Laser-induced Shock Wave Research Articles

Background Oriented Schlieren Technique With Reconstruction Of Three-Dimensional Vector Fields For Pressure Measurement Of Laser Induced Underwater Shock Waves

We propose a novel reconstruction of the three-dimensional vector field (VR) of an axisymmetric target to overcome the problems that existing background-oriented schlieren (BOS) techniques based on reconstruction of three-dimensional scalar field (SR-BOS). This approach is based on an approximate relation between the projection of the axisymmetric vector field and the reconstructed vector field. The remarkable feature of the proposed technique is the reconstruction of an axisymmetric vector field with nonzero divergence, such as the field of a laser-induced underwater shock wave. Reconstruction of 3D vector fields, such as conventional vector tomography, is usually used in the field of electromagnetics with vector fields of zero divergence, and its application to fluid phenomena has been di fficult. We have successfully applied this technique to BOS (VR-BOS) and measured the pressure fields of underwater shock waves. Using the experimental data and previous research data, the superiority of VR-BOS was clarified by comparing the measurement results of SR-BOS and VR-BOS, for example, measurement accuracy, in better convergence, less dependence on the spatial resolution of the acquired images, and lower computational cost. In particular, the fact that the measurement accuracy of VR-BOS is almost independent of resolution is expected to expand the maximum pressure values that can be measured by the BOS technique. We also used synthetic data to investigate the effect of noise on VR-BOS. If the synthetic displacement contains noise, the noise is amplified in the vicinity of the axisymmetry axis in the reconstruction distribution calculated by VT. If the distance from the axisymmetry axis is more than 1/4 of the measurement range, the measurement accuracy of the reconstructed distribution can be sufficiently guaranteed. This proposed technique can be applied fluid dynamical fields, as well as other axisymmetric targets with vector reconstruction fields in other areas, such as electromagnetics and material mechanism.

Removal mechanism of liquid-assisted nanosecond pulsed laser cleaning TA15 titanium alloy oxide film

Liquid-assisted pulsed laser cleaning is achieved by coating kerosene on titanium alloy oxide film, and the surface was processed to form micro- and nanostructured amorphous-nanocrystalline phases oxidized layer after laser cleaning. The surface morphology is acquired by changing the laser fluence (F) and spot overlap ratio (Ux = Uy) with field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The surface composition change was analyzed by X-ray photoelectron spectroscopy (XPS). The transmission electron microscopy (TEM) was used to analyze the sample section. The surface quality is optimal when the laser fluence is between 5.095 J/cm2 and 6.37 J/cm2 at the spot ratio of 70%. The ridge structure appears on surface when the laser fluence exceeds 7.644 J/cm2. The original oxide film consisted of titanium oxides (Ti4+ and Ti3+) and C-containing contaminants. Ti0 peak appears on the surface at 6.37 J/cm2 and TiC might be formed on the surface for new peak (281.6 eV). The martensitic heat-affected layer was formed about 2.33 um at 6.37 J/cm2, and surface oxide layer is only about 12 nm. The removal mechanisms of liquid-assisted laser cleaning titanium alloy oxide film are laser-induced shock wave, laser ablation and phase blasting at difference fluence. Liquid-assisted laser cleaning technology can provide an effective surface cleaning and material processing process.

Effect of laser peen forming process parameters on bending and surface quality of Ti-6Al-4V sheets

Laser peen forming (LPF) is a metal forming process that utilizes laser-induced mechanical shock waves to form desired shapes or modify bent structures. The present work focuses on the applicability of LPF to Ti-6Al-4V sheets, to identify an optimal LPF process parameter window and achieve desired bending without compromising the surface quality within the peened region. The effect of LPF process parameters, i.e. laser power density, overlap, type of sacrificial overlay, and the number of peening sequences was investigated for specimens with different thicknesses. The laser power density and number of peening sequences were the most influential parameters that affect the bending of the specimens. Using sacrificial overlay has a significant effect on the bending and surface quality of the specimens. Surface quality after LPF was assessed by measuring the roughness in the peened region. In experiments without a sacrificial overlay, a black titanium oxide residue on the peened region was observed and additionally, small micro-cracks were found in the near surface region. Further characterization of the peened region revealed that the average crack length increased with increase in laser power density. Two possible LPF process parameter combinations were identified to obtain bending in the peened region, where LPF with sacrificial overlay resulted in no surface damage. Furthermore, residual stresses were determined at various LPF process parameters by incremental hole-drilling method in the peened region.

Method improving low Signal-to-noise ratio of velocity test signals for Laser-induced shock waves

When laser-induced shock (LIS) wave characteristics testing is carried out using laser interference methods, the electrical noise, vibration noise, jitter or offset of the diffuse reflective surface during high-speed motion and other factors can lead to superposition or offset of the noise signal frequency with the low-frequency phase of the useful signal. As a result, problems such as phase shift of the useful signal and difficulty in extracting the initial position of the wavelet ridge may appear, and the initial position cannot be zeroed. By introducing a measurement factor through multi-resolution singular value decomposition and adaptively determining the optimal number of decomposition layers, zero-phase offset filtering is achieved and the signal-to-noise ratio (S/N) of the data is improved. According to the waveform decay characteristics of laser-induced shock wave velocity signals, the Shannon wavelet entropy method is proposed to accurately optimise the Morlet wavelet basis parameters to enhance the aggregation of wavelet scale spectrum time–frequency distribution. Simulation and test results show that: the proposed method overcomes the background noise interference and enables the initial velocity to be zeroed; the relative singular value points of the pulse are accurately symmetrically distributed and no phase shift occurs; the denoising performance is better than that of the wavelet method, the root mean square error is only 0.046, and the repeatability is strong; the error of the elastic wave velocity of the laser-induced shock wave at different power densities is within 3.2%; the relative velocity resolution remains constant at low/high frequencies, effectively avoiding false peaks and misclassification of the velocity profile.

Fabricating micro-dent on the surface of ultrafine-grained pure copper by laser-induced shock wave and its deformation behavior

Fabricating micro-dent on the surface of ultrafine-grained pure copper by laser-induced shock wave and its deformation behavior

The cause of acute lethality of mice exposed to a laser-induced shock wave to the brainstem.

Air embolism is generally considered the most common cause of death within 1 h of a blast injury. Shock lung, respiratory arrest, and circulatory failure caused by vagal reflexes contribute to fatal injuries that lead to immediate death; however, informative mechanistic data are insufficient. Here we used a laser-induced shock wave (LISW) to determine the mechanism of acute fatalities associated with blast injuries. We applied the LISW to the forehead, upper neck, and thoracic dorsum of mice and examined their vital signs. Moreover, the LISW method is well suited for creating site-specific damage. Here we show that only mice with upper neck exposure, without damage elsewhere, died more frequently compared with the other injured groups. The peripheral oxygen saturation (SpO2) of the former mice significantly decreased for < 1 min [p < 0.05] but improved within 3 min. The LISW exposure to the upper neck region was the most lethal factor, affecting the respiratory function. Protecting the upper neck region may reduce fatalities that are related to blast injuries.

Identification of Characteristic Values in Impulse-Based Processes Using Small Specimens

Suitable approaches are needed for rapid and cost-efficient materials development. High-throughput experimentation reduces the identification time of suitable material compositions. One approach is to use small specimen geometries to save additional production costs. Hence, research is continuously being conducted on a new hardness test based on laser-induced shock waves. Thus far, characteristic values from the induced indentations have been extracted, which correlate with hardness and tensile strength. However, the indentation result varies in dependence of the specimen size and mass. This condition hinders the correlation between characteristic values and material properties. Thus, the goal was to induce similar indentation results to minimum specimen size. Herein, different mounting materials and methods were investigated. The created indentations were compared with those induced in large specimens. Essential mounting parameters were derived from the findings. Consequently, small specimens can be used for material characterization by considering these mounting parameters.

The influence of particle size on the fluid dynamics of a laser-induced plasma

The interaction of a laser-induced shock wave with nanoparticles and microparticles of aluminum oxide is investigated through experiments and modeling. The chemistry and physics of the interaction between the particles and plasma generated from laser ablation shows similarities and discrete differences for the two particle sizes. For both particle sizes, early stage (&amp;lt;10 μs) ionization was dominant and evidenced by higher concentrations of Al II. While both sizes exhibit ionization over the same duration, the intensity of emission was greater for nanoparticles indicating greater concentrations of ionized species. Moreover, the dispersion of species was notably more elongated for microparticles while radial dispersion was more pronounced for nanoparticles with elevated drag forces. At later stages (i.e., &amp;gt;10 μs), oxidation reactions were dominant for both particle sizes, but the same distinctions in flow field were observed and attributed to particle drag. In all stages of interaction, microparticles expand axially with less drag that suppresses their radial expansion. As a result, the dispersion of reactive species was mapped over an up to 80% larger area for nanoparticles relative to microparticles. Results shown here can be applied toward advancing experimental diagnostics and particle-shock wave modeling and simulation efforts for energetic materials.

Toward a Better Understanding of Shock Imprinting with Polymer Molds Using a Combination of Numerical Analysis and Experimental Research.

In the last decade, a new technique has been developed for the nanoimprinting of thin-metal foils using laser-induced shock waves. Recent studies have proposed replacing metal or silicone molds with inexpensive polymer molds for nanoimprinting. In addition, explosive-derived shock waves provide deeper imprinting than molds, greatly simplifying the application of this technology for mass production. In this study, we focused on explosive-derived shock waves, which persist longer than laser-induced shock waves. A numerical analysis and a set of simplified molding experiments were conducted to identify the cause of the deep imprint. Our numerical analysis has accurately simulated the pressure history and deformation behavior of the workpiece and the mold. Whereas a high pressure immediately deforms the polymer mold, a sustained pressure gradually increases the molding depth of the workpiece. Therefore, the duration of the pressure can be one of the conditions to control the impact imprint phenomenon.

Spallation damage of 90W–Ni–Fe alloy under laser-induced plasma shock wave

Laser shock loading is a more promising technology for investigating spallation damage in materials under shock-wave loading. In this paper, shock-induced spallation in a 90W–Ni–Fe alloy at an ultrahigh tensile strain rate of 106 s−1 is investigated using a superintense ultrafast laser facility. The spallation of the 90W–Ni–Fe alloy was dominated by a transgranular fracture of tungsten(W) particles with a high spall strength of 6.46 GPa. Here, we found an interesting phenomenon that the formation of nanograins inside W particles leads to a new mode of transcrystalline fracture of W particles during the laser shock loading. Futhermore, most voids were nucleated inside the W particles rather than at the W/γ-(Ni, Fe) matrix-phase interface. This result contradicts the fracture theory under quasi-static loading, which posits that the W/γ-(Ni, Fe) matrix-phase interface is not the preferred site for the initial failure under shock loading.

Defect detection of concrete in infrastructure based on Rayleigh wave propagation generated by laser-induced plasma shock waves

Defects in large concrete structures, including critical infrastructure such as bridges and tunnels, are traditionally detected by hammering tests by inspectors. However, the results depend on the inspectors’ skills. Additionally, hammering tests are impractical to thoroughly inspect massive structures in a short time. Herein defect detection in concrete structures is realized by telemetry using shock waves generated by laser-induced plasma (LIP). LIP is generated by irradiating a Nd:YAG pulsed laser near the concrete surface. As LIP spreads to the surrounding area at an ultrafast speed, a spherical shock wave is generated. This shock wave realizes a non-contact, non-destructive impulse excitation force on a concrete structure. The response of the concrete structure is measured with a laser Doppler vibrometer (LDV). Applying this method to detect defects in an artificial concrete specimen can identify the natural frequencies and modes of the defects. By measuring the propagation of the Rayleigh waves generated by the LIP shock waves, defects and the approximate location of the boundary between the defective part and the defect-free part are detected.

Investigation of an incremental dual Laser Powder Bed Fusion strategy for improving the quality of up-facing inclined surfaces

Additive manufacturing can generate parts with complex shapes such as industrial mould components. However, the resulting surface texture often does not fulfil the industrial requirements, thus requiring costly post-processing. This paper further develops a dual laser Powder Bed Fusion technique for in-situ improving the quality of inclined surfaces. It combines selective powder removal by laser-induced shock waves and subsequent laser remelting. This method is applied to a novel tool steel (M789) suitable for mould applications with the objective of up-scaling to steeper and larger surfaces. In the first step a design of experiments focused on the laser remelting process was carried out. In the second step the dual laser processing was applied at defined height steps during building, investigating the effect on the surface texture for inclinations up to 60°. Finally, aforementioned strategies were applied to manufacture a real mould component with significantly improved as-produced surface quality. Consequently, the traditionally required post-processing steps can be considerably reduced.

Dynamics of Laser-Induced Shock Wave and Cavitation During Ultrashort Laser Ablation of Aliphatic Nitroalkanes

Dynamics of Laser-Induced Shock Wave and Cavitation During Ultrashort Laser Ablation of Aliphatic Nitroalkanes

Two-Component Seedless Velocimetry Utilizing Laser-Induced Shockwaves

Two-Component Seedless Velocimetry Utilizing Laser-Induced Shockwaves

Spectro‐Microscopic Characterization of Elastomers Subjected to Laser‐Induced Shock Waves

AbstractThe core of this research is separated into three domains, the ultrahigh strain rate response of elastomeric polymers, laser‐induced shock waves , and terahertz time‐domain spectroscopy (THz‐TDS). Elastomers, e.g., polyurea, constitute an advance class of materials suitable for many applications, specifically in high impact loading scenarios, thus, a laser‐induced shock wave (LSW) experimental technique is used to investigate the mechanical response of shock‐loaded polyurea. LSW can submit samples to a strain rate exceeding 106 s−1 at low strains, enabling determination of material intrinsic failure modes. The large deformation induced during shock loading may alter the macromolecule structure, which can only be detected spectroscopically. Therefore, this research incorporated terahertz bulk spectroscopy to detect and report molecular conformational changes. Microscopy techniques were also used to elucidate changes in the microscale properties, morphology, and topography. The interpretation of the results explicated brittle failure in terms of partial and total spallation and, remarkably, ductile failure leading to plastic deformation, including plastic bulging and adiabatic shearing, not previously associated with LSW technique. Furthermore, spectral changes found in the terahertz regime substantiated the validity of terahertz spectroscopy in elucidating the underlying mechanism associated with the impact mitigating properties of dynamically loaded polyurea.

Hybrid dual laser processing for improved quality of inclined up-facing surfaces in laser powder bed fusion of metals

Additive manufacturing techniques such as laser powder bed fusion (LPBF) enable production of components with highly complex geometries. However, their as-built surface quality and geometrical precision remain insufficient for high-end applications, requiring further post-processing. Unlike horizontal surfaces, inclined up-facing surfaces cannot be improved by in situ remelting, as they remain covered with powder. In this research a dual laser setup replaces the traditional single-laser layout. Surface enhancement is achieved in two steps: a nanosecond pulsed laser is used to remove powder from selected areas via laser-induced shock waves (LISW), so that these surfaces can be remelted with a continuous wave laser. This work presents an investigation of in situ remelting (laser polishing) of inclined surfaces in their as-built orientation, after powder removal by brushing or using LISW. Particular focus is given on the effect of process parameters (laser power, scanning speed, number of scans) on the surface and subsurface quality and dimensional accuracy. As a result, this in situ processing leads to a comparable surface texture on horizontal and inclined surfaces, without introducing any major subsurface defects.

Numerical and experimental investigation of laser shock microhydroforming technology for microforming thin-walled LA103Z alloy foil with complex microfeatures

In this work, laser shock microhydroforming technology was further investigated. This technology employs liquid as the laser-induced shock wave transmission medium to microform a thin-walled LA103Z magnesium–lithium (Mg-Li) alloy foil with six bump features. A finite element model established by Hypermesh/LS-DYNA was used to illustrate the dynamic transmission of the liquid shock wave and the dynamic plastic deformation of the workpiece. The mechanism by which the liquid shock wave interacted with the closed liquid chamber to generate a homogenised shock wavefront was revealed. The dynamic loading form of the liquid shock wave and the characteristics of the microfeatures of the workpiece resulting from this dynamic loading form were also analysed mechanically. A comparison of the effects of liquid type and laser energy in this process showed that using castor oil as the liquid medium at threshold laser energy achieved the best plastic forming effect of the workpiece. Analysis of the thickness distribution of the workpiece revealed that the rupture was concentrated in the shear thinning region at the edge of the bump features caused by the transient large stress when the laser energy exceeded the threshold value.

Velocity measurements in a supersonic wind tunnel with a novel calibration-free seedless velocimetry utilizing laser-induced shockwaves and a double-line probe system

A novel calibration-free seedless velocimetry technique utilizing a shockwave generated by a laser-induced plasma (LIP) is proposed and used for measuring velocity profile in a supersonic flow. A nanosecond laser pulse of 532-nm wavelength is focused in the test section of a supersonic wind tunnel to generate the LIP that emanates a shockwave. A continuous-wave laser beam of 658-nm wavelength traverses the region adjacent to the LIP twice between a knife-edge mirror and a retroreflecting mirror. The probe beam is monitored by a fast photodiode after the second reflection from the knife-edge mirror, to record the arrival of the LIP shockwaves at the two probe beam locations; the laser beam deflects when the shockwave reaches the beam, and this deflection is registered on the photodiode signal read by an oscilloscope. It is shown that the shockwave-arrival time is highly sensitive to the convection speed of the flow that carries the shockwave. An empirical point-explosion blast model, determining time-dependent shockwave location, is used to calculate the average convection velocity between the LIP and the farthest probe beam 4.5 mm from the LIP with a precision of better than 8 m/s (1.5%). The empirical model contains two coefficients, a shockwave-propagation coefficient and the flow velocity, which are determined by the shockwave-arrival times and the locations of the two probe beams relative to the LIP. The LIP is positioned along a span-wise direction of the supersonic wind tunnel to measure the velocity profile; the probe beams are parallel with the spanwise direction. Uniform flow velocity is found within the core-flow of the wind tunnel, which well agrees with the velocity estimated by the pressure and temperature measured in the tunnel.

Hydrodynamic simulation of laser-induced shock waves using the Turbulence Problem Solver software package

The problem of irradiation of a semi-infinite bulk metal target by a single laser pulse, being important for many lase technology applications, is considered. The model for simulating the propagation and attenuation of shock waves, occurring when metals are irradiated with single laser pulse, is presented. The paper describes the computational algorithm based on the model and features of its implementation in C++ in a software package Turbulence Problem Solver (TPS) for modeling a wide class of problems described by hyperbolic equations. The problem is modeled using different approaches and the results are compared. The object-oriented polymorphic architecture of the software complex accelerates and facilitates the process of model development and calibration, allowing for flexible configuration and combination of elements of numerical models: Riemann solvers, various types of and reconstructions, and equations of state of matter.

Measurements of S0 mode Lamb waves using a high-speed polarization camera to detect damage in transparent materials during non-contact excitation based on a laser-induced plasma shock wave

The demand for transparent materials has been expanding due to their ubiquity in products such as solar panels, tablet terminals, and smartphones. To mass produce high-quality products, quickly detecting damage on the µm scale and evaluating the quality are critical. Herein an Nd:YAG pulsed laser with a nanosecond order is used to generate a shock wave by laser induced plasma, which is subsequently used as a non-contact, non-destructive excitation force for transparent materials. Then, a high-speed polarization camera measures the generated Lamb wave. In this experiment, an impulse input is generated via a laser-induced plasma shock wave and the phase velocity of the generated Lamb wave in the polycarbonate plate is measured by a high-speed polarization camera. We found that this Lamb wave was in the S0 mode. Observing its propagation can detect scratches on the order of several hundred µm on the surface of a transparent plate in a non-contact, non-destructive manner.