High-speed Camera Images Research Articles

Dynamic response and fracture analysis of basalt fiber reinforced plastics and aluminum alloys adhesive joints

Recently, the use of environmentally friendly natural fibers as reinforcement materials has attracted much attention to achieve lightweight structures. Basalt fiber reinforced plastic (BFRP) was widely used as an alternative material with low price and excellent performance. In this study, the effects of loading rates and different material properties on mechanical properties and fracture behavior of BFRP and aluminum (Al) single-lap adhesive joints were investigated. The fracture processes and strain evolution were recorded and analyzed by high speed camera and digital image correlation (DIC) technique. The microscopic appearance of fracture surface was also investigated to explore the effects of loading rates and material types on the failure modes. Results showed that all specimens were sensitive to loading rates. The average peak load increased by 50% at the speed of 5 m/s compared with quasi-static condition of 2 mm/min and BFRP/BFRP joints had higher strength than BFRP/Al joints. The stiffness of adherends influenced the failure behavior of the joints. Adhesive failure occurred at the edge of the substrate with large plastic deformation and the ratio decreased with the loading rates increasing. Fiber tear occurred in all BFRP adherends because of the increase in loading speed and the poor bonding between basalt fibers and matrix. Meanwhile, there was a corresponding relationship between the loading rates and the change of failure mode and stress distribution.

Microfluidic protein detection and quantification using droplet morphology

Sensitive, inline detection of proteins is required for post-chromatographic analyses in proteomics, cell-based assays, and drug discovery workflows. Among the common inline methods, post-column derivatization requires chemical labels, while label-free methods are either expensive (mass spectrometry) or have limited sensitivity at small length scales (UV–Vis). This paper presents a label-free detection technique based on the concept that dissolved proteins can function as surfactants and decrease the dynamic interfacial tension (IFT) of an immiscible (water–oil) interface. Existing methods for measuring IFT, such as axisymmetric drop shape analysis (ADSA), operate in batch mode and are not suitable for continuous detection. Here we show that a microfluidic flow-focusing droplet generator operating at a frequency of > 100 Hz can track IFT changes continuously, with high temporal resolution and small detection volumes. Variations in protein concentration alter the size and shape of the drops/plugs formed, and these changes can be quantified in time using a high-speed camera and in-house image processing software. Moreover, the continuously refreshing interface alleviates issues related to surface aging. Two applications are demonstrated: (1) direct injection of a single protein into a microfluidic chip. (2) post-column detection of protein mixtures separated by high performance size exclusion chromatography (SEC HPLC). Of interest, the dynamic range of protein (bovine serum albumin, BSA) was 50 -104 μg/ml without using HPLC unit. The lowest limit of detection without HPLC unit was ~ 1 μg/ml of thyroglobulin protein in a 1 nl droplet, which equates to 1 fg of total protein. When used as a detector, the aforementioned detection method offered a sensitivity of six orders of magnitude higher than conventional UV–VIS detectors.

A Contactless Approach to Monitor Rail Vibrations

Continuous welded rails (CWR) are subjected to thermal effects that may lead to buckling or fracture during warm or cold seasons, respectively. The modal characteristics (frequency and mode shapes) of CWR may reveal important information about the thermal stress that can be used to prevent rail failures. The primary objective of this study is to prove a contactless method to monitor the vibration and to extract the modal characteristics of rails using a high-speed camera and advanced image processing. This study is the first step towards a general noninvasive monitoring paradigm aimed at measuring axial stress in CWR. To prove the principles of the proposed paradigm, a finite element model of an unrestrained rail segment under varying length, boundary conditions, and axial stresses was formulated. The results of the model were then used to interpret the experimental results relative to a 2.4 m-long rail subjected to compressive loading–unloading cycles. During the experiment, the rail was subjected to the impact of an instrumented hammer and the triggered vibration was recorded with a high-speed camera. The videos were then processed using the phase-based displacement extraction, motion magnification, as well as dynamic mode decomposition techniques to extract the modal characteristics of the specimen. The results show that the frequencies extracted from the images matched well those obtained with two conventional accelerometers bonded to the rail while the mode shapes extracted from the videos matched those predicted numerically. Additionally, the numerical analysis enabled the interpretation of some unexpected experimental results. The results presented here proved that the proposed method to infer axial stress in CWR requires proper modeling in order to link the modal characteristics of the rails to the axial stress. In the future, the finite element formulation presented here will be expanded to model CWR under given cross-ties and fasteners conditions in order to link the modal characteristics of the rail of interest to its axial stress.

Numerical Study on Powder Stream Characteristics of Coaxial Laser Metal Deposition Nozzle

A 3D model was established to accurately simulate the internal and external powder stream characteristics of the coaxial discrete three-beam nozzle for laser metal deposition. A k-ε turbulence model was applied in the gas flow phase, and powder flow was coupled to the gas flow by a Euler-Lagrange approach as a discrete phase model. The simulated powder stream morphology was in good agreement with the experimental results of CCD and high-speed camera imaging. The simulation results showed that the length, diameter and shrinkage angle of the powder passage in the nozzle have different effects on the velocity and convergence characteristics of the powder stream. The influence of different particle size distribution and the inner laser shielding gas on the powder stream were also discussed in this study. By analyzing the powder stream caused by different incident directions of powder passage, and the collision process between powder and the inner wall, the basic principle of controlling powder stream convergence was obtained.

Alcohol-thermal synthesis of approximately core-shell structured Al@CuO nanothermite with improved heat-release and combustion characteristics

Metastable intermolecular composites (MICs), also called nanothermites, are attracting wide attention mainly due to the increased interfacial contact area of the fuels and oxidants therein. In this study, an alcohol-thermal technique is adopted to synthesize approximately core-shell structured Al@CuO MICs, in which process copper acetate is used as the precursor of CuO while Al powder with a wide size distribution (50 nm−5 µm in diameter) is used as cores. The energy-release characteristics of the materials are studied by performing thermal analysis, constant-volume combustion cell tests, and high-speed camera imaging of the combustion process. The results show that Al nanoparticles are surrounded by CuO nanoparticles that are with an average diameter of about 10 nm, while micron-sized Al particles are coated by a continuous layer of nanoplatelets that are assembled from CuO nanoparticles. Benefiting from the enhanced interfacial contact compared with that of ultrasonically mixed counterpart, Al@CuO shows a lower apparent activation energy of solid-state interfacial reaction, a higher light intensity, a shorter burning time, a larger pressure output, and a higher pressurization rate. It is also found that equivalent ratio has a great effect on the combustion process, and a slightly fuel-rich formulation is preferred in this study. The synthesis method developed in this study may also be adapted to the preparation of other core-shell structured MICs.

Mechanical properties and fracture behavior of flawed granite under dynamic loading

The dynamic mechanical properties and fracture process of rock materials with pre-existing flaws are crucial for explaining the failure mechanisms of deeply buried constructions. To investigate the mechanical properties and fracture behavior of flawed granite subjected to dynamic loading, dynamic experiments were conducted on granite specimens by using a split-Hopkinson pressure bar test system. The effects of strain rate and flaw inclination angle on granite strength and deformation properties were investigated. The white patch initiation, micro-crack propagation, and failure mode were analyzed using a high-speed camera and digital image correlation technology. The fracture mechanism was revealed by quantitatively analyzing the localized strain field and fractal features. The results demonstrated that the dynamic strength and deformation of the flawed specimen compared to an intact specimen had more obvious strain rate effects. The development of white patches determined the direction and path of crack propagation. The fracture process and shear-tensile failure mode were closely correlated with the evolution of the strain field. Furthermore, the fractal characteristics indicated that a larger fractal dimension resulted in a more complex fracture distribution and mixed failure modes. These results provide a reference for the construction design and safety control of deep rock engineering.

Stochastic Uncertainty in a Dam-Break Experiment with Varying Gate Speeds

Uncertainties inherent in gate-opening speeds are rarely studied in dam-break flow experiments due to the laborious experimental procedures required. For the stochastic analysis of these mechanisms, this study involved 290 flow tests performed in a dam-break flume via varying gate speeds between 0.20 and 2.50 m/s; four pressure sensors embedded in the flume bed recorded high-frequency bottom pressures. The obtained data were processed to determine the statistical relationships between gate speed and maximum pressure. The correlations between them were found to be particularly significant at the sensors nearest to the gate (Ch1) and farthest from the gate (Ch4), with a Pearson’s coefficient r of 0.671 and −0.524, respectively. The interquartile range (IQR) suggests that the statistical variability of maximum pressure is the largest at Ch1 and smallest at Ch4. When the gate is opened faster, a higher pressure with greater uncertainty occurs near the gate. However, both the pressure magnitude and the uncertainty decrease as the dam-break flow propagates downstream. The maximum pressure appears within long-period surge-pressure phases; however, instances considered as statistical outliers appear within short and impulsive pressure phases. A few unique phenomena, which could cause significant bottom pressure variability, were also identified through visual analyses using high-speed camera images. For example, an explosive water jet increases the vertical acceleration immediately after the gate is lifted, thereby retarding dam-break flow propagation. Owing to the existence of sidewalls, two edge waves were generated, which behaved similarly to ship wakes, causing a strong horizontal mixture of the water flow.

Experimental study on the effect of centrally positioned vertical baffles on sloshing noise in a rectangular tank

Liquid sloshing in fuel tanks has become a major source of noise in hybrid vehicles under various driving conditions. Flow dampening devices like baffles proved to be a good solution to control sloshing induced loads on the tank walls. But their effect on sloshing noise is yet to be understood clearly. This study provides insights into generation mechanisms of sloshing noise in a rectangular tank, in the presence of vertical baffles, in different sloshing regimes. Three types of vertical baffle configurations, placed at the center of the tank are considered for this study, which are immersed bottom-mounted, flush mounted and surface-piercing bottom mounted baffle configurations. Different sloshing regimes are created by applying longitudinal periodic excitation at various frequencies to the tank. The synchronized measurement of parameters like dynamic pressure on the tank walls and sound pressure level is done along with high-speed camera images of fluid motion. This study helps in understanding the effect of vertical baffles on sloshing noise, for various fill levels in different sloshing regimes.

Observation of frost growth on the metal surface with a high-speed thermal imaging camera

Observation of frost growth on the metal surface with a high-speed thermal imaging camera

Ignition of particles of finely dispersed fuel mixtures based on coal and fine wood

The purpose of this work is to experimentally determine conditions and characteristics of floating particles ignition of fuel mixtures based on coal dust with characteristic sizes of less than 80 μm and fine wood (typical sizes up to 200 μm) at various mass concentrations of fuel components. The experiments were conducted in a medium heated to high temperatures of still air (from 600 °C to 1200 °C). Record of the processes of heating of the researched fuel mixture that is quite promising for heat power engineering, their subsequent ignition and combustion was performed by a high-speed video camera (image format - 1024 × 1024 pixels, frame rate - up to 105 per second). It has been established that increase in the proportion of relatively large particles (compared to coal) of the wood component in the fuel mixture does not significantly affect the values of ignition delay times of all investigated wood-coal mixtures based on two typical grades of coal (long-flame and lean).

Experimental and Numerical Investigation of Gas-Focused Liquid Micro-Jet Velocity

Compressible multiphase numerical simulations of gas-focused micro-jets are compared with the experimental data obtained with the dual pulse imaging laser-induced fluorescence drop velocimetry. Such jets, originating from a 3D printed gas dynamic virtual nozzle into a low-vacuum (150 Pa) environment, are increasingly being used for sample delivery in serial femtosecond crystallography. The distance traveled by a detaching drop from the jet is measured between the two consecutive illumination pulses with a known time delay at the positions 200 µm and 450 µm from the nozzle. Additionally, the high-speed camera images are used to analyze the shape of the jet. An axisymmetric, compressible, Newtonian two-phase helium-water mixture model is numerically solved within the framework of the volume of fluid and the finite volume method. The experimental and the computational studies are performed with a constant volumetric liquid flow rate of 14 μl/min and the gas mass flow rate in the range from 4.6 mg/min to 20 mg/min. The related jet Reynolds number ranges from 120 to 220 and Weber number from 30 to 150. The maximum difference between the measurements and the results of the numerical model in terms of the droplet velocity and jet diameter is within 10 %. The study provides new information on the jet velocities for micron-sized gas-focused nozzles. The validated numerical model can be used as a design tool for the nozzles dedicated to the specific needs of the femtosecond crystallography experiments.

Towards system pressure scaling of gas assisted coaxial burner nozzles – An empirical model

The present study investigates the influence of system pressure, gas velocity, and annular gas gap width on the resulting droplet size. Three external-mixing twin-fluid atomizers are operated at a constant liquid mass flow. The nozzle geometry is kept similar, except that the annular gas gap width is changed. At every system pressure level (1 – 21 bar), three different gas velocities were investigated by changing the gas mass flow. High-speed camera images are used for observation of primary breakup and discussed with regard to local measurements of droplet size performed by a phase Doppler anemometer. The gas momentum flux as well as the gas momentum flow were applied to describe the atomization process under varying operating conditions. Finally, an empirical model is derived, enabling the system pressure scaling of external-mixing twin-fluid atomizers for the range of gas momentum flow under investigation.

Real-time Measurement of Projectile Velocity in a Ballistic Fabric with a High-frequency Doppler Radar

Because of technical limitations, most experimental studies on the energy-absorbing properties of ballistic fabrics are limited to discrete evaluations based on impact and residual velocities. Consequently, the continuous interaction between a projectile and a target material is still commonly assessed with analytical models or numerical simulations, the validation of which is based on the aforementioned discrete values. The present document aims at describing and validating a new experimental method to make it possible to evaluate the continuous evolution of the projectile velocity during penetration into a fabric material. The method is based on the Doppler effect and a specific and complex nonstationary signal treatment. A high-frequency continuous-wave Doppler radar was adapted to assess the continuous evolution of the velocity of a projectile penetrating a fabric material. Based on two ballistic-grade fabric configurations, a perforating and a nonperforating case were described and evaluated. The instantaneous Doppler frequency was extracted based on the Hilbert-Huang transform. A validation of the proposed method was performed based on high-speed camera images, giving the displacement of the apex of the deformation pyramid of the fabric with time. Additionally, a Weibel® Doppler radar was used to measure the impact velocity. Based on instantaneous frequencies deduced from the high-frequency radar signal analysis, Doppler theory and high-speed camera images, velocity–time and displacement–time plots were obtained. Additionally, the evolution of the fabric deformation (pyramid morphology) was recorded from the high-speed camera images. Comparisons between the data assessed with the high-frequency Doppler radar and those deduced from the high-speed camera indicated that good agreement exists between the two methods. The new Doppler radar method seems to be a promising complementary tool for measuring the continuous interaction between a projectile and a fabric target material.

Dynamic Metal Transfer Behavior in Double-Wire DP-GMAW of Aluminum Alloy Under Different Pulse Phases

Abstract The effects of pulse phase and pulse stage on the metal transfer characteristics in double-wire double pulse gas metal arc welding (DP-GMAW) of aluminum (Al) alloy were studied using high-speed camera images and current and voltage waveforms. In addition, the effects of various forces on dynamic metal transfer behavior were analyzed under different pulse phases and pulse stages. The results show that the spray transfer mode can be obtained in both the alternating pulse phase (APP) and synchronous pulse phase (SPP). The transfer pattern of the leading and trailing droplets is alternating in the APP, but changes to simultaneous metal transfer in the SPP, mainly owing to influence of the pulse phase on droplet growth. The transfer type is one drop double pulse (ODDP) during the strong pulse stage and one drop triple pulse (ODTP) during the weak pulse stage, regardless of the pulse phase. The pulse phase does, however, affect the Lorentz force between the leading and trailing droplets, causing droplet collision in the SPP, which results in a poorer weld bead appearance compared with in the APP. Finally, the droplet diameter was found to be similar during different pulse phases and pulse stages.

Dynamic compressive properties of Kalgoorlie basalt rock

In this study, the basalt rock extracted from the Kalgoorlie region of Western Australia is intensively studied on its compressive properties under both static and dynamic loads covering strain rate between 2.22 × 10-6/s to 408/s. The ultimate compressive strength and corresponding failure strain are quantified. The test results show that Kalgoorlie basalt rock exhibits high sensitivity to strain rate effect on its compressive strength especially above 100/s and dynamic increment factor up to 2.3 at strain rate 403/s. The failure strain also shows dependency to high strain rate. Discussion is made on fragment analysis which found the natural heterogeneous and anisotropic of WA basalt rocks cause variations on its compressive strength and dependent on the failure angle of the joints (layer formation). The dynamic increase mechanism on material compressive properties is observed to be correlated to the failure crack path formation, which can be explained through the fracture process captured from high-speed camera images analysis. Comparisons are also made on rock strengths with others’ test data. A novel method based on numerical modelling is introduced which removes the influence of lateral inertia effect and specimen end friction effect out of the laboratory testing results. The true dynamic increase factor (DIF) for Kalgoorlie basalt rock at different strain rates are derived for more accurate analysis and design.

Crack Propagation Velocity Determination by High-speed Camera Image Sequence Processing.

The determination of crack propagation velocities can provide valuable information for a better understanding of damage processes of concrete. The spatio-temporal analysis of crack patterns developing at a speed of several hundred meters per second is a rather challenging task. In the paper, a photogrammetric procedure for the determination of crack propagation velocities in concrete specimens using high-speed camera image sequences is presented. A cascaded image sequence processing which starts with the computation of displacement vector fields for a dense pattern of points on the specimen’s surface between consecutive time steps of the image sequence chain has been developed. These surface points are triangulated into a mesh, and as representations of cracks, discontinuities in the displacement vector fields are found by a deformation analysis applied to all triangles of the mesh. Connected components of the deformed triangles are computed using region-growing techniques. Then, the crack tips are determined using the principal component analysis. The tips are tracked in the image sequence and the velocities between the time stamps of the images are derived. A major advantage of this method as compared to the established techniques is in the fact that it allows spatio-temporally resolved, full-field measurements rather than point-wise measurements. Furthermore, information on the crack width can be obtained simultaneously. To validate the experimentation, the authors processed image sequences of tests on four compact-tension specimens performed on a split-Hopkinson tension bar. The images were taken by a high-speed camera at a frame rate of 160,000 images per second. By applying the developed image sequence processing procedure to these datasets, crack propagation velocities of about 800 m/s were determined with a precision in the order of 50 m/s.

Development of a deep learning-based image processing technique for bubble pattern recognition and shape reconstruction in dense bubbly flows

This work presents a Convolutional Neural Network (CNN) based method for the shape reconstruction of bubbles in bubbly flows using high-speed camera images. The bubble identification and shape reconstruction adopted a methodology based on a set of anchor points and boxes, where a single anchor point is used for different anchor boxes with various sizes. These anchor points are determined, based on the internal features of the bubble images, which are more easily identifiable, in particular, in regions of the images with high bubble overlapping. This makes possible the application of the procedure to high void fraction bubbly flows. For a given anchor point, different ellipsoidal shapes are suggested as bubble shape candidates and are then correctly chosen by a trained CNN. The CNN training used labeled images from air–water system data set and a hyper-parameter analysis was performed to find the best configuration of the CNN architecture. From this optimal CNN architecture, different high-speed camera acquisitions of bubbly flows were analyzed by the CNN-based bubble shape reconstruction method. In order to gain a better comprehension of the method, experiments were conducted in two gas–liquid systems, air–water and air-aqueous glycerol solution, which resulted in different image parameters, such as brightness, contrast and edge definition. The CNN method trained only with air–water data, showed excellent performance in the cases with air-aqueous glycerol, demonstrating its generalization capability. In addition, the results showed that the deep learning method used in this work is able to detect most of the bubbles present in the high-speed camera images, even in dense bubbly flow configurations. The method developed in this work can be used to further analyze bubbly flows and generate experimental data for the implementation and validation of CFD models.

Morphology and flow behavior of buoyant bubble plumes

The morphological and hydrodynamic behavior of bubbles in air–water plumes was studied by employing Particle Tracking Velocimetry (PTV) techniques. Individual bubbles were identified by analyzing the instantaneous high-speed camera images of the plume. The bubble velocities were then obtained by Lagrangian tracking of each bubble, based on its size and location. The nature of bubble coalescence and breakup was also investigated based on the change in the projected area of the parent and daughter bubbles. The effect of the bubble position and the air inflow rate on the bubble properties like diameter, aspect ratio, velocity, number flux, etc., and on the coalescence and breakup behavior was investigated. A novel correlation is proposed to estimate the bubble aspect ratio as a function of bubble Reynolds number and Eotvos number, for a wide range of flow conditions, and is compared with existing correlation from the literature.

Experimental study of the flow structure around Taylor bubbles in the presence of dispersed bubbles

This work presents an experimental study focusing on the interactions between the dispersed and Taylor bubbles in a slug flow pattern. In order to better investigate these interactions, a quasi-real slug flow was studied, where a single Taylor bubble is injected in a bubbly flow background stream, under controlled and repeatable conditions, controlling the bubbly flow superficial gas and liquid velocity, and also the Taylor bubble length. In this way, ensemble averaged results can be obtained over several Taylor bubbles. The flow was analyzed through the laser diode photocell, high-speed camera imaging and PIV techniques. Results revealed that the terminal Taylor bubble velocity is affected by the gas volume fraction of the bubbly background flow. Ensemble average velocity profiles are presented for Taylor bubble nose and tail regions. Since the experimental apparatus used in the present work allowed a large number of instantaneous PIV acquisitions, the turbulent statistics around the Taylor bubble can also be calculated. These experimental results can be used for the implementation and validation of multidimensional CFD models for flows with different interface length scales.

A Simplified Numerical Approach to Examine the Sensitivity of Two-Electrode Capacitance Sensor Orientation to Capture Different Gas-Liquid Flow Patterns in a Small Circular Pipe.

This work involved the simulation of both a multiphase gas–liquid flow and the electromagnetic field representing a two-electrode capacitance sensor in a circular pipe. The simulation investigates in particular the sensitivity of the sensor orientation around the pipe (i.e., top-to-bottom or side-to-side) that best capture the induced flow patterns. The presented numerical work is a simplified simulation by COMSOL multi-physics which was validated by a systematic and an extensive experimental work, using (a) a specially designed simple capacitance sensor (i.e., concave two electrodes), (b) different gas–liquid superficial velocity combinations, (c) different flow section inclinations, and (d) high-speed camera images. The numerical modelling capacitance values were validated against the experimentally measured values which shows a satisfactory level of agreement with a deviation of less than ±2%. The quantity of finite points was between 280,000 and 340,000, which was influenced by the simulated flow pattern. The simulated cases captured the generated flow patterns and their variation inside the pipe, which was in a good agreement when compared to the experimental work as time-dependent values. It was found that the best orientation for the utilized two-electrode capacitance sensor was the top-to-bottom configuration. This is because the sensor’s electrical field distribution was more sensitive, and capable of detecting a greater range of capacitance values. The sensitivity of the top-to-bottom configuration was 1.25–1.64 times greater than that of the side-to-side configuration. Therefore, for design purposes and performance optimization, it is recommended to use the top-to-bottom configuration.