Prabhu Rajagopal

Bio: Prabhu Rajagopal is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Ultrasonic sensor & Guided wave testing. The author has an hindex of 18, co-authored 129 publications receiving 1035 citations. Previous affiliations of Prabhu Rajagopal include Imperial College London & Indian Institutes of Technology.

Deep sub-wavelength ultrasonic imaging

Abstract: There is much interest in improving the resolution of ultrasonic inspection, which suffers from large wavelengths typically in the range of millimeters, due to low value of speed of sound in solid media. The authors are interested in achieving this through holey structured metamaterial lenses, and have recently demonstrated an experimental subwavelength resolution of λ/25. However the previous work was in through-transmission mode with reception using Laser Doppler Vibrometer (LDV), which may not be suitable for practical applications. This paper discusses the use of optimized holey structured metalens to achieve a deep sub-wavelength imaging up to λ/18 in through-transmission mode, but using commercially available piezoelectric ultrasonic transducers for both generation and reception of ultrasound.There is much interest in improving the resolution of ultrasonic inspection, which suffers from large wavelengths typically in the range of millimeters, due to low value of speed of sound in solid media. The authors are interested in achieving this through holey structured metamaterial lenses, and have recently demonstrated an experimental subwavelength resolution of λ/25. However the previous work was in through-transmission mode with reception using Laser Doppler Vibrometer (LDV), which may not be suitable for practical applications. This paper discusses the use of optimized holey structured metalens to achieve a deep sub-wavelength imaging up to λ/18 in through-transmission mode, but using commercially available piezoelectric ultrasonic transducers for both generation and reception of ultrasound.

T-BERT -- Model for Sentiment Analysis of Micro-blogs Integrating Topic Model and BERT

Abstract: Sentiment analysis (SA) has become an extensive research area in recent years impacting diverse fields including ecommerce, consumer business, and politics, driven by increasing adoption and usage of social media platforms. It is challenging to extract topics and sentiments from unsupervised short texts emerging in such contexts, as they may contain figurative words, strident data, and co-existence of many possible meanings for a single word or phrase, all contributing to obtaining incorrect topics. Most prior research is based on a specific theme/rhetoric/focused-content on a clean dataset. In the work reported here, the effectiveness of BERT(Bidirectional Encoder Representations from Transformers) in sentiment classification tasks from a raw live dataset taken from a popular microblogging platform is demonstrated. A novel T-BERT framework is proposed to show the enhanced performance obtainable by combining latent topics with contextual BERT embeddings. Numerical experiments were conducted on an ensemble with about 42000 datasets using this http URL platform with a hardware configuration consisting of Nvidia Tesla K80(CUDA), 4 core CPU, 15GB RAM running on an isolated Google Cloud Platform instance. The empirical results show that the model improves in performance while adding topics to BERT and an accuracy rate of 90.81% on sentiment classification using BERT with the proposed approach.

Structural health monitoring of reinforced concrete structures using guided waves

Abstract: Ultrasonic guided waves provide a highly efficient method for non-destructive evaluation and the structural health monitoring (SHM) of solids with finite-cross-sectional dimensions (waveguides). Guided waves are widely used for the inspection of structures such as pipelines, annular tank plates, aircraft wing assemblies, composite radius fillers and wind turbines. They are also attractive for long-term structural health monitoring due to their ability to provide long-range and feature-rich through-structure information from a single transducer location. The use of guided waves in reinforced concrete structures is limited to the monitoring of setting and curing of concrete due to the heterogeneous nature of concrete and its high acoustic impedance which is comparable to the steel reinforcement bars. This leads to a significantly high energy leakage into the embedding concrete making it impossible to use guided waves for the inspection of reinforced concrete structures. Corrosion resistant epoxy coatings on reinforcement coatings are widely used for structure exposed to harsh environmental conditions such as high temperatures, humidity, water, acids, solvents, salts and other chemicals. In this work, the attenuation of the L (0,1) guided waves modes propagating along a 20 mm diameter mild steel is monitored using finite element simulation for a fusion bonded epoxy (FBE)rebar and for plain rebar both embedded in an infinite concrete medium. The attenuation in the epoxy coated rebar is shown to be substantially reduced compared to the plain rebar embedded in concrete. This reduction in guided wave attenuation can make it possible to use guided waves to conduct structural health monitoring of concrete structures.

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.

Probability and Statistical Inference

Abstract: The Foundations of Statistics By Prof. Leonard J. Savage. (Wiley Publications in Statistics.) Pp. xv + 294. (New York; John Wiley and Sons, Inc.; London: Chapman and Hall, Ltd., 1954.) 48s. net.

X-ray computed tomography for additive manufacturing: a review

Abstract: In this review, the use of x-ray computed tomography (XCT) is examined, identifying the requirement for volumetric dimensional measurements in industrial verification of additively manufactured (AM) parts. The XCT technology and AM processes are summarised, and their historical use is documented. The use of XCT and AM as tools for medical reverse engineering is discussed, and the transition of XCT from a tool used solely for imaging to a vital metrological instrument is documented. The current states of the combined technologies are then examined in detail, separated into porosity measurements and general dimensional measurements. In the conclusions of this review, the limitation of resolution on improvement of porosity measurements and the lack of research regarding the measurement of surface texture are identified as the primary barriers to ongoing adoption of XCT in AM. The limitations of both AM and XCT regarding slow speeds and high costs, when compared to other manufacturing and measurement techniques, are also noted as general barriers to continued adoption of XCT and AM.