Full-wave modeling of ultrasonic scattering for non-destructive evaluation

The physical modeling and simulation of nondestructive evaluation (NDE) measurements has a major role in the advancement of NDE and structural health monitoring (SHM). In ultrasonic NDE (UNDE) simulations, evaluating the scattering of ultrasound by defects is a computationally-intensive process. Many UNDE system models treat the scattering process using exact analytical methods or high-frequency approximations such as the Kirchhoff approximation (KA) to make the simulation effort tractable. These methods naturally have a limited scope. This thesis aims to supplement the existing scattering models with fast and memory-efficient full-wave models that are based on the boundary element method (BEM). For computational efficiency, such full-wave models should be applied only to those problems wherein the existing approximation methods are not suitable. Therefore, the adequacy of different scattering models for representing various test scenarios has to be studied. Although analyzing scattering models by themselves is helpful, their true adequacy is revealed only when they are combined with models of other elements of the NDE system, and the resulting predictions are evaluated against measurements. Very few comprehensive studies of this nature exist, particularly for full-wave scattering models. To fill this gap, two different scattering models-- the KA and a boundary-element method-- are integrated into a UNDE system model in this work, and their predictions for standard measurement outputs are compared with experimental data for various benchmark problems. This quantitative comparison serves as a guideline for selecting between the KA and full-wave scattering models for performing UNDE simulations. In accordance with theoretical expectations, the KA is shown to be inappropriate for modeling penetrable (inclusion-type) defects and non-specular scattering, such as diffraction from thin cracks above certain angles of incidence. A key challenge to the use of full-wave scattering methods in UNDE system models is the high computational cost incurred during simulations. Whereas the development of fast finite element methods (FEM) has inspired various applications of the FEM for ultrasound modeling in 3D heterogeneous and anisotropic media, very few applications of the BEM exist despite the progress in accelerated BEMs for elastodynamics. The BEM is highly efficient for modeling scattering from arbitrary shaped 3D defects in homogeneous isotropic media due to a reduction in the dimensionality of the scattering problem, and this potential has not been exploited for UNDE. Therefore, building on recent developments, this work proposes a fast and memory-efficient implementation of the BEM for elastic-wave scattering in UNDE applications. This method features three crucial elements that provide robustness and fast convergence. They include the use of (1) high-order discretization methods for fast convergence, (2) the combined-field integral equation (CFIE) formulation for overcoming the fictitious eigenfrequency problem, and (3) the multi-level fast-multipole algorithm (MLFMA) for reducing the computational time and memory resource complexity. Although numerical implementations based on a subset of these three elements are reported in the literature, the implementation presented in this thesis is the first to combine all three. Some numerical examples are presented to demonstrate the importance of these elements in making the BEM viable for practical applications in UNDE. This thesis contains the first implementation of the diagonal-form MLFMA for solving the CFIE formulation for elastic wave scattering without using any global regularization techniques that reduce hypersingular integrals into less singular ones

Cooperative diversity techniques for high-throughput wireless relay networks

Relay communications has attracted a growing interest in wireless communications with application to various enhanced technologies. This thesis considers a number of issues related to data throughput in various wireless relay network models. Particularly, new implementations of network coding (NC) and space-time coding (STC) techniques are investigated to offer various means of achieving high-throughput relay communications. Firstly, this thesis investigates different practical automatic repeat request (ARQ) retransmission protocols based on NC for two-way wireless relay networks to improve throughput efficiency. Two improved NC-based ARQ schemes are designed based on go-back-N and selective-repeat (SR) protocols. Addressing ARQ issues in multisource multidestination relay networks, a new NC-based ARQ protocol is proposed and two packet-combination algorithms are developed for retransmissions at relay and sources to significantly improve the throughput. In relation to the concept of channel quality indicator (CQI) reporting in two-way relay networks, two new efficient CQI reporting schemes are designed based on NC to improve the system throughput by allowing two terminals to simultaneously estimate the CQI of the distant terminal-relay link without incurring additional overhead. The transmission time for CQI feedback at the relays is reduced by half while the increase in complexity and the loss of performance are shown to be negligible. Furthermore, a low-complexity relay selection scheme is suggested to reduce the relay searching complexity. For the acknowledgment (ACK) process, this thesis proposes a new block ACK scheme based on NC to significantly reduce the ACK overheads and therefore produce an enhanced throughput. The proposed scheme is also shown to improve the reliability of block ACK transmission and reduce the number of data retransmissions for a higher system throughput. Additionally, this thesis presents a new cooperative retransmission scheme based on relay cooperation and NC to considerably reduce the number of retransmission packets and im- prove the reliability of retransmissions for a more power efficient and higher throughput system with non-overlapped retransmissions. Moreover, two relay selection schemes are recommended to determine the optimised number of relays for the retransmission. Finally, with respect to cognitive wireless relay networks (CWRNs), this thesis proposes a new cooperative spectrum sensing (CSS) scheme to improve the spectrum sensing performance and design a new CSS scheme based on NC for three-hop CWRNs to improve system throughput. Furthermore, a new distributed space-time-frequency block code (DSTFBC) is designed for a two- hop nonregenerative CWRN over frequency-selective fading channels. The proposed DSTFBC design achieves higher data rate, spatial diversity gain, and decoupling detection of data blocks at all destination nodes with a low-complexity receiver structure

Probabilistic Modeling Approach to Crack Nucleation from Forging Flaws

Der Prozess der Rissbildung aus Schmiedefehlern ist in der Literatur kaum beschrieben. In dieser Arbeit wurde dies durch umfangreiche Experimente und verschiedene Modellierungsans ätze gründlich untersucht. Sie ist eine Fortsetzung des Probabilistic Fracture Mechanics (ProbFM) Projektes, welches eine konservative Bewertung des Versagensrisikos basierend auf dem Wachstum von Ermüdungsrissen aus inhärenten Schmiedefehlern in großen Hochleistungsgasturbinen ermöglicht. Der Fokus der Versuchsreihe lag auf der Charakterisierung von typischen Schmiedefehlern und der Quantifizierung der Nukleationslebensdauer durch zyklische Belastung des Materials. Die Nukleationslebensdauer wurde definiert als die Anzahl der Belastungszyklen, die erforderlich sind, um einen scharfen Riss aus einem anfänglichen Fehler zu erzeugen. Dieser Teil des Lebenszyklus wird normalerweise beim Design ignoriert und konservativ gleich null Zyklen angenommen. Aus den Experimenten wissen wir, dass dies nicht stimmt und dass oft etwa 50% der Lebensdauer auf den Nukleationsprozess entfallen. Das Hauptziel der Untersuchungen ist die Entwicklung eines effektiven Modellierungsansatzes zur Quantifizierung der Nukleationslebensdauer von Fehlern unter variierenden Belastungsbedingungen. Dieser Ansatz muss dann in ein Engineering- Tool eingebettet werden, welches zum Design realer Komponenten verwendet wird. Es wurden drei verschiedene Modellierungsansätze untersucht, von denen einer in ProbFM implementiert wurde. Im ersten Ansatz wird der Fehler als eingebettetes Ellipsoid innerhalb des Matrixmaterials mit einer Finite-Elemente-Methode modelliert. Das Dehnungsfeld um das Ellipsoid, das sich aus den angelegten Spannungen und Temperaturen ergibt, dient als Eingabe für die lokale probabilistische LCF Bewertung. Bei dieser Bewertung wird die Wahrscheinlichkeit für Rissbildung als Oberfl¨achenintegral der lokalen Hazarddichte berechnet. Die zugrunde liegende Modellannahme ist, dass die lokale LCF-Lebensdauer Weibull verteilt ist. Der zweite und dritte Versuch basieren auf der Annahme eines Fehlers, der als flacher Oberflächenbereich modelliert ist. Auch hier wird die lokale probabilistische LCF-Lebensdauer berechnet und, in einem Fall, mit einem Lebensdauermultiplikationsfaktor w und, in dem anderen Fall, mit einem Spannungskonzentrationsfaktor Kt korrigiert. Das flächenbasierte Modell mit dem kalibrierten Spannungskonzentrationsfaktor passt am besten zu den experimentellen Ergebnissen und wird zur Implementierung vorgeschlagen. Der Konservatismus wurde während der gesamten Analyse berücksichtigt, um ein zuverlässiges Modell zu gewährleisten, das für eine technische Anwendung geeignet ist. Das resultierende Modell wurde in einer Zuverlässigkeitsbewertung eines realen Rotorscheibendesigns vorgestellt. Die erhaltenen Ausfallwahrscheinlichkeiten sind geringer, wenn die Nukleationslebensdauer berücksichtigt wird, und die Lebensdauer der Komponenten kann potenziell verlängert werden

Mechanisms and interfaces for software-extended coherent shared memory

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1994.Includes bibliographical references (p. 140-146).by David L. Chaiken.Ph.D

Computational Methods for Crashworthiness

Presentations and discussions from the joint UVA/NASA Workshop on Computational Methods for Crashworthiness held at Langley Research Center on 2-3 Sep. 1992 are included. The presentations addressed activities in the area of impact dynamics. Workshop attendees represented NASA, the Army and Air Force, the Lawrence Livermore and Sandia National Laboratories, the aircraft and automotive industries, and academia. The workshop objectives were to assess the state-of-technology in the numerical simulation of crash and to provide guidelines for future research

Review of NASA Sponsored Research at the Experimental Astronomy Laboratory

Technical details reviewed on NASA sponsored research at Experimental Astronomy Laborator

ICASE

This report summarizes research conducted at the Institute for Computer Applications in Science and Engineering in the areas of (1) applied and numerical mathematics, including numerical analysis and algorithm development; (2) theoretical and computational research in fluid mechanics in selected areas of interest, including acoustics and combustion; (3) experimental research in transition and turbulence and aerodynamics involving Langley facilities and scientists; and (4) computer science