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Pakrashi, Vikram
Preferred name
Pakrashi, Vikram
Official Name
Pakrashi, Vikram
Research Output
Now showing 1 - 10 of 86
- PublicationDirect and Probabilistic Interrelationships between Half-Cell Potential and Resistivity Test Results for Durability Ranking(2012-07-21)
; ; Tests related to durability studies on structures often feature half-cell potential and resistivity data. An approximately linear relationship between half-cell potential testing and resistivity data has been discussed and well-researched. In spite of criticisms related to environmental sensitivity of resistivity tests it remains as a popular choice for investigations into durability of structures. This paper investigates the correlation between half-cell potentials and resistivity tests on reinforced concrete from field data from tests on six bridges. The empirical interrelationships from the six bridges with widely varying environmental exposure conditions and the variation of such interrelationships are observed. Similar investigations are carried out on different elements of bridges. The paper then discusses problems related to the interpretation and practical application of correlations carried out on absolute values and advocates the use of statistical measures obtained from test data. The percentile correlations are observed to be helpful when considering exceedances of different threshold values. A customised use of such data in an empirically correlated probabilistic format with can be useful in durability ranking and infrastructure maintenance management. The studies presented in this paper emphasize the advantages of using probabilistic formats over traditional formats when interpreting or quantitatively establishing field relationships between half-cell potential and resistivity data. The ability of this empirically correlated probabilistic format to support structure-specific thresholds of serviceability limit states is discussed. The need for a shared repository for the improvement of accuracy of such correlations and for the use of such correlations as a surrogate for other structures is emphasized.319 - 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.176 - 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.275 - PublicationA bespoke signal processing algorithm for operational modal testing of post-tensioned steel and concrete beamsThe extraction of modal properties, specifically natural frequency, damping ratio and mode shape is a difficult task, especially when output-only data is measured. The accuracy of the estimation these modal properties is compromised by noisy signals, and signal filtering is required to suppress unwanted frequency content. Care is required however to avoid over-filtering of the output data, which can eliminate valid structural frequency content if required care is not exercised. This paper describes the development of a bespoke signal processing algorithm to extract the modal properties of both simply supported post-tensioned steel and concrete sections. Dynamic impact testing was conducted on a series of different post-tensioned steel rectangular hollow sections, and 9 different post-tensioned concrete beams, each with differing straight profiled post-tensioning strand eccentricities. Acceleration time-history data was recorded for each of the steel and concrete beams via an accelerometer. This data was subsequently processed, first centring the acceleration-time history using a moving average filter, and subsequently removing any zero drift in the accelerometer via a second order low pass Butterworth filter. Electrical noise was then removed via a notch filter. The accelerometer data was then smoothed in the time domain. The Fast Fourier Transform (FFT) was applied to the signal to convert into the frequency domain and finally a bespoke peak-picking algorithm was invoked to extract the natural frequencies of the beams. A comparison is subsequently made between the accuracy of the estimation of the modal properties of the steel and concrete beams for filtered and unfiltered data, and a sensitivity analysis of the filtering and peak picking parameters is conducted to determine the effect that this has on the accuracy of the estimation of the modal parameters. The results show the effectiveness of the bespoke signal processing algorithm in increasing the accuracy of the estimation of the modal properties as opposed to the raw unprocessed signals.
379 - 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.83 - 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.
78 - 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.212 - PublicationReliability of extreme wave prediction methods(2015-07-15)
; ; Extreme wave parameters are used for engineering design in our seas and oceans, yet the methods used to determine them are non-standardized and can give highly variable output. With increased commercial activity in the marine sector, the importance of accurate extreme wave parameter determination has become increasingly apparent. This is particularly the case for marine renewable structures where even small over-predictions in design parameters can affect the whole feasibility of the project. This paper addresses the methods of extreme wave prediction currently in use, with a view to selecting the optimal method for the prediction of extreme wave conditions (Hs, Hmax and Tz) in coastal Irish waters. The paper identifies pitfalls and drawbacks of current extreme prediction methods, with particular attention given to the use of limited in time buoy data from coastal locations where development is to take place. In addition a new methodology of determining extreme wave periods, that is the wave periods occurring coincidentally with the most extreme wave heights, is established. This is important as the destructive energy of a wave is dependent on the wave period. By estimating the extreme wave energy and significant wave height, it is possible to formulate a method of reliably approximating the likely coincident wave period.81 - 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.
195 - 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.161