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.

A new phased array inspection scheme, called Technique to Image using Virtual Array Sources (TIVAS), is proposed to image defects at deeper locations with good lateral resolution. An array of virtual focal spots using electronic beam forming and the expanded aperture through electronic linear scanning was employed to achieve significant increase in the focal depths, with improved SNR, when compared to single element array imaging or the conventional phased array focusing technique. The image reconstruction was performed using the well known synthetic focusing approach. In this paper, the role of the depth of focus and the focal spot size on the performance of TIVAS were studied semi-analytically and through FDTD simulations. It was observed smaller virtual focal spot size provides improved lateral resolution and there is an optimum depth of focus for the smallest focal spot size achievable for a array transducer. Experiments were carried out to validate these findings. © 2011 American Institute of Physics.

A novel ultrasonic waveguide based technique for measuring the moduli of elastic isotropic material as a function of temperature using ultrasonic guided wave modes is presented here. This technique can be utilized for measuring Young’s modulus (E) and Shear Modulus (G) of multiple material using the guided ultrasonic L(0, 1) and T(0, 1) wave modes respectively over a wide range of temperatures (demonstrated here from room temperature to 1200 °C). The specimens used in the experiments here have special embodiments (for instance, a bend) at one end of the waveguide and an ultrasonic guided wave generator (transducer) at the other end for obtaining reflected signals in a pulse-echo mode. The far end of the waveguides with the embodiment is kept inside a heating device such as a temperature-controlled furnace. The time of flight difference (δTOF) were used to measure the moduli at different temperatures. Several materials were tested and the comparison between literature values and measured values were found to be in agreement, for both elastic moduli (E and G) measurements, as a function of temperature. This technique provides significant reduction in time, effort and cost over conventional means of measurement of temperature dependence of elastic moduli.

This paper reports the simultaneous generation of multiple fundamental ultrasonic guided wave modes L(0,1), T(0,1), and F(1,1) on a thin wire-like waveguide (SS-308L) and its interactions with liquid loading in different attenuation dispersion regimes. An application towards liquid level measurements using these dispersion effects was also demonstrated. The finite element method (FEM) was used to understand the mode behavior and their dispersion effects at different operating frequencies and subsequently validated with experiments. In addition, the ideal configuration for the simultaneous generation of at least two modes (L(0,1), T(0,1), or F(1,1)) is reported. These modes were transmitted/received simultaneously on the waveguide by an ultrasonic shear wave transducer aligned at 0◦/45◦/90◦ to the waveguide axis. Level measurement experiments were performed in deionized water and the flexural mode F(1,1) was observed to have distinct dispersion effects at various frequency ranges (i.e., >250 kHz, >500 kHz, and >1000 kHz). The shift in time of flight (TOF) and the central frequency of F(1,1) was continuously measured/monitored and their attenuation dispersion effects were correlated to the liquid level measurements at these three operating regimes. The behavior of ultrasonic guided wave mode F(1,1) when embedded with fluid at three distinct frequency ranges (i.e., >250 kHz, >500 kHz, and >1000 kHz) were studied and the use of low frequency Regime-I (250 kHz) for high range of liquid level measurements and the Regime-II (500 kHz) for low range of liquid level measurements using the F(1,1) mode with high sensitivity is reported.

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. © 2007 American Institute of Physics.

Tubes used in aerospace and petrochemical industries for fluid transport and are subjected to high temperature and stress levels. Over a period of time, these tubes are expected to degrade owing to different creep mechanisms, sometimes leading to catastrophic failure. The damages ranging from formation of isolated cavities to presence of micro cracks are expected as result of creep mechanism. The existing method of ultrasonic inspection involves the generation of a longitudinal wave at appropriate angle of incident in order for the mode to travel across the chord of the tube and then received using an identical transducer. This technique has been shown to be un-reliable due to the variations in the type and orientation of the cracks. Here, a new multi-mode tandem configuration ultrasonic technique, that utilizes the two bulk wave modes in a tandem configuration, i.e. both Longitudinal and Transverse modes propagating along the chord of the tube, has been employed in order to improve the reliability of inspection. The transverse waves were found to be more sensitive to flaw orientations that were more circumferential. Eight different configuration of inspection were carried out using the multi mode inspection technique. Experimental and FEM model validate improved sensitivity and consequently validate the reliability of the tandem approach. © 2013 IEEE.

Continuous structural health monitoring (SHM) uses permanently mounted sensor networks on critical locations of a structural component. In-situ wired sensors require a large amount of cabling for power and data transfer, which can drive up costs of installation and maintenance. Hence the need for developing wireless sensors for SHM. The major obstacles preventing the widespread use of wireless sensor networks (WSN) for SHM is the availability of portable, low cost, low powered, low footprint, and high SNR based instrumentation. This paper presents a wireless sensor system that could be interfaced with piezoelectric transducers for the identification of anomalous events using ultrasonic techniques. Power aware algorithms are used to coordinate the actuator-sensor network interaction with a central processing server, where appropriate signal processing techniques are used to quantify the damage in terms of severity. © 2012 American Institute of Physics.

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. © 2007 American Institute of Physics.

Numerical simulation of ultrasonic wave propagation using methods such as finite element or finite difference is computationally expensive particularly when (a) structural dimensions are high, (b) inspection at higher frequencies (due to short wavelengths), and (c) in complex materials that are not isotropic. This paper discusses a numerical technique, which is similar to FEM, but works in frequency domain and has advantage of more accurate results in quick computational time called the spectral element method (SEM). When the second order partial differential wave equation transformed to frequency domain by Continuous Fourier Transform, the wave equation transforms to ordinary differential equation (ODE) that has exact solution. This paper discusses simulation of Lamb wave modes and Time of Flight Diffraction Technique in isotropic plates. © 2006 American Institute of Physics.

Structural Health Monitoring (SHM) of aircrafts is of great relevance in the present age aircraft industry. The present study demonstrates three techniques that have the potential for the SHM of multi-layered composite structures. The first technique is based on multi-transmitter-multireceiver (MTMR) technique with tomographic methods used for data reconstruction. In the MTMR, the possibility of SHM using algebraic reconstruction techniques (ART) for tomographic imaging with Lamb wave data measured in realistic materials is examined. Defects (through holes and low velocity impact delaminations) were synthetic and have been chosen to simulate impact damage in composite plates. The second technique is a single-transmitter-multi-receiver (STMR) technique that is more compact and uses reconstruction techniques that are analogous to synthetic aperture techniques. The reconstruction algorithm uses summation of the phase shifted signals to image the location of defects, portions of the plate edges, and any reflectors from inherent structural features of the component. The third technique involves a linear array of sensors across a stiffener for the detection of disbanded regions.