Partial self-centring dampers for wind turbine structures: Experimental study and structural behaviour

Fluid-Structure Coupling Effects on the Dynamic Response of Pump-Turbine Guide Vanes

Passive control methods reduced the vulnerability of structures to earthquakes by decreasing the seismic demand and improving structural plasticity. One of the passive control systems is the eccentrically braced frame with a vertical shear link (V-EBF). The present study aims to direct the damage to the absorbing plates of the vertical link beam to allow the structure’s appropriate seismic performance and reparability. Yielding dampers are one of the most widely used types in systems and can provide perfect vibration control if used optimally. Different types of dampers were introduced and used; how to use them depends on the shape and the way they connect to the structure. This research investigates a new type of damper called box damper, an improved type of shear panel damper. The improvement in the way of connecting to the braced frame and the ease of using this damper in different situations are the features of this new damper. This research investigated the mechanism of these yielding dampers in structures and their strengths and weaknesses. In the next step in this study, a V-EBF with plates of thickness 4, 6, and 8 mm was analysed in the finite element software ABAQUS using the nonlinear static analysis and cyclic loading conditions. Some examples of this damper were attached to the braced frames to investigate the effect of using this damper on the seismic behaviour of the braced structures. The results show that the shear link performs like an electrical fuse absorbing all damage and plastic hinges so that other elements of the braced frame remain in their nonlinear elastic region. By increasing the thickness of the damper from 2 to 8 mm, the resistance increased by two times, and the flexibility of the structure had a noticeable change with the rise in thickness from 2 mm to 8 mm. Ductility increased from 38 to 75 mm.

The complex domain structure in ferroelectrics gives rise to electromechanical coupling, and its evolution (via domain switching) results in a time-dependent (i.e. viscoelastic) response. Although ferroelectrics are used in many technological applications, most do not attempt to exploit the viscoelastic response of ferroelectrics, mainly due to a lack of understanding and accurate models for their description and prediction. Thus, the aim of this thesis research is to gain better understanding of the influence of domain evolution in ferroelectrics on their dynamic mechanical response. There have been few studies on the viscoelastic properties of ferroelectrics, mainly due to a lack of experimental methods. Therefore, an apparatus and method called Broadband Electromechanical Spectroscopy (BES) was designed and built. BES allows for the simultaneous application of dynamic mechanical and electrical loading in a vacuum environment. Using BES, the dynamic stiffness and loss tangent in bending and torsion of a particular ferroelectric, viz. lead zirconate titanate (PZT), was characterized for different combinations of electrical and mechanical loading frequencies throughout the entire electric displacement hysteresis. Experimental results showed significant increases in loss tangent (by nearly an order of magnitude) and compliance during domain switching, which shows promise as a new approach to structural damping. A continuum model of the viscoelasticity of ferroelectrics was developed, which incorporates microstructural evolution via internal variables and associated kinetic relations. For the first time, through a new linearization process, the incremental dynamic stiffness and loss tangent of materials were computed throughout the entire electric displacement hysteresis for different combinations of mechanical and electrical loading frequencies. The model accurately captured experimental results. Using the understanding gained from the characterization and modeling of PZT, two applications of domain switching kinetics were explored by using Micro Fiber Composites (MFCs). Proofs of concept of set-and-hold actuation and structural damping using MFCs were demonstrated.

Experimental and numerical analysis of GFRP frame structures. Part 2: Monotonic and cyclic sway behaviour of plane frames

Ultra High-Performance Fibre-Reinforced Concrete (UHPFRC) combines high strength, deformation capacity and low permeability. This makes UHPFRC particularly suitable as a strengthening and protective layer for application to existing reinforced concrete structures (RC-UHPFRC composite members). The structural behaviour of UHPFRC layers in composite members is investigated with a comprehensive experimental program. In a systematic approach the behaviour of plain UHPFRC and reinforced UHPFRC (R-UHPFRC), the bond behaviour of reinforcement in UHPFRC and the flexural behaviour of composite beams are analysed. The study is limited to thin UHPFRC layers (≤ 50 mm) and small diameter bars. Special attention is given to the representativeness of the experimental specimens. In order to avoid preferential fibre orientations, all UHPFRC and composite specimens are cut from larger elements. The high scatter of the tensile behaviour of plain UHPFRC, quantified by the testing of a high number of specimens, confirms the reasonableness and necessity of additional bar reinforcement. The additional reinforcement increases not only the resistance but also improves the deformation capacity and strain hardening behaviour of UHPFRC. The use of reinforcing bars with different steel grades and surface characteristics, namely ribbed and smooth bars, improves the current knowledge base on the bond behaviour. It is shown that the pre-peak behaviour is independent of the bond strength and that the crack spacing is dominated by the fibre reinforcement. The reinforcing bars with low bond strength allow for large post-peak deformations and avoid the localised stress concentration in the UHPFRC macro-crack. The use of high yield strength steel is beneficial in view of the increased deformation capacity of R-UHPFRC. The apparent strain hardening of UHPFRC is significantly increased by the reinforcement. The strengthening of reinforced concrete beams with a R-UHPFRC layer is characterised by the significant increase of the resistance and stiffness (depending on the reinforcement) combined with an important reduction of the rotation capacity. With a good balance between resistance increase and loss of rotation capacity the smooth reinforcing bars of the UHPFRC layer represent an interesting alternative to conventional reinforcement. The use of high yield strength steel proves to be advantageous due to the higher deformation capacity. Through the use of an analytical model for plain UHPFRC the origin of the scatter of the tensile behaviour and the conditions for the occurrence of strain hardening are identified. The modelling of R-UHPFRC gives an insight into the interaction of fibres and reinforcing bars and the apparent UHPFRC tensile behaviour. Further, it is shown that the characteristic structural behaviour of composite sections can be described with the extended sectional analysis. Finally, the main experimental and analytical results are summarised in recommendations for the application of R-UHPFRC to existing structures. This thesis contributes to the understanding of the behaviour of R-UHPFRC and composite members and emphasises the necessity of additional reinforcing bars in UHPFRC in order to achieve a reliable structural behaviour.

In the case of aluminium structures, welding may introduce weak zones, so-called heat-affected zones (HAZ), close to the weld. Such zones should be accounted for when predicting the overall behaviour of structures since the deformation tends to localise within such weak zones leading to the initiation of failure. Whereas the effect of welding on mechanical properties such as yield stress and work hardening is known, the ductility of a HAZ and how to model it in numerical simulations remain uncertain. To address this challenge, this article presents a combined experimental and numerical study on ductile fracture modelling of an aluminium alloy AA6063-T6 exposed to complex heat treatments. The first part includes preparation of specimens and uniaxial tension testing. A Gleeble machine is used to generate well-defined heat treatments involving high peak temperatures and heating rates. Subsequently, uniaxial tension tests at low and high strain rates are conducted. The results show the effect of the complex heat treatments on strength, strain-rate sensitivity, and fracture. The numerical part of this study concerns two widely used damage models, namely the Cockcroft–Latham fracture criterion and the Gurson–Tvergaard model. Calibration of material models and a parameter study are carried out based on analyses of the tension tests. To this end, a case study is conducted elaborating on the importance of the damage parameters within an idealised heat-affected zone and plane-strain tension. The main objective of this study is to obtain a more general understanding of ductile fracture modelling of aluminium alloys exposed to complex heat treatments representative of welding.

The objective of this research is to develop a numerical model of one widely used bolted beam splice connection that dissipates energy through structural damping. The reference experimental setup is carefully designed to obtain the highly nonlinear dynamic response due to the suddenly released load. The fact that the monolithic beam with welded connection has a linear response is utilized for the initial calibration of the numerical and experimental models. Then, the numerical model of bolted beam splice connection is verified and adopted through an iterative process. The influences of time and spatial integration, bolt load application, element type, contact formulation, bulk viscosity, and mass scaling are discussed. A special attention is given to the load application and load release functions. After the verification, the Abaqus/Explicit numerical model is validated through the comparison with experimental data, where an appropriate friction coefficient is adopted. It is demonstrated that the nonlinear structural damping occurs due to the complex micro slip behavior at the contact interface.

As wind turbine blades tend to be long and narrow, the demand for its properties of bending resistance and vibration suppression is improving day by day. Mimicking the three-layer structure (the outer skin, the middle, the inner skin) of bamboo, this paper puts forward a new damping structure with the blade as a basic layer, one transitional layer and one free damping layer added to it. The formula of the structural loss factor is deduced by vibration differential equations. Then combined with the formula, the models of airfoil flutter and damping vibration suppression and related data, the numerical simulation is done using Matlab/Simulink Software. Consequently, the comparison diagrams of swing and wave velocity responses and their displacement responses between conditions with and without the new damping structure are gained, which indicates that the property of vibration suppression of wind turbine blades with the new damping structure is significantly improved.

Soil damping calculation of the offshore wind turbine supported by wide-shallow bucket foundation

A general multidisciplinary procedure to approach the study of heritage building is synthesized in this PhD thesis considering the main features necessary to identify the structural behaviour of such type of buildings (historical inquires, geometric survey, material identification, in situ inquires) according to qualitative and quantitative methodologies. An in-depth survey has been carried out on the masonry of the Bell Tower of the “Santa Maria del Carmine” Church in the city of Naples, Italy. The analyses followed a multidisciplinary approach to identify its architectural and structural features, to identify the materials used and the state of damage and interventions made in various periods, to assess the stress state in the masonry under gravity loads, and to define its dynamic behavior. The information gathered represents the starting point for designing a seismic upgrade to meet the standards applied to historic monuments. A detailed historical, architectonical and structural inquires have been developed on the Bell Tower of “Santa Maria del Carmine” sited in Naples. These inquires have been preliminary to the study of the Tower by a finite element model for static and dynamic loads. The Bell Tower of “Santa Maria del Carmine” is an ancient (XVII century) historical masonry building located in city of Naples (Italy). The Tower is 68m high and is made by faced yellow tuff masonry along the initial 41 meters of height and by clay bricks along the upper levels. Internal and external walls are covered by decorative dark stone along the first nine meters. Geometrical and material surveys were performed to evaluate the actual damage state, the sequence of changes in the structures. Detailed experimental in situ investigations have been performed to define the geometry, to find the composing materials, to characterize their mechanical properties, to analyze damage condition and to identify the dynamic behaviour of the structure. The experimental results have been successively used to develop a numerical 3-D FE model. This model has been used to perform a linear static analysis under gravity loads to calibrate the mechanical parameters given by experimental results (i.e. the density of materials) and to check the safety condition for the vertical loads usually acting on the structure. Then, a linear dynamic analysis has been also accomplished to assess the values of Young’s modulus of materials and the constraint conditions offered by the surrounding buildings, based on parametric comparisons with the experimental results (natural frequencies and corresponding modal shapes obtained by dynamic in situ tests). This last study has been useful to understand the behaviour of the Tower under horizontal actions like an earthquake, considering that the structure is located in a medium risk seismic area. Therefore with the assumption of a ‘global behaviour’ of the Tower, a non linear static analysis (Push - over) was carried out based on the results of the previous inquiries, pointing out a low risk condition for the expected seismic actions (peak ground acceleration of 0.168g). Considering that previous numerical analysis evidenced that the Tower satisfied global checks for the expected seismic actions, in this paper attention is focussed on the study of local out-of-plane mechanisms activated by those actions. To prevent the activation of local mechanisms an innovative and low invasive intervention with FRP materials has been designed in the areas where local checks were not satisfying.

Historical timber frame structures constitute a huge part of the cultural heritage. The preservation of such structures requires a good understanding of both the original constructional methods and of the load-bearing principles of the structure. Carpentry connections are crucial for the behaviour of the structures, and a good understanding of them is of basic importance to lead a correct structural analysis. The present work introduces the basic load-bearing principles of the stop-splayed and tabled scarf joint with key (Jupiter joint) and the halved undersquinted joint; two among the basic types of lengthening joints used in traditional half-timbered and frame structures. The halved joint was used as a reference for developing an overview on the general structural behaviour. Therefore, the problem was examined in depth with the experimental and numerical analysis of the halved undersquinted scarf. Basic analytical linear-elastic models were developed. Tests were conducted under static loads in order to describe the maximum load, failure mechanisms, position of the forces along the loading process, rotational stiffness, friction mechanisms and the influence of the geometry on the load-bearing behaviour. The analytical model, based on the initial assumptions was calibrated on the experimental results. The influence and function of both the sloped surfaces and the wedge is investigated through some intermediate geometries. Finally, in order to give general remarks about the behaviour of the splayed and tabled scarf with key joint (Jupiter joint) along the strong and weak axis, a small series of specimens were tested, in order to describe the interaction between normal force (N) and bending moment (M) in a N-M interaction diagram and to record the failure modes along the strong and weak axis. The experimental outputs were compared with the numerical results obtained from the structural analysis of the dome of Santiago de Cuba’s cathedral to verify the reliability of the analytical model and the experimental results. The case study was chosen because the inner timber framed structure of the building demonstrated reliability against earthquakes and hurricane along the centuries. Here, the Jupiter joint was employed in the dome’s structure. The description of such carpentry connections through a static model is of importance for the structural analysis of old timber structures with the aim of restoration and reinforcement of them.

This paper investigates mechanical eigenmodes and eigenfrequencies of a linear transverse flux machine (LTFM), using experimental, numerical, and analytical methods. A state-of-the-art analytical model, based on Euler-Bernoulli beam theory, is adapted for the specific characteristics of linear stator structures and compared with finite element simulations and experimental modal analysis. The LTFM investigated in this study poses unique challenges due to its elongated structure and specific coupling effects introduced by the test bench design. These factors amplify the potential for vibration-related issues, emphasizing the importance of reliable vibration analysis. The study reveals some discrepancies between analytical and experimental eigenfrequencies, mainly due to structural simplifications and boundary conditions. Despite these deviations, the analytical model proves promising adaptability, providing close approximations of key vibration characteristics, particularly for lower-order modes. This adaptation represents a novel approach in applying beam theory to linear stator structures of electrical machines. These findings provide a basis for refining the analytical model to better align with experimental methods and evaluate the models integration with numerical electromagnetic simulations. By balancing predictive accuracy with computational efficiency, the analytical approach holds significant potential as a tool for the design and optimization of LTFMs.

In this study, a new damping correction factor is proposed to provide an accurate design response spectrum for megawatt (MW) class wind turbines. The predicted response spectra show good agreement with those by time history analysis (THA). An equivalent modal damping of wind turbine support structures is then proposed for response spectrum method (RSM) based on the modal decomposition method for non‐classically damped structures. Seismic loadings on wind turbine towers and footings by RSM with the proposed damping correction factor and equivalent modal damping are also investigated and compared with those by THA. Finally, a quantile value between 0.5 and 0.85 in the damping correction factor is calibrated to ensure the same reliability level as evaluated by THA currently used for estimation of seismic loadings on support structures.

An experimental set-up has been constructed for medium scale impact tests. The main objective of this effort is to provide data for the calibration and verification of numerical models of a loading scenario where an aircraft impacts against a nuclear power plant. One goal is to develop and take in use numerical methods for predicting response of reinforced concrete structures to impacts of deformable projectiles that may contain combustible liquid (“fuel”). Loading, structural behaviour, like collapsing mechanism and the damage grade, will be predicted by simple analytical methods and using non-linear FE-method. In the so-called Riera method the behavior of the missile material is assumed to be rigid plastic or rigid visco-plastic. Using elastic plastic and elastic visco-plastic material models calculations are carried out by ABAQUS/Explicit finite element code, assuming axisymmetric deformation mode for the missile. With both methods, typically, the impact force time history, the velocity of the missile rear end and the missile shortening during the impact were recorded for comparisons.

This thesis presents both experimental and theoretical studies on the structural behaviour of externally confined concrete-filled-steel-tube (CFST) short columns under uni-axial compression. External confinement in the form of tie bars, steel rings, spirals and jackets is proposed. From the study, it can be concluded that the external confinement is highly effective in improving the uni-axial behaviour of CFST columns by improving confining stress and suppressing inelastic local buckling.
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\nThe first part of the thesis covers the experimental behaviour of concentrically loaded confined CFST columns. The experimental programme involved testing a total of 208 specimens that consisted of 46 hollow-steel-tube columns with or without external confinement and CFST columns: 28 unconfined, 25 tie-confined, 77 ring-confined, 24 spiral-confined and 8 jacket-confined columns. The test results are summarized and discussed. From analysing the obtained experimental results, a better understanding of the structural behaviours, such as steel-concrete interface bonding, stress state of steel tube, behaviour of core concrete of CFST columns can be obtained. Besides, the test results obtained are useful for deriving and verifying theoretical models in this study.
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\nIn the second part of the thesis, a theoretical stress-strain model is proposed based on the obtained test results. This model is obtained through a four-step procedure. As the first crucial step, a new and accurate hoop strain equation is proposed and verified based on experimental results. Having validated the assumption of stress path independence of the core concrete in CFST columns under monotonic uni-axial compression, an actively confined concrete constitutive model is adopted to predict the axial stress-strain response of the core concrete. Next, with the aid of Generalized Hooke’s Law and Prandtl-Reuss theory, the complicated stress-state of steel tube is accurately modelled. Lastly, by using the newly proposed hoop strain equation together with the interaction models among the core concrete, steel tube and external confinement, the uni-axial behaviours of CFST columns can be predicted. The validity of this model is subsequently verified with the author’s and published experimental results of CFST columns and FRP-confined concrete.
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\nFinally, a parametric study using the proposed model is conducted in this thesis, involving a wide range of parameters, i.e. steel tube yield strength, concrete compressive strength, steel ratio (steel tube outer diameter / thickness) and external confinement (types and arrangements). From the parametric study, two sets of critical steel ratios based on two levels of ductility at different steel yield and concrete compressive strengths are established. Moreover, the maximum column strengths of CFST columns obtained from the model and previous experimental studies are compared with the design strengths calculated using different design codes and the results indicate that the design codes cannot give reasonably accurate predictions. Therefore, new design equations are proposed in this thesis to predict the maximum strength of the CFST columns.