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.

Topographic metamaterials for ultrasonic non-destructive evaluation

Abstract: Ultrasound is mainly used in Non-Destructive Evaluation of industrial structures and non-invasive structures and non-invasive diagnostics in medicine, owing to attractive features such as absence of radiation and affordability of associated electronics. Recently there has been much interest in improving the resolution of ultrasonic evaluation using various metamaterials concepts. Phononic crystals and resonant structures have demonstrated the negative refraction and focusing, at different scales. The drawbacks of these are complex design and manufacturing constraints. This paper studies the potential of Topographical Waveguides (TW) for improving the resolution of Ultrasonic inspection, by manipulating the wave field using backward propagating waves. The existence of backward waves in elastic wave guides such as plates, is well known from literature. Due to topographical change, the guided elastic waves in plates, undergo refraction and mode conversion; the forward propagating wave gets converted to backward wave. The phase and group velocity of backward propagating waves are antiparallel. Recent research has studied the phenomenon of anomalous refraction and mode focusing of Zero Group Velocity (ZGV) modes in topographies. The ZGV in thin plates occurs at a specific point, where the group velocity goes to zero while the phase velocity remains finite. At the same wave number the forward and backward propagating waves will interfere, and at that point group velocity becomes zero. This work studies the feasibility of TW lenses for improved resolution of Ultrasonic NDE by using ZGV mode. An aluminium plate, with a step thickness change is considered to show the concept of ZGV. Finite Element (FE) simulation results are presented. Experiments are carried over to validate.

Guided waves for online monitoring of composites

Abstract: Aerospace & Automotive industries make extensive use of composite structures, often with complex geometries. Ultrasonic guided waves are attractive for long-range inspection of large-scale structures. However surface based transducers that are typically used, lead to two-dimensional wave propagation, limiting the applicability of guided wave methods. This work explores the use of embedded waveguides for monitoring delamination type defects in composites. These waveguides confine the wave transmission in one dimension and waves leak only through the opening provided, which enhances the capability to inspect large composite structures with very low attenuation rate. Inaccessible areas can be inspected and inter-laminar delamination detection can be achieved. Live monitoring and assessment of discontinuities can be accomplished effectively by using this mechanism.

Feasibility of ultrasonic guided waves for detecting surface-breaking cracks in laminated composite plate structures

Abstract: Ultrasonic guided waves are attractive for rapid inspection of laminated composite structures, where the cracks developed transverse to the loading direction are the serious type of damage. In this context, the paper presents studies of guided wave interaction with transverse cracks in symmetric and non-symmetric cross-ply composite laminates. In view of this, a two-dimensional finite element approach is used to gain physical insight into the reflection regime. Reflection ratios for GFRP/CFRP cross-ply laminates with various surface-breaking/transverse crack sizes are calculated, and results are presented in frequency spectrum, which shows that the minimum crack depth of 0.0093 of the operating wavelength has been successfully detected. Effect of different ply layup orientations is also considered. This work will be useful for practical guided wave-based inspection of composite plate structures. These points have been added at the abstract section of the revised manuscript.

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.