Showing papers by "Prabhu Rajagopal published in 2022"

Statistics-based baseline-free approach for rapid inspection of delamination in composite structures using ultrasonic guided waves

Abstract: Delamination in composite structures is characterized by a resonant cavity wherein a fraction of an ultrasonic guided wave may be trapped. Based on this wave trapping phenomenon, we propose a baseline-free statistical approach for the identification and localization of delamination using sparse sampling and density-based spatial clustering of applications with noise (DBSCAN) technique. The proposed technique can be deployed for rapid inspection with minimal human intervention. The Performance of the proposed technique in terms of its ability to determine the precise location of such defects is quantified through the probability of detection measurements. The robustness of the proposed technique is tested through extensive simulations consisting of different random locations of defects on flat plate structures with different sizes and orientation as well as different values of signal to noise ratio of the simulated data. The simulation results are also validated using experimental data and the results are found to be in good agreement.

Application of Selected Fractal Geometry Resonators in Microstrip Strain Sensors

Abstract: Until now, strain gauges, fiber optic sensors, magnetic sensors, and piezoelectric sensors have been used for strain measurements. In recent years, the idea of using microstrip antennas in strain measurement applications has emerged. One of the key requirements for deformation sensors is the ability to mount them on small-size elements (therefore the size of these sensors is crucial). The possible way to miniaturize microstrip sensors is the application of appropriate patch geometry, e.g. fractal geometry. In this article application of various fractal patch geometries in microstrip strain sensors were investigated. All sensors were designed for the same operating frequency ${f}_{\textbf {r}} =2.725$ GHz and on the same laminate. This approach showed how the influence of a specific geometry affects the sensitivity and the transducer size. The Sierpinski carpet, the Sierpinski triangle, the Sierpinski curve, and the combination of the Koch curve and the Sierpinski carpet were used to obtain the fractal geometry and miniaturize the sensor dimensions. They have been compared with the widely used rectangular patch. Numerical and experimental analyzes for the proposed sensors were carried out. The best solution to this problem was to use a combination of the Koch curve and the Sierpinski carpet. This fractal geometry, compared to a rectangular patch, enables the reduction of the patch size by 70% while reducing the sensitivity by 26%.

Implementing an Adaptive Thrust Distribution Algorithm on the Robust Control System for Serial Split-Hull Underwater Vehicles

Abstract: This paper discusses designing and implementing an adaptive thrust distribution algorithm on the already robust control system for serial split-hull underwater vehicles to maximise operational efficiency. Split-hull underwater vehicles are gaining popularity in subsea operations due to their flexible hyper-redundant design and enhanced manoeuvrability. These features facilitate them to replace conventional solutions, including torpedo-shaped Autonomous Underwater Vehicles (AUVs) and Remotely Operated Vehicles (ROVs). The field of research is currently undergoing a considerable transition toward operations on subsea activities that are more economical and efficient. Thus, implementing a robust and energy-efficient control system is of utmost importance. The robust Super Twisting Algorithm (STA) based control system is used on the vehicle, and an adaptive thrust distribution algorithm is proposed and implemented to optimise the energy efficiency further. The work involves modelling the vehicle, fluid forces, torque effects, and thruster configuration matrix and implementing the adaptive weight distribution on the STA-based control system. With the help of simulations, the improvements in the control system are proved.

Laser spot thermography for defect detection on mild steel at higher temperatures (30–600 °C)

Abstract: This paper studies the performance of laser spot thermography (LST) in defect detection on a mild steel sample at different temperatures (30–600 °C). In laser spot thermography, a laser spot is used to scan over the surface of the sample and the thermal profile is monitored using a thermal camera. For checking the feasibility/capability of laser spot thermography in defect detection under higher temperatures, a three-dimensional numerical model is developed using a commercial FE software package. This FE model is used to understand the heat transfer phenomenon during defect detection at higher temperatures, even after oxidation temperature. The influence of the oxide layer formation (scale) at higher temperatures (above 500 °C) in defect detection is established using the validated model. The thermal properties and optical properties of this oxide layer are different from those of the base metal; this will lead to a drastic variation in the thermal profile after the oxidation temperature. An oxide layer at 600 °C is introduced in the improved numerical model as a thin resistive layer at the top surface with a thickness of 50 μm. The thermal properties of the layer are assigned as the scale properties. FE modeling results show better agreement with experimental results even at 600 °C. Thus, the applicability of LST in high temperatures is experimentally proved.

Non-Destructive Testing of Underwater Concrete Structures Using Remotely Controlled Drones

Abstract: Concrete is a fundamental component in civil, commercial, and industrial infrastructure. Periodic inspection is mandated to ensure safe operation during the structure’s lifetime. Recent innovations in inspection technology have enabled rapid characterization of the interior of a concrete specimen with techniques like ground penetrating radar and phased array ultrasonic tomography. A universal limitation of these conventional methods is that they are best documented for in-air application, with comparatively scant literature available for underwater deployment. Visual inspections and diver deployed semi-destructive tests are the main modes of underwater concrete inspection. Underwater, concrete structures encounter harsher conditions and larger weathering effects compared to those above. Underwater inspections sites often have poor visibility, strong currents and debris, making manned inspections risky. Divers also have limited inspection time and depth, decreasing scope and increasing asset downtime. To overcome those limitations, this paper proposes the use of remotely operated underwater vehicles (ROVs) to perform quantitative non-destructive tests on underwater concrete structures. ROVs developed by Planys Technologies are compact, lightweight, and versatile. They are capable of extended inspection times, and operational depths of up to 200 m. These ROVs can be deployed by a crew of 2-3 personnel and are remotely controlled from a safe location above water. One of the most well-known techniques, ultrasonic pulse velocimetry, was adapted for the marine environment. While foundational when compared to the state-of-the-art in-air techniques, it is still capable of providing a quantitative measure of a concrete structure’s integrity. The paper describes experimental results from in-lab and in-field testing, as well as limitations to practical applications.

A Study on the Influence of Wave Scattering in Metamaterial-Based Super-Resolution Imaging of Defects in Materials

Abstract: Recently there is much interest in metamaterial based super resolution imaging. Several demonstrations have been reported using sources or slits as targets for imaging. However, in the context of non-destructive evaluation, imaging of defects and discontinuities within a sample are of more interest. Such defects, unlike sources or slits, induce wave scattering which could potentially impact image generation. This paper studies the effects of wave scattering by subwavelength spaced defects in holey structured metamaterial based super resolution imaging using numerical (finite element) models. In these models, the ultrasonic waves are assumed to impinge on the defects in a normal incidence through transmission configuration, and a line-scan image at the receiver location is generated based on the captured waves past the metamaterial. The influence of defect position within the specimen sample (object plane) and the receiver location (image plane) with respect to the metamaterial on the output images are investigated. The results show that the defect-induced wave scattering processes produce intensity and spatial artefacts that have a signature on imaging. For various parametric cases, the changes in the output images are quantified and discussed in the context of metamaterial based super resolution imaging in the field of non-destructive evaluation and non-invasive diagnostics.

Non-Destructive Testing of Quay Walls Using Submersible Remotely Operated Vehicles (ROV) In Waterways Around the North Sea Coast

Abstract: The North Sea, with a coast length of 1590 km, is amongst the most active commercial shipping route in the world, and is subject to harsh environmental conditions throughout the year. Coastal protecting measures defend waterways around the North Sea coast against storms, rising sea levels, and corrosion. Quay walls, one of the most widely used coastal protection system, have been one of the fundamental building blocks of the economic system. They are designed to enable ship moorings and absorb impacts from vessels. Further, quay walls are used to allow shipment of goods to the vessels using heavy equipment. They must withstand varying water levels, environmental conditions, loads being carried in and out of the ports. Periodic asset maintenance of quay walls is therefore crucial to ensure uninterrupted services and minimum downtimes, and to prevent catastrophic damages and loss of lives.Quay walls are subject to numerous structural damages including interlock damage, scouring, impact damage, and corrosion of concrete, reinforcement, and steel components. Continuous inspection programs are required to timely maintain, and repair associated infrastructure. Conventionally, the underwater sections of the quay wall are inspected by professional divers carrying underwater cameras and sensors to perform visual and ultrasonic thickness measurement. However, diver-based methodologies are often restricted by limited operational durations, depths, and safety risks. Further, the waterways connected to the North Sea are subjected to numerous challenges including as dense marine growth, dynamic variations in water visibility, underwater currents or flow, and operation in confined spaces. To overcome these challenges and inherent risks associated with manned underwater operations, an alternative inspection approach using remotely operated robotic vehicles (ROV) indigenously developed by Planys BV is proposed in this paper. ROVs have several key advantages in comparison to the conventional-manned operations including unlimited endurance, and depth of operation, live video streaming, improved data quality, and defect geotagging to improve result repeatability.These ROVs are controlled remotely from a safe location by a pilot. They are also compact and lightweight, deployable by a crew of two personnel. They carry underwater cameras and lights for live visual inspection, sonic and ultrasonic transducers for sonar surveys and ultrasonic thickness measurement, respectively. This paper presents a case study of inspecting sheet pile infrastructure using ROVs in waterways connecting the North Sea in Western Europe. Key details explaining the components of the ROV, and methodology of operation are discussed. Exemplary results from visual inspection and ultrasonic thickness measurement are depicted, showing satisfactory quality of data collected. Furthermore, details of data analysis and representation implementing state-of-the-art digital analysis dashboard to enable rapid and accurate interpretation, are presented.Key limitations of using freely swimming ROVs are described from the viewpoint of operational efficiencies and rate of inspection. Recommendations on development of next generation of robotic crawler devices for enhanced commercially viable inspection routines are discussed. The work presented in the paper may be of interest to authorities, asset owners and managers of marine assets, operating both, inland and nearshore.

Quantitative Non-Destructive Testing (NDT) of Submerged Civil Concrete Structures Using Remotely Operated Robotic Drones

Abstract: Periodic inspection is of paramount importance to assess structural integrity and safety. Conventional concrete inspection procedures ranging from rebound hammer, to state-of-the-art ground penetrating radar and ultrasonic tomography focus on superstructures. Often, the part of the structure underwater, is subject to significant levels of corrosion. Today, qualitative visual inspection of the submerged surface can be performed by diving teams, wherein internal defects often go undetected. Furthermore, divers are limited to shallow depths and face several risks, especially when visibility is poor and operating in confined spaces. Presently, there are few commercially available quantitative NDT techniques for submerged structures. This paper presents an alternate solution to inspection of submerged concrete structures — using lightweight remotely operated vehicles (ROV) manufactured by Planys Technologies to perform concrete NDT underwater. Rebound hammer and ultrasonic pulse velocity techniques, although elementary, have been redesigned for use on a submersible drone. Several challenges were addressed including long-range communication and data acquisition, marinizing transducers and pulser-receivers, obtaining measurements in water currents and over deformed surfaces. Planys’ ROVs carry payloads including high-definition cameras, lasers, sonar, and underwater positioning systems to improve data-acquisition quality and location tagging of defects. Key results from experimental studies in a laboratory on canonical specimens are presented. Furthermore, the technology has been successfully tested in a port — details of the structure, operational arrangement and sample results from the field trial are also presented. Following calibration procedures, ROV based underwater concrete NDT was used to acquire data from the structure in grids and subsequently, identify zones of potential non-homogeneity. Numerous challenges, including interpretation of the time traces obtained, and correlation of results to structural integrity and to structures above water, are discussed in the paper. This work will be of interest to asset owners in marine, water management and power generation industries.

Influence of duty ratio of a pattern source on laser generation of Lamb waves

Abstract: This work explores the generation of Lamb waves and their higher harmonics using a pattern source. An array of laser sources, when used with a short-pulsed laser, generate multiple Lamb modes in a plate-like component. The variation in duty ratio of the pattern source is found to influence the generation of fundamental modes and their harmonics. A two-dimensional finite element model is developed to simulate the narrowband Lamb wave generation process, and validated against targeted experiments. For a ‘square-like’ spatial profile of energy distribution, the second harmonic of the fundamental wave modes is fully suppressed at the duty ratio of 0.5, and the third harmonic of the fundamental wave modes is fully suppressed at duty ratios of 1/3 and 2/3. As the duty ratio increased from 0.2 to 0.8, the variation in amplitude of the fundamental modes is greater than that of the higher order modes. For duty ratio values less than 0.25 and greater than 0.75, the second harmonic component of the fundamental waves had approximately the same amplitude as the fundamental wave modes. Similarly, the third harmonic of the fundamental wave modes had the same amplitude or slightly higher amplitude than the higher order Lamb modes for duty ratio values less than 0.25 and greater than 0.75.

Far-field ultrasonic imaging using hyperlenses

Abstract: Abstract Hyperlenses for ultrasonic imaging in nondestructive evaluation and non-invasive diagnostics have not been widely discussed, likely due to the lack of understanding on their performance, as well as challenges with reception of the elastic wavefield past fine features. This paper discusses the development and application of a cylindrical hyperlens that can magnify subwavelength features and achieve super-resolution in the far-field. A radially symmetric structure composed of alternating metal and water layers is used to demonstrate the hyperlens. Numerical simulations are used to study the performance of cylindrical hyperlenses with regard to their geometrical parameters in imaging defects separated by a subwavelength distance, gaining insight into their construction for the ultrasonic domain. An elegant extension of the concept of cylindrical hyperlens to flat face hyperlens is also discussed, paving the way for a wider practical implementation of the technique. The paper also presents a novel waveguide-based reception technique that uses a conventional ultrasonic transducer as receiver to capture waves exiting from each fin of the hyperlens discretely. A metallic hyperlens is then custom-fabricated, and used to demonstrate for the first time, a super-resolved image with 5X magnification in the ultrasonic domain. The proposed hyperlens and the reception technique are among the first demonstrations in the ultrasonic domain, and well-suited for practical inspections. The results have important implications for higher resolution ultrasonic imaging in industrial and biomedical applications.

Assessment of Structural Integrity of Submerged Concrete Structures Using Quantitative Non-Destructive Techniques Deployed from Remotely Operated Underwater Vehicles (ROV)

Abstract: Marine assets in ports, harbors, and those which are part of our civil infrastructure including dams and bridges undergo persistent structural deterioration over time. Several national and international disasters, in recent times, have proved that these ageing infrastructures pose high risk from both social and economic perspectives. Therefore, periodic inspection of marine assets is essential to detect early deterioration, to schedule maintenance and restoration plans to prolong their life, prevent catastrophic damages and loss of lives.Non-destructive testing of concrete structures is performed using numerous techniques including ultrasonic tomography, ground penetrating radar, and radiography to identify structural anomalies and predict remaining life. However, conventional inspection procedures are limited to areas of the structures which are easily accessible and safe to work. Concrete structures operating underwater, are subject to rates of corrosion and impact damage which are often higher than the part of the structure above water. However, today, there are few quantitative NDT techniques available to inspect submerged concrete assets — qualitative visual inspection by professional divers is the most widely used technique in the industry. Diver-based visual inspection operations are subject to inherent risks, especially when operating under poor water visibility, underwater currents, and confined spaces. They also often mandate shutdown of operations of the asset. Manned underwater operations are also often inadequate to confidently comment on the assets’ integrity. Furthermore, often, several critical defects in concrete structures do not manifest on the surface to be visible in visual inspection routines until their severity is significant. Therefore, to mitigate these risks, this paper introduces a first-of-its-kind quantitative NDT technique for concrete structures operating underwater using robotic remotely operated vehicles (ROV) indigenously by Planys Technologies, an IIT Madras incubated company.The remotely operated vehicle (ROV) can be controlled from a safe location by a pilot. The ROV can be thought of as a platform which can carry payloads to perform operations as per the requirements of the application. ROV Beluga, described in this paper, carries numerous payloads including marine cameras to provide live visual feed for visual inspection and navigation, lasers for quantification of defect dimensions, sonars to conduct waterbed surveys, and transducers to perform NDT. ROV-based operations offer numerous advantages over diver-based operations including unlimited endurance, enhanced visual inspection in turbid waters, improved systems for data acquisition and repeatability.Widely accepted concrete NDT techniques, Schmidt hammer and ultrasonic pulse velocimetry were chosen to be reengineered for underwater inspection on a robotic drone. Key constraints including operational complexity, power and communication, and data acquisition and processing requirements were considered in this process. Preliminary experimental studies performed in a lab-scaled facility, and in real field conditions, described in this paper, demonstrate the potential of ROV-based quantitative NDT of underwater concrete structures. Key realizations and limitations to practical implementation are also described. The work presented in this paper will be of interest to asset owners and managers, enabling them to make confident post-inspection decisions.