Showing papers by "Prabhu Rajagopal published in 2016"
TL;DR: In this paper, a holey structured metamaterial lens was used for ultrasonic characterization of subwavelength subsurface defects in metallic components. Butler et al. used numerical simulation to investigate the parameters that can help improve the resolution performance of the metammaterial lens, particularly, the addition of end-conditions.
Abstract: This paper presents the implementation of holey structured metamaterial lens for ultrasonic characterization of subwavelength subsurface defects in metallic components. Experimental results are presented, demonstrating ultrasound-based resolution of side drilled through-holes spaced (λ/5) in an aluminum block. Numerical simulation is then used to investigate the parameters that can help improve the resolution performance of the metamaterial lens, particularly, the addition of end-conditions. This work has important implications for higher resolution ultrasonic imaging in the context of practical non-destructive imaging and non-invasive material diagnostics.
43 citations
TL;DR: Semi-Analytical Finite Element simulations revealing the capability of energy confinement carried by two types of guided modes in 90° carbon fiber/epoxy (CF/EP) bends and the feasibility of using such confined guided waves for rapid inspection of bent composite plate structures is discussed.
29 citations
TL;DR: Semi-Analytical Finite Element (SAFE) method simulations reveal the existence of a new family of Shear Horizontal (SHB) type of FGW mode in 90° bends in plate structures, demonstrating the SH-like nature of a bend-confined mode identified in studies of power flow across the bend.
26 citations
TL;DR: In this paper, a single reconfigurable ultrasonic wire waveguide that is configured in the form of a helical spring is used for multi-level temperature measurement with multiple sensing levels.
Abstract: This paper introduces a novel technique for multi-level temperature measurement using a single reconfigurable ultrasonic wire waveguide that is configured in the form of a helical spring. In this embodiment, the multiple sensing levels located along the length of the helical waveguide wire can be repositioned by stretching or collapsing the spring to provide measurements at different desired spacing in a given area/volume. This method can measure over a wide range of temperatures. The transduction is performed using Piezo-electric crystals that are attached to one end of the waveguide which act as transmitter as well as receiver. The wire will have multiple reflector embodiments (notches was used here) that allow reflections of input L(0,1) mode guided ultrasonicwave, in pulse echo mode, back to the crystal. Using the time of fight measurement at multiple predefined reflector locations, the local average temperatures are measured and compared with co-located thermocouples. The finite element modeling simulation was used to study the effect of excitation frequency and the mean coil diameter of the “spring-like” waveguide. This technique improves on the limitations of a straight waveguide technique earlier reported.
18 citations
TL;DR: Shear horizontal guided waves are attractive for structural health monitoring applications in view of the non-dispersive behaviour of the fundamental mode, possibly higher frequencies of operation as mentioned in this paper. But they are difficult to use in practice.
Abstract: Shear horizontal guided waves are attractive for structural health monitoring applications in view of the non-dispersive behaviour of the fundamental mode, possibly higher frequencies of operation ...
17 citations
TL;DR: An ultrasonic torsional mode based technique, configured in the form of a helical “spring-like” waveguide, for multi-level temperature measurement is introduced in this article.
Abstract: This paper introduces an ultrasonic torsional mode based technique, configured in the form of a helical “spring-like” waveguide, for multi-level temperature measurement. The multiple sensing levels can be repositioned by stretching or collapsing the spring to provide simultaneous measurements at different desired spacing in a given area/volume. The transduction is performed using piezo-electric crystals that generate and receive T(0,1) mode in a pulse echo mode. The gage lengths and positions of measurements are based on machining multiple reflector notches in the waveguide at required positions. The time of fight (TOF) measurements between the reflected signals from the notches provide local temperatures that compare well with co-located thermocouples.
17 citations
TL;DR: Semi-analytical finite element simulations are used to demonstrate the existence of a longitudinal guided ultrasonic mode confined or trapped in the Noodle regions, and reveal that this mode has attractive properties for rapid screening of Noodle joints, including strong energy concentration, low dispersion, and attenuation.
Abstract: This paper studies the feasibility of using ultrasonic guided waves for fast inspection of conformal deltoid radius filler or “Noodle” regions of joints in stringer composite structures. Semi-analytical finite element simulations, supported by experiments and three-dimensional finite element models, are used to demonstrate the existence of a longitudinal guided ultrasonic mode confined or trapped in the Noodle regions. Studies reveal that this mode has attractive properties for rapid screening of Noodle joints, including strong energy concentration, low dispersion, and attenuation. Discussing the physics of mode confinement in light of material differences and geometry, the phenomenon is shown to be related to feature-guiding effects noted in literature recently.
13 citations
TL;DR: In this paper the techniques which are already used or are implementable for non-destructive examination of wind turbine blades will be presented shortly and new electromagnetic techniques like active IR testing or terahertz imaging will be more widely presented and some exemplary results of inspections will be shown.
Abstract: Wind turbines are complex composite structures, which condition should be constantly tested. In this paper the techniques which are already used or are implementable for non-destructive examination of wind turbine blades will be presented shortly. New electromagnetic techniques like active IR testing or terahertz imaging will be more widely presented and some exemplary results of inspections will be shown. Streszczenie. Turbiny wiatrowe są złożonymi strukturami kompozytowymi, których stan powinien być stale badany. W niniejszym opracowaniu krótko przedstawiono metody nieniszczącego badania łopat turbin wiatrowych, które są obecnie stosowane lub są możliwe do wdrożenia. Nowe techniki wykorzystujące zjawiska elektromagnetyczne, jak aktywna inspekcja w podczerwieni, czy obrazowanie terahercowe, zostaną szerzej omówione. Zaprezentowane zostaną również przykładowe wyniki badań. Metody nieniszczącego badania łopat turbin wiatrowych
11 citations
10 Feb 2016
TL;DR: In this paper, the authors demonstrate an approach for rapid noncontact air-coupled ultrasonic inspection of composite ailerons with complex cross-sectional profile including thickness changes, curvature and the presence of a number of stiffeners.
Abstract: This paper demonstrates an approach for rapid non-contact air-coupled ultrasonic inspection of composite ailerons with complex cross-sectional profile including thickness changes, curvature and the presence of a number of stiffeners. Low-frequency plate guided ultrasonic modes are used in B-scan mode for the measurements in pitch-catch mode. Appropriate probe holder angles suitable for generating and receiving lower order guided wave modes are discussed. Different embodiments of the pitch-catch tandem positions along and across stiffener and curved regions of the test sample enable a rapid test campaign capturing the feature-rich sample profile. Techniques to distinguish special features in the stiffener are presented.
6 citations
01 Jan 2016
TL;DR: In this paper, 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 [1] or imaging in liquid media [2]. 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. Finite Element (FE) simulations are used to study longitudinal wave interaction with ideal cracks in isotropic elastic materials. Holeystructured meta-lenses are then used to transmit the scattered waves. We experimentally demonstrate a subwavelength resolution of λ/7 with a sub-wavelength notch in an aluminium block, which to the best of our knowledge this is the highest resolution achieved in the ultrasonic regime.
01 Jan 2016
TL;DR: In this article, the feasibility of using noncontact infrared thermography as a potential tool to monitor the CMT welding process is explored, and a correlation between measured online thermal indications with the weld anomalies is successfully attempted and the results are compared with the conventional post-weld NDT inspection methods.
Abstract: In this study, the feasibility of using non-contact Infrared thermography as a potential tool to monitor the CMT welding process is explored. The presence of internal defects such as porosity, lack of filler material deposition and formation of improper weld bead produce perturbations in the surface temperature which can be identified using an Infrared thermography technique. We present recent results obtained from online monitoring of the the dissimilar joining using CMT weld brazing of Aluminum and Steel using a transmission mode measurement approach. The effect of loss of zinc coating on the weldability of the cold metal transfer joining of aluminum to galvanised steel was investigated. A correlation between measured online thermal indications with the weld anomalies is successfully attempted and the results are compared with the conventional post-weld NDT inspection methods.
01 Jan 2016
TL;DR: In this paper, the axial pipe HOMC is generated using comb transduction and evaluated by calculating reflection co-efficient from canonical defects using Finite Element models, which is a novel approach for generation of a new type of high-order guided wave modes.
Abstract: Guided wave non-destructive evaluation (NDE) is the most widely used technique to perform long range inspection. Higher Order Modes Cluster (HOMC), discovered at the author’s research group [1] consist of multiple higher order guided wave modes that travel together as a cluster and without any 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 circumference, and in view of the higher frequencies, they can offer improved resolution over conventional low-frequency guided waves. This paper introduces a novel approach for generation of a new type of HOMC, namely axial pipe HOMC, using comb transduction [2]. Generation of these wave-modes is simulated using Finite Element models. Sensitivity of axial pipe HOMC is evaluated by calculating reflection co-efficient from canonical defects. Advantages and limitations for practical realization of the above approach have also been discussed.
10 Feb 2016
TL;DR: In this paper, the authors explored the feasibility of using feature guided waves (FGW) for rapid screening of 90˚ composite laminated bends for high performance structures such as aircraft and large wind turbine blades.
Abstract: Fiber reinforced composite materials have been increasingly used in high performance structures such as aircraft and large wind turbine blades. 90◦ composite bends are common in reinforcing structural elements, which are prone to defects such as delamination, crack, fatigue, etc. Current techniques are based on local inspection which makes the whole bend area scanning time consuming and tedious. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of 90◦ composite laminated bends. In this study, the behavior of the bend-guided wave in the anisotropic composite material is investigated through modal studies by applying the Semi-Analytical Finite Element (SAFE) method, also 3D Finite Element (FE) simulations are performed to visualize the results and to obtain cross validation. To understand the influence of the anisotropy, three-dimensional dispersion surfaces of the guided modes in flat laminated plates are obtained, showing the dependence of the phase velocity with ...