Thesis (Engineer)--California Institute of Technology, 1973. / Advisor names found in the Acknowledgments pages of the thesis. Title from home page. Viewed 02/18/2010. Includes bibliographical references.

Lightweight cellular materials such as foams exhibit excellent energy absorption characteristics and are widely used for impact mitigation in a variety of applications. In this study a modeling framework is developed in order to investigate the crushing behavior of Al-alloy open-cell foams under quasi-static and dynamic loadings. Quasi-static crushing produces a response that exhibits a relatively stiff linearly elastic regime that terminates into a load maximum; it is followed by an extended load plateau during which localized cell crushing initiates and gradually spreads throughout the specimen. When most of the cells are crushed the densified material stiffens again. Quasi-static compression is simulated using micromechanically accurate foam models. Skeletal random models are generated from soap froth using the Surface Evolver software. The linear edges of the skeletal microstructure are then dressed with appropriate distributions of solid to match those of ligaments in the actual foams and their relative density. The ligaments are modeled as shear-deformable beams with variable cross sections discretized with beam elements in LS-DYNA, while the Al-alloy is modeled as a finitely deforming elastic-plastic material. Utilization of the beam-to-beam contact algorithm of the code is an essential component of the simulation of crushing. Such models are shown to reproduce all aspects of quasi-static crushing faithfully. Dynamic crushing experiments on the same foam have shown that specimens impacted at velocities of 60 m/s and above develop nearly planar shocks that propagate at well-defined velocities crushing the specimen. The same modeling framework is used to simulate these impact experiments. It is demonstrated that random foam models reproduce essentially all aspects of the dynamic crushing behavior observed experimentally. This includes the formation and propagation of shocks, the stresses at both ends, the Hugoniot strain, and the linear relationship of shock front vs. impact velocities. The same models are also used to examine the transition from quasi-static to shock front type crushing. In addition, a detailed parametric analysis is performed to examine the effect of relative density on the crushing response, from the quasi-static initiation and plateau stresses to the formation of shocks and the associated Hugoniot. / text

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published_or_final_version / abstract / toc / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

In electronic packaging, solder joints in surface mount technology are used for not only electrical connections, but mechanical connections as well. Due to the mismatch of the coefficients of thermal expansion of different components in chip-substrate systems, solder joints under thermal cycles could develop thermal stress inside and therefore experience fatigue failure after a certain number of load cycles. In this work, the disturbed state concept (DSC) model, a unified and hierarchical approach to model a variety of materials such as soils, rocks, ceramics, metals, and alloys, was appropriately modified to characterize a 63Sn-37Pb solder. This includes a modified hardening function that eliminates some inconsistency in the HISS-delta0 model when the bonding stress is nonzero, and a different fully adjusted state that is properly assumed from test data on the 63Sn-37Pb solder. A generalized computer procedure was then developed for 3-D constitutive level back prediction with the DSC model. In addition, a modified computer procedure for parameter determination was proposed and implemented to calculate the relative intact stress-strain curve for simple shear test data automatically. The above procedures for parameter determination and back prediction were used to model simple shear tests of the 63Sn-37Pb solder at different temperatures and strain rates. Based on material properties determined from test data at different combinations of temperature and strain rate, constitutive level back predictions were performed for each test data set using (1) specific material properties and (2) temperature and rate dependent material properties. Further, a 3-D DSC FEA (finite element analysis) program was used to simulate the same stress-strain behaviors of solder joints at different temperatures and strain rates. The results from back prediction and 3-D FEA simulation show that the test data have been better characterized by the modified DSC model. Moreover, 2-D and 3-D DSC FEA programs were employed to study the fatigue failure of a 144-Pin PBGA solder ball under cyclic thermomechanical loading. An accelerated-approximate procedure was incorporated into the 3-D FEA program to reduce the computational effort for fatigue analysis. Results of 3-D FEAs show that the 3-D geometry of a solder joint has significant influences on its fatigue life. The comparison of 2-D and 3-D results with the test results for the 144-pin PBGA solder ball indicates the FEA results are consistent with the initiation of failure observed in laboratory test. In addition, failure criteria based on fractional volume were also proposed for 2-D and 3-D FEAs by calibration of the available test results.

No description available.

In this research the cylindrically guided wave inspection technique is proposed for detecting the anomalies in a pipe. Efficient inspection of pipelines for internal and external damages is a challenging task in the chemical and power industries where long pipelines are used and the pipes are coated by insulating materials. Under traditional methods insulation coatings are removed at selected places, then the pipe wall thickness at these spots is measured by ultrasonic transducers. This is a time-consuming and expensive operation since the operation requires point-to-point examination. Guided wave ultrasonics, proposed in this research, is a much more efficient technique because by this technique long pipes can be inspected by removing insulation at only limited places. Detecting anomalies inside the pipe wall at a specific depth can be realized by correctly selecting a cylindrical guided wave and propagating that mode through the pipe. A new transducer holder mechanism has been designed and fabricated for pipe inspection by cylindrical guided waves. A number of advanced coupling mechanisms developed recently for large plate and pipe inspection require the presence of a coupling fluid between the ultrasonic transducer and the pipe or plate specimen. These mechanisms can be used for inspecting horizontal pipes and plates. Commercially available ultrasonic transducers have been used to generate compressional ultrasonic waves in the coupling medium. Those waves are converted to cylindrical guided waves in the pipe by the new coupling mechanism. The new coupling mechanism presented in this research uses solid material as the coupler and can be used equally well for inspecting horizontal as well as inclined or vertical pipes. The new coupling mechanism has been designed to generate efficiently different guided wave modes in the pipe. Different kinds of anomalies in pipes have been successfully inspected. The preliminary results show that a number of Lamb modes when generated properly by the new coupling mechanism are very sensitive to pipe defects. These experimental results along with the new design of the coupling mechanism are presented in this dissertation.

Uniformly sampled filter-bank transforms and their inverses are introduced and the conditions to obtain perfect reconstruction upon inversion are explored. It is shown that perfect reconstruction requires both filter addition and multiplication and the necessary and sufficient conditions for these operations are given. Examples indicate how to use the conditions to construct perfect-reconstruction synthesis filters from a given set of analysis filters. Additionally, an iterative scheme is presented that achieves exact inversion to an arbitrary accuracy. The methods to obtain synthesis filters are applied to discretizations of the continuous wavelet transform using both finite and infinite impulse response filters. If exact reconstruction is not a requisite, it is possible to improve imperfect-reconstruction filter banks so that their inverse is closer to the input signal. Two methods to achieve such improvement are described. To better understand the discretizations, one has to look at the continuous case. Therefore the discrete-time filter-bank transforms definitions are extended to continuous-time signal processing. It is shown that the Gabor and continuous wavelet transforms are special cases of the continuous-time extension. The methods introduced in the discrete-time case are used to derive all the linear time-invariant synthesis functions of these two transforms. A straightforward generalization of the Gabor and wavelet transforms generates filter banks whose bandwidths can vary arbitrarily with center frequency. These filters are used to create a cochlear transform, i.e., a "mixed" transform that behaves like a Gabor transform at low center frequencies and like a continuous wavelet transform at high center frequencies. The methodology described in this thesis is implemented in a set of algorithms whose complete documentation are given in chapter 4.

Large civil infrastructure systems all over the world have become an integral part of our civilization. The inspection and maintenance of these structures for public safety is a difficult task. The assessment of integrity of such huge structures due to local damages is even more difficult to deal with. The conventional inspections are performed manually, generally by visual examination and sometimes by more advanced techniques like ultrasonic, electromagnetic and fiber optic techniques. These inspections involve human interventions, depend on individual inspector's experience, and are time consuming. Such inspection methods may not be very useful for real time health assessment of a structure in service and as a result are not very helpful in preventing any disastrous situation through early warning. Therefore, it is very important to look for a comprehensive strategy of global integrity monitoring infused with information about local damages in the structure. For local damage assessment the current state of the health monitoring technology lacks a generalized and definitive approach to the identification and localization of damage. In past decades several signal processing tools have been used for solving different health monitoring problems but the commutability of the tools between different problems has been restricted. Fundamental reasons for this shortcoming have never been investigated in detail. In this dissertation an investigation has been carried out employing almost all promising feature extraction tools on a representative problem - a plate with rivet holes. The problem considered has radial cracks around rivet holes in a joint panel of a steel truss bridge. Such defects are very difficult to detect. Although well established, Lamb wave based nondestructive evaluation techniques are revisited and new tools are developed to address this issue. Simulation of the scattered ultrasonic wave field is carried out using the finite element method. This ultrasonic wave field is further analyzed to evaluate the integrity of the structure using various feature extraction (FE) techniques. Joint time-frequency-energy representation is obtained from ultrasonic signals recorded at various locations on the plate (joint panel) and used to extract damage sensitive features. Those features were then used to formulate a new Damage Parameter (DP) for better visualization of the crack. Results are shown to demonstrate the comparative effectiveness of these techniques. It is concluded that any particular FE technique cannot detect all possible sizes and orientations of the crack. It is suggested that the statistical occurrence and pattern of the crack must be visualized through a few selective FE techniques in a sequence. Modeling of the wave scattering phenomenon by conventional numerical techniques such as finite element method requires very fine mesh at high frequencies necessitating heavy computational power. Distributed point source method (DPSM) which is a recently developed semi-analytical technique, is applied to model the scattering of ultrasonic wave field on representative problem geometries and the results are used to diagnose structural damages. DPSM is a newly developed robust mesh-free technique for simulating ultrasonic, electrostatic and electromagnetic field problems. In most of the previous studies the DPSM technique has been applied to model two dimensional surface geometries and relatively simple three dimensional scatterer geometries. It has been very difficult to perform the wave scattering analysis for very complex three-dimensional geometries. This technique has been extended to model wave scattering in an arbitrary geometry. The simulation has been carried out with and without the presence of cracks near the rivet holes.