Krishnan Balasubramanian

Surface temperature mapping is crucial for the monitoring and control of an object of interest, such as furnace, reactor pipes carrying hot fluids, or a component under a temperature-dependent process. While the use of waveguides for temperature measurement is well documented in literature, the attachment of the waveguide to a metallic component poses challenges. These include the relationship between the local waveguide temperature and that of the metal component, and wave leakage into the component. In this paper, the authors study the propagation of shear horizontal (SH) guided wave in a strip waveguide and its interaction with the notch embodiments in the waveguide. The effects of the type of notch and its depth on the SH mode characteristics are investigated through simulation studies. The mode of attachment of the waveguide to the metal component is by means a slot made in the component. The area of contact between the waveguide and metal component is optimized such that there is minimum wave leakage into the bulk material. Based on the simulation results, a waveguide strip is fabricated and used to monitor the local surface temperature of a test metal component. The waveguide is calibrated by correlating the time of flight (ToF) shift in the waveforms against reference temperature values. Thereafter, the instantaneous temperature of the metal component is determined from the calibration equations. A set of experimental trials are performed to check for repeatability. The experiments are conducted in near steady-state conditions for better accuracy in the measurements.

In the present study, two goniometry based immersion techniques have been compared for the measurement of elastic constants in isotropic and transversely isotropic materials. Measurements were carried out on transversely isotropic unidirectional (glass-epoxy and graphite-epoxy) composite materials. From the measured velocity data, the elastic constants were determined through a numerical inversion. These techniques (through-transmission and back-reflection) were verified using contact testing, mechanical testing, rule of mixtures estimation and the data provided by the manufacturers. The results obtained and the limitations of each of these techniques are discussed. © 2004 Elsevier Ltd. All rights reserved.

Low frequency guided wave based inspection is an extensively used method for asset management with the advantage of wide area coverage from a single location at the cost of spatial resolution. With the advent of high frequency guided waves, short range inspections with high spatial resolution for monitoring corrosion under pipe supports and tank annular plates has gained widespread interest and acceptance. One of the major challenges in the application of high frequency guided waves in a short range inspection is to attain the desired modal displacements with respect to the application. In this paper, an investigation on the generation and formation of fundamental S0 mode is carried out through numerical simulation and experiments to establish a prenatal zone for guided waves. The effect of frequency, thickness of the plate and frequency-thickness (f∗d) is studied. The investigation reveals the existence of a rudimentary form with similar modal features to the fully developed mode. This study helps in the design and development of a high frequency guided wave generator for particular applications which demands waves with very less sensitivity to the surface and loading during the initial phase which immediately evolves to a more sensitive wave towards the surface on propagation for the detection of shallow defects.

This paper studies the effect of axially uniform eccentricity on the modal structures and velocities of the lower order axisymmetric guided wave mode L(0,2) in circular tubes or pipes. The semi-analytical finite element method is mainly used, supported by fully three-dimensional finite element models and validated using experiments. The studies show that even a small eccentricity in the pipe can cause a loss in the L(0,2) mode axisymmetry, leading to its confinement in the thinned side of the pipe cross-section and also a reduction in mode velocities. The physics of this phenomenon is related to the feature-guiding and mode confinement effects noted in recent years in the literature, particularly studies on waveguides with local cross-section variations and curvature.

The magnetostriction means of generation and reception of ultrasonic guided waves is being used for the longrange inspection of pipelines for in-service damage such as corrosion. The generation of higher-strength ultrasonic guided waves in pipes using a magnetostrictive transducer, due to increased magnetostriction using a flux concentrator, is reported in this paper. A soft magnetic ribbon with negligible magnetostriction was used as a flux concentrator in the magnetostrictive sensor. The amplitude of the signal from a sensor with the flux concentrator at a power level of 10% (of the maximum instrument power level) was found to be comparable with the signal obtained from the sensor without flux concentrator at power levels of 50% of the maximum. The effect of the flux concentrator on different magnetostrictive sensors is also reported in this paper.

Thickness reduction arising due to corrosion or in any other ways can cause severe damages to structures. Hence regular inspection of structures is vital in many industries. One of the factors deciding the replacement of a structure is the minimum thickness remaining in the structure. A novel ultrasonic method is introduced in this paper to precisely evaluate the minimum remnant thickness of structures. This method utilized the cut-off property of SH1 guided wave mode. A continuous broadband SH1 wave mode was required for establishing this method. This was achieved by exciting a range of wavelength and a range of frequency corresponding to the desired SH1 generation region. The range of wavelength generation was achieved by appropriately selecting the magnet spacings in a Lorentz force EMAT. The range of frequency input was achieved using a linear frequency modulated chirp signal. Finite Element models and experiments were conducted to demonstrate the method. These results are in close agreement.

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.

A multi-frequency nonlinear ultrasonic measurement is used to characterize grain size variations and distributions unambiguously. The ultrasonic nonlinearity parameter varies linearly with grain size in the Rayleigh scattering regime but deviates from linear behavior at the Rayleigh-to-stochastic transition zone. Frequency dependence of this parameter is found to be a reliable tool for rapid screening of materials where grain size varies widely.

In this paper, it is aimed to identify flaws in glass fiber reinforced polymer composites by smart skin graphene nano platelet (GNP) spray coating in infrared thermography technique. The initial resistance of GNP was made to 1 kΩ. Characterization of sensor and beam was done with scanning electron microscopy and computed tomography (CT) respectively. The thermo-elastic behaviour was evaluated in uniaxial test. The surface temperature was studied with IR camera and it was observed that the surface coated GNP sensor upon a damage and without the damage specimen retains heat than without coating the sensor. Hereafter testing with 0.1 mm/min, 0.5 mm/min and 1 mm/min, it was found that without damage specimen, the temperature increased to 112.5%, 13.3% and 40% respectively. And temperature increased to 93.2%, 36.7% and 76.4% in the specimen with the damage. Specimen were also tested for spectrum fatigue cyclic load at 0.1 Hz and 1 Hz. Failure peak of laminates has been analyzed with optical microscopy and CT which was correlated with temperature rise. For 0.1 Hz spectrum loading, the specimen with the damage, with and without GNP coated, temperature rose to 2040% after first laminate failure. Similarly, for 0.1 Hz specimen temperature rose to 15,637.5% in case of without damage specimen, with GNP coated than without GNP coated. And in case of 1 Hz spectrum loading with damage specimen, the temperature rose to 105.73% after GNP coated. Similarly, at 1 Hz loading, the temperature rose to 143.07% in case of without damage specimen after GNP coated. GNP skin coated nano-sensor helps in early detection of temperature signals.

Lamb wave tomography offers a new dimension to the challenging field of in situ health monitoring of structures. The possibility of constructing tomograms from a network of sensors generating and sensing Lamb waves in thin, multi-layered, anisotropic composite plates is explored in the present study. It is shown that improved tomograms result when the anisotropic and attenuative characteristics of composite plates are accounted for by (a) using the newly identified energy of the earliest Lamb wave signals as the reconstruction parameter, (b) modifying the sensor configuration from conventional geometries (and thereby also optimizing the number of sensors), and (c) normalizing the Lamb wave energy data of the defective sample with respect to that of the defect-free sample.