Damage and condition identi&amp / #64257 / cation of existing structures using non-destructive tests is a common challenge that has been worked on for a long time. In this study, two di&amp / #64256 / erent methods were developed to &amp / #64257 / nd existing force on cables as well as determine bending characteristics (EI coe&amp / #64259 / cients) of beam like structures (such as bridges). Comparing forces in symmetrically placed cables or against values obtained from design drawings would indicate structural imbalance as well as &amp / #64257 / nding EI coe&amp / #64259 / cients at a number of segments on a bridge girder might indicate weak regions that might possibly have undergone structural damage, having weak connections, lost composite action etc. With the help of the proposed algorithm, the sti&amp / #64256 / ness parameters of bridges can be assessed and the location of any damage that is in the magnitude which can a&amp / #64256 / ect displacement behavior of system can be located. The developed methods are demonstrated using the values analytically obtained from the created models and the e&amp / #64256 / ectiveness of the algorithm is criticized. Furthermore, several damage scenarios on a scaled lab beam was used to test the application using real experimental data / including tests on undamaged beam (for identi&amp / #64257 / cation) and tests on the damaged beam. Additional experiments were conducted on a cable stretched in the laboratory instrumented using a load cell to measure instantaneous axial load on the cable and compare these values against the values obtained from the developed tension measurement device. The results are compared and conclusions are derived.

Master of Science / Department of Civil Engineering / Hani G. Melhem / The objective of this research is to use structure ambient random vibration response to detect damage level and location. The use of ambient vibration is advantageous because excitation is caused by service conditions such as normal vehicle traffic on a highway bridge, train passage on a railroad bridge, or wind loads on a tall building. This eliminates the need to apply a special impact or dynamic load, or interrupt traffic on a bridge in regular service. This research developed an approach in which free vibration of a structure is extracted from the response of this structure to a random excitation in the time domain (acceleration versus time) by averaging out the random component of the response. The result is the free vibration that includes all modes based on the sampling rate on time. Then this free vibration is transferred to the frequency domain using a Fast Fourier Transform (FFT). Variations in frequency response are a function of structural stiffness and member end-conditions. Such variations are used as a measure to identify the change in the structural dynamic properties, and ultimately detect damage. A physical model consisting of a 20 × 20 × 1670 -mm long steel square tube was used to validate this approach. The beam was tested under difference supports conditions varying from a single- to three-span continuous configuration. Random excitation was applied to the beam, and the dynamic response was measured by an accelerometer placed at various locations on the span. A numerical model was constructed in ABAQUS and the dynamic response was obtained from the finite element model subjected to similar excitation as in the physical model. Numerical results were correlated against results from the physical model, and comparison was made between the different span/support configurations. A subsequent step would be to induce damage that simulates loss of stiffness or cracking condition of the beam cross section, and that would be reflected as a change in the frequency and other dynamic properties of the structure. The approach achieved good results for a structure with a limited number of degrees of freedom. Further research is needed for structures with a larger number of degrees of freedom and structures with damage in symmetrical locations relative to the accelerometer position.

L’interaction dynamique entre le sol et les structures (IDSS) a fait l’objet de nombreuses études sous l’hypothèse de l’élasticité linéaire, bien que les effets de l’IDSS puissent être différents entre un système élastique et un système inélastique. De fait, les méthodologies usuelles développées à partir des études élastiques peuvent ne pas être adaptées aux bâtiments conçus pour dissiper de l’énergie par de l’endommagement lors de séismes sévères. De plus, il est bien connu que la limite d’élasticité du sol est normalement atteinte même pour de séismes relativement faibles. En conséquence, si les effets inélastiques de l’IDSS sont négligés, les études d’endommagement sismique des bâtiments peuvent être très inexactes. L’objectif de ce travail est de développer une stratégie générale pour l’étude du problème de l’IDSS non-linéaire dans le contexte de l’analyse de la vulnérabilité sismique des bâtiments. Ainsi, des modèles d’éléments finis réalistes sont développées et appliquées à des problèmes d’IDSS non-linéaires. Les modèles couvrent une large gamme des conditions pour le sol et des typologies de bâtiments soumis à plusieurs bases de données sismiques. Une stratégie de modélisation a été développée et validée afin de réduire significativement le coût numérique. Pour cela, un modèle 2D équivalent a été développé, implanté dans GEFDyn et utilisé pour effectuer une importante étude paramétrique. De nombreux indicateurs de comportement non-linéaire de la structure et du sol ont été proposés pour synthétiser leur fonctionnement lors du chargement sismique. De surcroît, une stratégie d’évaluation de la vulnérabilité sismique basée sur l’information apportée par une base des données sismiques a été développée. De façon, générale, les résultats ont mis en évidence une réduction de la demande sismique lorsque les effets inélastiques de l’IDSS sont pris en compte. Cette réduction est liée fondamentalement à deux phénomènes : l’amortissement par radiation et l’amortissement hystérétique du sol. Ces deux effets ont lieu simultanément pendant le mouvement sismique. Il est alors très difficile d’isoler l’influence de ces deux phénomènes. En effet, le mouvement effectif transmis à la structure n’est pas le même que celui en champs libre du aux effets d’interaction, ainsi qu’à la modification locale du comportement du sol fortement lié aux poids du bâtiment. Une série de mesures de sévérité sismique et des mécanismes de dissipation d’énergie au niveau du sol et du bâtiment a été introduite dans le but d’analyser ces effets. Cependant, ces résultats sont en général très irréguliers et leur généralisation a été très difficile. Néanmoins, ces résultats mettent en évidence l’importance de la prise en compte des effets du comportement inélastique du sol. La plupart des cas étudiés ont montré un effet favorable de l’IDSS non-linéaire. Mais, en général, l’IDSS peut augmenter ou diminuer la demande sismique en fonction de la typologie de la structure, des caractéristiques du mouvement sismique et des propriétés du sol. Tout de même, il y a une justification économique pour étudier les effets du comportement non-linéaire du sol sur la réponse sismique. / The dynamic interaction of the soil with a superstructure (DSSI) has been the subject of numerous investigations assuming elasticity of both, superstructure and soil foundation behavior. Nevertheless, the effect of DSSI may differ between elastic and inelastic systems. Thus, the current interaction methodologies based on elastic response studies could not be directly applicable to structures expected to behave inelastically during severe earthquakes. Additionally, the soil is known to exhibit inelastic behavior even for relatively weak to moderate ground motions. Consequently, ignoring these characteristics in studying DSSI could lead to erroneous predictions of structural damage. The main purpose of this work is to develop a general strategy to address the full DSSI problem in the context of the seismic vulnerability analysis of structures. Thus, realistic Finite Elements models are constructed and applied in a practical way to deal with these issues. These models cover a large range of soil conditions and structural typologies under several earthquake databases. Some modelling strategies are introduced and validated in order to reduce the computational cost. Therefore, an equivalent 2D model is developed, implemented in GEFDyn and used in the large parametric study conducted. Several indicators for both structural and soil responses are developed in order to synthesize their behavior under seismic loading. Additionally, a vulnerability assessment strategy is presented in terms of measures of information provided by a ground motion selection. According to the investigation conducted in this work, there is in general a reduction of seismic demand or structural damage when non-linear DSSI phenomenon is included. This reduction can be associated fundamentally to two phenomena: radiative damping and hysteretic damping due to non-linear soil behavior. Both effects take place simultaneously during the dynamic load and it is extremely difficult to separate the contribution of each part in reducing seismic demand. Indeed, effective motion transmitted to the superstructure does not correspond to the free field motion because of the geometrical and inertial interactions as well as the local modification of soil behavior, specially due to the supplementary confinement imposed by the superstructure’s weight. A series of strong-motion severity measures, structural damage measures and energy dissipation indicators have been introduced and studied for this purpose. Nevertheless, results are erratic and consequently, generalization was extremely difficult. Despite these difficulties, the results illustrate the importance of accounting for the inelastic soil behavior. The major part of the studied cases show beneficial effects such as the decrease of the maximum seismic structural demand. However, the non-linear DSSI could increase or decrease the expected structural damage depending on the type of the structure, the input motion, and the dynamic soil properties. Furthermore, there is an economic justification to take into account the modification effects due to inelastic soil behavior.

The thesis addresses the problem of structural damage detection using inverse sensitivity based methods. The focus here is on characterization with regard to identification, location, and, quantification of structural damage in linear time invariant (LTI) systems, using vibration data. The study encompasses both analytical and experimental methods. A suite of five algorithms for damage detection, namely, inverse eigensensitivity method that is refined to account for cross orthogonality between distinct modes, damping dependent eigensolutions, and sensitivity with respect to points of antiresonance and minima, inverse FRF method that includes refinements in terms of inclusion of second order sensitivity, response function method (RFM) based on first order Taylor’s expansion, a newly proposed inverse sensitivity method based on singular values of FRF matrix, and method based on response time histories, are presented. The scope of these methods vis-à-vis the need for model reduction, ability to deal with incomplete data, ill-posedness of governing equations and the need for regularization, sensitivity with respect to measurement noise, ability to identify damping characteristics, the highest and lowest magnitudes of changes in structural properties, and the ability to characterize systems with closely spaced natural frequencies that the methods can detect are discussed. The performance of proposed procedures is illustrated by considering a five degrees-of-freedom (dof) mass-spring-dashpot system and subsequently applied on three archetypal structural systems using analytical and experimental methods. In the examples presented, factors, such as, completeness of measured data in time and frequency, nature (proportional/non-proportional) and magnitude of damping, levels of changes in structural properties, modal truncations, number of governing equations for system parameters, and efficacy of regularization techniques are investigated. The study also highlights the difficulties in implementing the damage detection algorithm based on real life noisy vibration data. A comparative study on the suitability of each of these methods in locating and quantifying of different damage scenarios has been reported. A critical review of performance of the various methods is presented. The thesis concludes with a summary on the contributions made and also deliberates on future avenues for research and development in this area of research.

This thesis analyses the impact of granular aggregates with structures using experiments and numerical simulations. Original contributions include an insight into multiple factors affecting the loading and damage to the structures, along with study of numerical parameters important for realistic prediction of the interaction between the granular media and structures. It extends the current understanding related to such interactions, with an underlying motivation to guide strategies in order to reduce the structural damage. The response of structures impacted by granular media (sand or soil) is of significant research interest for many applications. One of the applications is for landmine explosions which causes ejection of soil from ground and damage to structures impacted by this ejected soil. Experimentation is done in a laboratory setting where the cylindrical sand slugs are generated at high speed using an impulse provided by a piston. This induces a velocity gradient along the slug, because of which the slug expands during the flight before impacting the target. Deformable as well as rigid flat targets are considered in two orientations relative to the incoming slug: perpendicular (i.e. normal orientation) and inclined at an angle of 45°. The targets are supported by force transducers to capture the loading from the slug. Simulations are performed using a combination of discrete particle and finite element schemes, which enables the analysis of the fully coupled interaction between the flowing granular media and the structure. A contact model involving multiple parameters is used for inter-particle and particle-target contact. Firstly, a numerical analysis is performed to characterise the temporal evolution of slugs and their impact on monolithic beams constrained at the ends. Out of all the parameters used for inter-particle contact definition in discrete particle method, only the contact stiffness is found to effect the velocity gradient in the slug before it impacts the target. Other factor influencing the gradient is the acceleration provided by the piston. A strong dependence of beam deflection on the stand-off distance is observed due to the velocity gradient in the slugs. As the second step, the effect of target surface properties on the transmitted momentum is analysed. Experiments are done by applying coatings of different hardness and roughness on the target surface impacted by sand slugs. For normally oriented targets, the transmitted momentum is observed to be insensitive to the change in surface coating. In contrast, for inclined targets, a significant influence of coatings is observed. Additionally, the momentum transmitted to the inclined targets is always less than that for normal targets. Numerical analysis of this surface effect reveals that assuming the slug particles to be spherical shape in simulations does not capture the particle/target interactions accurately and under-predicts the frictional loading on the target. Following this, a detailed numerical study is done to understand the effect of the shape of particles in the slug. Simple shaped non-spherical particles are constructed by combining spherical sub-particles. With increasing angularity of particles in the slug, the frictional loading on the target is shown to increase. This results in an increase of momentum transmitted to inclined targets. For normally oriented targets however, the particle shape does not affect the overall transmitted momentum, which is a behaviour similar to that observed when studying the effect of target surface properties. In addition, effect of fracture of particles in the slug is analysed by using beam connections between sub-particles that break during the impact with the target. If the fracture results in increasing particle angularity, the transmitted momentum increases, whereas the situation reverses if fracture results on more spherical shaped particles. Lastly, a strategy to reduce the loading on the targets is analysed by using sacrificial coating on the target surface. In experiments, this coating is placed on the rigid target surface using a lubricant at their interface. When impacted by the slug, this coating slides on the target surface, resulting in a reduction of frictional loading on the target. If the friction at the coating/target interface vanishes, the transmitted momentum approaches the theoretical minimum value. Simulations are used to first validate the experimental observations and then to extend the concept of sliding coatings using deformable targets. Both the transmitted momentum and deflections depended on the thickness of the target and coating. When a coating is used, the deflections increase due to reduction in target thickness. It is found that the best strategy to reduce the damage to the target is to use least possible thickness of the coating and minimise the friction at the interface between the coating and the target. The presented work examines many of the factors that affect the loading on the target impacted by granular slugs, in addition to characterising the expansion of slugs before the target impact. The analysed factors include those already known such as target stand-off distance, inclination and unveils others such as target surface properties and granular properties. The numerical analysis discloses important parameters and shows the effect of particle shape, highlighting the shortcomings of widely used spherical particle assumption in the numerical studies. A strategy using a sacrificial coating to reduce damage to the target is also analysed.

The problem of vibration induced structural faults has been a real one in engineering over the years. If left unchecked it has led to the unexpected failures of so many structures. Needless to say, this has caused both economic and human life losses. Therefore for over forty years, structural damage identification has been one of the important research areas for engineers. There has been a thrust to develop global structural damage identification techniques to complement and/or supplement the long-practised local experimental techniques. In that respect, studies have shown that vibration-based techniques prove to be more potent. Most of the existing vibration-based techniques monitor changes in modal properties like natural frequencies, damping factors and mode shapes of the structural system to infer the presence of structural damage. Literature also reports other techniques which monitor changes in other vibration quantities like the frequency response functions, transmissibility functions and time-domain responses. However, none of these techniques provide a complete identification of structural damage. This study presents a damage detection technique based on operational response monitoring, which can identify all the four levels of structural damage and be implemented as a continuous structural health monitoring technique. The technique is based on monitoring changes in internal data variability measured by a test statistic <font face="symbol">c</font>2Ovalue. Structural normality is assumed when the <font face="symbol">c</font>2Om value calculated from a fresh set of measured data is within the limits prescribed by a threshold <font face="symbol">c</font>2OTH value . On the other hand, abnormality is assumed when this threshold value has been exceeded. The quantity of damage is determined by matching the <font face="symbol">c</font>2Om value with the <font face="symbol">c</font>2Op values predicted using a benchmark finite element model. The use of <font face="symbol">c</font>2O values is noted to provide better sensitivity to structural damage than the natural frequency shift technique. The analysis carried out on a numerical study showed that the sensitivity of the proposed technique ranged from three to thousand times as much as the sensitivity of the natural frequencies. The results from a laboratory structure showed that accurate estimates of damage quantity and remaining service life could be achieved for crack lengths of less than 0.55 the structural thickness. This was due to the fact that linear elastic fracture mechanics theory was applicable up to this value. Therefore, the study achieved its main objective of identifying all four levels of structural damage using operational response changes. / Dissertation (MSc (Mechanics))--University of Pretoria, 2007. / Mechanical and Aeronautical Engineering / unrestricted

Existing methods of evaluating the structural system of a building after a seismic event consist of removing architectural elements such as drywall, cladding, insulation, and fireproofing. This method is destructive and costly in terms of downtime and repairs. This research focuses on removing the guesswork by using forced vibration testing (FVT) to experimentally determine the health of a building. The experimental structure is a one-story, steel, bridge-like structure with removable braces. An engaged brace represents a nominal and undamaged condition; a dis-engaged brace represents a brace that has ruptured thus changing the stiffness of the building. By testing a variety of brace configurations, a set of experimental data is collected that represents potential damage to the building after an earthquake. Additionally, several unknown parameters of the building’s substructure, lateral-force-resisting-system, and roof diaphragm are determined through FVT. A suite of computer models with different levels of damage are then developed. A quantitative analysis procedure compares experimental results to the computer models. Models that show high levels of correlation to experimental brace configurations identify the extent of damage in the experimental structure. No testing or instrumentation of the building is necessary before an earthquake to identify if, and where, damage has occurred.

Composite structures are extremely useful for aerospace, automotive, marine and civil applications due to their very high specific structural properties. These structures are subjected to severe dynamic loading in their service life. Repeated exposure to these severe loading conditions can induce structural damage which ultimately may precipitate a catastrophic failure. Therefore, an interest in the continuous inspection and maintenance of engineering structures has grown tremendously in recent years. Sensitive aerospace applications can have small design margins and any inadequacy in knowledge of the system may cause design failure. Structures made from composite materials posses complicated failure mechanism as compared to those made from conventional metallic materials. In composite structural design, it is hence very important to properly model geometric intricacies and various imperfections such as delaminations and cracks. Two important issues are addressed in this thesis: (1) structural modeling of nonlinear delamination and uncertainty propagation in nonlinear characteristics of composite plate structures and (2) development of a model based damage detection system to handle uncertainty issues. An earlier proposed shear deformable C0 composite plate finite element is modified to alleviate modeling uncertainty issues associated with a damage detection problem. Parabolic variation of transverse shear stresses across the plate thickness is incorporated into the modified formulation using mixed shear interpolation technique. Validity of the proposed modification is established through available literature. Correction of the transverse shear stress term in the formulation results in about 2 percent higher solution accuracy than the earlier model. It is found that the transverse shear effect increases with higher modes of the plate deformation. Transverse shear effects are more prominent in sandwich plates. This refined composite plate finite element is used for large deformation dynamic analysis of delaminated composite plates. The inter-laminar contact at the delaminated region in composite plates is modeled with the augmented Lagrangian approach. Numerical simulations are carried out to investigate the effect of delamination on the nonlinear transient behavior of composite plates. Results obtained from these studies show that widely used unconditionally stable β-Newmark method presents numerical instability problems in the transient simulation of delaminated composite plate structures with large deformation. To overcome this instability issue, an energy and momentum conserving composite implicit time integration scheme presented by Bathe and Baig is used for the nonlinear dynamic analysis. It is also found that a proper selection of the penalty parameter is very crucial in the simulation of contact condition. It is shown that an improper selection of penalty parameter in the augmented Lagrangian formulation may lead to erroneous prediction of dynamic response of composite delaminated plates. Uncertainties associated with the mathematical characterization of a structure can lead to unreliable damage detection. Composite structures also show considerable scatter in their structural response due to large uncertainties associated with their material properties. Probabilistic analysis is carried out to estimate material uncertainty effects in the nonlinear frequencies of composite plates. Monte Carlo Simulation with Latin Hypercube Sampling technique is used to obtain the variance of linear and nonlinear natural frequencies of the plate due to randomness in its material properties. Numerical results are obtained for composite plates with different aspect ratio, stacking sequence and oscillation amplitude ratio. It is found that the nonlinear frequencies show increasing non-Gaussian probability density function with increasing amplitude of vibration and show dual peaks at high amplitude ratios. This chaotic nature of the dispersion of nonlinear eigenvalues is also revealed in eigenvalue sensitivity analysis. For fault isolation, variations in natural frequencies, modal curvatures and curvature damage factors due to damage are investigated. Effects of various physical uncertainties like, material and geometric uncertainties on the success of damage detection is studied. A robust structural damage detection system is developed based on the statistical information available from the probabilistic analysis carried out on beam type structures. A new fault isolation technique called sliding window defuzzifier is proposed to maximize the success rate of a Fuzzy Logic System (FLS) in damage detection. Using the changes in structural measurements between the damaged and undamaged state, a fuzzy system is generated and the rule-base and membership functions are generated using probabilistic informations. The FLS is demonstrated using frequency and mode shape based measurements for various beam type structures such as uniform cantilever beam, tapered beam in single as well as in multiple damage conditions. The robustness of the FLS is demonstrated with respect to the highly uncertain input information called measurement deltas (MDs). It is said, if uncertainty level is larger than or close to the changes in damage indicator due to damage, the true information would be submerged in the noise. Then the actual damaged members may not be identified accurately and/or the healthy members may be wrongly detected as damaged giving false warning. However, this being the case, the proposed FLS with new fault isolation technique tested with these noisy data having large variation and overlaps shows excellent robustness. It is observed that the FLS accurately predicts and isolates the damage levels up-to considerable uncertainty and noise levels in single as well as multiple damage conditions. The robustness of the FLS is also demonstrated for delamination detection in composite plates having very high material property uncertainty. Effects of epistemic uncertainty on damage detection in composite plates is addressed. The effectiveness of the proposed refined Reddy type shear deformable composite plate element is demonstrated for reducing the modeling or epistemic uncertainty in delamination detection.

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Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro / Neste trabalho são utilizados a técnica baseada na propagação de ondas acústicas e o método de otimização estocástica Luus-Jaakola (LJ) para solucionar o problema inverso relacionado à identificação de danos em barras. São apresentados o algoritmo algébrico sequencial (AAS) e o algoritmo algébrico sequencial aperfeiçoado (AASA) que modelam o problema direto de propagação de ondas acústicas em uma barra. O AASA consiste nas modificações introduzidas no AAS. O uso do AASA resolve com vantagens o problema de identificação de danos com variações abruptas de impedância. Neste trabalho são obtidos, usando-se o AAS-LJ e o AASA-LJ, os resultados de identificação de cinco cenários de danos. Três deles com perfil suave de impedância acústica generalizada e os outros dois abruptos. Além disso, com o objetivo de simular sinais reais de um experimento, foram introduzidos variados níveis de ruído. Os resultados alcançados mostram que o uso do AASA-LJ na resolução de problemas de identificação de danos em barras é bastante promissor, superando o AAS-LJ para perfis abruptos de impedância. / In this work the techniques based on the wave propagation approach and the Luus- Jaakola optimization method to solve the inverse problem of damage identification in bars are applied. The sequential algebraic algorithm (SAA) and the improved sequential algebraic algorithm (ISAA) that model the direct problem of acoustic wave propagation in bars are presented. The ISAA consists on modifications of the SAA. The use of the ISAA solves with advantages the problem of damage identification when the generalized acoustical impedance variations are abrupt. In this work the results of identification of five damage scenarios are obtained using the SAA and the ISAA. Three of them are smooth impedance profiles and the other two are rough ones. Moreover, to simulate signals obtained experimentally, different noise levels were introduced. It is shown that using the ISAA-LJ in solving problems of damage identification in bars is quite promising, furnishing better results than the SAA-LJ, specially when the impedance profiles are abrupt.