Vamshi Krishna Chillara

Bio: Vamshi Krishna Chillara is an academic researcher from Los Alamos National Laboratory. The author has contributed to research in topics: Guided wave testing & Harmonics. The author has an hindex of 11, co-authored 40 publications receiving 569 citations. Previous affiliations of Vamshi Krishna Chillara include Pennsylvania State University.

Guided wave mode conversions across waveguide transitions: A study using frequency domain finite element method

Abstract: The use of frequency domain finite element method (FDFE) for studying the mode conversions across wave guide transitions is presented. Specific examples of shear-horizonatal (SH) and Rayleigh-Lamb (RL) mode excitations are chosen to demonstrate the application of the approach. Frequency domain finite element approach is applied to study the mode conversions across three different wave guide transitions. The stress and displacement field characteristics obtained for each of the above cases is compared to the corresponding results obtained using the time domain finite element approach. Potential advantages of the frequency domain finite element approach for guided wave mode selection are presented.

Higher harmonic guided waves in isotropic weakly non-linear elastic plates

Abstract: Use of nonlinear guided waves appears to be a promising option for material characterization, but appropriate guided wave mode selection is a critical issue. To that end, an outline of a comprehensive analysis as to which guided wave modes have the capability to generate cumulative second harmonic guided waves is presented. The problem of higher harmonic guided wave generation in plates is studied and an approach to predict the nature of higher harmonics is presented.

Coupled electromechanical modeling of piezoelectric disc transducers for low-frequency ultrasonic collimated beam generation

Abstract: Low-frequency ultrasonic collimated beam generation from radial modes of piezoelectric disc transducers is studied using a coupled electromechanical finite element approach. First, resonance and vibration characteristics of the radial modes of the disc transducers are obtained using an eigenfrequency analysis. The vibration patterns obtained from numerical simulation are compared with those obtained from experiments and are in good agreement. Next, ultrasonic beam profiles in water generated from the radial modes of a piezo-disc are studied. It was found that a free piezo-disc generates a Bessel-beam with multiple side-lobes. In contrast, clamping the lateral edges of the piezo-disc results in a well-collimated central beam with reduced side-lobes. This provides a novel transducer design for low-frequency collimated beam generation for imaging through highly attenuating materials

Investigation of resonance enhancement through non-uniform piezoelectric polarization for information storage methodology

Abstract: Non-traditional information storage has become increasingly ubiquitous as a means of providing interactive, environment-specific information. Two popular examples of this are Optical Quick Response (QR) codes and Radio-Frequency Identification (RFID) tags, which are used in product tags, anti-theft systems, tamper-evident seals, logistics, etc. From a security point-of-view though, these two examples have unique drawbacks. Optical QR codes can be easily viewed and replicated because they are limited to the surface of objects, and RFID tags are prone to remote RFID skimming. With this in mind, we are investigating a PZT-based information storage method which has visibly indistinguishable features, is incompatible with skimming, and can be used with a QR-type encoding scheme. This information storage method involves a frequency-dependent array of transducers with engineered resonance profiles. In this proceedings, we investigate methods of engineering these resonance profiles for the case of a square PZT wafer. Specifically, we investigate the use of non-uniform poling to enhance specific resonances. Simulation results and a discussion of changes to the resonance characteristics resulting from non-uniform poling are included.

Nonlinear Guided Waves for Continuous Material Microstructure State Awareness

Abstract: This paper summarizes the characteristics of nonlinear ultrasonic guided waves and demonstrates the use of the nondispersive fundamental shear horizontal wave mode to generate a third harmonic that is sensitive to plastic strain and fatigue. Experimental results for an aluminum alloy plate are presented within the context of the current understanding of nonlinear guided waves. Due to the sensitive nature of higher harmonic measurements, instrumentation nonlinearities are discussed. In two different sets of experiments, the third harmonic generation is shown to be dependent upon (i) the plastic strain and (ii) the number of fatigue cycles. Thus, higher harmonic generation can provide material state awareness throughout the service life, especially before a macroscale crack initiates.Copyright © 2014 by ASME

Review of nonlinear ultrasonic guided wave nondestructive evaluation: theory, numerics, and experiments

Abstract: Interest in using the higher harmonic generation of ultrasonic guided wave modes for nondestructive evaluation continues to grow tremendously as the understanding of nonlinear guided wave propagation has enabled further analysis. The combination of the attractive properties of guided waves with the attractive properties of higher harmonic generation provides a very unique potential for characterization of incipient damage, particularly in plate and shell structures. Guided waves can propagate relatively long distances, provide access to hidden structural components, have various displacement polarizations, and provide many opportunities for mode conversions due to their multimode character. Moreover, higher harmonic generation is sensitive to changing aspects of the microstructures such as to the dislocation density, precipitates, inclusions, and voids. We review the recent advances in the theory of nonlinear guided waves, as well as the numerical simulations and experiments that demonstrate their utility.