从而使粒子速度的计算更加准确. 对实验的改进方向进行了比较明确的分析. 所提供的数据和分析过程对该领域的研究人员有一定的参考价值.     

纳秒激光冲击加载的全过程诊断

 与传统的冲击加载方式相比,激光驱动试样具有微尺寸（直径小于1 mm,厚度约10 μm）、超短作用过程（纳秒量级）等特点,但其速度变化历史的实时诊断颇为困难,因此发展适用于激光驱动的高时空分辨率的实时测试技术是十分重要和有价值的。采用桌面式脉冲Nd:YAG激光器作为加载平台,发展了激光加载下的小焦点多普勒光纤探针测量系统（焦斑直径约200 μm,时间分辨力约50 ps）,成功实现了从激光脉冲驱动微尺寸飞片飞行直至撞击Z-切石英试样的全过程实时诊断。实验结果显示,将6 μm 厚Al箔飞片驱动至2.48 km/s时,撞击Z-切石英试样的粒子速度为1.27 km/s,与Hugoniot理论计算结果相符,表明该测试技术是可靠、有效的;多层薄膜靶结构设计（基底/烧蚀层/硅油/Al箔）可提升激光与靶物质的能量耦合效率,使飞片保持更好的宏观完整性。为开展超短脉冲激光加载下材料动态特性研究提供了一种有效的技术途径。     

脉冲激光等离子体冲击波碎石机理分析

利用自行研制的Nd:YAG激光器产生的纳秒和微秒激光脉冲,分别在水中和空气中对几种人体结石进行了粉碎实验,激光被耦合到芯径为400μm的石英光纤中,耦合效率达70%.实验研究和分析计算表明脉冲激光等离子体冲击的产生是激光碎石的主要动力,而热分解作用只是促进了等离子体的形成并不是粉碎结石的主要原因,体液冲流和周围介质对等离子体的约束加速了碎石过程,脉冲激光碎石的过程是这几个方面共同作用的结果。     

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N₂ gas.     

基于速度干涉仪的冲击波精密调速实验技术研究

冲击波精密调速技术是惯性约束聚变研究的关键技术之一. 针对冲击波精密调速诊断技术的要求, 以神光III原型上脉宽为3 ns的两台阶整形激光脉冲为源, 用石英晶体做窗口材料, 模拟了双冲击在石英晶体中传输和追赶的过程. 利用两发典型双冲击波调速的实验数据, 验证了神光III原型具有的精密调速能力. 实验结果表明, 在驱动源、 靶和诊断系统参数一致的条件下, 两发实验获得的实验结果中两次冲击出现的时刻, 冲击波速度都很一致. 通过精细分析, 发现在两次冲击交会的时刻, 干涉条纹清晰度和完整性变化最大, 最终数据处理获得的冲击波速度偏差也最大. 同时, 在二次冲击的冲击波速度有较大增加的条件下, 并没有观察到冲击波阵面的反射率的明显增加. 本文的实验结果作为一个数据依据, 为全面开展冲击波调速实验提供了有效的方法.     

大气环境下飞秒激光对铝靶烧蚀过程的研究

利用时间分辨的光阴影成像技术研究了在大气环境下飞秒激光烧蚀铝靶的动态过程. 在入射激光能量为4 mJ, 激光光斑超过1 mm时, 激光烧蚀区表面物质以近似平面冲击波形式向外喷射; 在同样激光能量下、激光光斑较小时(约0.6 mm), 激光烧蚀区以近似半球型冲击波形式向外喷射. 当激光能量比较大时(7 mJ), 发现空气的电离对于激光烧蚀靶材有着重要影响. 在光轴附近烧蚀产生的喷射物具有额外的柱状和半圆型的结构, 叠加在平面冲击波结构上.     

Effect of laser energy on the deformation behavior in microscale laser bulge forming

Microscale laser bulge forming is a high strain rate microforming method using high-amplitude shock wave pressure induced by pulsed laser irradiation. The process can serve as a rapidly established and high precision technique to impress microfeatures on thin sheet metals and holds promise of manufacturing complex miniaturized devices. The present paper investigated the forming process using both numerical and experimental methods. The effect of laser energy on microformability of pure copper was discussed in detail. A 3D measuring laser microscope was adopted to measure deformed regions under different laser energy levels. The deformation measurements showed that the experimental and numerical results were in good agreement. With the verified simulation model, the residual stress distribution at different laser energy was predicted and analyzed. The springback was found as a key factor to determine the distribution and magnitude of the compressive residual stress. In addition, the absorbent coating and the surface morphology of the formed samples were observed through the scanning electron microscope. The observation confirmed that the shock forming process was non-thermal attributed to the protection of the absorbent coating.     

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.     

Shadowgraph investigation of plasma shock wave evolution from Al target under 355-nm laser ablation

The propagation of a plasma shock wave generated from an Al target surface ablated by a nanosecond Nd:YAG laser operating at 355 nm in air is investigated at the different focusing positions of the laser beam by using a time-resolved shadowgraph imaging technique. The results show that in the case of a target surface set at the off-focus position, the condition of the focal point behind or in front of the target surface greatly influences the evolution of an Al plasma shock wave, and an ionization channel forms in the case of the focal point set in front of the target surface. Moreover, it is found that the shadowgraph with the evolution time around 100 ns shows that a protrusion appears at the front tip of the shock wave if the focal point is at the target surface. In addition, the calculated results of the expanding velocity of the shock wave front, the mass density, and pressure just behind the shock wave front are presented based on the shadowgraphs.     

Non-contact sound speed measurement by optical probing of beam deflection due to sound wave

We report a non-contact and non-invasive method of sound speed measurement by optical probing of deflected laser beam due to normally incident degenerated shock wave. In this study the shock wave from an exploding wire was degenerated to an ordinary sound wave at the distance exceeding 0.23 m. Temporal resolution of the deflected beam signal was improved by passing through an adequate electronic high-pass filter, as a result we obtained a better temporal resolution than that of the acoustic pressure detection by PZT transducer in terms of rising time. The spatial resolution was improved by passing the refracted beam signal into the edge of focusing lens to make a larger deflection angle. Sound speed was calculated by monitoring the time of flight of transient deflected signal at the predetermined position. Sound speed has been measured in air, distilled water and acryl, agreed well with the published values. The sound speed measured in the solution of glycerin, magnesium sulfate (MgSO4), and dimethylformamide with various mole fractions also agrees within 3% of relative error with those measured in the present work by ultrasonic pulse echo method. The results suggest that the method proposed is to be reliable and reproducible.     

An experimental investigation of underwater pulsed laser forming

Laser forming is a new forming technology, which deforms a metal sheet using laser-induced thermal stresses. This paper presents an experimental investigation of pulsed laser forming of stainless steel in water and air. The effects of cooling conditions on bending angle and morphology of the heat affected zone (HAZ) are studied. It is shown that the case of the top surface in air and the bottom surface immersed in water has the greatest bending angle based on the forming mechanism of TGM. The water layer above the sample decreases the coupling energy, leading to a small bending angle. For a thin water thickness (1 mm), the water effects on the HAZ are limited. As water layer thickness increases (5 mm), the concave shape of the HAZ is more remarkable and irregular because the shock waves by high laser energy heating water are fully developed. However, the area and the depth of the HAZ become less significant when water thickness is 10 mm due to the long pathway that laser undergoes.