Experimental study of inhomogeneous deformation in bulk metallic glass and their composite during wedge-like cylindrical indentation

Bulk metallic glasses (BMGs) are amorphous metals with impressive mechanical properties, such as high elastic strain up to 2%, high strength (up to 2% of Young\u27s modulus) and high hardness. Their weight normalized properties exceed the high strength to weight ratio of titanium alloys. Because of the lack of crystalline defects such as grain boundaries and dislocations, they have good corrosion resistance and good formability. The unique die molding properties of BMGs render them as excellent candidates for micro-scale machine parts, pressure sensor, golf clubs and casings. BMG\u27s also exhibit enhanced plastic creep resistance, since homogeneous plastic deformation is inhibited at room temperature. Below the glass transition temperature, BMGs exhibit inhomogeneous plastic flow through the formation of localized shear bands. Under unconfined loading geometry, BMGs fails in a brittle material manner with unstable propagation of a single shear band. However, under confined geometry, BMG\u27s show increased ductility due to the ability to nucleate and propagate multiple shear bands. This dissertation focuses on experimentally analyzing evolution and propagation of the shear bands in BMGs and their composites, by monitoring the deformation mechanisms at the scale of the shear band under confined geometry. Wedge-like cylindrical indentation has been used to provide a stable loading configuration for in-situ observation of the inhomogeneous deformation zone underneath the indenter. High resolution digital camera has been employed to capture surface images of the evolution of the process-zone. An in-house digital image correlation (DIC) program has been developed, utilizing MATLAB commercial software, to calculate the in-plane finite strain distribution at the scale of the shear band. First, the plastic deformation and flow field under the indenter are studied in both aluminum and copper alloys with different grain sizes to verify and validate the analysis protocol. The measured plastic zone size is comparable with the one predicted by the simplified cavity model and there is a unique correlation of the strain distribution along the radial line with different angular positions originating from the indentation center. The deformation zones developed under indenters with different radii are found to be self-similar. In the elastic domain, the measured strain distribution agree with FEM predictions; in the elastic-plastic domain, extra hardening is observed, which could be the result of constrained deformation. Second, the inhomogeneous deformation behavior of Vitreloy-1 bulk metallic glass is examined at room temperature. To overcome the resolution limit of the DIC technique to resolve the strain within a single shear band having 10-20nm width, an alternative method is implemented, addressing the strain jump within the band and the surrounding matrix. The results show that the BMG can deform homogenously to a large elastic strain level of about 4-6% before the onset of inhomogeneous deformation via localized shear bands. Such observation indicates the ability of BMG to withstand such high levels of stresses and strains if unstable shear band can be suppressed from the nucleation from the surface, such as the case of tension or bending. Following the perturbation analysis of Hwang et al (2004) and utilizing the same material parameters, it is found that homogenous nucleation strain is of the same order. The experimental measurements show more subtle details about the kinematics of shear band propagations. The shear band propagates intermittently at the expense of the surrounding matrix stored elastic strain energy. The surrounding matrix ceases to deform, during the activity of the shear band, however, no unloading is observed. The accumulated strain level inside of the shear band is about 3 orders higher than the one in the surrounding matrix. By tracking the strain increments of a single shear band and its surrounding matrix, the deformation filed has been shown to be self-similar, within the surrounding matrix. While the stress state at the observation point is defined by the global indentation filed, the local stress state within the shear band is a simple shear state, with respect to the band propagation direction. Relative to the band-propagation direction and the corresponding normal, the surrounding matrix deforms in a pure shear-state to accommodate shear band deformation. The experimental protocol is also utilized to study the kinematics of shear band initiation, propagation and arrest or hindrance by a secondary ductile phase. The deformation mechanisms in BMG composite with brass particles are examined. The composite is manufactured by warm extrusion of a mix of gas atomized powders of Ni-based BMG and brass. The resulting composite has an elongated particulate structure in the extrusion direction. The fracture toughness and toughening mechanism of the BMG composites are examined in the parallel and normal directions to the extrusion axis. This composite shows highly anisotropic properties along different loading directions. For the normal direction loading, brass reinforcements not only trigger the initial localized shear band, but also modify the crack propagation by crack bridging mechanisms. Also, microcracking is another important toughening mechanism. For the parallel direction loading, interface debonding is the main failure mechanism. Using FEM simulations, it is shown that local fracture is strain-controlled along the normal loading direction and stress controlled along the parallel loading direction. The proposed experimental framework is further extended for fracture match applications in forensic science. The likelihood of matching broken pieces, wherein a macroscopic crack trajectory cannot be established is analyzed via spectral analysis of the 3D fracture surfaces. The surface topographies are acquired using a non-contact 3D optical surface profilometer. A quantitative signature of the fracture surface, employing the different length scales of the fracture process zone is derived and used to establish class and sub-class matching. The details of the algorithm and its applications are detailed in the Appendix

Direct and Inverse Computational Methods for Electromagnetic Scattering in Biological Diagnostics

Scattering theory has had a major roll in twentieth century mathematical physics. Mathematical modeling and algorithms of direct,- and inverse electromagnetic scattering formulation due to biological tissues are investigated. The algorithms are used for a model based illustration technique within the microwave range. A number of methods is given to solve the inverse electromagnetic scattering problem in which the nonlinear and ill-posed nature of the problem are acknowledged.Comment: 61 pages, 5 figure

26th Symposium on Plasma Physics and Technology

List of abstract

TIME DOMAIN SIMULATION FOR SOUND PROPAGATION OVER VARIOUS OBJECTS AND UNDER VORTICAL BACKGROUND CONDITIONS

Acoustic wave propagations have been studied for a long time with both experimental and numerical methods. Most of the analytical solutions for wave propagations are considered for simple environments such as a homogeneous atmospheres. As a result, the analytical solutions are unable to be applied for complicated environments. Numerical methods have become more and more important in acoustics studies after decades of development. The finite difference time-domain method (FDTD) is one of the most commonly used numerical methods in wave propagation studies. Compared with the other methods, the FDTD method is able to include many aspects of sound wave behaviors such as reflection, refraction, and diffraction in the physical problems. In this thesis, the linearized acoustic Euler equations coupled with the immersed boundary method are applied to investigate the sound wave propagation over complex environments. For the three-dimensional simulations of sound wave propagation in long distance, the moving domain method and parallel computing techniques are applied. Based on these approaches, the computational costs are significantly reduced and the simulation efficiency is greatly improved. When looking into the effects of high subsonic vortical flow, a high order WENO scheme is applied for the simulation. In this way the simulation stability can be achieved and the sound scattering of vortical flow can be studied. Then, the numerical scheme is applied to simulate an ultrasonic plane wave propagating through biological tissue. The linearized Euler acoustic equations coupled with the spatial fractional Laplacian operators are used for numerical simulations. The absorption and attenuation effects of the biological lossy media are successfully observed from the simulation results. Throughout this thesis, the simulation results are compared with either experimental measurements or analytical solutions so that the accuracy of the implemented numerical scheme is validated

TIME DOMAIN SIMULATION FOR SOUND PROPAGATION OVER VARIOUS OBJECTS AND UNDER VORTICAL BACKGROUND CONDITIONS

Acoustic wave propagations have been studied for a long time with both experimental and numerical methods. Most of the analytical solutions for wave propagations are considered for simple environments such as a homogeneous atmospheres. As a result, the analytical solutions are unable to be applied for complicated environments. Numerical methods have become more and more important in acoustics studies after decades of development. The finite difference time-domain method (FDTD) is one of the most commonly used numerical methods in wave propagation studies. Compared with the other methods, the FDTD method is able to include many aspects of sound wave behaviors such as reflection, refraction, and diffraction in the physical problems. In this thesis, the linearized acoustic Euler equations coupled with the immersed boundary method are applied to investigate the sound wave propagation over complex environments. For the three-dimensional simulations of sound wave propagation in long distance, the moving domain method and parallel computing techniques are applied. Based on these approaches, the computational costs are significantly reduced and the simulation efficiency is greatly improved. When looking into the effects of high subsonic vortical flow, a high order WENO scheme is applied for the simulation. In this way the simulation stability can be achieved and the sound scattering of vortical flow can be studied. Then, the numerical scheme is applied to simulate an ultrasonic plane wave propagating through biological tissue. The linearized Euler acoustic equations coupled with the spatial fractional Laplacian operators are used for numerical simulations. The absorption and attenuation effects of the biological lossy media are successfully observed from the simulation results. Throughout this thesis, the simulation results are compared with either experimental measurements or analytical solutions so that the accuracy of the implemented numerical scheme is validated

Aerospace medicine and biology: A continuing bibliography with indexes, supplement 218, April 1981

This bibliography lists 161 reports, articles, and other documents introduced into the NASA scientific and technical information system in March 1981

Wave Propagation

A wave is one of the basic physics phenomena observed by mankind since ancient time. The wave is also one of the most-studied physics phenomena that can be well described by mathematics. The study may be the best illustration of what is “science”, which approximates the laws of nature by using human defined symbols, operators, and languages. Having a good understanding of waves and wave propagation can help us to improve the quality of life and provide a pathway for future explorations of the nature and universe. This book introduces some exciting applications and theories to those who have general interests in waves and wave propagations, and provides insights and references to those who are specialized in the areas presented in the book