High-speed Camera Images Research Articles

Investigation on the formation mechanism of collapse defects, microstructural evolution and mechanical properties in full penetration laser welding of thick-section steel

In this paper, we described full penetration laser welding (FPLW), a process capable of achieving single-pass welding of thick-section steel. This work summarized a welding process window encompassing various typical formations, including those well-formed and collapse defects. Furthermore, collapse defects were categorized as “collapse formation with backside humping” and “collapse formation with underfill”. High-speed camera (HSC) images reveal that the formation mechanisms of these two defects are attributed to the accumulation of liquid metal at the rear edge of the bottom molten pool and the ejection of a large amount of liquid metal from the weld in a spatter form, respectively. A computational fluid dynamics (CFD) numerical simulation model well-aligned with the experimental results has been established. The analysis of the molten pool flow behavior and keyhole evolution revealed that during well-formed welding, the keyhole status in the welding process exhibits a periodic closure-collapse mode. This periodic mode can instantaneously release the pressure at the bottom of the keyhole, inhibiting the excessive accumulation of liquid metal at the bottom of the weld and thus inhibiting the formation of collapse defects. The cooling rate in the upper part of the weld exceeds that in the middle and lower parts, primarily resulting in the formation of fine acicular ferrite and martensite. The increased tensile strength exhibited by the welded joint relative to the base material predominantly emanates from the mechanisms of grain boundary reinforcement and dislocation strengthening, precisely caused by the refined microstructure.

Experimental investigation of wet particle flow characteristics in a pseudo two-dimensional fluidized bed

The flow characteristics of wet particles in a fluidized bed are extremely complicated due to the presence of moisture. It is essential to understand the complex behavior and flow characteristics of wet particles in the drying, coating and granulation process for optimizing the process efficiency and product quality. Thus, the flow characteristics of wet particles, such as pressure fluctuation, bubble size and bubble fraction, in a pseudo two-dimensional fluidized bed were investigated through the multi-resolution time-frequency analysis of the pressure signal combined with the high-speed camera images. Results show that the critical fluidization velocity increases after the particles are wetted, and it shows a slight increase with higher moisture content levels. Besides, there are two main characteristic frequencies, which correspond to the gas-solid flow state of the gas channel and the large bubble in the bed, respectively. Notably, higher gas velocities promote the dominance of large bubbles. And increasing particle moisture inhibits bubble expansion and reduces bubble size, while dense particle areas expand vertically due to enhanced bonding. The bed height increases with the increase of moisture, and the fluctuation of bubble fraction decreases. In the drying experiment, two characteristic frequencies also appear and the pressure drop reduce slightly due to the reduction of water. During the drying process, both the average bubble fraction and fluctuation intensity increase.

Effect of intended shift in propagation direction of cavitation generation position on coagulation position in bubble-enhanced ultrasonic heating

High-intensity focused ultrasound (HIFU) is a noninvasive cancer treatment that uses powerful focused ultrasound energy to coagulate tissue. However, due to its small focal region, this treatment is time-consuming. Therefore, we have focused on the heating effect of cavitation bubbles. The safe utilization of bubbles requires that they are consistently generated and maintained in the treatment area. In this study, the effects of the HIFU focal shift in the direction of ultrasound propagation on the bubble and coagulation regions were investigated by high-speed camera and ultrasound imaging. As a result, the appropriate focal shift of pulses that generates the bubbles prevented expansion of bubble regions and generated coagulation regions with good reproducibility near the treatment target.

Squeezing Droplet Formation in a Flow-Focusing Micro Cross-Junction.

Motivated by the increasing need of optimised micro-devices for droplet production in medical and biological applications, this paper introduces an integrated approach for the study of the liquid-liquid droplet creation in flow-focusing micro cross-junctions. The micro-junction considered is characterised by a restriction of the channels cross-sections in the junction, which has the function of focusing the flow in the region of the droplet formation. The problem is studied numerically in the OpenFOAM environment and validated by a comparison with experimental results obtained by high-speed camera images and micro-PIV measurements. The analysis of the forces acting on the dispersed phase during the droplet formation and the diameter of the droplets obtained numerically are considered for the development of a model of the droplet breakup under the squeezing regime. On the basis of energy balancing during the breakup, a relation between interfacial tension, the size of the cross-sections in the junction, and the time interval needed for droplet creation is obtained, which yields a novel correlation between the dimensionless length of the droplet and the dimensionless flow rate. This research expands our knowledge of the phenomenon of drop creation in micro-junctions with restrictions providing new aid for the optimal design of micro-drop generators.

A new rapid-release SMA-activated micropump with incorporated microneedle arrays and polymeric nanoparticles for optimized transdermal drug delivery

Recent research in Transdermal drug delivery (TDD) systems has predominantly focused on microneedles (MNs) and nanoencapsulated drugs, which offer painless, gradual drug release but may not cater to quick-release drug demands. This study introduces an innovative approach to expedite the TDD process by merging a new rapid-release micropump, MNs, and nanoencapsulation into a modular framework. The fabrication and evaluation of a 3D printed shape memory alloy (SMA)-activated micropump combined with ibuprofen-loaded polymeric nanoparticles delivered via integrated MNs is thereby discussed. The system utilizes an SMA spring wire to actuate a loaded spring which drives an elastic membrane into a chamber integrated with an array of stereolithography (SLA) 3D printed MNs, facilitating painless injection into the dermis layer of the skin. The MNs array is optimized with finite element analysis (FEA) to minimize discomfort from the needles while maximizing the drug load into the skin. The system’s performance is characterized through high-speed camera imaging, and an average membrane speed of 0.28 m/s is developed. The system’s effectiveness is finally evaluated via a short time frame (5 minutes) ex vivo skin deposition experiment using Franz diffusion cells and rat skin, revealing almost twice the ibuprofen deposition in skin layers with polymeric nanoparticles compared to an untreated solution. The rapid response and demonstrated efficacy of this concept aims to provide a framework for potentially extending this application for transdermal insulin injection.

Flame morphology of hydrous ethanol combustion under EGR dilution for port fuel injection and direct injection in a spark ignition optical engine

The increasing popularity of plug-in hybrid vehicles has prompted investigation of options such as range extender units, which may include small engines powered by biofuels. To minimize energy consumption, the best technologies must be chosen, including the use of exhaust gas recirculation (EGR, as a charge dilution technique) and the best alternative for fuel injection systems. Therefore, a combustion evaluation was conducted on an optically accessible engine fueled with hydrous ethanol to determine the effects of different injection modes, such as direct injection (DI) and port fuel injection (PFI), combined with EGR. The study employed high-speed camera imaging to analyze flame morphological characteristics and understand their impact on in-cylinder thermodynamics and engine emissions. The DI mode presented more stability than PFI, although the dilution limit was lower for DI.

Ultrahigh-speed imaging for high-impact concrete deformation analysis in pre- and post-cracking stages.

The evaluation of high-speed camera image sequence analysis results in concrete material testing under high-impact loading necessitates the consideration of the effect of the image quality on the measurement accuracy and thus on the potential of the geometric measurements derived from the image sequences. In this contribution, we evaluate the application potential of three ultrahigh-speed cameras with frame rates up to 10Mfps to analyze the deformation of concrete specimens before and after main crack formation in bending and compression tests. Specifically, we evaluate the Kirana 7M and Shimadzu HPV-X2 cameras with ISIS sensor architecture, and the Phantom TMX 7510 camera with BSI CMOS sensor technology. Three-point bending tests and split-Hopkinson pressure bar tests are performed on 160×40×40m m 3 cuboids and on 80mm long, 50mm diameter cylinders. Prior to main crack formation, the displacement vector field represents the specimen deformation, with higher values indicating the position where main cracks will initiate and propagate. Deformations of 80µm in 54µs for a bending test and of 154µm in 36.67µs for a compression test could be measured. The main cracks are then detected using displacement vector field discontinuity analysis techniques, and their evolution is followed to estimate the crack propagation velocity. Average velocities in bending tests between 603 and 854m/s have been determined over a time interval up to 40µs. An investigation of the camera sensor operation of the three optical devices is presented to assess their suitability for deformation analysis. Laboratory tests and real experimental results show that the quality of the propagation vector field, the crack detection, and the crack tip tracking are obviously affected by the image quality, but more significantly by the spatial and temporal resolution due to the small relative step deformations.

Stability Analysis of Metal Active-Gas Welding Short-Circuiting Transfer Based on Input Pulsating Energy.

In this study, a platform for a welding experiment, used to collect input and output electrical signals, was constructed, and the algorithm for the input pulsating energy interpolation line (IPEI) was given. Experiments with MAG surface straight line welding were conducted at various voltages. Analysis of the IPEI in relation to the welding current was performed while combining real-world welding occurrences with high-speed camera images of droplet transfer. It was established that the IPEI can be employed as a characteristic parameter to assess the stability of the short-circuiting transfer process in MAG welding. The three criteria for assessing the stability were the spectrum, approximation entropy, and coefficient of variation. A comparative analysis was conducted on each of these approaches. It was determined that the most effective technique is approximation entropy. The approximation entropy of the welding current and IPEI are also highly consistent, with a correlation coefficient as high as 0.9889.

Trumpet-shaped diffuse plasma jet in a semi-confined environment

Abstract This paper reports a unique trumpet-shaped diffuse plasma jet (TDPJ) with a caliber exceeding 30 mm in a semi-confined environment, which is larger than the diameter of existing atmospheric pressure plasma jets (APPJs) in an open environment. The morphology of the TDPJ is attributed to the influence of the electric field on the development characteristics of ionization waves in the ambient environment. High-speed ICCD camera images show that the plasma bullets in the TDPJ have a completely different propagation trajectory compared to the APPJ. Different shapes of diffuse plasma jets can also be formed by adjusting the doping gas, which is due to the Penning ionization and consistent with the impact of the electric field. This research reveals the long-neglected effect of the electric field on ionization waves and provides a way to manipulate the morphology of plasma jets efficiently. The generation mechanism of the TDPJ can provide preliminary guidance for the effective medical treatment of plasma jets in semi-confined environments, such as human internal organs.

HIGH-SPEED VIDEO IMAGE ANALYSIS OF LIQUID METAL ATOMIZATION PROCESS

Gas-assisted atomization is used to produce metal powders for additive manufacturing. In these systems the primary atomization governs the powder size distribution, but it is experimentally challenging to investigate. In this study, the primary atomization of a molten metal spray was monitored with a calibrated high-speed camera and digital image processing. The technique enabled the generation of spatio-temporal maps at different distances from the nozzle. Among the characteristics of the spray, mean values and standard deviation of the spray width along the spraying direction, and the frequency of the oscillation of the spray boundaries are presented. Statistically relevant spray asymmetries were identified, which can be caused by faulty spray operation in form of a localized metal solidification at the pouring nozzle. By systematically varying the gas temperature and the gas atomization pressure, it is shown that heating of the gas up to 573 K stabilizes the spraying process and enhances the liquid dispersion in the recirculation zone.

Effect of Phase-Change Nanodroplets and Ultrasound on Blood-Brain Barrier Permeability In Vitro.

Phase-change nanodroplets (PCND;NDs) are emulsions with a perfluorocarbon (PFC) core that undergo acoustic vaporisation as a response to ultrasound (US). Nanodroplets change to microbubbles and cavitate while under the effect of US. This cavitation can apply forces on cell connections in biological barrier membranes, such as the blood-brain barrier (BBB), and trigger a transient and reversible increased permeability to molecules and matter. This study aims to present the preparation of lipid-based NDs and investigate their effects on the brain endothelial cell barrier in vitro. The NDs were prepared using the thin-film hydration method, followed by the PFC addition. They were characterised for size, cavitation (using a high-speed camera), and PFC encapsulation (using FTIR). The bEnd.3 (mouse brain endothelial) cells were seeded onto transwell inserts. Fluorescein with NDs and/or microbubbles were applied on the bEND3 cells and the effect of US on fluorescein permeability was measured. The Live/Dead assay was used to assess the BBB integrity after the treatments. Size and PFC content analysis indicated that the NDs were stable while stored. High-speed camera imaging confirmed that the NDs cavitate after US exposure of 0.12 MPa. The BBB cell model experiments revealed a 4-fold increase in cell membrane permeation after the combined application of US and NDs. The Live/Dead assay results indicated damage to the BBB membrane integrity, but this damage was less when compared to the one caused by microbubbles. This in vitro study shows that nanodroplets have the potential to cause BBB opening in a similar manner to microbubbles. Both cavitation agents caused damage on the endothelial cells. It appears that NDs cause less cell damage compared to microbubbles.

Research on molten pool flow characteristic in all-position LP-LMH welding

ABSTRACTThe flow characteristics of keyhole and molten pool impact the real-time process stability and seam formation severely in all-position low-power laser MAG hybrid (LP-LMH) welding. To probe into the molten pool’s real-time dynamics at various spatial welding positions, the high-speed camera images of metal transfer and molten pool were sensed. Meanwhile, a novel spatial flow model of molten pool was developed, and the functions of spatial position and gravity on the molten pool flow were investigated. The results proved that as the spatial position increases, the location of where droplet striking molten pool changes accordingly, further disturbing the keyhole’s stability. Meanwhile, the spatial position seriously affects the molten pool’s flow. The driving forces of molten pool flow at various spatial welding positions are basically the same, and gravity affects the overall movement of molten pool, thereby affecting the vortexes and heat transmission inside the molten pool, as reflected in weld penetration.

Numerical modelling of high-power laser spot melting of thin stainless steel

Numerical modelling is an important scientific tool in laser materials processing to study different melting, evaporation, and solidification phenomena. It can assist in understanding why certain defects are formed, and thus may provide solution paths for how to decrease certain defects and to increase the quality of a product. Laser spot melting in heat conduction mode represents a simple case and is an excellent first step to build and test solvers prior to moving on to more complicated cases such as laser keyhole welding of thick plates. Modelling of laser spot melting requires only a limited computational domain and allows more complex physics to be added gradually. In this work, a thin 3.0 mm thick stainless steel plate was melted with high-power fiber laser and numerically simulated using a native and custom-build solver based on the VOF method within OpenFOAM. The process was captured with a high-speed imaging camera and simulation results are compared with the experimental results. It was found that temperature-dependent surface tension plays a vital role in controlling melt flow directions within melt pool.

Artificial intelligence methods for in-process high-speed image analysis in laser beam welding of hairpins

In the production of modern electric drives for battery electric vehicles, hairpin technology is used to increase the copper fill factor in the stator of a permanently excited synchronous machine. A central process in the production of these stators is the contacting of the hairpin ends by means of laser beam welding. This welding process is characterized by geometric and process-related deviations from previous process steps, which influence the result of the welded joint. It is desirable to find an in-process method for monitoring. As part of the process monitoring of welded joints, high-speed camera images are often used to detect weld spatter. These can be detected by a program based on a static algorithm. For this reason, a feasibility analysis is performed regarding the application of AI for the detection of spatters, in which the methods of semantic segmentation and single-image classification prove to be useful. In a preliminary experiment, three base networks for each of the two methods are evaluated with respect to the best training results. The single-image classification method will then be extended by a subsequent static algorithm, so that a hybrid use of AI and static algorithm will be investigated. The evaluation and final comparison of all evaluation methods is performed using data from a welding experiment. It turns out that the hybrid approach of single-image classification and static algorithm has numerous advantages in the detection of spatter compared to semantic segmentation and the static algorithm.

Experimental and numerical study of disk specimen with rough structural plane under splitting test

To study the effects of the anisotropic matrix and structural planes on the splitting strength and failure mode of rocks, Brazilian splitting tests were carried out with seven different loading angles on specimens of rock-like materials with rough structural planes. The surface strains of the samples during the failure process were monitored and analysed with the help of a high-speed camera and digital image correlation (DIC) technology. The test results showed that the Brazilian splitting strength (BSS) decreased gradually with an increased loading angle. According to the crack morphology, the samples showed three failure modes, and the structural plane and the loading angle (θ) had an important effect on the failure mode. When θ < 75°, the sample failure was mainly affected by the matrix, and when θ > 75°, the sample failure was mainly controlled by the structural plane. The numerical simulation of the sample with a structural plane was carried out by the PFC2D particle flow program, the micro parameters were calibrated using a back propagation (BP) neural network model. The internal cracks of the sample under a splitting load were mainly matrix tensile microcracks and structural plane shear microcracks, and the tensile microcracks in the side with the weak matrix appeared significantly earlier than those in the side with the strong matrix. With increasing loading angle, the proportion of tensile microcracks in the matrix increased, while the proportion of shear microcracks in the matrix decreased, especially in the weak matrix. The microcracks at the structural plane mainly changed from tensile microcracks to shear microcracks, and the development degree of microcracks along the structural plane was more significant than that on the weak matrix with increasing loading angle. The results of the study can provide a reference for rock stability evaluation and utilization.

Tungsten dust injection into the STOR-M tokamak

Controlled tungsten (W) dust injection experiments have been performed in the STOR-M tokamak. Tokamak discharges were initiated after a known delay time with respect to the injection of W micro-spheres. The plasma parameters were detrimentally affected, proportional to an increased amount of dust within the plasma volume. High speed camera imaging of the injected dust revealed that the dust particles experience a strong force in the counter-plasma current direction. This force decreased for high dust number density due to the associated decline of plasma parameters.

Experimental study on turbulent characteristics of dispersed bubbly flow in a narrow rectangular channel by two-phase PIV

Narrow rectangular channels are widely used in high heat flux applications, due to the high heat transfer efficiency and compact structure. However, a rare understanding was known about the two-phase turbulent characteristics. In this study, an experimental investigation of bubble and turbulent characteristics of dispersed bubbly flow in the narrow rectangular channel was carried out by using the high-speed camera and two-phase particle image velocimetry. The bubble size effect was studied independently using the continuous spectrum bubble generator. According to the results, the bubble size distribution conforms to a Weibull distribution, and the wall-peak void fraction distributions were found. Both turbulent enhancement and suppression were observed in current conditions. Large bubbles enhance turbulence intensity, while small bubbles suppress it. The eddy viscosity ratio ϕ characterizes the modification of turbulence intensity by bubbles, with a critical value of ϕ=1.

Sidewall rockburst of highly stressed D-shape hole tunnel triggered by impact load: An experimental investigation

This study aims to clarity the failure mechanism of sidewall rockburst of highly stressed D-shape tunnel triggered by impact load. Using the biaxial Hopkinson pressure bar (BHPB) system, we have developed experimental capabilities to study the sidewall rockburst of D-shape tunnel by applying various prestresses, including horizontal and vertical static stresses, to sand prefabricated D-shape hole specimen, followed by impact loads. High-speed (HS) camera and digital image correlation (DIC) were used to tracked the process and strain field of the sidewall rockburst. The test results reveal that the process of sidewall rockburst can be summarized as: calm stage, crack initiation, propagation, and coalesce stage, spalling stage and rock fragments ejection stage. During the rockburst process, the surrounding rock experienced spalling and violent ejection, involving both tensile and tensile-shear failure. The mechanism of sidewall rockburst under the coupled of static stress and impact loads has been elucidated, i.e., the static prestress determines the initial stress and strain distribution, and the horizontal prestress influences the affected range and strain values of strain concentration zone; the impact load disrupts the original static stress equilibrium, inducing alterations in the stress and strain of the surrounding rock and triggering sidewall rockburst.

Experimental study on passive plate pitching and coherent structures in a square channel by TR-PIV

The interaction of plate pitching and vortex may damage the accumulator isolation passive valve in nuclear power plant. The plate pitching and turbulent flow in a square channel were studied by high-speed camera (HSC) and time-resolved particle image velocimetry (TR-PIV) at Reynolds numbers (Re) from 1474 to 58952.The plate angle captured by HSC were analyzed by edge detection algorithm. The plate is inclined in hydraulic equilibrium without oscillation when Re is less than 14738. The plate pitching start to be periodic at Re = 22107, and then the periodicity weakens with Re increasing from 22,107 to 51583. The time-averaged pitching angle increases from 0.08467 rad to 1.454 rad with Re increasing from Re = 1474 to Re = 51583. The standard deviation of pitching angle increases from zero with Re less than 14,738 to 0.1294 rad at Re = 22107, and then it decreases to 0.006536 rad at Re = 51583. The peak frequency of plate pitching is the same as that of turbulent flow extracted from the power spectral density of instantaneous velocity downstream of the pitching plate, indicating the passive plate pitching is determined by turbulence. The plate pitching is driven by lift force torque due to the leading-edge-vortex shedding from the pitching plate.The interaction between passive plate pitching and fluid generates complicated turbulence. The time-averaged flow structures are typical shear-flow due to small plate inclination angle without oscillation when Re is less than 14738. The Reynolds stresses are highly enhanced in the shear flow. At Re = 22107, the time-averaged flow structures behave as vortex bubble over the plate and shear flow surrounding the plate, and the Reynolds stresses downstream of the plate are obviously enhanced by shedding vortices from the plate. With Re increasing from 22,107 to 58952, the time-averaged vortex bubble becomes smaller and shifts downwards to the plate trailing end, while the Reynolds stresses with similar distribution patterns decrease fast. The peak frequency of vortex shedding increases with Re increasing. The large spatial and temporal length scales are induced by shear flow at low Re less than 14738. The large length scales are dominated by von Kármán street shedding from the plate at Re from 22,107 to 58952, which are identified by proper orthogonal decomposition (POD) analysis, because the first two modes are the same mode of von Kármán street with a phase difference.The experimental work improves understandings of plate-fluid-interaction of the isolation passive valve and supports validations of numerical simulations.

Microplastic and natural sediment in bed load saltation: Material does not dictate the fate.

Microplastic (MP) pollution is a well document threat to our aquatic and terrestrial ecosystems, however, the mechanisms by which MPs are transported in river flows are still unknown. The transport of MPs and natural sediment in aquatic flows could be somewhat comparable, as particles are similar in size. However, it is unknown how the lower density of MPs, their shape and their different material properties impact transport dynamics. To answer this, novel laboratory experiments on bed load saltation dynamics in an open-channel flow, using high-speed camera imaging and the detection of 11,035 individual saltation events were used to identify the similarities and differences between spherical MPs and spherical natural sediments transport. The tested MPs and sediment varied in terms of size and material properties (density and elasticity). Our analysis shows that the Rouse number accurately describes saltation length, height, transport velocity and collision angles equally well for both MPs and natural sediments. Through statistical inference, the distribution functions of saltation trajectory characteristics for MPs were analogous to natural sediment with only one sediment experiment (1.4% of cases) differing from all other plastic experiments. Similarly, only nine experiments (9.3% of cases) showed that collision angles for MPs differed from those of natural sediment experiments. Differences observed in terms of restitution become negligible in overall transport dynamics as turbulence overcomes the kinetic energy lost at particle-bed impact, which keeps particle motion independent from impact. Overall, spherical MP particles behave similarly to spherical natural sediments in aquatic environments under the examined experimental conditions. This is significant because there is an established body of knowledge for sediment transport that can serve as a foundation for the study of MP transport.