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.

Multiple temperature sensors embedded in an ultrasonic “spiral-like” waveguide

Abstract: This paper studies the propagation of ultrasound in spiral waveguides, towards distributed temperature measurements on a plane. Finite Element (FE) approach was used for understanding the velocity behaviour and consequently designing the spiral waveguide. Temperature measurements were experimentally carried out on planar surface inside a hot chamber. Transduction was performed using a piezo-electric crystal that is attached to one end of the waveguide. Lower order axisymmetric guided ultrasonic modes L(0,1) and T(0,1) were employed. Notches were introduced along the waveguide to obtain ultrasonic wave reflections. Time of fight (TOF) differences between the pre-defined reflectors (notches) located on the waveguides were used to infer local temperatures. The ultrasonic temperature measurements were compared with commercially available thermocouples.

Fiber bragg grating based detection of part-thickness cracks in bent composite laminates using feature-guided waves

Abstract: Composite structures with bends are widely used in aerospace and industrial sectors. However health monitoring of such structures is challenging due to their complex topographical features. Recent literature shows that bends in composite laminates can confine and guide ultrasonic energy along their length, known as feature-guided waves (FGW). This article demonstrates a fiber Bragg grating (FBG) based technique using FGW modes for defect detection and identification in bent composite laminates. In addition, the effects of defect depth and excitation frequency on the FGW mode reflection coefficient are reported using 3D finite element simulations. Physical insight into the reflection behavior is discussed based on an analysis of mode interaction with part-thickness cracks.

Characterization of Deep Sub-wavelength Sized Horizontal Cracks Using Holey-Structured Metamaterials

Abstract: Image resolution in classical wave applications is limited by the diffraction limit which corresponds to half the operating wavelength (λ). To achieve higher resolution, ways of overcoming the natural diffraction limits are of interest. In this paper, an experimental demonstration of deep sub-wavelength resolution in the ultrasonic regime using a metamaterial lens is presented. Metamaterial lenses effectively transfer the evanescent waves to the imaging plane, which carry the details of the sub-wavelength features. The successful transmission of the decaying evanescent waves which contain much larger wave vectors than the propagating waves enables to overcome the diffraction limit set by the operating wavelength. We report an imaging technique using optimized holey-structured metamaterial lens to characterize a horizontal crack of size λ/25 in a layered aluminium sample.

Probability of Detection (PoD) Curves Based on Weibull Statistics

Abstract: Probability of detection (PoD) curves are a popular metric for the reliability assessment of Nondestructive Testing (NDT) procedures. However, the classical Berens method for signal response PoD analysis strongly relies on the hypothesis of Gaussian residuals which can be violated in practical conditions. In particular, data from sparse field trials can be scattered and or skewed. Hence, this paper studies the feasibility of assuming a Weibull distribution, which is known for versatility in representing several fundamental statistical states, for regression residuals without modifying the overall Berens framework for PoD curve determination. The proposed ‘Weibull-Berens’ PoD statistics is first shown to compare well with the classical Berens method for an ideal case of Gaussian residuals. The advantages of the method are further demonstrated using a synthesised dataset, as well as a practical case of non-Gaussian residuals arising from reduced number of experimental trials.

Additively manufactured integrated slit mask for laser ultrasonic guided wave inspection

Abstract: Guided ultrasonic waves are attractive for inspection of additively manufactured plate-like components. Illumination of a slit mask by a pulsed laser is one method by which guided ultrasonic waves can be generated. This work proposes a method for generating narrowband ultrasonic guided waves using an additively manufactured slit mask that is integrated onto the component during selective laser melting (SLM) process. Multiple guided wave modes with a dominant wavelength but with different frequencies were generated using the slit mask fabricated using AlSi12 material. The generated modes were identified using the time frequency response of the received signals and dispersion plots. Identifying the modes and its characteristics (frequency, wavelength, phase and group velocity) beforehand facilitates material and defect characterization. A multiphysics numerical model was developed to simulate laser generation of ultrasound and the model was validated using experimental results. The numerical model developed aided in understanding the physics of line arrayed laser ultrasonic generation and was used as a tool to optimize laser parameters. The developed model was used to study the effect of pulse width of the laser on Lamb wave mode generation. It was observed that a pulse width of 100 ns reduced the overall ultrasonic bandwidth to 4.5 MHz thereby limiting the modes to the fundamental modes A0 and S0 for the given wavelength of 0.8 mm. Rayleigh wave studies using a slit mask showed that the rate of decay of the fundamental frequency component was steeper than the rate of decay of the second harmonic component.

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.