Location Of Voids Research Articles

Ballastless track mortar layer void detection by high-order statistical analysis of axle box acceleration

A scheme based on high-order statistical analysis of vertical axle box acceleration (ABA) is proposed to locate and identify mortar voids in ballastless tracks quickly. The ABA signal near a void area contains abnormal non-Gaussian components, but are easily buried by random noise. A 4th order cumulative time-varying kurtosis difference (TKD-cum4) method is designed to locate void areas. Twenty-four edge-type and concealed-type mortar void cases are identified and classified by bispectral analysis of ABA signal segments. The measured and simulated data verify the proposed scheme's CAM void location and recognition effect. The results show that the maximum void area location error along the railway line is 0.137 m. The recognition accuracy of unidirectional void level is 92%. The simultaneous recognition rate of void type and damage degree is nearly 90%. The good noise resistance of the proposed detection scheme is verified.

Morphology Engineering in Multicomponent Hollow Metal Chalcogenide Nanoparticles.

Hollow metal chalcogenide nanoparticles are widely applicable in environmental and energy-related processes. Herein, we synthesized such particles with large compositional and morphological diversity by combining scanning probe block copolymer lithography with a Kirkendall effect-based sulfidation process. We explored the influence of temperature-dependent diffusion kinetics, elemental composition and miscibility, and phase boundaries on the resulting particle morphologies. Specifically, CoNi alloys form single-shell sulfides for the synthetic conditions explored because Co and Ni exhibit similar diffusion rates, while CuNi alloys form sulfides with various types of morphologies (yolk-shell, double-shell, and single-shell) because Cu and Ni have different diffusion rates. In contrast, Co-Cu heterodimers form hollow heterostructured sulfides with varying void numbers and locations depending on synthesis temperature and phase boundary. At higher temperatures, the increased miscibility of CoS2 and CuS makes it energetically favorable for the heterostructure to adopt a single alloy shell morphology, which is rationalized using density functional theory-based calculations.

Evaluation on ventilation and traffic pollutant dispersion in asymmetric street canyons with void decks.

With continuous global warming, growing urban population density, and increasing compactness of urban buildings, the "void deck" street canyon design has become increasingly popular in city planning, especially for urban streets located in tropical areas. Nevertheless, research on traffic pollutant dispersion in street canyons with void decks (VDs) is still at its early stage. This study quantitatively evaluates the effects of void deck height and location on the canyon ventilation and pollutant dispersion in asymmetric street canyons with void decks, and the pollutant exposure risk level for pedestrians and street dwellers. Void decks introduce more fresh air, thereby greatly improving the ventilation properties of the asymmetric canyon. The air exchange rate (ACH: 147.9%, 270.9%) and net escape velocity (NEV*: 416.7%, 915.8%) of the step-up and step-down canyons with VDs (3m high at full scale) at both buildings are higher than those of regular asymmetric canyons. Moreover, the mean dimensionless pollutant concentration (K) on the building wall and pedestrian respiration plane in which VDs are located stands at a low level, because pollutants are removed by the airflow entering or exiting through the void decks. Increased VD height (4.5m at full scale) enhances the strength of airflow flowing into and out of the canyon, significantly increasing ACH (177.3%, 380.9%) and NEV* (595.2%, 1268.4%) and decreasing the mean K on both pedestrian respiration planes and canyon walls. In particular, the K values on both pedestrian respiration planes and both walls are almost zero for the canyons with VDs at both buildings. Therefore, among the three VD locations, both VDs provide the best living environment for pedestrians and near-road residents. These findings can help to design urban street canyons for mitigating traffic pollution risk and improving ventilation in tropical cities.

On cavitation in the radial flow of a thin lubricating film between two overlying disks

This study is focused on the characterization and modeling of aviation fuel cavitation physics in radial flow in a thin layer between two disks—a geometry highly relevant to aviation fuel pump systems. This involves a lower circular disk with a centrally located fuel injection port and a matching disk resting on top of the lower disk. In the described experiments, we have quantified and compared cavitating disk behavior for distilled water and JP-5 fuel at various inlet supply pressures via high-speed imaging and radial pressure measurements. High-speed imaging data were used to quantify the radial collapse location of cavitation voids. An enhanced gradient shadowgraphy method was employed to obtain the spatial–temporal evolution of propagating bubbly shock waves. This technique revealed unsteady shock waves propagating in a spiral motion in JP-5 fuel, while a standing bubbly shock was observed in water. In our modeling efforts, the Rayleigh–Plesset equation was adapted to a spatial form in order to predict the radial location of cavitation bubble collapse. Further work incorporated the spatial Rayleigh–Plesset equation into the barotropic model that has been used previously for cavitating nozzle flows and generalized it to radial flow geometry in order to reproduce radial pressure profiles obtained from the experiment. The model predictions of the radial location of bubble collapse and the radial pressure profiles are shown to be in excellent agreement with the experiments. This approach will be valuable for predicting aviation fuel cavitation in complex fuel system geometries.

Impact of processing defects on microstructure, surface quality, and tribological performance in 3D printed polymers

Additive manufacturing (AM), also known as three-dimensional (3D) printing, of polymer-based materials is growing as a time-efficient, economical, and environmentally sustainable technique for prototype development in load-bearing applications. This work investigates the defects arising from the processing in material extrusion-based AM of polymers and their impact on the part performance. The influence of raster angle orientation and printing speed on tribological characteristics, microstructure, and surface finish of acrylonitrile butadiene styrene (ABS) fabricated in a heated build chamber was studied. Comprehensive analysis with fractography and tomography revealed the formation, distribution, and locations of internal voids, while surface defects were studied with the topography analysis of as-printed surfaces. Surface roughness and tribological results show that printing speed can be optimally increased with a minimal impact on interlayer bonding and part performance. Increased printing speed allowed up to 58% effective reduction in printing time obtaining comparable mechanical properties at varying process parameters. 3D printed ABS exhibited dry sliding friction coefficients in the range of 0.18–0.23, whilst the maximum specific wear rate was 6.2 × 10−5 mm3/Nm. Higher surface roughness and increased printing speed exhibited delayed running-in during dry sliding, while insignificant influence was observed for steady-state friction and wear behaviors. The findings indicate that improved surface finish and reduced internal defects can be achieved with a controlled build environment allowing for higher printing speed. The observations in this study are evidence that 3D printing can be adapted for the sustainable manufacturing of polymeric components for tribological applications.

Leveraging cross-correlations and linear covariance-based filtering for line-intensity map reconstructions at linear scales

We explore the possible application of linear covariance-based (LCB) filtering to line-intensity mapping (LIM) signal reconstructions. Originally introduced for reconstruction of the integrated Sachs-Wolfe effect in the cosmic microwave background, the LCB filter is an optimal map estimator that extends the simple Wiener filter by leveraging external correlated data. Given a detectable strong LIM-galaxy or LIM-LIM cross power spectrum, we show recovery of high-redshift, large-scale line-intensity fluctuations---even in the presence of bright interloper emission---in simulations of a futuristic [C ii] LIM survey as well as simulated future iterations of the CO Mapping Array Project. With sufficient galaxy abundances or low LIM survey noise, normalized cross-correlation between the LCB reconstruction and the true signal reaches 70%--90% on large, linear comoving scales corresponding to $k\ensuremath{\sim}0.1\text{ }\text{ }{\mathrm{Mpc}}^{\ensuremath{-}1}$. This suggests the possible use of such signal reconstructions in astrophysical or cosmological contexts that require identifying the locations of line emissivity peaks and voids, although clear shortcomings exist on smaller scales. The successful application of the LCB filter in simulated LIM contexts highlights the importance of cross-correlations to studies of the reionizing and reionized high-redshift universe with LIM and other large-scale structure surveys.

Study on the Effect of Void Geometry and Location on Electric Field Distribution and Partial Discharges in HVDC Cables

Study on the Effect of Void Geometry and Location on Electric Field Distribution and Partial Discharges in HVDC Cables

Punching Shear Capacity of RC Biaxial Voided Slab – Prediction by Critical Shear Crack Theory

In this study the applicability of critical shear crack theory (CSCT) to biaxial voided slab is investigated, which predicts the punching shear capacity of solid slab. Based on the experimental results of biaxial voided slab available in the literature (40 specimens) the research is carried out. The experimental results showed that the load-rotation behaviour of biaxial voided slab is remain same as that of solid slab except for specimens with void located within 0.5d from face of the column. Further, the punching shear capacity of slab specimens with voids located beyond 2d distance from column face is almost the same as that of the solid slab. Depends on the available load-rotation data modified CSCT was developed depends on the location of the void from face of the column. The predictions were observed to be very scattered with a mean and standard deviation of 0.752 and 0.412, respectively. This is reasonable as the modified CSCT estimates the capacity with respect to the location of voids and ignores the area of void at the section considered. The CSCT approach needs to be further modified by including location and area of void to predict the punching shear capacity of biaxial voided slab with reasonable accuracy.

Analysis of the influence of L-PBF porosity on the mechanical behavior of AlSi10Mg by XRCT-based FEM

This work aims to present an XRCT-based FEM methodology to assess the influence of porosity defects on the mechanical behavior and failure of L-PBF AlSi10Mg components. Hence, the influence of defects on performance is estimated by means of XRCT scanning and analysis of porosity features, followed by mechanical FEM modelling of digitalized components. For this purpose, some test specimens with induced artificial porosity were manufactured by L-PBF, according to ASTM E8/E8M. Once this is done, inspection and defect characterization were carried out by means of XRCT. Porosity features such as void size, shape and location, apart from porosity percentage, were analyzed. Then, tensile tests were carried out, followed by further inspections by XRCT in order to research the relationship between the fracture level and the porosity of the as-built sample. Afterwards, virtual tensile tests of XRCT-ed specimens were modeled by FEM technique. Results obtained by the FEM model were well-correlated with the tensile experimental results to predict both ultimate tensile strength and elongation at fracture of each specimen, showing errors below 5.2%. However, the results of the final fracture level predicted by FEM analysis reveal that possibly underestimated and/or non-detected defects on the AlSi10Mg specimens seem to influence the accuracy of the fracture level prediction in some samples. Thus, XRCT arises as an ideal technology for the assessment of L-PBF-ed components.

A comparative study of void characteristics on the mechanical response of unidirectional composites

Defects in composite structures, such as voids, cannot entirely be avoided during the manufacturing, and have detrimental effect on the damage behavior and mechanical properties. The main characteristics of voids in composites are void content, shape, size, location and distribution. To investigate the effect of void characteristics on the failure mechanisms and mechanical properties of unidirectional composites, a non-conservative modeling method based on separation axes is proposed to generate representative volume element with arbitrary local microstructures, then the effect of void characteristics on the mechanical responses of unidirectional composites is studied by computational mechanics. Four types of typical voids are investigated to reveal the effect of void shapes and locations on failure mechanism, the damage evolution processes and mechanical response under various loading cases are analyzed comprehensively. It was found that the presence of interfacial voids modified the trend of the stress–strain curve under shear loading and almost all voids led to a significant reduction in the failure strength under transverse and shear loadings. Besides, the influence of void characteristics in the onset and propagation of damage throughout the microstructure is obviously different. Finally, the predictions by present model are compared with analytical method and available experimental data, good overall agreement is found.

Study on the damage evolutional process of secondary tunnel lining under rheological effect of surrounding rock

Cracks are prone to occur in the secondary tunnel lining with the action of rheological deformation of surrounding rock, which will cause severely structural defects and adversely affect the long-term safety of tunnels. In this article, similar model tests and numerical simulations were conducted to investigate this issue. First, the overloading method was adopted as a simplified way to investigate the failure evolution process of lining in the model test. Then, the long-term safety of lining under the influence of geo-stress field, the amount and location of voids were revealed through numerical simulations. The results show that: (1) The variation of inner force with loading steps has two distinct stages that are similar to those for lining under the creep effect of rocks, which validates the availability of the overloading method preliminarily. (2) Sections of lining near the void bend outward, and the axial force in these sections is smaller than that of the other sections. (3) The failure process for lining with no void shows evidently progressive feature with the occurrence of two stress adjustment processes before its failure, while the existence of void accelerates the failure process of lining. (4) A crack near the void appears in the lining when the lining reaches its ultimate limit state of carrying capacity, and the crack occurs earliest compared to other cracks.

Effects of void deck on the airflow and pollutant dispersion in 3D street canyons.

In general, urban canyons are the areas most clearly affected by traffic pollutants since the ability of the canyon to self-ventilate is inhibited due to blockage of buildings or other urban structures. However, previous studies have aimed to improve the pedestrian-level wind speed with void deck in single buildings or short canyons. This study investigated the effects of void deck height and location, and the building height on the airflow field and the traffic pollutant diffusion in a long canyon with L/H = 10, validated by wind-tunnel experiment data. The results show that the void decks have a significant effect on the airflow and pollutant distribution inside the canyon. Air exchange rates (ACH) of the canyons with the void deck are much larger than that of regular canyons, and the perturbation changes of turbulence (ACH′) decrease. For the windward void deck, purging flow rate (PFR) and normalized net escape velocity (NEV*) increase by 6.4 times compared to the regular canyon, and for the leeward void deck, increase by 13 times. In particular, when the void decks are at both buildings, they are increased by 38.3 times. Also, for the canyons with the void deck, traffic pollutants are removed out of the canyon by the strong airflow through the void deck. Therefore, unlike the regular canyons, as the void deck and the building height increases, the strength of the airflow through the void deck becomes stronger, and as a result, the mean pollutant concentration is significantly reduced at both walls and the pedestrian respiration level. The mean pollutant concentration on the wall of the building with the void deck and on the pedestrian respiration plane close to it is near zero. These findings can help ease traffic pollution inside the street canyons composed of high-rise buildings, especially in tropical cities.

Damage Detection Using Ultrasonic Techniques in Concrete-Filled Steel Tubes (CFSTs) Columns.

Concrete-filled steel tubes (CFSTs) are structural elements that, as a consequence of an incorrect elaboration, can exhibit internal defects that cannot be visualized, being usually air voids. In this work, the detection of internal damage in CFST samples elaborated with a percentage of contained air voids in concrete, was carried out by performing a complete ultrasound scan using an immersion tank. The analysis of the ultrasound signals shows the differences presented in the amplitude of the fundamental frequency of the signal, and in the Broadband Ultrasound Attenuation (BUA), in comparison with a sample without defects. The main contribution of this study is the application of the BUA technique in CFST samples for the location of air voids. The results present a linear relationship between BUA averages over the window of the CFSTs and the percentage of air voids contained (Pearson’s correlation coefficient r = 0.9873), the higher percentage of air voids, the higher values of BUA. The BUA algorithm could be applied effectively to distinguish areas with defects inside the CFSTs. Similar to the BUA results, the analysis in the frequency domain using the FFT and the STFT was sensitive in the detection of internal damage (Pearson’s correlation coefficient r = −0.9799, and r = −0.9672, respectively). The results establish an improvement in the evaluation of CFST elements for the detection of internal defects.

Molecular dynamics study on mechanical behaviors of Ti/Ni nanolaminate with a pre-existing void

Metallic nanolaminated materials possess excellent mechanical properties due to their unique modulation structures and interfacial properties. However, how microdefects affect their mechanical properties is still uncertain. To evaluate the influences of void location (in the crystalline layer and the Ti/Ni interface), void diameter ( d ) and thickness of the intermediate layer ( h ) on overall tensile behaviors, various types of defective Ti/Ni nanolaminates with pre-existing void are established by the molecular dynamics method in this work. The results indicate that the strength and plastic deformation mechanisms are strongly dependent on those determinants. Yield stresses of Ti/Ni nanolaminates decrease distinctly with increasing void diameter, while peak stresses with a void in the crystalline layer decrease with increasing d / h . Different void locations lead eventually to disparate initial plastic deformation carriers around the void, and various evolutions in the microstructure of the defective Ti/Ni nanolaminates. The Ti/Ni interface plays a significant role in the tensile process. The semi-coherent interface impedes new grains and lattice dislocations from passing across the interface, while the incoherent interface facilitates dislocations generating and sliding along the interface, and absorbs the dislocations moving to the interface. The results also indicate that the strain rate significantly affects the evolution of the microstructure and the tensile properties of defective Ti/Ni nanolaminates.

Characterization of Continuous-Wave Laser Crystallized Silicon Thin Films with Hole-Patterns

Single crystal grain low-temperature polycrystalline thin-film transistors (LTPS-TFTs) fabricated on a glass substrate have been the key target to develop transparent electronics such as monolithic three-dimensional integrated circuits (3DICs) and glass sheet computers because of their ultrahigh-performances. This study proposed a simple method to pre-define single-crystal grains with hole-patterned polycrystalline silicon (poly-Si) thin film. The crystal growth and the temperature distribution were clarified by observing the characterization of a hole-patterned poly-Si thin film. The relationship between the temperature gradient, crystal growth, and void formation in the poly-Si thin film was discussed. The location of voids was controllable in the hole-patterned poly-Si thin film. These pre-defined single-crystal grains are promising to fabricate ultrahigh-performance LTPS-TFTs.

Mechanism investigation of pre-existing void-induced multi-modal electro-migration behavior

A multi-modal time-to-failure distribution for an electro-migration (EM) structure has been observed and studied from long durationin-situ EM experiment, for which the failure mechanism has been investigated and discussed comprehensively. The mixed EM failure behavior strongly suggest that the fatal voids induced EM failure appear at various locations along the EM structure. This phenomenon is believed to be highly related to the existence of pre-existing voids before EM stress. Meanwhile, the number and location of the pre-existing voids can influence the EM failure mode significantly. Based on our research, a potential direction to improve the EM lifetime of Cu interconnect is presented.

Shape identification of scatterers Using a time-dependent adjoint method

This paper deals with the inverse problem of accurately identifying the shape, size and location of 2D objects or voids in a given medium via the measurement of waves scattered from them. Examples of scatterers include an obstacle in a 2D acoustic medium or a small hole in a membrane. To fix ideas, the latter example, governed by the scalar wave equation, serves here in demonstrating the method. In the proposed method, a known source generates waves that propagate in the membrane, and these time-dependent waves are measured by a small number of sensors located at chosen points. Based on these measurements, an iterative process is performed to find the closed curve that represents the boundary of the scatterer. Mathematically, the iterative process aims at minimizing a cost functional that represents the difference between the measured wave signals and the wave signals obtained in the presence of a candidate scatterer. The gradient of this functional, which represents its sensitivity to the scatterer geometry, is calculated efficiently using the adjoint method. Finite elements are used to discretize the spatial domain, the Newmark method is used for time-stepping, and a smoothed gradient method is used for the functional minimization. The unknown boundary of the scatterer is discretized from the outset, which allows a simpler derivation of the adjoint procedure compared to a fully variational treatment. The unknown curve is defined by a parametric representation, that is completely general and does not make any preliminary assumptions about the scatterer geometry. Multiple sources are used to enhance the identification. The performance of the proposed method is demonstrated via numerical examples. It is shown that excellent identification is obtained even in the presence of noise, with a small number of sensors, and with no need for regularization (other than that induced by the discretization).

Effects of voids on the dynamic response of tunnels under train-induced vibration loads

This paper presents results of the influence of voids on the dynamic response of tunnel linings and surrounding strata acquired in series of model tests. An experimental model of a tunnel was tested with a void of varying size (ratio of void width-to-tunnel diameter of 0.13, 0.27 and 0.4) and location (at tunnel apex, haunch and invert). During the tests, the tunnel invert was applied with three types of vibration loads: a sinusoidal load with a fixed frequency, a sinusoidal sweep load and a simulated train load. The tunnel lining and soil response were recorded by accelerometers, with results for analysis presented in both time and frequency domains. The experimental results manifest that the voids can have a distinct influence on the tunnel and soil dynamic response, with a clear amplification of tunnel and soil response for certain cases. The test results also demonstrate that the tunnel and soil response are sensitive to the void size and location in the near field. However, with increased distance from the excitation source, the void effects on the tunnel and soil dynamic response gradually declines.

Predicting short beam shear strength reduction in carbon/epoxy laminates containing voids

The presence of voids in composite materials reduces the material’s strength, particularly matrix-dominated properties such as short beam shear (SBS) strength. Until now there has been no satisfactory way to reliably predict the effect of voids on SBS strength. Instead, there has been a reliance on using average void content combined with extensive testing to assess the linear reduction of strength with increasing void content to determine a 'knock-down' factor. Here, the reduced cross-sectional area is used to predict SBS strength of laminates containing voids. A detailed characterisation of the internal void location and morphology for numerous samples was obtained through X-ray CT scanning, prior to mechanical testing. The ‘critical plane’ in each sample, i.e. the length–width plane with the maximum cross-sectional area of voids was located and used to predict SBS strength. To additionally account for non-uniformity in the shear stress distribution, a stress scaling factor (SSF) is introduced.

Void Content Determination of Carbon Fiber Reinforced Polymers: A Comparison between Destructive and Non-Destructive Methods.

The properties of composite materials are highly dependent on the fiber and matrix fraction and on the porosity resulting from micro voids. This paper addresses void content characterization and the constituent content of composite materials by resorting to a comparison of destructive and non-destructive methods. The work presents the detailed procedures of two destructive methods, using acid digestion of epoxy resins matrices, and compares their processes. It also presents the results of a non-destructive method, by means of Micro Computed Tomography (MicroCT). The results of both destructive and non-destructive methods are compared, and a recommendation is made based on the application and the type of composite being analyzed. The MicroCT showed better and more consistent results in detecting voids in the material, while the acid digestion tests provided better results about the fiber and matrix percentage. Exported results from the MicroCT scanning with actual locations of voids were used in numerical analysis, to examine the feasibility of using them, whether by developing models that map damage in the proximity of the void, or by developing models that predict the properties of the entire material with respect to the content, shape, and distribution in the material.