Krishnan Balasubramanian

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.

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.

Inverse problem solutions in NDE can be broadly classified as model-based approach and system-based approach. In model-based approach an accurate forward model is used in an iterative framework to provide a defect shape that minimizes the error between the measured signal and a simulated signal. However this approach results in repeated executions of a three dimensional forward model in each iteration, making it computationally demanding. This paper presents a direct approach to inversion using principles of time reversal. The feasibility of the approach is demonstrated via application to microwave NDE data. A two-dimensional finite difference time domain model for simulating the propagation of forward and time reversed wave fields is first developed. The key advantage of the approach is that it provides a model-based inversion method that is not iterative. Simulation and experimental results validating the approach are presented. © 2011 American Institute of Physics.

Research in quick Non-Destructive Evaluation methods for field maintenance of aircraft has become a focus of attention due to increase in the use of co-cured skin-stiffened composite structures. When Lamb wave propagates over skin-stiffened structure, the occurrence of multiple modes is unavoidable due to structural features such as stringers and stiffeners, which in turn makes the interpretation of the received wave difficult and limits the defect-detection ability of Lamb waves. Using Finite Element simulations, the propagation of incident S0 Lamb mode in a typical composite T-joint with delamination between the flange and skin (interface) is investigated. Arrival time delays of the out-of-plane mode-converted wave packets are found to be promising indicator for quantitative detection and sizing of the delamination. A novel experimental technique of combining a liquid couplant-free transduction scheme using dry-coupled roller transducer (contact probe) for generating S0 mode and an air-coupled transducer (non-contact probe) as a receiver is proposed. This transduction scheme helps in selectively separating mode-converted wave packets. This investigation also establishes quantitative B-scan imaging method for characterization of delamination in the T-joint. Experimental results show a good agreement with Finite Element predictions.

Online ultrasonic NDE at high-temperature is of much interest to the power, process and automotive industries in view of possible savings in downtime. This paper describes a novel approach to developing ultrasonic transducers capable of high-temperature in-situ operation using the principle of magnetostriction. Preliminary design from previous research by the authors [1] is extended for operation at 1 MHz, and at elevated temperatures by amorphous metallic strips as the magnetostrictive core. Ultrasonic signals in pulse-echo mode are experimentally obtained from the ultrasonic transducer thus developed, in a simulated high-temperature environment of 350°C for 10 hours. Advantages and challenges for practical deployment of this approach are discussed.

This paper presents a method to identify the material parameters of a thermo-mechanical constitutive model using the temperature profile observed in a material during fatigue loading. The temperature profile observed during loading is a function of the damage accumulated in the material and can provide valuable information for the prognosis of material state. Accumulated low cycle fatigue tests were carried out on stainless steel SS304 specimens and the temperature was measured using infrared thermography. The experimental data was then used to show the change in material state through the identification of material parameters. © 2011 American Institute of Physics.

When subjected to continuous tensile loads, a material cools in the elastic regime and heats up in the plastic regime. Temperature changes are also observed when metals are subjected to cyclic stresses of amplitude below the yield strength. The insights into the use of thermomechanical phenomenon as a NDE technique are presented here based on experimental and computational evidence. Infrared thermal camera has been used to monitor the temperature changes in the materials. Measured temperature changes for tensile load on a material subjected to different levels of plastic deformation and for cyclic load on material are presented. Existing theoretical basis to explain the thermomechanical response is discussed in terms of (a) thermoelastic effect for the elastic regime in tensile load, (b) Taylor–Quinney coefficient for the plastic regime in tensile load, and (c) phenomenological models for the cyclic load. For the case of monotonic tensile loading, the extent of initial plastic deformation of a material is experimentally correlated with the observed decrease in temperature in the elastic regime. For the case of cyclic loading leading to stresses below the yield strength, the Kelvin–Voigt model has been found to be sufficient to explain the temperature changes. Amount of plastic deformation accumulated in the material can be deduced based on the decrease in temperature during tensile loading. Closeness between the parameters of Kelvin–Voigt model and that of the grain make this model suitable for understanding the thermomechanical response of polycrystalline materials subjected to stresses below the yield strength. To fully explain the thermoplastic effect beyond the Taylor–Quinney coefficient, a phenomenological model that accounts for the grain resizing, along with rotation and sliding, during the plastic deformation needs to be developed.