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Pakrashi, Vikram
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Pakrashi, Vikram
Official Name
Pakrashi, Vikram
Research Output
Now showing 1 - 10 of 86
- PublicationDamage Assessment of the Built Infrastructure using Smartphones(University College Dublin, 2018-08-30)
; ; ; The use of image-processing and machine-learning algorithms for road condition monitoring has attracted considerable interest in recent years. This surge in popularity has been propelled by advances in camera technology and the emergence of state-of-the-art deep learning techniques which have allowed inspectors to obtain high-quality imagery on a consistent basis, and then use efficient techniques to recognise road defects with credibility. A wide variety of road defect detection techniques have been proposed, however, the influence that different image acquisition devices have on the accuracy of defect detection has not been studied despite being a key component. The use of smartphone cameras as an inspection tool is of particular interest as they have become ubiquitous in recent times and the built-in cameras have progressed significantly. These cameras are now capable of producing perfectly acceptable images, yet they are still not well compared against established benchmarks. In this paper, the qualities of smartphones are explored and compared against a dedicated DSLR (Digital Single-Lens Reflex) camera. An experiment was designed that involved capturing video footage of a road surface using a smartphone (Samsung Galaxy S7) and a dedicated imaging device (Canon 600D DRSL). A deep learning based crack detection method was applied to imagery from the smartphone and the DRSL cameras and the performances levels were subsequently compared. The results indicate that smartphones are a viable, low-cost method for executing quick assessments of the road integrity. The evaluation of smartphone cameras also addresses the ongoing uncertainty around the level of performance that can be achieved using cheaper sensors for quantitative purposes. Such findings provide reassurance for inspectors wishing to use their smartphones for simple monitoring tasks.117 - PublicationEffects of Turbidity and Lighting on the Performance of an Image Processing based Damage Detection Technique(Taylor & Francis, 2014-02-10)
; ; ; Measuring the true extent of damage in a structure remains a difficult task for inspectors. For visual inspections, an accurate assessment of the damage state is often subjective in nature and prone to error, especially when an inspection is conducted in hostile surroundings or when there are challenging environmental conditions present. While incorporating some form of Non-Destructive Technique (NDT) is generally useful for the inspection process, its performance may similarly degrade in the presence of environmental conditions. It is thus of great practical importance to have a measure of the performance of an NDT for a host of varying conditions, thereby allowing the inspector to determine whether it could be successfully applied in a given situation. In this paper, a measure of the effectiveness of an NDT is probabilistically determined for various environmental conditions through the use of Receiver Operating Characteristic (ROC) curves. ROC curves offer a convenient way of characterizing and comparing the performance of an NDT under various conditions. The NDT considered in this paper is an image processing based damage detection technique which uses texture information in conjunction with Support Vector Machine (SVM) classification to identify damaged regions. The variability of this technique is evaluated for various damage forms that are subjected to two changing parameters; turbidity and lighting. There were three set levels (low, medium, high) for each parameter. The conditions that were conducive to good detection were isolated and ranked using the α-δ method as part of the ROC analysis. The technique is applied to standard dynamic range (SDR) images and high dynamic range (HDR) images in order to assess their respective sensitivities to the changing parameters.214 - PublicationThe Dynamics Effects of Marine Growth on a Point Absorbing Wave Energy ConverterWave energy convertors have the potential to generate a sizeable proportion of Ireland’s energy needs. Such platforms will be susceptible to bio fouling over their design life with marine growth capable of altering the hydrodynamic loading. Marine growth causes member effective diameter, mass, drag coefficients, force and hydrodynamic added mass to increase. In this paper, marine growth of various thickness and surface roughness is numerically modelled on two torus shaped point absorbers moving against a monopile in operational conditions using combined potential flow boundary element method and Morison equation viscous drag. Marine growth thickness and surface roughness have a notable effect on the platform hydrodynamic forces. However, the power matrix remains generally similar. The smaller WEC shows the greatest change with a 20 % increase in draft and 5 % increase in power.
88 - PublicationEvaluation of Camera Calibration Techniques for Quantifying Deterioration(Civil Engineering Research Association of Ireland, 2016-08-30)
; ; ; Imaging systems offer an efficient way of obtaining quantitative information on the health status of structural components. They hold particular value for underwater inspections as they can be easily adapted for underwater use and they enable physical information to be captured from a scene for the purpose of later analysis. In order to make the visual data a part of a quantitative assessment, it is necessary to calibrate the imaging systems so that photographed instances of damage can be expressed and measured in physically meaningful real world units, such as millimetres, which can then be used by engineers in subsequent analyses. The imaging system employed in this study is a stereo rig. It consists of two synchronised cameras that capture images of the scene from slightly different perspectives, thereby encoding depth information. This paper evaluates and compares two main approaches for calibrating such a stereo systems, namely, the classical checkerboard procedure and self-calibration based on Kruppa’s equations. Conventional checkerboard calibration must be carried out on-site by photographing a planar checkerboard pattern that is held at multiple random poses, while self-calibration can be carried out after-the-fact and relies only on the static scene acting as a constraint on the camera parameters. The performance of each approach is assessed through a set of experiments performed on controlled real-world specimens as well as on synthetic data. Results indicate that checkerboard calibration is slightly more accurate than self-calibration; however, the practical advantages of using self-calibration may outweigh this reduction in accuracy. An understanding of the advantages and limitations associated with each camera calibration allows inspectors to rationalise the use of either approach as part of their inspection regime, and it helps them to fully capitalise on the benefits of image-based methods.100 - PublicationA comparison of image based 3D recovery methods for underwater inspectionsOffshore structures can be subjected to millions of variable amplitude load cycles during their service life which is the primary cause of structural deterioration. Such fatigue loading is exacerbated by marine growth colonization which changes the surface roughness characteristics and increases the diameter of structural members. Having an accurate knowledge of these parameters is essential for analyzing the increased hydrodynamic forces acting on the structure. This paper addresses the issue of acquiring shape information by comparing two popular classes of image based shape recovery techniques; stereo photography and Structure from Motion (SfM). Stereo photography utilises a dual camera set-up to simultaneously photograph an object of interest from slightly different viewpoints, whilst SfM methods generally involve a single camera moving in a static scene. In this paper, these techniques are performed on a controlled shape in an underwater setting, as well as synthetic data which allows for an irregular shape typical of marine growth to be tested whilst still having knowledge of the exact geometrical shape. The results reveal that the self-calibrated stereo approach fared well at getting an appropriately scaled full metric reconstruction, whilst the SfM approach was more susceptible to breaking down.
37 - PublicationNumerical Modelling of a Combined Tension Moored Wind and Wave Energy Convertor System(European Wave and Tidal Energy Conference, 2017-01-01)
; ; The offshore wind industry is moving to deeper water sites which generally have more severe environmental conditions. This presents not only a challenge to deploying devices in such conditions but also an opportunity for developing combined hybrid platforms that harvest both wind and wave energy. This paper introduces the novel tension moored combined wind and wave energy converter (WEC): TWindWave. Numerical modelling of the system in power production and novel survival modes is undertaken. Hydrodynamic interaction is analysed and shown to reduce the overall WEC power production. The addition of the WECs onto platform characteristics is described. Novel survival modes which involve ballasting the WEC floats and sinking them along the platform columns, before they rotate around the column and pontoon connecting bends are described and analysed. Future work incorporating these results and more advanced techniques are proposed.90 - PublicationGuidelines and Recommendations from COST TU 1406(2019-03-29)
; Asset management depends on well-defined rules and standards for general application. Research results often represent the state of science in specific sections of specific cases. Standardization needs a downgrade of scientific work towards generally applicable specifications. This often leads to irritation in the scientific community fearing that their most interesting results are not specifically appreciated. This conflict makes standardization sometimes difficult. COST action TU1406 is devoted to harmonizing the state of science and technology and to bring all involved disciplines into the process. The results produced are of value for a number of standards on ISO, CEN and National level. This represents another difficulty, namely, to find the right place in any of the organizations and to identify the most suitable technical committee and code. This contribution reports on the experiences made with various committees and the results achieved. It soon became clear that specific parameters for bridge assessment cannot be standardized because they are too specific for a small sector. The way out is formulating the principles how to define them, to make a framework under which the detail results can be applied, and to define the interfaces to other necessary disciplines. Sustainability as a headline was found to be suitable and TC59 of ISO was the committee of choice. Frameworks set in ISO55000 (asset management) and ISO31000 (risk management framework) are successfully addressed. A final draft of ISO 21929-2 is to be expected by early 2019.210 - PublicationExperimental Detection of Sudden Stiffness Change in a Structural System Employing Laser Doppler Vibrometry(Erredi Grafiche Editoriali, 2012-06-20)
; ; ; Sudden changes in the stiffness of a structure are often indicators of structural damage. Detection of such sudden stiffness change from the vibrations of structures is important for Structural Health Monitoring (SHM) and damage detection. Non-contact measurement of these vibrations is a quick and efficient way for successful detection of sudden stiffness change of a structure. In this paper, we demonstrate the capability of Laser Doppler Vibrometry to detect sudden stiffness change in a Single Degree Of Freedom (SDOF) oscillator within a laboratory environment. The dynamic response of the SDOF system was measured using a Polytec RSV-150 Remote Sensing Vibrometer. This instrument employs Laser Doppler Vibrometry for measuring dynamic response. Additionally, the vibration response of the SDOF system was measured through a MicroStrain G-Link Wireless Accelerometer mounted on the SDOF system. The stiffness of the SDOF system was experimentally determined through calibrated linear springs. The sudden change of stiffness was simulated by introducing the failure of a spring at a certain instant in time during a given period of forced vibration. The forced vibration on the SDOF system was in the form of a white noise input. The sudden change in stiffness was successfully detected through the measurements using Laser Doppler Vibrometry. This detection from optically obtained data was compared with a detection using data obtained from the wireless accelerometer. The potential of this technique is deemed important for a wide range of applications. The method is observed to be particularly suitable for rapid damage detection and health monitoring of structures under a model-free condition or where information related to the structure is not sufficient.41 - PublicationStructural health monitoring of reinforced concrete beam using piezoelectric energy harvesting system(INRIA Rennes - Bretagne Atlantique, 2014-07-11)
; ; ; ; There has been focus in recent times in the creation of smart, wireless sensor networks for the purposes of Structural Health Monitoring of large scale civil infrastructure. However, the power requirements of such networks are dependent on finite batteries, which limit the effectiveness of such a system. The use of energy harvesters, however, offers a viable and attractive solution to this problem. This paper investigates the use of such energy harvesters not only to power wireless sensor nodes, but to also act in the process as a damage detection tool. The properties and creation of such energy harvesters is detailed in full. The effects of damage on a simply supported reinforced concrete beam are investigated through finite element analysis. The use of the energy harvesters for damage detection is subsequently investigated and the feasibility of using such harvesters is experimentally validated. The simultaneous power of wireless sensor nodes by the harvesters is determined and an energy harvesting circuit is examined in this regard. This paper establishes the basis and viability of using an energy harvesting system for use in this dual role.68 - PublicationROC-based Performance Analysis and Interpretation of Image-based Damage Diagnostic Tools for Underwater Inspections(Taylor & Francis, 2018-10-10)
; ; ; It is of practical importance for inspectors to have knowledge of the efficiency of Non-Destructive Testing (NDT) tools when applied commercially. It has become common practice to model the performance of NDT tools in a probabilistic manner in terms of Probability of Detection (PoD), Probability of False Alarm (PFA) and eventually by Receiver Operating Characteristic (ROC) Curves. Traditionally, these quantities are estimated from training data, however, there are often doubts about the validity of these estimates when the sample size is small. In the case of underwater inspections, the scarcity of good quality training data means that this scenario arises more often than not. Comprehensive studies around the on-site performance of image-based damage diagnostic tools have only recently been made possible through the availability of online resources such as the Underwater Lighting and Turbidity Image Repository (ULTIR), which contains photographs of various damages forms captured under controlled visibility conditions. This paper shows how meaningful information can be extracted from this repository and used to construct ROC curves that can be related to the on-site performance of image-based NDT methods for detecting various damage forms and under a range of environmental conditions. The ability to draw connections between image-based techniques applied in real underwater inspections with ROC curves that can be constructed on-demand provide the engineer/ inspector with a clear and systematic route for assessing the reliability of data obtained from image-based methods. As a case study, the general approach has been applied to characterise the performance of image-based techniques for identifying instances of corrosion and cracks on marine structures. A discussion around how the results can be used for further analysis is provided. This includes looking at how the results can be fed into in the decision chain and can be used for risk analysis, intervention and work scheduling, and eventually understanding the value of information.120