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Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Science. / Microfiche copy available in Archives and Science. / Vita. / Bibliography: leaves 345-370. / Ph.D.

Four different thermoviscoplastic relations, namely, the Litonski-Batra, the Johnson-Cook, the Bodner-Partom and the power law are used to model the thermoviscoplastic response of a material. Each one of these relations accounts for strain hardening, strain-rate hardening and thermal softening of the material. The material parameters in these relations are found by solving an initial-boundary-value problem corresponding to simple shearing deformations so that the computed effective stress vs. the effective plastic strain curves match closely with the experimental data of Marchand and Duffy who tested thin-walled HY-100 steel tubes in torsion. These four viscoplastic relations are used to analyze dynamic thermomechanical deformations of a prenotched plate impacted on the notched side by a cylindrical projectile made of the same material as the plate. The impact loading on the contact surface is simulated by prescribing the time history of the normal component of velocity and null tangential tractions. A plane strain state of deformation is assumed to prevail in the plate and its deformations are studied for different values of the impact speed. The in-house developed finite element code employs constant strain triangular elements, one point integration rule, and a lumped mass matrix. The Lagrangian description of motion is used to describe deformations of the plate. The coupled nonlinear partial differential equations are first reduced to coupled nonlinear ordinary differential equations (ODEs) by using the Galerkin approximation. The ODEs are integrated by using the stiff solver, LSODE, which adaptively adjusts the time step size and computes the solution within the prescribed accuracy. Results computed with the four constitutive relations are found to be qualitatively similar to each other and the general trends agree with the experimental observations in the sense that at low speed of impact, a brittle failure ensues at a point on the upper surface of the notch tip. However, at high impact speeds, a ductile failure in the form of a shear band initiates first from a point on the lower surface of the notch tip. The predicted speed at which the failure mode transitions from brittle to ductile is different for the four viscoplastic relations. Results have been computed using the Bodner-Partom law to study the effects of the notch tip radius and the presence of a circular hole ahead of the notch-tip. For sharp elliptic notch tips, it is found that there is no failure transition speed and the ductile failure always preceeded the brittle failure for the range of the impact speeds studied. For the hole located on the axis of the circular notch tip, the brittle failure always preceeded the ductile failure and it initiated at a point on the lower surface of the circular hole. / Ph. D.

Previous mathematical modeling work has produced a transfer function that relates otoconial layer displacement to stimulus acceleration. Due to the complexity of this transfer function, time domain solutions may be obtained only through numerical methods. In the current work, several approximations are introduced to the transfer function that result in its simplification. This simplified version can be inverted to yield analytic time domain solutions. Results from a frequency response analysis of the simplified transfer function are compared with the same results from the complete transfer function, and with mammalian first-order neuron frequency response data. There is good agreement in the comparisons. Time domain solutions of the approximation are compared to numerical solutions of the full transfer function, and again there is a good match. System time constants are calculated from the simplified transfer function. A 2-D finite element model of a mammalian utricular macula is presented. Physical dimensions used in the model are taken from mammalian anatomical studies. Values for the material properties of the problem are not readily available; however, ranges are chosen to produce realistic physiologic behavior. Deflections predicted by this model show that a single value for hair bundle stiffness throughout the organ is inadequate for the organ to respond to the entire range of human acceleration perception. Therefore, it is necessary for a range of hair bundle stiffnesses to exist in each organ. Natural frequencies calculated in this model support previous studies on vestibular damage due to low frequency sound. Divers exposed to high-intensity underwater sound have experienced symptoms attributed to vestibular stimulation. An in-water video-oculography (VOG) system was developed to monitor divers' eye movements, particularly torsional, during exposure to varying underwater sound signals. The system included an underwater closed-circuit video camera with infrared lights attached to the diver's mask with an adjustable mounting bracket. The video image was sent to a surface control room for real-time and post-experiment processing. Six divers at 60 feet in open water received 15 minutes daily cumulative exposure of 240-320 Hertz underwater sound at 160 dB re 1 mPa for 10 days. No spontaneous primary position nystagmus, horizontal, vertical or torsional, was detected in any diver. This experiment was the first successful attempt to record and analyze eye movements underwater. / Ph. D.

Multidisciplinary interactions are expected to play a significant role in the design of future high-performance aircraft (Blended-Wing Body, Truss-Braced wing, High Speed Civil transport, High-Altitude Long Endurance aircraft and future military aircraft). Also, the availability of supercomputers has made it now possible to employ high-fidelity models (Computational Fluid Dynamics for fluids and detailed finite element models for structures) at the preliminary design stage. A necessary step at that stage is to calculate the wing angle-of-attack at which the wing will generate the desired lift for the specific flight maneuver. Determination of this angle, a simple affair when the wing is rigid and the flow regime linear, becomes difficult when the wing is flexible and the flow regime non-linear. To solve this inherently nonlinear problem, a Newton's method type algorithm is developed to simultaneously calculate the deflection and the angle of attack. The present algorithm requires the sensitivity of the aerodynamic pressure with respect to each of the generalized displacement coordinates needed to represent the structural displacement. This sensitivity data is easy to determine analytically when the flow regime is linear. The present algorithm uses a finite difference method to obtain these sensitivities and thus requires only the pressure data and the surface geometry from the aerodynamic model. This makes it ideally suited for nonlinear aerodynamics for which it is difficult to obtain the sensitivity analytically. The present algorithm requires the CFD code to be run for each of the generalized coordinates. Therefore, to reduce the number of generalized coordinates considerably, we employ the modal superposition approach to represent the structural displacements. Results available for the Aeroelastic Research Wing (ARW) are used to evaluate the performance of the modal superposition approach. Calculations are made at a fixed angle of attack and the results are compared to both the experimental results obtained at NASA Langley Research Center, and computational results obtained by the researchers at NASA Ames Research Center. Two CFD codes are used to demonstrate the modular nature of this research. Similarly, two separate Finite Element codes are used to generate the structural data, demonstrating that the algorithm is not dependent on using specific codes. The developed algorithm is tested for a wing, used for in-house aeroelasticity research at Boeing (previously McDonnell Douglas) Long Beach. The trim angle of attack is calculated for a range of desired lift values. In addition to the Newton's method algorithm, a non derivative method (NDM) based on fixed point iteration, typical of fixed angle of attack calculations in aeroelasticity, is employed. The NDM, which has been extended to be able to calculate trim angle of attack, is used for one of the cases. The Newton's method calculation converges in fewer iterations, but requires more CPU time than the NDM method. The NDM, however, results in a slightly different value of the trim angle of attack. It should be noted that NDM will converge in a larger number of iterations as the dynamic pressure increases. For one value of the desired lift, both viscous and inviscid results were generated. The use of the inviscid flow model while not resulting in a markedly different value for the trim angle of attack, does result in a noticeable difference both in the wing deflection and the span loading when compared to the viscous results. A crude (coarse-grain) parallel methodology was used in some of the calculations in this research. Although the codes were not parallelized, the use of modal superposition made it possible to compute the sensitivity terms on different processors of an IBM SP/2. This resulted in a decrease in wall clock time for these calculations. However, even with the parallel methodology, the CPU times involved may be prohibitive (approximately 5 days per Newton iteration) to any practical application of this method for wing analysis and design. Future work must concentrate on reducing these CPU times. Two possibilities: (i) The use of alternative basis vectors to further reduce the number of basis vectors used to represent the structural displacement, and (ii) The use of more efficient methods for obtaining the flow field sensitivities. The former will reduce the number of CFD analyses required the latter the CPU time per CFD analysis. NOTE: (03/2007) An updated copy of this ETD was added after there were patron reports of problems with the file. / Ph. D.

Characterization of composite materials, such as Asphalt Concrete (AC) and other engineering materials is required to provide data for design and construction. This is usually carried out through various performance tests, which are always time consuming for specimen preparation, equipment calibration and test setting up. For materials with time- and temperature-dependent properties, this procedure requires fabrication of a large number of specimens in order to get reasonably comprehensive results. Furthermore, for materials that consist of phases with significant differences in properties, macroscopic homogeneous assumption or microscopic statistic approximation will lead to complex correction schemes. This will add complexity in material characterization. On the other hand, the homogeneity based interpretation of test results makes it difficult to understand the interaction between different components. The objective of this research is to develop a numerical testing method for material characterization based on x-ray tomography and finite element method. The introduction of tomography technology, such as x-ray tomography into engineering field makes it possible to obtain material microstructure without disturbing the phase configuration. Along with the development of image analysis technology, image data can be manipulated to obtain digitalized sample reconstruction and to build finite element geometric model. Based on well developed material models that sufficiently capture the essential behavior of individual material component, we developed a framework of numerical tests for characterization of composite material. The geometric model imports the microstructural data of the sample, the configuration of aggregates, voids and flakes, through x-ray tomography and image processing. The voids distribution as well as density variation was quantified to verify the model microscopic characteristics. FORTRAN programs were developed to automatically achieve data transfer and model generation, e.g. boundary identification and ABAQUS simulation model generation. Material model was studied and selected for different material components. Viscoplastic material models were evaluated and calibrated in ABAQUS. Monotonic loading and repeated loading were considered in the study to validate the model for most characterization needs. The digital model was validated through small sample tests and was implemented and used in various material characterizations. For the wood panel characterization, the anisotropic elastic properties were studied while the viscous and plastic responses were studied for asphalt concrete. Factors affecting the accuracy and the limitations of the application were determined. It is worth noting that further advance and data collection will make the calibration of material model more accurate. Nevertheless, the work can be extended to other regimes, such as high speed impact especially where the actual testing is complicated to setup. / Ph. D.

Hair cells are transducers found found in the inner ear of vertebrates. They convert a mechanical signal, detected by the deflection of a bundle of cilia extending from their top surface, into an electrochemical signal. This dissertation studies the mechanical influence of the structure and materials on the function of the cells. I introduce two methods to conduct the mechanical analysis. The first uses strength of materials formulae to solve the simplified hair cell bundle models. The second is a finite element analysis, used to better account for the observed complexity of the structure. I then use these two techniques to build a fundamental understanding of the hair cell bundle structure. By first studying simplified models, then adding complexity, the effects of geometric and material variation can be deduced. I then study three actual bundles. These are all taken from vestibular organs of turtles, two from the posterior semicircular canal and one from the utricle. I present estimations of stiffness, tip link tensions, and nonlinear response. Finally, I investigate a single cilium forced by a fluid flow. The problem is solved by finite difference technique. Three different initial conditions are solved. / Ph. D.

Due to problems associated with welded moment connections uncovered after the Northridge earthquake, large bolted connections are becoming a much more attractive alternative for design in seismic regions. However, stringent design requirements established by the AISC Seismic Provisions for Structural Steel Buildings (1997) make current moment end-plate configurations and design procedures inadequate for multi-story buildings. This dissertation first examines and critiques current seismic design philosophies as applied to moment end-plate connections. Next, the finite element method is used to develop much-needed design procedures for large moment end-plate connections, and to improve the understanding of the role of geometric parameters (e.g., bolt pitch and stiffener locations) in the response of these connections. Finally, single-story and multi-story frames incorporating large moment end-plate connections with known moment-rotation characteristics are considered under seismic loading to determine the effectiveness of these systems in dissipating energy caused by the ground motion. / Ph. D.

There have been several design models for single plate shear connections in the past 20 years. The current design model states that the bolt shear rupture strength of a connection is a function of the number of bolts and the a-distance, which is the distance from the weld line to the bolt line. The evaluation of this design model demonstrates inconsistent predictions for the strength of the connection. The finite element program ABAQUS was used throughout the research to study single plate shear connections. Finite element analyses included model verification and investigations of parameters, including the effect of a-distance, plate thickness, plate material, and the position of a connection with respect to a beam neutral axis. In addition, double-column bolt connections were studied. The results show that bolt shear rupture strength of a connection is not a function of the a-distance. Plate materials and thicknesses that do not satisfy ductility criteria result in connections with significant horizontal forces at the bolts. This horizontal force reduces the shear strength of a bolt group and creates a moment that must be considered in design. The magnitude of the force depends on the location of the bolt with respect to the beam neutral axis. A new design model for single plate shear connections with bolts in a single column is proposed. It was found that in double-column bolt connections, force redistribution among the bolt columns occurs. Force redistribution does not occur when thick plates are used, resulting in bolts in the outer column (from the support) fracturing while bolts in the inner column resist much less force. Further study is needed for double-column configurations. The study of plate behavior shows that the shear stress distribution when a plate reaches the strain hardening stage is not constant throughout the cross section. A relationship for calculating plate shear yielding strength based on this shear distribution is proposed. / Ph. D.