Browsing by Author "Mohammad N. Noori, Committee Member"

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    Asymptotic Wave Solutions for Euler-Bernoulli and Timoshenko Beam by Ray Method and Stationary Phase Method
    (2003-11-18) Liang, Aihua; Taofang Zeng, Committee Member; Mohammad N. Noori, Committee Member; Fuh-Gwo Yuan, Committee Chair
    The behavior of flexural (bending, transverse) waves in beam structures is of basic importance in the stress wave theory and has been studied for many years. Many methods have been attempted to understand the flexural waves in beams based on Euler-Bernoulli and Timoshenko beam theories. Accurate numerical evaluations for transient waves in structures are usually difficult due to the complexity of the governing equations. The present research was aimed at developing asymptotic wave solutions that can effectively describe one-dimensional flexural waves based on Euler-Bernoulli beam theory and Timoshenko beam theory. Two methods were introduced to obtain the asymptotic wave solutions for Timoshenko and Euler-Bernoulli beams. One is stationary phase method, which derives the asymptotic solution directly from the wave solution that can be expressed in terms of the Fourier integral. From the features obtained from the stationary phase method, the ray method is introduced to seek the asymptotic wave solutions from the governing equations of wave motion. The asymptotic solutions obtained by the two methods, as expected, are consistent, and the attributes of the wave motion are very clear to comprehend in the asymptotic solutions. The amplitude function of the wave is composed of an arbitrary function, which can be determined by initial and/or boundary conditions, and a time decay rate proportional to t -1/2. The wave motion depends on one parameter t along the ray x/t=c8. The numerical calculations were then applied to initial-value problems, boundary-value problems. The results show that the asymptotic solutions predict the transient wave behavior very closely for the long time response in beam structures.
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    A Computationally Efficient Free Energy Model for Shape Memory Alloys - Experiments and Theory
    (2004-12-26) Heintze, Olaf; Mohammad N. Noori, Committee Member; Ralph C. Smith, Committee Member; Richard F. Keltie, Committee Member; Stefan Seelecke, Committee Chair
    Shape memory alloys (SMA) belong to the class of active materials and have recently been considered as novel actuation and damping mechanisms in micro- and macro-scale applications. Combined with their advantageous lightweight and high work output characteristics is a complex, highly non-linear and hysteretic material behavior, which is also thermo-mechanically coupled. Due to this complexity, model development for SMA material behavior is a challenging task, and experimental data in particular about the inner hysteresis loops is necessary to gain further understanding and successfully design applications. In this thesis, a single crystal material model is presented and subsequently extended to the more realistic polycrystalline case considering material inhomogeneities, grain impurities and lattice imperfections. A first implementation, based on a stochastic homogenization procedure, provides a very accurate description of the observed phenomena, but also requires very high computation times. A reformulation of the underlying concept leads to a parameterization method, which preserves the advantages of the original method, but dramatically reduces the computation times. It is shown that the material behavior prediction of both models are identical, and the parameterization method is compared extensively to data from tensile experiments with a pseudoelastic SMA wire. Remarkably, the model is able to capture all facets of the material behavior including rate-dependence and minor loops. The versatility of the model also allows for the simulation of SMA actuator behavior including the electrical resistance. Finally, a MEMS device using polycrystalline SMA thin film actuators is experimentally investigated. As a first step, the material behavior of the SMA thin films is presented using strain-temperature and resistance-temperature measurements. Secondly, the performance of the MEMS device was determined for different driving frequencies.