Dissertations / Theses on the topic 'Structural Engineering and Mechanics'

Methods for modeling structural failure with applications for fluid structure interaction (FSI) are developed in this work. Fracture as structural failure is modeled in this work by both the extended finite element method (XFEM) and element deletion. Both of these methods are used in simulations coupled with fluids modeled by computational fluid dynamics (CFD). The methods presented here allow the fluid to pass through the fractured areas of the structure without any prior knowledge of where fracture will occur. Fracture modeled by XFEM is compared to an experimental result as well as a test problem for two phase coupling. The element deletion results are compared with an XFEM test problem, showing the differences and similarities between the two methods.

A new method for modeling fracture is also proposed in this work. The new method combines XFEM and element deletion to provide a robust implementation of fracture modeling. This method integrates well into legacy codes that currently have element deletion functionality. The implementation allows for application by a wide variety of users that are familiar with element deletion in current analysis tools. The combined method can also be used in conjunction with the work done on fracture coupled with fluids, discussed in this work.

Structural failure via buckling is also examined in an FSI framework. A new algorithm is produced to allow for structural subcycling during the collapse of a pipe subjected to a hydrostatic load. The responses of both the structure and the fluid are compared to a non-subcycling case to determine the accuracy of the new algorithm.

Overall this work looks at multiple forms of structural failure induced by fluids modeled by CFD. The work extends what is currently possible in FSI simulations.

The focus of this work is to advance the theoretical and modeling techniques for the fields of hybrid simulation and multi-slider friction pendulum systems (MSFPs). Hybrid Simulation is a simulation technique involving the integration of a physical system and a computational system with the use of actuators and sensors. This method has a strong foundation in the experimental mechanics community where it has been used for many years. The hybrid simulation experiments are performed with the assumption of an accurate result as long as the main causes of error are reduced. However, the theoretical background on hybrid testing needs to be developed in order validate these findings using this technique. To achieve this objective, a model for hybrid simulation is developed and applied to three test cases: an Euler-Bernoulli beam, a nonlinear damped, driven pendulum, and a boom crane structure. Due to the complex dynamics that these three test cases exhibit, L² norms, Lyapunov exponents, and Lyapunov dimensions, as well as correlation exponents were utilized to analyze the error in hybrid simulation tests. From these three test cases it was found that hybrid simulations are highly dependent on the natural frequencies of the dynamical system as well as how and where the hybrid split is located. Thus, proper care must be taken when conducting a hybrid experiment in order to guarantee reliable results.

Multi-stage friction pendulum systems (MSFPs), such as the triple friction pendulum (TFP), are currently being developed as seismic isolators. However, all current analytical models are inadequate in modeling many facets of these devices. Either the model can only handle uni-directional ground motions while incorporating the kinetics of the TFP system, or the model ignores the kinetics and can handle bi-directional motion. And in all cases, the model is linearized to simplify the equations. The second part of this dissertation presents an all-in-one model that incorporates the full nonlinear kinetics of the TFP system, while allowing for bi-directional ground motion. In this way, the model presented here is the most complete single model currently available. It was found that the non-linear model can more accurately predict the experimental results for large displacements due to the nonlinear kinematics used to describe the system. The model is also able to successfully predict the experimental results for bi-directional ground motions.

The work presented in this dissertation describes a general algorithm and its Finite Element (FE) implementation for performing concurrent multiple sub-domain simulations in linear structural dynamics. Using this approach one can solve problems in which the domain under analysis can be selectively discretized spatially and temporally, hence allowing the user to obtain a desired level of accuracy in critical regions whilst improving computational efficiency globally. The mathematical background for this approach is largely derived from the fundamental principles of Domain Decomposition Methods (DDM) and Lagrange Multipliers, used to obtain coupled equations of motion for distinct regions of a continuous domain. These methods when combined together systematically yield constraint forces that not only ensure conservation of energy, but also enforce continuity of field quantities across sub-domain interfaces. Multiple Grid (MG) coupling between conforming or non-conforming sub-domains is achieved in the form of linear multi-point constraints that are modeled using Mortar Finite Element Method (M-FEM); whereas coupled Multiple Time-scale (MT) equations are derived for the classical Newmark integration scheme and its constituent algorithms. A rigorous proof of stability is provided using Energy Method and necessary conditions for enforcing energy balance are discussed in reference with field variables that are selected to enforce sub-domain interface continuity. Fully discretized equations of motion for component sub-domains, augmented with an interface continuity condition are then solved using block elimination method and Crout factorization. A step-by-step solution approach, utilizing recursive black box sub-routines, is modeled in order to allow efficient implementation within existing finite element frameworks.

Proposed MGMT Method and corresponding solution algorithm is systematically implemented by using the finite element approach and programming in FORTRAN 90. Resulting in-house code - FEAPI (Finite Element Analysis Programming Interface) is capable of solving linear structural dynamics problems that are modeled using independently discretized sub-domains. Auxiliary sub-routines for defining pre simulation parameters and for viewing global/component sub-domain results are built into FEAPI and work in conjugation with GiD; a universal, adaptive and user-friendly pre and post-processor. Overall stability, numerical accuracy and computational efficiency of MGMT Method is evaluated and verified using a series of benchmark examples. Verification matrices take into consideration performance evaluation factors such as energy balance (at global and component-sub-domain levels), interface continuity, evolution/distribution of kinematic quantities and propagation of structural waves across connecting sub-domains. Assessment of computational efficiency is derived by comparing the size of respective FE problems (nodes, elements, number of equations, skyline storage requirements) and the required computation times (CPU solution time). Discussed examples highlight the greatest advantage of MGMT Method; which is significant gain in simulation speedups (at the cost of reasonably small errors).

This thesis aims toward developing strategies in the area of structural optimization and to implement these strategies in design processes. At

. A Structural Optimization group has great potential of closing the gap between structural designers and structural analysis engineers which in long terms mean that better structures can be developed in less time. To be competitive in the automotive industry these are two of the most important factors for being successful.

In traumatic brain injury (TBI), the skull-brain interface, composed of three meningeal layers: the dura mater, arachnoid mater, and pia mater, along with cerebrospinal fluid (CSF) between the layers, plays a vital role in transmitting motion from the skull to brain tissue. Magnetic resonance elastography (MRE) is a noninvasive imaging modality capable of providing in vivo estimates of tissue motion and material properties. The objective of this work is to augment human and phantom MRE studies to better characterize the mechanical contributions of the skull-brain interface to improve the parameterization and validation of computational models of TBI. Three specific aims were to: 1) relate 3D skull kinematics estimated from tri-axial accelerometers to brain tissue motion (rigid-body motion and deformation) estimated from MRE, 2) modify existing MRE data collection methods to capture simultaneous scalp and brain displacements, and 3) create cylindrical and cranial phantoms capable of simulating a CSF interface and dural membranes. Achievement of these aims has provided new quantitative understanding of the transmission of skull motion to the brain.

A common technique to improve the performance of shell and tube heat exchangers (STHE) is by redirecting the flow in the shell side with a series of baffles. A key aspect in this technique is to understand the interaction of the fluid dynamics and heat transfer. Computational fluid dynamics simulations and experiments were performed to analysis the 3-dimensional flow and heat transfer on the shell side of an STHE with and without baffles. Although, it was found that there was a small difference in the average exit temperature between the two cases, the heat transfer coefficient was locally enhanced in the baffled case due to flow structures. The flow in the unbaffled case was highly streamed, while for the baffled case the flow was a highly complex flow with vortex structures formed by the tip of the baffles, the tubes, and the interaction of flow with the shell wall.

This dissertation is primarily concerned with the development of an effective methodology for reducing structure-borne sound radiation from an arbitrarily shaped vibrating structure. There are three major aspects that separate the present methodology from all the previous ones. Firstly, it is a non-contact and non-invasive approach, which is applicable to a class of vibrating structures encountered in engineering applications. Secondly, the input data consists of a combined normal surface velocity distribution on a portion of a vibrating surface and the radiated acoustic pressure at a few field points. The normal surface velocities are measured by using a laser vibrometer over a portion of the structural surface accessible to a laser beam, while the field acoustic pressures are measured by a small array of microphones. The normal surface velocities over the rest surface of the vibrating structure are reconstructed by using the Helmholtz Equation Least Squares (HELS) method. Finally, the acoustic pressures are correlated to structural vibration by decomposing the normal surface velocity into the forced-vibro-acoustic components (F-VAC). These F-VACs are mutually orthogonal basis functions that can uniquely describe the normal surface velocity. The weightings of these F-VACs represent the relative contributions of structural vibrations into the sound radiation. This makes it possible to suppress structure-borne acoustic radiation in the most cost-effective manner simply by controlling the key F-VACs of a vibrating structure. The effectiveness of the proposed methodology for reducing structure-borne acoustic radiation is examined numerically and experimentally, and compared with those via traditional experimental modal analyses. Results have demonstrated that the proposed methodology enables one to reduce much more acoustic radiation at any selected target frequencies than the traditional approach.