The main purpose of this project was the creation of a tool, based on the research performed by Scheldbergen [25] and Roscher [24], capable of performing Finite Element Analysis (FEA) on a Horizontal Axis Wind Turbine (HAWT) blade and minimize its weight by altering the various thicknesses locally, always satisfying a number of structural constraints. The first goal was the development of a geometry creation algorithm based on a Non Uniform Rational Basis Splines (NURBS) algorithm developed by Ferede [14], with a more complex spanwise/chordwise discretization technique in order to more accurately design a discretized HAWT blade. The second goal was the alteration of the material creation algorithm developed by Scheldbergen [25], so that different stacks at various blade locations can be assigned. Moreover, the aerodynamic loads generation algorithm of Scheldbergen [25] was modified to apply for different airfoils along the blade span. Also, Blade Element Momentum (BEM) theory was used for the calculation of the aerodynamic loads and the software XFOIL was used for the aerodynamic analysis of the airfoils (pressure distribution on the airfoil at corresponding angles of attack and Re numbers). An initial HAWT blade design was created, matching the SANDIA 5 MW wind turbine blade [23] geometry and materials. Furthermore, the apwise/edgewise stiffness, moments of inertia, mass and mode shapes were also validated. Additionally, the SANDIA blade was analyzed in DNV GL Bladed in order to validate the aerodynamic loads on the blade. Following, the tool was designed to process the geometry, the materials and the gravitational/ centrifugal/aerodynamic loads of the blade and translate them to an input for the FEA software MSC Nastran for Linear Static Analysis (SOL 101), Buckling Analysis (SOL 105) and Modal Analysis (SOL 103). The post processing algorithm of Scheldbergen [25] was modified to apply for a HAWT (instead of a VAWT), transforming the results of the FEA analysis into optimization constraints (ultimate load, buckling, tip deflection and fatigue). The final part of the algorithm involved the minimization of the blade's mass by altering the thickness at various locations, using the MATLAB function fmincon. Concluding, a case study of a smart rotor blade equipped with trailing edge ap (TRF) was examined. A _ 2 flap was used at 90-100% of the chord and 78-98% of the span. Two kinds of optimization were performed for the HAWT and smart rotor scenarios; one that included ultimate stress, tip deflection and fatigue constraints (UL-TD-FA) and one that also included buckling constraints (UL-BU-TD-FA). It was found that the buckling constraint was the limiting factor of the optimization and the fatigue constraint was the least dominant. The initial design of the smart rotor blade showed a 16.5% decrease of the maximum fatigue damage and a 2% decrease in the maximum stress. The optimized smart rotor blade mass was 0.5 % lower in the UL-TD-FA scenario and 2.5% in the UL-BU-TD-FA scenario compared to the HAWT blade. The tool created is capable of creating a flexible and fairly accurate representation of a HAWT and smart rotor blade. It also provides the ability to structurally analyze the blade under the most significant structural criteria and minimize the blade's weight under specific loading conditions. The tool constitutes a good starting point for more detailed structural analysis of smart rotors and more complex structural optimization of HAWT and smart rotor blades.

Fiber-reinforcedplastic (FRP) is an upcoming material in the construction industry due tocharacteristic material properties such as its high resistance to corrosion andhigh strength to density ratio. Also, it is often claimed that structures fromFRP have lower life-cycle costs and eco burden compared to constructions madefrom steel, concrete, or wood; this can be attributed to the low amount ofrequired maintenance and longer life span of FRP. Therefore, FRP seems a verysuitable material in the harsh environments where hydraulic structures residecompared to conventional materials.No actual commercial jetties, besidessmall pedestrian jetties, are yet constructed from FRP: knowledge regarding thepotential financial savings or the environmental impact of such jetties are notwell known. Also, specific consequences of constructing a jetty from FRP areunknown, as well the ability of FRP jetties to maintain their structuralcapabilities over their entire life-time. Therefore, this thesis investigatesthe feasibility of FRP jetties and judges whether FRP jetties are betteralternatives than jetties constructed fromtraditional materials. In the scope of this thesis, the research is narrowed down to comparing FRP withreinforce concrete (RC).The main design challenge of FRP incivil engineering related structures is coping with the relatively lowstiffness of FRP, as this presumably determines the dimensions of thestructural elements and restrictions of the structure as a whole. Governingstructural safety criteria in steel and concrete are more often strengthrelated. The research rests on a case study of an RC jetty, which provides boundaryconditions and a program of requirements. An FRP jetty is designed whichcomplies with the structural criteria. These criteria were both extracted fromthe case study and provided by the CUR96, a Dutch design guideline for FRP incivil engineering practice. Most structural elements are designed from scratch:laminates are designed for the flanges and webs in a composite calculator namedeLamX2. The finite element method (FEM) software program SCIA Engineer is usedfor the structural analysis. One dimensional structural elements were firstvalidated before utilizing them in the FEM model. The pile properties anddimensions are based on contemporary literature and commercially availableproducts. The driveability of the FRP piles is researched by means of Wave EquationAnalysis of Piles (WEAP), for which the program AllwavePDP is utilized.Furthermore, sustainability aspects of both jetties are researched by means ofa Life Cycle Assessment (LCA). The LCA determines how much equivalentgreenhouse gases are expelled over the life-time of the jetties for a set ofimpact categories. These results are normalized by calculating the respectiveshadow costs for each impact category; this makes the total environmentalimpact of the structures comparable. The financial feasibility is the lastinvestigated topic; under various scenarios, life-cycle costs of both jettiesare investigated. The scenarios contained different variables such as estimatesof FRP raw material costs or assumed share of maintenance costs; end-of-lifecosts were not included in the analysis.The structural analysis of the FRPjetty indicated that both Serviceability Limit State (SLS) criteria and UltimateLimit State criteria (ULS) determine the dimensions of the structural elementsand the jetty design in general. The most crucial parts are partially embeddedFRP piles, which are prone to buckling. Initially, the FRP piles in thedetailed design were to be installed to a depth of 13 meter below ground level,but the results from the WEAP indicated that the piles refused duringinstallation before reaching this level. An analysis indicated that drivingshorter piles to a depth of 8 meter is possible: at this depth, the piles donot refuse and have accumulated sufficient bearing capacity by shaft frictionto support the superstructure. The eco burden of the FRP jetty was foundsignificantly higher compared to the RC jetty: in the base case LCA, therelative difference is 365 percent higher for the FRP variant. After asensitivity analysis, the relative difference is still 59 percent higher when comparingthe best-case scenario of the FRP jetty with the worst-case scenario of the RCjetty. The RC jetty also performed better than the FRP jetty regardinglife-cycle costs in various considered scenarios. The relative difference inlife-cycle costs for the most favorable scenario of the FRP jetty is still 28 %higher compared to the life-cycle costs of the RC jetty.Due to the poorer performance of theFRP jetty regarding the life-cycle costs and environmental burden, it isconcluded that FRP jetties, for the time being, are not better alternativesthan RC jetties. Regarding the type of jetty, the conclusion can begeneralized. The jetty is designed for the turnover of liquid bulk; imposed loadsare generally lower than loads on Ro-Ro, solid bulk, or container transfer jetties.It therefore seems unlikely that FRP does seem to be a better alternative forthose cases. Regarding the material choice, the conclusion cannot begeneralized. The FRP jetty was compared to an RC jetty. Jetties made from steelor wood are likely more vulnerable to degradation in harsh conditions. Thedurability properties of FRP might be more beneficial to the assessment of FRP jettiesin these cases. Certain future developments might affect the conclusion.Innovation in manufacturing techniques and an increase of market demand for FRPcould lower the price. Besides, biodegradable FRP materials are being developedwhich potentially may reduce the environmental burden of FRP. Keywords: FRP, composite design,hydraulic structures, jetty, pile driving, LCA, life-cycle costs

Efficient design of woven composite structures is limited by a knowledge gap in predictive modeling of their behaviour. Fatigue modeling in particular is difficult and limited by a lack of experimental data. For compression-compression (C-C) or tension-compression (T-C) fatigue, experimental results in literature are uncommon. In this master thesis, a test set-up using digital image correlation was developed to effectively capture evolution of properties such as stiffness and permanent strain for C-C and T-C experiments of woven carbon-epoxy material through a test program covering relevant load cases.

Pushing the envelope of aerospace structures requires the complete exploitation of their potential in terms of load-carrying capacity per unit weight for both economic and ecological reasons: the two most important being the reduction of fuel consumption and greenhouse gas emissions. A key approach involves allowing structures like stiffened panels to function within the post-buckling range domain while in service. To do so, the finite element approach allows a broad design space for researching the post-buckling behaviour of such structures. Accurately representing post-buckling behaviour in finite element models requires accounting for geometric and loading imperfections. The present study explores their effects on the post-buckling behaviour of a composite L-stiffened panel. A finite element model is created and validated based on an experimental case. This is then further modified to incorporate imperfections. Geometric imperfections are modelled using linear eigenvalue modes, while loading imperfections are introduced via a rigid loading plate making contact at an angle. The research showed that both first and higher eigenmode combinations for geometric imperfections influence post-buckling behaviour. Their shape and amplitude impact the transition into post-buckling and their ultimate loads. Similar behaviour was also observed for loading imperfections. Additionally, their configuration also showed an offset in axial displacement results. These insights emphasise the need for precise imperfection modelling to promote safer and more efficient post-buckling design of aerospace structures.

Offshore wind turbines are used more and more for the production of our electricity. The wind turbines are located in a remote and harsh environment, and are subject to heavy cyclic loading, which may cause fatigue in the support structures. Periodic inspections are required to assess the structural health of the wind turbines. Acoustic Emission Monitoring is the technique of acquiring and processing of the high-frequency sound waves emitted during crack growth. Accurate processing of these signals can lead to detection and localization of fatigue cracks, and further reduction of the need for costly periodic inspections. The factors that determine the coverage area of acoustic emission sensor nodes are source signal strength, attenuation of signal in the medium, and onsite noise level. Together with a cost benefits analysis, this leads to insight into the feasibility of this technique for offshore wind turbines. Investigation of the attenuation is done using a higher-order Spectral Element Method with the input acquired from experiments. The signal that is detected at the sensor has to have sufficiently higher intensity than the surrounding noise. The most severe sources of noise are rain drops hitting the water surrounding the turbine, for which a setup is used in the laboratory, and noise from rotating equipment, for which a measurement has been performed at an on shore wind turbine. The localization accuracy of 5 – 10 percent was shown to be achievable in a laboratory setup. With the coverage area determined, a monitoring strategy for a single wind turbine is proposed, as well as how an acoustic emission monitoring system can be efficiently implemented at a larger scale for a wind farm. It is concluded that acoustic emission monitoring is generally feasible for this application, yet further testing is required in order to decrease the uncertainties and to demonstrate the capabilities to potential operators.

Straight fiber variable stiffness laminates use multiple overlapping patches to tailor the fiber orientations in particular regions of the laminate. A novel, automated framework for the design of manufacturable laminate has been developed using different stages. In the first and second stage an optimal stiffness distribution is determined using lamination parameters and converted to stacking sequences on a per region basis. In the third and fourth stages cellular automata and network theory are used to introduce cohesion between regions and comply with current manufacturing standards. Plates in uniaxial compression and cylinders in bending show an increase in buckling performance of 42% and 30% respectively. Such improvements do allow weight savings between 7-11% for these types of structures. When the additional cost of manufacturing and the cost-to-weight ratio for airlines are considered, these laminates already have the potential to significantly reduce the cost of air travel.

Induction welding is an effective technique for joining unidirectional carbon fiber reinforced thermoplastic composites and L-joints can be produced through quick and cost-effective processing steps. However, due to high localized stresses in the skin-stiffener interface, these L-joints are often avoided in primary aircraft structures. Also, no international testing standards have been developed for testing of such joints. A method was developed for the implementation of a neat thermoplastic resin fillet between the L-joint skin and stiffener web using the induction welding process in an attempt to remove the high stress concentration at this location. A 35.4% increase in quasi-static pull-off strength was measured with a weight penalty of less than 0.5%. This result was compared with a similar autoclave co-consolidated joint, which showed an 80.9% improvement. An ANSYS Parametric Design Language finite element model was developed based on the virtual crack closure technique and it showed that the joint pull-off performance is strongly dependent on geometric parameters such as the skin and stiffener thickness. Also, a new test setup was developed, which reduced internal stresses created by the setup compared to those commonly used in literature. By further improving the method through which the fillet is joined to the induction welded L-joint, a performance increase similar to that of the co-consolidated joint should be achievable. Test results have shown that the use of this type of fillet can lead to the skin-stiffener interface no longer being the critical failure point for realistic joints in primary aircraft structures.

In this thesis, a methodology is presented to predict impact damage on next-generation (composite) aircraft based on maintenance data of in-service (metal) aircraft. To achieve this conversion, an analytical model is developed to Model Impact Damage on Aircraft Structures (MIDAS) for composite (-C ) and metal (-M ) aircraft. The model characterizes impact threats based on damage dimensions in two steps. First, for a given specific threat an impact event is approximated, and the corresponding damage (i.e. the permanent dent) is estimated. Second, the analytical model is reverse engineered to deduce the impact threat characteristics from the permanent damage. MIDAS-M implements a new transition region between the local and global deformation modes based on penetration limits, while composite variant (MIDAS-C) provides a novel approach for the permanent indentation in the post fiber breakage region.

The current generation of commercial aircrafts extensively use composite materials such as Carbon Fibre Reinforced Polymers (CFRP) in both exposed and primary structures. These materials lack through-thickness reinforcement and are hence susceptible to out-of-plane impact damages. Barely visible impact damage caused by low energy impacts poses a unique problem, since delaminations, de-bonding and cracking may be present below the surface layers without any indication of damage on the surface. The focus of this research is on one such scenario, multiple site low energy hail impacts, while the aircraft is on the ground. Taking into account the relevant parameters, a hail impact envelope was established both in terms of the initial kinetic energy and peak impact force. Further, contradictions found in literature over the influence of the compressive strength of hailstones were resolved. These were accomplished with the aid of a state-of-the-art finite element model and experiments in the laboratory. With help from a custom designed and assembled impact force measurement experimental setup a relation was established between impacts carried out with steel impactors and those with simulated hail ice impactors. Based on this relation, predictions are made on which hailstones have the potential to cause damages to CFRP structures.

Fibre-reinforced composites are increasingly being used in maritime structures. Piezoelectric sensors can be used to monitor both loads acting on a fibre-reinforced composite structure and elastic waves emitted from damage initiation within the structure. In certain marine applications, due to a harsh environment or for hydrodynamic reasons, piezoelectric sensors cannot be placed on the surfaces of the structure. Fortunately fibre-reinforced composite materials allow piezoelectric sensors to be embedded inside the structure. The focus in this research is on embedded piezoelectric sensor design, performance and behaviour. Carbon fibre beam specimens with embedded piezoelectric sensors have been manufactured and subject to low-frequency bending, high-frequency elastic wave emission, electrical excitation and monotonic bending to failure. The low-frequency bending test has given insight in the effects of sensor discharge, that is, decreasing and distorting the sensor response. Also it has been used to check consistency between different specimens. The effect of electrical discharge is practically absent around 1.8Hz but becomes apparent at lower frequencies. The test was simulated numerically. The simulation captures the effects of electrical discharge but underestimated sensor response amplitude by 40% to 65% for the small and large sensor respectively. High-frequency elastic wave emissions, originating from fibre-reinforced composite failure during monotonic bending, have been measured by an embedded piezoelectric sensor. The signals measured by the embedded piezoelectric sensor have a median signal-to-noise-ratio of 17dB. Using the spectral element method, a parameter study has been performed for a similar, two-dimensional setup. Embedded piezoelectric sensor length and embedment location in the plate thickness has been varied. Both a symmetric and antisymmetric elastic wave have been excited. It has been found that increasing sensor size has a negative effect on the sensitivity of the sensor. The antisymmetric elastic wave is better captured by the sensor response than the symmetric elastic wave. Effects of electromechanical resonance were not noticeable. Monotonic four-point bending failure tests were performed to compare structural integrity of a carbon fibre-reinforced beam specimen with an embedded piezoelectric sensor to a specimen without sensor. With embedded specimens, an average stiffness drop from 3 to 8% is noted. Specimens with small sensors on the tension side of the composite had a decrease lower than 4% in ultimate strain and at maximum 8% reduction in ultimate load and failure toughness. For specimens with embedded sensors on the compression side, the decline was larger, with the larger sensor outperforming the smaller sensor.

Composite lattice structures (CLS) offer high performance and demonstrate significant mass savings in space structure applications. Local modifications of regular lattice designs have the potential to further improve the lattice performance. This research explored the micro-structural quality features of CLSs manufactured with pre-preg fibre-placement, and how quality is impacted by lattice modifications. Modifications to a regular lattice for a representative case of an attachment point load are identified using a topology optimisation tool. Three lattice modifications techniques were identified: rib width variations, rib angle variations and additional ribs. A sample lattice with modifications was manufactured and evaluated by C-scan, CT techniques and micro-sections. An explanation of quality features in CLS is described and quantified using a presented characterisation model based on node transition waviness. Using the presented explanation, a quantitative quality model was developed to relate lattice design geometry to manufactured quality for this process. The model was used to design and manufacture a second panel with improved implementation of lattice modifications. The quality model was improved and shown to explain more than 75% of all observed quality variation in the two panels with a linear regression. Future work and limitations the of the quality model are discussed.

The use of Grid-stiffened structures has been increasing in the space industry over the last decade. Thanks to it is high specific mechanical properties and excellent damage tolerance characteristics, researchers also started looking into the potential of using them for aeronautical applications. However, not much has been done to investigate the reduction in strength after barely visible impact damage and visible impact damage, and how much energy is needed to cause such damage, which is an important requirement for aircraft certification. In this project, a grid-stiffened replica of an existing reference cargo drone is created, and the design is tested to check its compliance with the requirements. This involved modelling of the design, manufacturing of test coupons, impact and mechanical tests, and model validation. The results showed a possible weight reduction, with no major reduction in strength observed with impacting using the cut-off energy level set by the manufacturer.

Fibre-reinforced polymer composite laminates are used extensively nowadays in various applications due to their superior mechanical properties. However, a great weakness of composite laminates is their susceptibility to out-of-plane concentrated loading (which can be broadly divided into quasi-static indentation and impact) owing to their weak interlaminar strength. Delamination is a typical damage type for composite laminates under such loading conditions. The occurrence of delamination in composite laminates could significantly degrade their load-carrying capabilities, especially the compressive performance. Principles of strength of materials and fracture mechanics can be adopted to apply specific delamination initiation and propagation prediction methods to composite laminates. It is known that fracture mechanics methods have advantages in addressing delamination growth problems. On the other hand, it is also shown in the literature that strength of materials methods are generally best suited for quasi-static delamination growth. This implies that the growth of low-velocity impact or quasi-static indentation delaminations in composite laminates could also be predicted with an appropriate strength of materials approach. It is therefore necessary to comprehensively assess the ability of strength of material approaches to predict delamination of a composite laminate under out-of-plane concentrated loading. Since a stable stress field is the basic condition for the application of the strength of materials methods, the out-of-plane quasi-static indentation loading condition is first considered in this thesis.@en

The safety of offshore assets such as wind turbines, offshore cranes, and turret mooring systems partly relies on the integrity of heavy-duty bolted joints. These critical connections can be of different dimensions and exposed to monotonic and cyclic loading. Despite being small components of a structure, bolts need to be maintained appropriately to ensure structural safety and reliability. The integrity of bolted joints can be assured by checking for adequate preload. In addition, fatigue failure resulting from preload loss is of primary concern, as it can occur suddenly without any visible changes. Hence, bolts should be re-tightened periodically to ensure sufficient preload. However, in offshore conditions, this countermeasure leads to high maintenance costs while exposing the crew to unfavourable conditions and risks. This research focuses on the feasibility of condition-based maintenance of bolts using ultrasonic waves. An energy attenuation method has been implemented for this feasibility research. The main objective is to establish a proper methodology and hypotheses for preload detection. Furthermore, along with preload detection, the feasibility of crack detection was investigated. Finally, the proposed methodology and hypotheses were validated by performing experiments and numerical simulations. The experimental and numerical results verify the proposed methodology by showing an increasing trend in the energy and the power of the transmitted ultrasonic wave for increasing preload. Also, the feasibility of crack detection using the same setup has been positively evaluated. The obtained results suggest that the ultrasonic waves can be employed to monitor bolts for condition-based maintenance. Additionally, a number of relevant research activities are recommended based on this study.