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.

Towards a micro-mechanics based understanding of ultrasonic higher harmonic generation

Abstract: The need for micro-mechanics based understanding leading to meso-scale models for understanding relation between microstructure and ultrasonic higher harmonic generation is emphasized. Three important aspects of material behavior, namely tension-compression asymmetry, shear-normal coupling and deformation induced anisotropy that are relevant to ultrasonic higher harmonic generation are identified. Of these, the role of tension-compression asymmetry in micro-scale material behavior on ultrasonic higher harmonic generation is investigated in detail. It is found that the tension-compression asymmetry is directly related to ultrasonic even harmonic generation and an energy based measure is defined to quantify the asymmetry. Using this energy based measure, a homogenization based approach is employed to quantify the acoustic nonlinearity in material with micro-voids and the findings are discussed.

Ultrasonic sensing for noninvasive characterization of oil-water-gas flow in a pipe

Abstract: A technique for noninvasive ultrasonic characterization of multiphase crude oil-water-gas flow is discussed. The proposed method relies on determining the sound speed in the mixture. First, important issues associated with making real-time noninvasive measurements are discussed. Then, signal processing approach adopted to determine the sound speed in the multiphase mixture is presented. Finally, results from controlled experiments on crude oil-water mixture in both the presence and absence of gas are presented.

Simultaneous measurement of ultrasonic longitudinal wave velocities and thicknesses of a two layered media in the absence of an interface echo

Abstract: A measurement technique has been developed to extract the phase information of successive echoes for the simultaneous estimation of thicknesses and ultrasonic velocities of individual layers in a two layered media. The proposed method works in the absence of an interface echo and requires the total thickness of the sample to be known. Experiments have been carried out on two layered samples of white cast iron and gray cast iron with layer thickness variation in the range of 2-8 mm for total thickness variation in the range of 12-13 mm. Comparison with micrographs of a few samples confirmed the model predictions. The model is found to be sensitive to the total sample thickness but fairly insensitive to noise in the data.

Effect of Load and Temperature Changes on Nonlinear Ultrasonic Measurements: Implications for SHM

Abstract: Advances in ultrasonic guided waves make it possible to combine the penetration power of guided waves with the early damage detection capabilities of nonlinear ultrasound to interrogate a fairly large area. However, there is a need to re-examine the definition of the relative nonlinearity parameter in the context of SHM. It is known that the definition of the relative nonlinearity parameter relies on the amplitudes of the fundamental and higher harmonic modes and a change in any of these can affect the relative nonlinearity parameter. For SHM methodologies incorporating nonlinear ultrasound, factors such as transducer operating conditions and environmental effects come into play and may affect the nonlinearity parameter even without significant microstructural changes in the material being interrogated. To bridge this gap, the current study aims to investigate the effect of load and temperature changes on guided wave third harmonic generation from shear horizontal (SH) waves in aluminum plates. SH waves were excited using magnetostrictive transducers and third harmonic measurements were made under increasing static-tensile loads at constant temperature and increasing temperature under no load. The results obtained, along with the implications for SHM incorporating nonlinear ultrasound are discussed. doi: 10.12783/SHM2015/99

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.