Showing papers by "Prabhu Rajagopal published in 2017"
TL;DR: A combination of plate-guided ultrasound and air-coupled ultrasonics is used to locate and visualize delaminations in composite laminates using wave visualization method as mentioned in this paper.
Abstract: This article reports on the characterization of delamination damages in composite laminates using wave visualization method. A combination of plate-guided ultrasound and air-coupled ultrasonics is used to locate and visualize delaminations. The study focuses on the physics of Lamb wave propagation and interaction with delaminations at various through-thickness locations and positions. Three-dimensional finite element simulations are used to study, in detail, the changes in wave features such as mode velocity, wavelength and wave refraction in the delamination region. These wave features provide information on the location, position and orientation of the delamination. These studies are validated by experimental measurements. The influence of position of source and delamination on wave refraction in the delamination region is examined. This method also correlates the results obtained from experiments and finite element simulations to theoretical dispersion curves in order to distinctly determine the delami...
38 citations
TL;DR: The experimental demonstration of deep subwavelength ultrasonic imaging of defects in metallic samples with a feature size of λ/25 using holey-structured metamaterial lenses shows how the extraordinary transmission capacity of holey structured metam material comes about by the coupling of higher frequencies in the incident ultrasonic wave field to resonant modes of the lens.
Abstract: This paper reports the experimental demonstration of deep subwavelength ultrasonic imaging of defects in metallic samples with a feature size of λ/25 using holey-structured metamaterial lenses. Optimal dimensions of the metamaterial’s geometric parameters are determined using numerical simulation and the physics of wave propagation through holey lenses. The paper also shows how the extraordinary transmission capacity of holey structured metamaterials comes about by the coupling of higher frequencies in the incident ultrasonic wave field to resonant modes of the lens.
36 citations
TL;DR: Novel multiple bent waveguide method proposed to measure temperature in a chamber using two configurations of bent wires used with single transducer and electronics to improve upon the earlier reported studies using straight waveguides.
21 citations
TL;DR: In this article, a fiber Bragg grating based technique was proposed to detect anomalies or defects in plate structures with transverse bends, both in the case of defect-free and transverse defects.
Abstract: Harnessing of ultrasonic guided waves confined in local features such as bends and welds, known as feature-guided waves, has emerged as a promising technique for non-destructive testing and structural health monitoring of industrial and aerospace structures. This article introduces a fiber Bragg grating based technique which uses feature-guided waves to detect anomalies or defects in plate structures with transverse bends. We are able to obtain good consistency between simulation and experimental results, both in the case of defect-free bent plates and those with transverse defects. Such results establish fiber Bragg gratings as a viable alternative to conventional techniques for structural health monitoring of bent plates.
20 citations
07 Dec 2017
TL;DR: In this paper, a sleeved waveguide is proposed to confine guided waves in one dimension, with leakage to the surrounding media only through specially created openings, thus enhancing the capability to inspect large structures.
Abstract: This article reports the development of a novel embedded acoustic waveguide sensor concept for monitoring the curing process and online health of composite structures. A sleeved waveguide embedded in the composite is proposed to confine guided waves in one dimension, with leakage to the surrounding media only through specially created openings, thus enhancing the capability to inspect large structures. The method is first developed using a rectangular copper strip embedded in an epoxy plate structure having an artificial delamination-type defect. Finite element simulations are used to gain insights on parameters and limitations. The approach is also demonstrated on a more practical bi-layer composite plate with an artificial delamination and an embedded wire waveguide sensor.
12 citations
06 May 2017
TL;DR: In this paper, the authors used the Semi Analytical Finite Element (SAFE) method to study the characteristics of guided wave modes supported by bone-like multi-layered tubular structures.
Abstract: Numerical models based on the Semi Analytical Finite-Element method are used to study the characteristics of guided wave modes supported by bone-like multi-layered tubular structures. The method is first validated using previous literature and experimental studies on phantoms mimicking healthy and osteoporotic conditions of cortical bone, and later used to study a trilayer marrow–bone–tissue system at varying mechanical degradation levels. The results show that bone condition strongly affects the modal properties of axially propagating guided waves and indicates that L(0,3) and F(1,6) are suitable modes for assessing the mechanical condition of the bone. The work here reports suitable modal selection and their dispersion properties which would the aid in development of a transduction mechanism for mechanical assessment of bones.
10 citations
16 Feb 2017
TL;DR: In this paper, a new design of Electromagnetic Acoustic Transducer (EMAT) that can generate a T(0, 1) as a single transducer, rat...
Abstract: Guided waves inspection is a well-established method for the long-range ultrasonic inspection of pipes. Guided waves, used in a pulse-echo arrangement, can inspect a large range of the pipe from a single point as the pipe structure carries the waves over a large distance due to the relatively low attenuation of the wave modes. However, the complexity of the dispersion characteristics of these pipe guided wave modes are well known, and can lead to diffculty interpreting the obtained results. The torsional family of guided wave modes are generally considered to have much simpler dispersion characteristics; especially the fundamental T(0,1) mode, which is nominally non-dispersive, making it particularly useful for guided wave inspection. Torsional waves have been generated by a circumferential ring of transducers to approximate an axi-symmetric load to excite this T(0, 1) mode. Presented here is a new design of Electromagnetic Acoustic Transducer (EMAT) that can generate a T(0, 1) as a single transducer, rat...
10 citations
TL;DR: In this article, the authors studied the propagation of ultrasound in spiral waveguides towards distributed temperature measurements on a plane, where the finite element (FE) approach was used for understanding the velocity behaviour and consequently designing the spiral 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.
9 citations
16 Feb 2017
TL;DR: In this article, a nonlinear mixing technique of two co-directionally travelling Rayleigh surface waves generated and detected using laser ultrasonics is presented, where the optical generation of Rayleigh waves on the specimen is obtained by shadow mask method.
Abstract: This research presents the nonlinear mixing technique of two co-directionally travelling Rayleigh surface waves generated and detected using laser ultrasonics. The optical generation of Rayleigh waves on the specimen is obtained by shadow mask method. In conventional nonlinear measurements, the inherently small higher harmonics are greatly influenced by the nonlinearities caused by coupling variabilities and surface roughness between the transducer and specimen interface. The proposed technique is completely contactless and it should be possible to eliminate this problem. Moreover, the nonlinear mixing phenomenon yields not only the second harmonics, but also the sum and difference frequency components, which can be used to measure the acoustic nonlinearity of the specimen. In this paper, we will be addressing the experimental configurations for this technique. The proposed technique is validated experimentally on Aluminum 7075 alloy specimen.
7 citations
16 Feb 2017
TL;DR: In this article, the sensitivity of axial plate HOMC to notch-like defects, evaluated by calculating wave reflection co-efficient, was investigated using finite element models validated by experiments.
Abstract: Guided ultrasonic waves are widely used for long range inspection. Higher Order Modes Cluster (HOMC), discovered at the author’s research group [1-3] consist of multiple higher order guided wave modes that travel together as a single wave-packet and without appreciable dispersion for distances in the range of meters. These waves not only propagate along the length of the structure but also cover the entire thickness, and in view of the higher frequencies, they can offer improved resolution over conventional low-frequency guided waves. This paper studies the sensitivity of axial plate HOMC to notch-like defects, evaluated by calculating wave reflection co-efficient. The studies are carried out using finite element models validated by experiments. Analysis is presented for better understanding of wave-defect interaction. Advantages and limitations for practical realization of the above approach are also discussed.
4 citations
16 Feb 2017
TL;DR: In this article, a novel approach to developing ultrasonic transducers capable of high-temperature in-situ operation using the principle of magnetostriction has been described, which is extended for operation at 1'o'MHz and at elevated temperatures by amorphous metallic strips as the magnetostrictive core.
Abstract: Online ultrasonic NDE at high-temperature is of much interest to the power, process and automotive industries in view of possible savings in downtime. This paper describes a novel approach to developing ultrasonic transducers capable of high-temperature in-situ operation using the principle of magnetostriction. Preliminary design from previous research by the authors [1] is extended for operation at 1 MHz, and at elevated temperatures by amorphous metallic strips as the magnetostrictive core. Ultrasonic signals in pulse-echo mode are experimentally obtained from the ultrasonic transducer thus developed, in a simulated high-temperature environment of 350 °C for 10 hours. Advantages and challenges for practical deployment of this approach are discussed.
16 Feb 2017
TL;DR: In this paper, the 3D scattering of the Lamb mode S0 from delimitations in composite plates is studied and the effect of ply-layup orientation and through-thickness delamination location in fiber composite laminate has been analyzed.
Abstract: This paper addresses a gap in the literature on the 3-dimensional scattering of the fundamental symmetric Lamb mode S0 from delimitations in composite plates. We study the scattering of low-frequency S0 Lamb mode from a delamination in a stiffened 4-ply CFRP composite plate with [0/0]S ply orientation. Far field scattering coefficients for the S0 Lamb mode are plotted as a function of circumferential position around the delamination using 3D FE simulations. Results show that the delamination size has less influence on S0 Lamb wave scattering in the low-frequency regime where the S0 mode is non-dispersive. Further analysis was done using two-dimensional FE simulation for different ply-layup orientations with S0 Lamb mode. This study shows that ply-layup orientation and through-thickness delamination location in fiber composite laminate have a significant influence on S0 Lamb mode interaction. We also analyzed the interaction of A0 Lamb mode for a few cases. This work will be useful for practical Lamb wave ...
23 Apr 2017
TL;DR: In this article, the authors demonstrate a technique to monitor cross-sectional irregularities in pipes by detecting the feature-guided waves using fiber Bragg grating sensors, which is in good agreement with the results reported in elastic wave literature.
Abstract: Cross-sectional irregularities such as eccentricity are an important problem for pipe and tubing infrastructure. Recent work by the authors shows that such axially extended pipe irregularities can cause confinement and feature-guiding of lower order ultrasonic guided waves. In this paper, we demonstrate a technique to monitor such irregularities in pipes by detecting the feature-guided waves using fiber Bragg grating sensors. Our experimental results are in good agreement with the results reported in elastic wave literature.
22 May 2017
TL;DR: In this paper, the feasibility of TW lenses for improved resolution of ultrasonic NDE by using zero group velocity (ZGV) modes in topographies has been investigated, where an aluminium plate, with a step thickness change is considered to show the concept of ZGV.
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.
16 Feb 2017
TL;DR: In this article, the authors explored the use of embedded waveguides for monitoring delamination type defects in composites, which 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.
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.
TL;DR: A key feature of this design is that the reconfigurability is achieved without additional actuators - helpful in the development of autonomous swarm robots with good maneuverability and efficiency.
Abstract: Maneuverability and propulsive efficiency are of much interest in autonomous underwater robots. In this paper, we present a novel underwater robot design with two reconfigurable and detachable swimming modules that would be capable of offering both maneuverability and propulsive efficiency. They are also capable of reconfiguring automatically to take two different orientations favoring reduced drag in the swimming direction. A key feature of this design is that the reconfigurability is achieved without additional actuators - helpful in the development of autonomous swarm robots with good maneuverability and efficiency.
01 May 2017
TL;DR: In this article, the authors demonstrate the application of a holey-structured metamaterial lens for sub-wavelength imaging of defects in a metallic sample, in the ultrasonic regime.
Abstract: In this paper we demonstrate the application of a holey-structured metamaterial lens for sub-wavelength imaging of defects in a metallic sample, in the ultrasonic regime. This type of lens, operating on the Fabry-Perot resonance principle, has earlier been demonstrated for super-resolution in the acoustic regime, and by the authors for nominal sub-wavelength resolution in the ultrasonic regime. Here we experimentally demonstrate a subwavelength imaging of an artificially created crack of size λ/15 in the aluminium sample: to our knowledge this is the highest resolution achieved in the ultrasonic regime. The subwavelength image obtained with the ultrasonic system incorporated with the holey-structured metamaterial is shown to compare favourably with corresponding results obtained using X-ray Computed Tomography (CT).
16 Feb 2017
TL;DR: In this article, a Holey-structured metamaterial lens was used for sub-wavelength imaging of defects in a metallic sample, in the ultrasonic regime, in order to study longitudinal wave interaction with ideal cracks in isotropic elastic materials.
Abstract: In recent years, various types of acoustic metamaterials have been proposed with capabilities for overcoming the diffraction limit. However, typically such developments only consider the acoustic regime or imaging in liquid media. In this paper we show the application of a holey structured metamaterial lens for sub-wavelength imaging of defects in a metallic sample, in the ultrasonic regime. Finite Element (FE) simulations are used to study longitudinal wave interaction with ideal cracks in isotropic elastic materials. Holey-structured meta-lenses are then used to transmit the scattered waves. We present a super resolution of λ/7 with a subwavelength crack in an aluminium sample, which to the best of our knowledge this is the highest resolution achieved in the ultrasonic regime.