Krishnan Balasubramanian

Tensile test experiments were in-situ monitored using Pulsed Eddy Current (PEC) NDE technique for metallic materials. Materials such as Manganese Stainless steel (Mn-SS), SS304, Copper and Aluminum were employed in this study. Experiments were carried out to characterize the changes in PEC response due to the effect of loading pattern either continuous or interrupted loading or due to the effect of loading rate. Plastic deformation induced in the material increases the PEC signal response for all materials studied. Mn-SS material provided the best PEC response due to its property of phase transformation from austenitic (paramagnetic) to martensitic (ferromagnetic) phase as the plastic deformation increases. The effect of loading rate does not appear to influence the PEC response of materials, when the data was analyzed as a percentage of fracture strain. The effect of prior cold work could be identified using the PEC technique by characterizing the slope of PEC signal response in the elastic region when the material was subjected to an interrupted loading/unloading pattern. Offline PEC Measurements were taken along the length of the failed and the plastically deformed specimens. It was observed that from the PEC measurements, the impending failure location could be ascertained. These results suggest that PEC technique could be used as a NDE technique for material characterization and failure location identification. © 2012 The authors and IOS Press. All rights reserved.

This paper describes a technique for measurement of temperatures at multiple locations using the multiple waveguide configurations. A single transducer has been used for transmitting and receiving the torsional wave T(0,1) mode in the waveguides. Here, a single transducer is attached to multiple waveguides of different lengths (each waveguide is designed with a single bend). This method improves upon the earlier reported studies using straight waveguides, where the non-consideration of the temperature gradient issues. The temperature measurement range is from room temperature to the maximum utility temperature of the waveguide material. The time of flight difference (δTOF) of reflected ultrasonic torsional guided wave modes (T(0,1)) from the bend, which is the reference signal, and another signal from the end of the waveguide is utilized to measure the corresponding temperature of the surrounding media. The T(0,1) wave mode is less dispersive as compared to L(0,1) mode in the same material from the early reported work. The wavelength of the T(0,1) mode is significantly smaller than that of L(0,1) mode due to torsional velocity is less than the longitudinal velocity of ultrasonic sensor. Hence, it can be improved the sensitivity of the temperature measurements. This temperature measurement technique is more interest in several industrial applications, where using the furnaces and melters.

Jacket-type steel members are widely used in near and offshore structures wherein tubular members are welded together to either form or protect the load-carrying member. Tubular joints are subject to damage as a result of fatigue, marine growth and corrosion from the environment. These structures are conventionally inspected for loss of wall thickness and pitting to prevent catastrophic damage and improve failure prediction systems using the conventional ultrasonic testing (UT). However, especially in the case of marine structures, direct access to the structure is hindered by marine growth, insulation or coating. Surface preparation is an essential step before conventional nondestructive testing modalities can be used. Marine growth is removed using powered brushes, high-pressure water jets or in some cases, manually using chisels causing the procedure to be time consuming and expensive. An alternative technology which can be used for wall thickness estimation without removing marine growth (that is thicker than 10 mm) is pulsed eddy current (PEC) which uses a stepped input signal to detect wall-thinning areas. In this paper, the authors present a methodology of rapidly estimating thickness of the steel members in the splash-zone and deeper underwater zones using PEC without removing marine growth or insulation on a remotely operated robotic vehicle (ROV). The results are compared to the conventional ultrasonic testing methodology performed both by professional divers and an ROV using a commercially available 2.25 MHz ultrasonic transducer. Key advantages and limitations of the ROV-based PEC system are discussed in detail.

Honeycomb sandwich structure is a regular and periodically repeated array of hexagonal cells used in automobile, aviation, marine, and construction industries. This structure offers a high strength-to-weight ratio, effective acoustic insulation, high energy absorption, and good thermal properties advantages. Variable load application and hazardous work conditions can lead to bond interface weakness in the honeycomb sandwich structure, leading to sudden failure. The existing non-destructive evaluation techniques cannot detect interfacial weakness effectively. This study aims to develop a shear-horizontal (SH) guided wave-based technique to evaluate the interfacial weakness in an aluminium honeycomb sandwich structure. The particle vibration in SH-guided waves is parallel to the adhesive-adherent interface, thus improving sensitivity in detecting interfacial properties. The interaction of SH-guided waves with different levels of interface shear stiffness is studied using a finite element model. A semi-analytical finite element model is solved in the frequency domain to simulate the wave propagation. The spring stiffness approach is adopted to model the interface stiffness. Different cases of interfacial adhesion, ranging from the perfect bond, intermediate and weak bonds, were modelled by changing the transverse spring stiffness constant. Frequency-wavenumber analysis reveals that the incident SH0 wave mode converts to SH0 and S0 modes. The presence of the S0 mode can be used as an indicator of the bond quality. Experiments were carried out on the aluminium-epoxy-aluminium honeycomb core joint using PPM-EMAT to verify the findings of the finite element simulations. The analysis shows that the proposed technique can detect the different adhesion levels rather than classify them as good or bad bonds.

The problem of reconstructing the shape of a 2-D Perfect Electric Conductor (PEC) cylinder from limited view scattered near-field data is considered in the frequency domain. Firstly a modified T-matrix method is proposed for direct scattering problem, which uses the concept of 2-D analogue of T-matrix method to obtain the surface currents and Fast Fourier transforms to speed up the computations. Substantial enhancement in the computation speed is seen compared to the classical T-matrix method. The inverse scattering problem is formulated as a non-linear optimization problem that seeks to minimize the difference between measured data and the simulated data. The proposed inverse methodology is applied for (a) the determination of whether a cylinder is corroded or not, and (b) the shape reconstruction of corroded cylinder over a range of size parameters using Transverse Electric (TE) and Transverse Magnetic (TM) polarization. Synthetic data generated from commercial package (COMSOL) representing pulse-echo mode, of ground penetrating radar (GPR) experiment, is used in lieu of measured data. To exploit the information of scattered TE and TM fields, the inverse problem is formulated using multi-objective optimization. Numerical results over a wide range of size parameter (ka) values show that error in reconstruction are within 0.5% in the range of 1.2

Ultrasonic SHM approaches using bulk or guided waves has been well documented. Here the waves are generate using PZT transduction that requires excitation inputs using either high voltage or high current or both. Several critical applications such as component in explosion susceptible environment, extreme temperature condition, etc. have limitations in deployment of high voltage/current excitation. In this paper, the use of wave-guide-based excitation is explored for plate and pipe like geometries. The waveguide help isolate the excitation part of the ultrasound generation, away from the component that may be in a hostile condition. The ultrasound wave modes excited in the waveguide is coupled to the structure and optimized to generate the desired guided wave modes in the geometry of the structures using Finite Element Wave Propagation models. The reception of these modes are also explored using similar ultrasonic waveguides. The detection of defects, in an SHM mode, using this remote generation approach is also demonstrated on a plate geometry component. This article demonstrates the detection of fundamental symmetric wave mode (S0), shear horizontal waves (SH0) and antisymmetric wave mode (A0) in an steel plate by placing a thin wire-like stainless steel waveguide sensor. In order to uniquely identify the three-fundamental plate-guided modes, we map the experimentally measured group velocities as detected by the waveguide sensor to theoretically obtained group velocity dispersion curves. The analytically obtained FEM results are evaluated experimentally using time-of-flight measurements. In future, this sensor arrangement may be utilized in areas like high-temperature furnaces, engines and aircrafts for detecting cracks.

A technique is proposed to predict crack growth for the estimation of Strain Energy Release Rate (SERR) of Double Cantilever Beam (DCB) bi-metallic specimen, employing ultrasonic Lamb waves. Techniques based on the Time-of-Flight (ToF) of the Turning Lamb Mode (TLM) and Direct Lamb Mode (DLM) explored to determine the crack growth. Sensitivity analysis revealed that the Lamb mode with low velocity is more sensitive to crack growth than that of the high velocity Lamb mode. © (2012) Trans Tech Publications, Switzerland.

Aircraft components exposed to high temperature are usually coated with an insulating layer to protect them from failure due to thermal fatigue. This thin layer, known as thermal barrier coating (TBC), has to be monitored regularly in order to check for its degradation and loss of thickness during operation. Inspection of such components poses a challenge owing to the chemical composition, unevenness, and porous nature of the coating. In this paper, the authors propose a methodology for the feasible implementation of the eddy current technique in order to perform a high-speed inspection of thermal barrier-coated components and evaluate their condition. A set of reference specimen with predetermined TBC thickness values is used for the trials. After preliminary simulation studies to select the optimum parameters, raster scanning is performed on the specimen using an experimental configuration assembled in-house. The results show a variation in TBC thickness within each specimen, which are validated using measurements from the THz time-domain spectroscopy (TDS) method. A thickness map of each specimen is generated after cross-correlation. Furthermore, this eddy current-based approach is implemented for quantitatively evaluating the condition of another batch of TBC samples that were subjected to varying in-service conditions. With a robust setup and proper calibration procedure, the proposed methodology has the potential to be implemented for the fast and reliable inspection of aircraft engine components to ensure structural integrity.

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.

Any NDE process may be considered to involve three systems, each having a unique set of parameters that define its characteristics viz. (a) The Input to the material, (b) The material itself, and (c) The output response measured by the NDE system. Traditionally, the input and the material parameters are assumed known and numerous Forward Models have been developed that predict or estimate the output response function. Over the years, forward models are very well established and serve the key purpose, for improved interpretation of the, as well as to optimize the input parameters to obtain the desired, output response. The other two scenarios i.e. if the output response function in the form of measured data is available, to obtain one of system parameters, i.e. either the input function or the material properties, while the other one is assumed to be known are classified as Inverse Problems. Due to the availability of computational resources, the inverse problem solutions are becoming increasingly feasible. Typical applications include measurement of material properties such as modulus, viscosity, temperature, hardness and stress profiles, etc. This paper will discuss the different techniques and the kinds of problems that have been successfully addressed in the area of NDE and their implications on the expanding horizons in NDE.