Metal mesh foil bearings (MMFBs) are a promising low cost gas bearing technology for support of high speed oil-free microturbomachinery. Elimination of complex oil lubrication and sealing system by installing MMFBs in oil free rotating machinery offer distinctive advantages such as reduced system overall weight, enhanced reliability at high rotational speeds and extreme temperatures, and extended maintenance intervals compared to conventional turbo machines. MMFBs for oil-free turbomachinery must demonstrate adequate load capacity, reliable rotordynamic performance, and low frictional losses in a high temperature environment. The thesis presents the measurements of MMFB break-away torque, rotor lift off and touchdown speeds, temperature at increasing static load conditions, and identified stiffness and equivalent viscous damping coefficients. The experiments, conducted in a test rig driven by an automotive turbocharger turbine, demonstrate the airborne operation (hydrodynamic gas film) of the floating test MMFB with little frictional loses at increasing loads. The measured drag torque peaks when the rotor starts and stops, and drops significantly once the bearing is airborne. The estimated rotor speed for lift-off increases linearly with increasing applied loads. During continuous operation, the MMFB temperature measured at one end of the back surface of the top foil increases both with rotor speed and static load. Nonetheless, the temperature rise is only nominal ensuring reliable bearing performance. Application of a sacrificial layer of solid lubricant on the top foil surface aids to reduce the rotor break-away torque. The measurements give confidence on this simple bearing technology for ready application into oil-free turbomachinery. Impact loads delivered (with a soft tip) to the test bearing, while resting on the (stationary) drive shaft, evidence a system with large damping and a structural stiffness that increases with frequency (max. 200 Hz). The system equivalent viscous damping ratio decreases from ~ 0.7 to 0.2 as the frequency increases. In general, the viscous damping in a metal mesh structure is of structural type and inversely proportional to the frequency and amplitude of bearing motion relative to the shaft. Impact load tests, conducted while the shaft rotates at 50 krpm, show that the bearing direct stiffness is lower (~25% at 200 Hz) than the bearing structural stiffness identified from impact load tests without shaft rotation. However, the identified equivalent viscous damping coefficients from tests with and without shaft rotation are nearly identical. The orbits of bearing motion relative to the rotating shaft show subsynchronous motion amplitudes and also backward synchronous whirl. The subsynchronous vibration amplitudes are locked at a frequency, nearly identical to a rotor natural frequency. A backward synchronous whirl occurs while the rotor speed is between any two natural frequencies, arising due to bearing stiffness asymmetry.

No / The causal and physically realizable Biot hysteretic model proves to be the simplest linear model able to describe the nearly rate-independent behaviour of engineering materials. In this paper, the performance of the Biot hysteretic model is analysed and compared with those of the ideal and causal hysteretic models. The Laguerre polynomial approximation (LPA) method, recently proposed for the time-domain analysis of linear viscoelastic systems, is then summarized and applied to the prediction of the dynamic response of linear hysteretic systems to deterministic and random excitations. The parameters of the LPA model generally need to be computed through numerical integrals; however, when this model is used to approximate the Biot hysteretic model, closed-form expressions can be found. Effective step-by-step procedures are also provided in the paper, which prove to be accurate also for high levels of damping. Finally, the method is applied to the dynamic analysis of a highway embankment excited by deterministic and random ground motions. The results show that in some cases the inaccuracy associated with the use of an equivalent viscous damping model is too large.

Steel plate shear walls (SPSWs) are an attractive option for lateral load resisting systems for both new and retrofit construction. They, however, present various challenges that can result in very thin web plates and excessively large boundary elements with moment connections, neither of which is economically desirable. Moreover, SPSW also suffers from buckling at small loads which results in highly pinched hysteretic behavior, low stiffness, and limited energy dissipation. To mitigate these shortcomings, a new type of SPSW has been developed and investigated. The buckling resistant steel plate shear wall (BR-SPSW) utilizes a unique pattern of cut-outs to reduce buckling. Also, it allows the use of simple shear beam-column connections and lends tunability to the shear wall system. A brief discussion of the concept behind the BR-SPSW is presented. A detailed parametric study is presented that investigates the sensitivity of the local and global system behavior to the geometric design variables using finite element models as the main tool. The key output parameters which define the system response are discussed in detail. Analytical solutions for some output parameters like strength and stiffness have been derived and resulting equations are proposed. Finally, preliminary suggestions have been made about how this system can be implemented in practice to improve the seismic resistance of the buildings. The proposed BR-SPSW system was found to exhibit relatively fuller hysteretic behavior with high resistance during the load reversals, without the use of moment connections. / Master of Science

A novel shear wall system for building structures has been developed that improves upon the performance of conventional steel plate shear walls by mitigating buckling. The new structural system, called the Ring Shaped - Steel Plate Shear Wall, was investigated and developed through experimental and computational methods. First, the plastic mechanism of the system was numerically derived and then analytically validated with finite element analyses. Next, five large-scale, quasi-static, cyclic experimental tests were conducted in the Thomas M. Murray Structures Laboratory at Virginia Tech. The large-scale experiments validated the system performance and provided data on the boundary frame forces, infill panel shear deformation modes, buckling mode shapes, and buckling magnitudes. Multiple computational modeling techniques were employed to reproduce different facets of the system behavior. First, detailed finite element models were constructed to accurately reproduce the cyclic performance, yielding pattern, and buckling mode shapes. The refined finite element models were utilized to further study the boundary element forces and ultra-low cycle fatigue behavior of the system. Second, reduced-order computational models were constructed that can accurately reproduce the hysteretic performance of the web plates. The reduced-order models were then utilized to study the nonlinear response history behavior of four prototype building structures using Ring Shaped - Steel Plate Shear Walls and conventional steel plate shear walls. The nonlinear response history analyses investigated the application of the system to a short period and a long period building configuration. In total 176 nonlinear response history analyses were conducted and statistically analyzed. Lastly, a practical design methodology for the Ring Shaped - Steel Plate Shear Wall web plates was presented. The experimental tests and computational simulations reported in this dissertation demonstrate that Ring Shaped - Steel Plate Shear Walls are capable of improving seismic performance of buildings by drastically reducing buckling and improving cyclic energy dissipation. / Ph. D.

O trabalho trata da formulação e implementação numérica de um modelo de dano para o concreto. O objetivo é o de reproduzir laços de histerese observados experimentalmente em diagramas de tensão-deformação uniaxiais quando, a partir de um certo nível de solicitação, e dano correspondente, passam-se a realizar ciclos de descarregamento e recarregamento. O modelo propõe que os laços de histerese resultam da dissipação conjunta decorrente do dano evolutivo e do fenômeno de fricção interna nas faces das fissuras. O fenômeno de fricção interna está relacionado à suposição de que as fissuras não se abrem simplesmente por separação formando faces com superfícies lisas, mas tendem a combinar modos de separação e deslizamento segundo superfícies com uma certa rugosidade. O deslizamento entre as faces das fissuras dá origem a uma deformação por escorregamento, assumida como responsável por um comportamento plástico com encruamento cinemático não-linear. A abordagem termodinâmica considerada no sentido de contemplar a danificação e a fricção interna leva à dedução de que a tensão total num ponto do meio com dano resulta dividida em uma parcela dita tensão elasto-danificada e em uma outra denominada tensão de escorregamento, também dependente do nível de danificação da estrutura. Dois tipos básicos de aplicações são realizadas: as primeiras visam avaliar a influência das variáveis de estado e dos parâmetros do material na forma dos diagramas de tensão-deformação. As outras aplicações são dedicadas à utilização do modelo na análise do comportamento dinâmico de uma viga em concreto armado submetida à vibração forçada. Destaca-se o efeito dos laços de histerese sobre o amortecimento estrutural. Mostra-se que a utilização deste modelo de dano leva à geração de um amortecimento estrutural que com os modelos de dano usuais somente poderia ser obtido a partir da utilização do modelo de amortecimento de Rayleigh, por exemplo, na equação do movimento. / This work deals with numerical formulation and implementation of a damage model to concrete. The aim is to reproduce hysteresis loops observed experimentally in uniaxial stress-strain diagram when cycles of unloading and reloading are produced. The model proposes that the hysteresis loops are due to the coupling between the dissipation of damage process and the internal friction phenomenon between the faces of crack lips. The phenomenon of internal friction is related to the supposition that the crack doesn\'t open itself by separation mode creating plane lips, but there is a tendency to combine opening and sliding modes across irregular surfaces. The sliding between the crack surfaces generates a sliding strain, which is assumed as responsible by a plastic behaviour with non-linear kinematics hardening. The thermodynamic approach considering damage and internal friction leads to the deduction that the total stress at a point is composted by one component named elasto-damage stress and another named sliding stress, both dependent of the structure damage level. Two basic kinds of simulation are performed: the first one aims to evaluate the influence of the state variables and the material parameters over the stress-strain diagrams. The following simulations are dedicated to the use of the model in dynamic analysis of a reinforced concrete beam subjected to vibration. It is emphasized the effect of the structural damping from hysteresis loops. It is showed that the present damage model generates structural damping that could only be obtained with conventional damage model by using the Rayleigh damping approach in the equation of motion.

O trabalho trata da formulação e implementação numérica de um modelo de dano para o concreto. O objetivo é o de reproduzir laços de histerese observados experimentalmente em diagramas de tensão-deformação uniaxiais quando, a partir de um certo nível de solicitação, e dano correspondente, passam-se a realizar ciclos de descarregamento e recarregamento. O modelo propõe que os laços de histerese resultam da dissipação conjunta decorrente do dano evolutivo e do fenômeno de fricção interna nas faces das fissuras. O fenômeno de fricção interna está relacionado à suposição de que as fissuras não se abrem simplesmente por separação formando faces com superfícies lisas, mas tendem a combinar modos de separação e deslizamento segundo superfícies com uma certa rugosidade. O deslizamento entre as faces das fissuras dá origem a uma deformação por escorregamento, assumida como responsável por um comportamento plástico com encruamento cinemático não-linear. A abordagem termodinâmica considerada no sentido de contemplar a danificação e a fricção interna leva à dedução de que a tensão total num ponto do meio com dano resulta dividida em uma parcela dita tensão elasto-danificada e em uma outra denominada tensão de escorregamento, também dependente do nível de danificação da estrutura. Dois tipos básicos de aplicações são realizadas: as primeiras visam avaliar a influência das variáveis de estado e dos parâmetros do material na forma dos diagramas de tensão-deformação. As outras aplicações são dedicadas à utilização do modelo na análise do comportamento dinâmico de uma viga em concreto armado submetida à vibração forçada. Destaca-se o efeito dos laços de histerese sobre o amortecimento estrutural. Mostra-se que a utilização deste modelo de dano leva à geração de um amortecimento estrutural que com os modelos de dano usuais somente poderia ser obtido a partir da utilização do modelo de amortecimento de Rayleigh, por exemplo, na equação do movimento. / This work deals with numerical formulation and implementation of a damage model to concrete. The aim is to reproduce hysteresis loops observed experimentally in uniaxial stress-strain diagram when cycles of unloading and reloading are produced. The model proposes that the hysteresis loops are due to the coupling between the dissipation of damage process and the internal friction phenomenon between the faces of crack lips. The phenomenon of internal friction is related to the supposition that the crack doesn\'t open itself by separation mode creating plane lips, but there is a tendency to combine opening and sliding modes across irregular surfaces. The sliding between the crack surfaces generates a sliding strain, which is assumed as responsible by a plastic behaviour with non-linear kinematics hardening. The thermodynamic approach considering damage and internal friction leads to the deduction that the total stress at a point is composted by one component named elasto-damage stress and another named sliding stress, both dependent of the structure damage level. Two basic kinds of simulation are performed: the first one aims to evaluate the influence of the state variables and the material parameters over the stress-strain diagrams. The following simulations are dedicated to the use of the model in dynamic analysis of a reinforced concrete beam subjected to vibration. It is emphasized the effect of the structural damping from hysteresis loops. It is showed that the present damage model generates structural damping that could only be obtained with conventional damage model by using the Rayleigh damping approach in the equation of motion.

The particular study case in this thesis is the shock test performed on the LISA Pathfinder satellite conducted in a laboratory environment on a dedicated test bed: Vega Shock Test Apparatus (VESTA). This test is considered fully representative to study shock levels produced by fairing jettisoning event at Vega Launcher Vehicle, which induces high shock loads towards the satellite. In the frame of this thesis, some transient response analyses have been conducted in MSC Nastran, and a shock simulation tool for the VESTA test configuration has been developed. The simulation tool is based on Nastran Direct Transient Response Analysis solver (SOL 109), and is representative of the upper composite of Vega with the LISA Pathfinder coupled to it. Post-processing routines of transient response signals were conducted in Dynaworks which served to calculate Shock Response Spectra (SRS). The simulation tool is a model of forcing function parameters for transient analysis which adequately correlates with the shock real test data, in order to understand how the effect of shock generated by the launcher is seen in the satellite and its sub-systems. Since available computation resources are limited the parameters for analysis were optimised for computation time, file size, memory capacity,  and model complexity. The forcing function represents a release of the HSS clamp band which is responsible for fairing jettisoning, thus the parameters which were studied are mostly concerning the modelling of this event. Among many investigated, those which visibly improved SRS correlation are radial forcing function shape, implementation of axial impulse, clamp band loading geometry and refined loading scheme. Integration time step duration and analysis duration were also studied and found to improve correlation.  From each analysis, the qualifying shock environment was then derived by linear scaling in proportion of the applied preload, and considering a qualification margin of 3dB. Consecutive tracking of structural responses along shock propagation path exposed gradual changes in responses pattern and revealed an important property that a breathing mode (n = 0) at the base of a conical Adapter translates into an axial input to the spacecraft. The parametrisation itself was based on responses registered at interfaces located in near-field (where the clamp band is located and forcing function is applied) and medium-field with respect to the shock event location. Following shock propagation path, the final step was the analysis of shock responses inside the satellite located in a far-field region, which still revealed a very good correlation of results. Thus, it can be said that parametrisation process was adequate, and the developed shock simulation tool can be qualified. However, due to the nature of shock, the tool cannot fully replace VESTA laboratory test, but can support shock assessment process and preparation to such test. In the last part of the thesis, the implementation of some finite element model improvements is investigated. Majority of the panels in spacecraft interior exhibited shock over-prediction due to finite element model limitation. Equipment units modelled as lump masses rigidly attached with RBE2 elements to the panel surface are a source of such local over-predictions. Thus, some of the units were remodelled and transient responses were reinvestigated. It was found that remodelling with either solid elements, or lump mass connected to RBE3 element and reinforced by RBE2 element, can significantly improve local transient responses. This conclusion is in line with conclusions found in ECSS Shock Handbook.