Vamshi Krishna Reddy Kommareddy

Air‐Coupled Ultrasonic Measurements in Composites

Air‐coupled ultrasound is a non‐contact technique and has clear advantages over water‐coupled testing. Research of air‐coupled ultrasonics, especially using capacitance and micromachined transducers, has been extensively reported in the literature. This paper reports our experience of applying piezoceramic air‐coupled transducers for nondestructive evaluation of composites. The beam profiles of air‐coupled piezoceramic transducers, with and without apodization, were mapped out. The transmission of air‐coupled ultrasonic energy through composite plates of different thickness was measured experimentally; model calculation of the transmission coefficient, taking into account the frequency bandwidth of the transducer, agreed with the measurement results. The occurrence of diffraction phenomenon (“Poisson bright spot”) while imaging flaws in composite laminates was investigated. The resolution of scanned images obtained with air‐coupled transducers was investigated for different frequency, focusing, and apodiz...

Finite Element Modeling of Guided Wave Propagation in Plates

This paper aims at developing a numerical model for guided wave propagation in plates and the interaction of modes with defects using Finite Element Modeling (FEM). Guided waves propagate as extensional, flexural and torsional waves. Theoretically, these modes are infinite in number, but only some of these propagate and the others are attenuated. The dispersion curves for a structure reveal the plausibility of these modes. In this paper, FEM is used to examine interaction of first few symmetric and anti‐symmetric modes independently with the cracks of various sizes in a plate. A time‐frequency representation of the acquired guided wave mode signals will be discussed to show the mode sensitivity with crack size.

Effect of Mass Damping and Stiffness Damping in Micromachined Air Coupled Capacitance Transducer

Performance of MEMS devices relies largely on the interaction between structural actuation membranes and fluids surrounding them. When optimizing these MEMS design it is importent to consider the general characteristics of this fluid‐structure interaction. Finite Element approach is used to study the effect of mass damping and stiffness damping arising due to micro fluid layers in Micro‐machined Air‐coupled Capacitance transducer. Transient dynamics of these systems are completely driven by this fluid‐structure interaction. The aim of this paper is to understand the variations in mass and stiffness damping with frequency of operation. Furthermore, the effect of the mass and stiffness damping with geometrical parameters and transient behavior of the system are discussed.

Sensitivity of Time‐Frequency Representations in the Presence of Noise in Guided Waves NDE

Time frequency representation (TFR) of ultrasonic signals plays an important role in describing the propagation and dispersive effects in thin walled structures. In this paper, Generalized time‐frequency representations viz. the Wigner‐Ville distribution, Choi‐William distribution and Short‐Time Fourier Transform are discussed in the context of mode analysis. A comparison on the estimation of various modes and the obscuring interference terms, due to bilinear structure of TFRs, is presented for lamb waves in aluminum plates. Studies on the performance of TFRs in the presence of strong colored noise are presented.

Finite Element Analysis of Capacitive Micromachined Ultrasonic Transducer (CMUT) for NDE Applications

This paper presents a comparison of three Finite Element approaches for modeling the behavior of a Capacitive Micromachined Ultrasonic Transducer (CMUT). CMUTs have become very popular over the last decade because of the comparable bandwidth, sensitivity and dynamic range with its piezoelectric counterparts. The ease of fabrication is an added advantage. Modeling of CMUTs is a coupled physics problem, which involves solving Electrostatics and Structural interactions simultaneous. Finite Element models of the CMUT are constructed using the commercial code ANSYS (9.0). Three different approaches of solving the coupled field problem are discussed and the results are compared for resonance frequency, collapse voltage, capacitance and electromechanical coupling coefficient. The approaches discussed involve sequentially coupled‐field analysis, direct coupled‐field analysis and reduced order modeling. Detailed results have been presented for the effect of variation in geometrical factors as predicted by the thre...