Showing papers by "Krishnan Balasubramaniam published in 2007"

Semiempirical Models of the Solar Atmosphere. II. The Quiet-Sun Low Chromosphere at Moderate Resolution

Abstract: We present a new, one-dimensional model of the solar atmosphere (called SRPM 305) at moderate angular resolution (~1''-2''). Key characteristics of the SRPM 305 model include (1) a minimum temperature of ~3800 K at a gas pressure of ~80 dyne cm-2 and (2) a rapid temperature rise above the temperature-minimum layer that results in substantial overionization of most elements when compared with LTE calculations. The model calculations reproduce the ~4300 K minimum brightness temperature of the UV continuum (between 1400-1500 A) observed by SUMER and the ~4400 K observed minimum radio-continuum brightness temperature (between wavelengths 0.01 and 100 mm). Neither the UV nor the radio continuum bear on the low-temperature minimum value because their broad intensity contribution functions cause the higher temperatures of the upper chromospheric layers to effectively hide the low minimum temperature region. The SRPM 305 model reproduces the observed intensities of CO lines at 4.466 μm, at both the disk center and near the limb, by using C and O abundances consistent with recent literature low values. The model also reproduces observed intensities of C I spectral lines at 5381 and 8337 A, CH lines at about 4306 A, the CN band head at 3883 A, and the O I lines at 7772, 7774, and 7776 A, respectively. Using the SRPM 305 model, we find no significant abundance variations between the photosphere and the low chromosphere. Consequently, the single-component model presented here matches several apparently contradictory observations and thereby resolves the controversy about the temperature minimum value.

SHM of pipes using torsional waves generated by in situ magnetostrictive tapes

Abstract: Long-range, torsional guided waves generated in pipes using magnetostrictive sensors (MsSs) have great potential for applications to the structural health monitoring (SHM) of hard-to-inspect pipes. This paper reports an improved MsS technique (when compared to related techniques currently used for the NDT of pipes) that uses polymeric magnetic tape material that is suitable for use in a variety of industries as an SHM tool for pipes. Improvements include increased efficiency, reduced cost and increased long-term survivability of the sensor system. Transduction efficiency was increased by reducing the sensor eddy current losses and by using a field concentrator strip. For long-term monitoring, a low-cost magnetic oxide based MsS material (video recording tape) having the required magnetic properties was used. The MsS strips were oriented to generate non-dispersive torsional guided ultrasonic waves that propagate long distances with minimal mode conversion. Further, considering both safety and long-term survivability of the sensor, low-power ultrasonic instrumentation was developed and tested. Measurements reported here demonstrate the sensitivity of this sensor to both radial notches (saw cuts) and drilled holes. Results also show that magnetic anisotropy of the strip plays a role in generating torsional waves. It is envisioned that results obtained from the present study will significantly enhance the ability to monitor the long-term structural health of piping systems.

Genetic algorithm based reconstruction of the elastic moduli of orthotropic plates using an ultrasonic guided wave single-transmitter-multiple-receiver SHM array

Abstract: The reconstruction of all nine unknown elastic moduli of orthotropic plate structures has been achieved using a single-transmitter-multiple-receiver (STMR) compact structural health monitoring (SHM) array. This method uses the velocity measurement of the fundamental guided Lamb wave modes (S0 and A0), generated from a central transmitter, and received by a sparse array of receivers that encircle the transmitter. The measured velocities are then used in an inversion algorithm based on genetic algorithms. A prototype compact STMR array was developed and used in the measurement. Simulated data were used to demonstrate the feasibility of the technique. Experiments were conducted on 3.15 mm graphite–epoxy composite plate using a PZT based STMR array as well as laser vibrometer based displacement measurement. Experimental Lamb wave velocity data were used to validate the present technique. This technique finds application in the areas of material characterization and SHM of anisotropic plate-like structures used in aerospace and automobile components made using fiber reinforced composites.

Genetic algorithm reconstruction of orthotropic composite plate elastic constants from a single non-symmetric plane ultrasonic velocity data

Abstract: This paper reports a Genetic Algorithm (GA) based reconstruction procedure to determine the elastic constants of an orthotropic plate from ultrasonic velocity data Phase velocity measurements are carried out using ultrasonic back-reflection technique on laminated unidirectional graphite–epoxy (0)16 and quasi-isotropic graphite–epoxy (+45, −45, 0, 90)7s fiber reinforced composite plates A forward model to generate the slowness curves from elastic constants has been used to verify the quality of the reconstruction The sensitivity of the chosen GA parameters is studied As expected, out of 9 orthotropic elastic constants to be determined, the C23 and C44 found to be insensitive The GA based reconstruction using data obtained from multiple planes were evaluated and it is shown that the single plane reconstruction at a non-symmetric plane was sufficient for the computation of the seven elastic constants

Are Moreton Waves Coronal Phenomena

Abstract: We report on permeability characteristics of the upper solar atmosphere due to the progression of a Moreton wave. An exceptional Moreton wave is tracked to cover most of the Sun, following an unusually large solar X-ray flare observed on 2003 October 29. Using Hα intensity and Doppler measurements, the Moreton wave is tracked for as long as 12 minutes. Moving outward, the wave circumnavigates strong-field active regions. The wave sweeps through solar magnetic neutral lines, disrupting material from filament and filament channels, thereby accentuating the visibility of the wave. We establish that the requirement for the visibility of a Moreton wave is the necessary presence of higher density material in the layers of the corona, besides reaffirming that Moreton waves are observed only when the speed of the disturbance exceeds Mach 2. We suggest that the cause can be a removal of significant amount of material from the solar upper atmosphere due to a coronal mass ejection.

Simulation of the TOFD technique using the finite element method

Abstract: The ultrasonic Time-of-Flight Diffraction (TOFD) technique is a well-known technique for defect sizing. This technique has been applied to thick sections (>15 mm). The application of the TOFD technique to thin sections like pressure vessels and piping requires simulation of this technique. Simulation gives ideas about the expected results from experiments where real experiments are not possible or are difficult to conduct. Further, simulation helps us to derive the optimum choice of experimental parameters. This paper discusses the application of the finite element technique to simulate the ultrasonic time-of-flight diffraction (TOFD) technique. The diffracted and reflected signals in TOFD techniques for vertical and inclined defects were simulated using plane strain elements. The simulated results are compared with the experimental observations. FEM simulation of wave propagation in complex joints such as a T-joint with embedded flaws is also discussed.

Simplified experimental technique to extract the acoustic radiation induced static strain in solids

Abstract: A simplified experimental technique to measure the acoustic radiation induced static strain during the longitudinal acoustic wave propagation in solids is proposed. Experiments have been carried out to extract the static displacement (dc) component without resorting to electronic filters, as in the case of previously reported experimental measurements. This prevents the influence of the filter time response characteristics on the measurement. The dependence of the static displacement amplitude on the square of the fundamental amplitude of the input wave, its being independent of the burst width of the tone burst, and its abnormal variation at low input amplitudes are reported.

Ultrasonic torsional guided wave sensor for flow front monitoring inside molds.

Abstract: Measuring the extent of flow of viscous fluids inside opaque molds has been a very important parameter in determining the quality of products in the manufacturing process such as injection molding and resin transfer molding. Hence, in this article, an ultrasonic torsional guided wave sensor has been discussed for monitoring the movement of flow front during filling of resins in opaque molds. A pair of piezoelectric normal shear transducers were used for generating and receiving the fundamental ultrasonic torsional guided wave mode in thin copper wires. The torsional mode was excited at one end of the wire, while the flowing viscous fluid progressively wet the other free end of the wire. The time of flight of the transient reflections of this fundamental mode from the air-fluid interface, where the wire enters the resin, was used to measure the position of the fluid flow front. Experiments were conducted on four fluids with different viscosity values. Two postprocessing algorithms were developed for enhancing the transient reflected signal and for suppressing the unwanted stationary signals. The algorithms were tested for cases where the reflected signals showed a poor signal to noise ratio.

Sizing of surface-breaking cracks in complex geometry components by ultrasonic Time-of-Flight Diffraction (TOFD) technique

Abstract: Sizing a surface-breaking crack in a complex geometry component, for example a steam turbine's solid rotor shaft, is a challenging inspection task. This paper deals with experimental work for the evaluation of the Time-of-Flight Diffraction (TOFD) technique for sizing of surface-breaking cracks in such a complex geometry component. The experimental results show that with the help of the mathematical model explained here, TOFD can size such cracks with reasonable accuracy.

Directional filter bank-based segmentation for improved evaluation of nondestructive evaluation images

Abstract: This paper introduces a directional filter bank (DFB) for segmentation of NDE images containing directional information. The DFB is used to split an image into a desired number of sub-band images with each sub-band image containing features belonging only to a given angular range. The DFB is a two-channel decomposition employing the Quincunx sampling matrix and the diamond half band filter pair. The DFB is also designed to incorporate the property of perfect reconstruction or alias free reconstruction. Applications of the DFB towards segmenting C-scan images of fiber-reinforced composites, magnetic flux leakage (MFL) images of seamless tubes, IR images of solar cell panels and optical images for the computation of area coverage in a shot-peening process are discussed.

Multiheight Properties of Moving Magnetic Features

Abstract: We report on spectropolarimetric and dynamical properties of a moving magnetic feature (MMF) around a disk-center sunspot observed using photospheric (Fe I λλ6301.5 and 6302.5) and lower chromospheric (Mg b2 λ5172.7) lines. We find that there are 33% fewer MMFs at the lower chromosphere compared to the photosphere, implying a sophisticated magnetic field geometry of tight low-level loops. A majority of bipolar MMFs are oriented with their neutral line perpendicular to the radial direction. Their "spot-ward" component has the same polarity as the sunspot. The magnetic filling factor is larger for all types of MMFs situated closer to the spot than those situated further away. Bipolar MMFs have a larger filling factor compared to the unipolar ones. Comparison of dI/dλ and Stokes V profile suggests a large magnetic filling factor within the MMFs in the photosphere. Traversing individual MMFs, the Stokes V profiles vary from normal antisymmetric structures to multilobed anomalous profiles. The chromospheric counterpart of multilobed and anomalous photospheric MMF Stokes V profiles are normal and antisymmetric. This suggests that magnetic loops corresponding to MMFs in the lower atmosphere are of mixed polarity and perhaps twisted while they are relatively relaxed in the corresponding upper atmosphere. The temporal evolution of the MMFs shows a transition between anomalous and normal Stokes V profiles.

Magnetic properties of Tb0.28Dy0.57Ho0.15Fe2−xMnx(x=0,0.05,0.1,0.15,0.2)

Abstract: The pseudobinary Laves phase R1R2Fe2 magnetostrictive compounds have one degree of freedom by which the largest anisotropy constant K1 is minimized to obtain the best performing material. However, by including a third rare earth element, additional degree of freedom is obtained, which allows a better way to choose a material for application. In the present study, Ho has been chosen as it exhibits positive magnetostriction, similar to Tb and Dy. Substitution of Mn for Fe leads to the realization of improved magnetostriction values and reduction in the anisotropy. In the present paper magnetic properties of Tb0.28Dy0.57Ho0.15Fe2−xMnx(x=0,0.05,0.1,0.15,0.2) are presented. The compounds have formed in single phase with cubic Laves phase structure. The lattice parameter is found to increase from 7.327to7.347A with increasing the Mn concentration to 0.2. The Fe∕Mn–Fe∕Mn distance is less than 2.66A, the critical distance below which the Fe and Mn moments could be antiferromagnetically ordered, leading to a decre...

Phased Array Ultrasonic Measurement of Fatigue Crack Growth Profiles in Stainless Steel Pipes

Abstract: This paper reports experimental sizing of fatigue crack profiles that are initiated from artificially made circumferential starter notches in stainless steel pipes of 169 mm outer diameter and 14.33 mm thickness, which were subjected to cyclic bending loads in a four point bending load arrangement using two nondestractive evaluation (NDE) methods: (a) phased array ultrasonic technique and (b) alternating current potential drop technique. The crack growth estimated using the two NDE techniques were compared with the beach marks that were present in the fracture surface. A simulation study using the ray tracing method was carried out to model the ultrasonic wave propagation in the test specimen, and the results were compared with the experimental results.

Defect Detection in Carbon-Fiber Composites using Lamb-Wave Tomographic Methods

Abstract: Lamb-wave tomography (LWT) offers a powerful nondestructive technique for the health assessment of large structures as their propagation properties depend on the thickness and the mechanical properties of the material. Development of a fast and accurate algorithm for defect detection is of paramount importance in any structural-health-monitoring (SHM) system. The present study explores the prospects of LWT as a SHM technique with an accent on developing a suitable algorithm for real-time inspection. Projection data is collected by electronically scanning an array of ultrasonic sensors arranged in a modified cross-hole geometry. The data thus collected is investigated to extract energy profile of the traveling waves. Multiplicative algebraic reconstruction technique (MART) algorithms are used as a tool for tomographic reconstruction from a set of multiple independent measurements. The performance of algorithms is evaluated from the point of view of the cost of algorithm, achievable resolution, and accuracy...

Finite difference time domain simulation of the time-of-flight diffraction technique for imaging and sizing cracks

Abstract: This paper reports on the numerical simulations of the Time-of-Flight Diffraction (TOFD) technique to image and size cracks in plate specimens, using the Finite Difference Time Domain (FDTD) method for modelling the ultrasonic wave propagation. Experiments were conducted to validate modelling results for pitch-catch simulations of transducers on samples with calibrated defects of different configurations and sizes in aluminium plate sections. Defects were imaged and sized taking into consideration the velocity of the wave and the times of arrival of the diffracted echoes. Experiments were carried out using a 5 MHz transmitter-receiver transducer pair mounted on suitable wedges to generate a beam of 50° refracted angle within the specimen to image and size cracks and hence validate the simulations. A front/back-wall correction algorithm coupled with a synthetic aperture focusing of the TOFD signals was also demonstrated for improved sizing of defects.

In‐Situ Damage Detection in Plate Structures Using PWAS and Non‐Contact Laser Doppler Velocimeter

Abstract: The capability of embedded piezoelectric wafer active sensors (PWAS) to perform in‐situ Nondestructive evaluation (NDE) for structural health monitoring (SHM) of fiber reinforced polymer (FRP) composite plate like structures is explored The basic principles of Lamb wave transmission and reception with PWAS transducers were verified with simple laboratory experiments, performed on both isotropic and anisotropic plates In the second case, Noncontact measurements for Lamb wave sensing using Laser Doppler Velocimeter were explored

Magnetostrictive Acoustic Transducer Based Torsional Wave Generation in Pipes Using Magnetic Tapes

Abstract: Long range torsional guided wave generation in pipes using magnetostrictive sensor (MsS), made of video magnetic tape material, that has a potential application in nondestructive pipe line testing in oil and gas industry is reported in this paper. Generally, magnetic metal strips of Nickel or amorphous Cobalt alloys are used as magnetostricitive strips in these types of sensors. Though in principle, the technique is an established one, there is still scope for improving its sensitivity by getting rid of eddy current losses in these metallic strips. And also corrosion and oxidation will pose a problem in the long run resulting in signal degradation. To avoid such problems, a magnetic oxide based Ms strip material is a good choice. Further, a field enhancer strip in between the coil and Ms strip can also be added to increase the magnetic field on the Ms strips thus reducing the power to drive the coils. The present sensor is designed to have a video magnetic tape as the magnetostrictive strip, and permalloy strip as field enhancer. Results obtained from a systematic study to assess the quality of the torsional wave generated by this sensor in terms of the amplitude and its frequency dependence with two different orientations of magnetostrictive strips and also the defect sensitivity of the sensor to radial notch and drill hole are reported in this paper.

Algorithm for Health Monitoring of Anisotropic Plates Using Flexible Ultrasonic Patches

Abstract: Fiber reinforced composite plate‐like structures have been used to achieve substantial reductions in the structural weight of both military and commercial aircrafts. For large area and layered structures, damage detection using any conventional testing methods is time consuming. Structural Health Monitoring (SHM) of such structures is seen as a new paradigm that will reduce maintenance costs and increase safety. The aim of the technology is to provide an early indication of physical damage. The early warning provided by an SHM system can then be used to define remedial strategies before the structural damage leads to failure. This paper describes the development and successful demonstration of a SHM system using Smart Flexible Sensor Patch (FSP) that has a built‐in network of Piezoelectric Wafer Active Sensors (PWAS) embedded on a thin film of dielectric material. These PWAS are arranged such that conventional cross‐hole tomography (CHT) as well as modified cross‐hole tomography (MCHT) can be carried out using Lamb waves. Using MCHT, the lateral extent of the damage for barely visible low‐velocity impact damages (BVID) on Composite structures like wing and aileron are were imaged. The conventional CHT was deployed in a region of large aspect‐ratio such as stiffeners. It was observed that disbonds of stiffener in Eleven and other structures may be effectively monitored by this method.

Chromospheric Heating and Low-Chromosphere Modeling

Abstract: Updated modeling of the “quiet” Sun low chromosphere based on existing observations show that at least all the data we examined in detail is consistent with a single model that has a very low temperature minimum and a sharp temperature increase above it. Such a model explains simultaneously the deep CO lines observed on the disk and off the limb, as well as the UV and radio continua and thus solves the controversy regarding the minimum temperature without resorting to “bifurcation”. This results simply from considering both: the spatial extent of the intensity contribution functions, and non-LTE. The model also shows that the structure of the low-chromosphere cannot be unambiguously inferred from any simple diagnostic but rather needs to be examined by forward modeling with consideration of full-NLTE radiative transfer and observations at many wavelengths. In addition, the characteristics of this model are consistent with the magnetic heating of the chromosphere. The mechanism proposed consists of small scale magnetic fields and sudden triggering of their free-energy dissipation by a plasma instability starting at the base of the chromospheric plateau. As a result of such a mechanism a complex spatial structure would result in the upper chromosphere that can hardly be explained in terms of shocks but instead shows magnetic patterns.

Impact Echo Array Technique for Concrete Structure NDT: Simulation Studies

Abstract: This paper simulates an impact‐echo array technique for the two‐dimensional imaging of defects in thick structures such as concrete. A Finite Difference Time Difference (FDTD) Model was employed for the forward model of the impact‐echo method. The absorbing boundary conditions were employed for the efficient modeling of the technique. The signals predicted using the FDTD models from a linear array of impact‐echo transmitter‐receiver system in a multiplexed configuration was used for 2‐D image reconstruction of the cross‐sectional region of the structure. The reconstruction algorithm uses time shifting (migration) of the signals based on point‐source assumption for the impact sources. The images are represented in a typical “B‐scan” representation. Several types of defects were simulated and parametric studies on the different parameters that influence the image quality during imaging of defects in concrete structures were examined.

The 2006 Ultrasonic Benchmark Problem — FDTD Simulations in 2D

Abstract: The 2006 ultrasonic benchmark problem involves pulse‐echo angle beam scanning of a notch located on an inclined planar back surface. The response from a side‐drilled hole is to be used as a reference. The models are to simulate (a) the peak‐to‐peak B‐scan P‐ and SV‐ responses of the slots normalized by the appropriate SDH response and (b) the maximum peak‐to‐peak corner response of the slots (either mode‐converted or not). At CNDE, several simulation tools are being developed to assess/predict UT response for various geometries. The Finite‐Difference‐Time‐Difference (FDTD) scheme is one such simulation tool that has been under development in 1D, 2D and 3D. The FDTD is an explicit time domain tool that can simulate pulse propagation characteristics in acoustic/elastic media. The computational domain is limited by implementing Perfectly Matched Layers (PMLs) at the domain boundaries. We present the results of calculations based on 2D FDTD to determine the response of rectangular shaped surface‐breaking defe...

Material Symmetries from Ultrasonic Velocity Measurements: A Genetic Algorithm Based Blind Inversion Method

Abstract: The determination of material symmetries and principle plane orientations of anisotropic plates, whose planes of symmetries are not known apriori, were calculated using a Genetic Algorithm (GA) based blind inversion method. The ultrasonic phase velocity profiles were used as input data to the inversion. The assumption of a general anisotropy was imposed during the start of each blind inversion. The multi‐parameter solution space of the Genetic Algorithm was exploited to identify the “statistically significant” solution sets of elastic moduli in the geometric coordinate system of the plate, by thresholding the coefficients‐of‐variation (Cv). Using these “statistically significant” elastic moduli, the unknown material symmetry and the principle planes (angles between the geometrical coordinates and the material symmetry coordinates) were evaluated using the method proposed by Cowin and Mehrabadi. This procedure was verified using simulated ultrasonic velocity data sets on material with orthotropic symmetry. Experimental validation was also performed on unidirectional Graphite Epoxy [0]7s fiber reinforced composite plate.