In a strive to further improve the performance of Aircraft movables the performance of Carbon fibre PolyEtherKetoneKetone (C/PEKK) on component level was assessed in this study. In this study 3 concepts were designed for a fictitious Next Signle Aisle aircraft (NSA) rudder to perform a trade-off between the two composite materials and the two stiffening concepts. An existing C/PPS multi-rib rudder concept is used as a reference. Specified requirements were elaborated to create a C/PEKK based multi-rib rudder redesign. Two multi-rib concepts, each with a different minimum skin thickness, were made and compared. The weight and cost of these two concepts was found to be nearly equal. Alternative stiffening concepts for the multi-rib were generated in collaboration with experts. Grid-stiffening (GS) of the skin using a short fibre reinforced composite was selected. The GS skin allowed most of the ribs to be omitted so that part count and assembly effort could be reduced. The multi-rib concept proved to be the lightest, regardless of the used material. The Technology Readiness Level (TRL) of some manufacturing processes for C/PEKK is low and requires more development. The C/PEKK rudder shall be a viable option in the near future when all envisioned manufacturing concepts and welding of C/PEKK tape become available. The GS rudder requires more research and development. The short fibre material and manufacturing process of the grid have a low TRL and are not yet qualified. A discrete model of the GS skin is also required to better understand the behaviour of the rudder. Thus the GS rudder is expected to be a viable solution in the long term future.

In multi-stage launch vehicles, the adjacent stage re-enters the Earth atmosphere. Once the stage is re-entering the atmosphere, the low-melting-point materials melt and the inner parts will be exposed to excessive heat. With heat increasing, the parts will break into debris. Some of the fragments will burn in the atmosphere, others will crash into the Earth’s surface. Since there is a lack of knowledge about the re-entering physics, Demise Observation Capsule (DOC) project is started. The DOC collects information about the re-entry progress of launch vehicle stage, where it is mounted on. Several launchers and stages are considered to investigate the effect of the location on the DOC’s configuration. In an ideal situation, the DOC can be placed on all stages/parts of launch vehicles and at the same time it is optimized for that location. A generalized design concept that fits on several locations and stages, will reduce the cost and effort. Therefore, the thesis objective is to determine whether it is feasible to have a generalized DOC or an optimized DOC per location and load case. Additionally, the current DOC is designed for four locations and the second objective of this thesis is to optimize the current DOC and shift its center of gravity to the nose of the DOC. For this reason, the main research questions are as follows: Is it feasible to generalize the DOC concept design so that it will fit on every ESA launcher and stage and meet the predefined requirements and will withstand environmental loads while general operability of the DOC is guaranteed? How can the center of gravity be shifted on the DOC to its nose under the current designed loads such that the DOC can be made more stable?

Automated tape or fiber placement (ATP/AFP) with in-situ consolidation has been identified as a promising manufacturing technique for thermoplastic (TP) composites, which are highly in-demand in the aerospace industry for future aircraft structural applications. This manufacturing technique is attractive since it has the potential to eliminate the energy and time consuming autoclave consolidation. However, low quality is one of the biggest challenges in the application of in-situ consolidation. It seems that pressure contribution to the bond quality has received less attention than thermal one. Therefore, this work is initiated to simulate the pressure distribution between the compaction roller and the mandrel.
The objective in this work is to predict the pressure distribution in the contact area between the rubber-covered roller and the mandrel in room temperature by finite element methods (FEM). Rubber material is characterized by mechanical testing and then modeled as hyperelastic material. 3rdd-order Ogden material model is found to well describe the strain- stress behavior of rubber material. The material constants are then implemented in FE models as an input. The pressure distribution is predicted by FE modes and the influence of compaction force, rubber thickness, pre-stretching force on the pressure distribution are discussed, followed by an optimization between compaction force and rubber thickness. Experiments are conducted to validate the FE models regarding the three influencing factors, that are: (a). compaction force, (b). with/without pre-stretching and (c). various rubber thickness. Rubber deformation can be captured by the digital image correlation (DIC) Pressure distribution is obtained from the Prescale pressure measurement film produced by Fujifilm®, which are the films that show different color densities under different pressures. Experimental results of strain and pressure distribution are compared with FEM results, followed by discussions and conclusions.
It can be concluded that pre-stretching force should be used to avoid rubber-roller separation and prevent rubber from moving out to the sides. Both of the compaction force and rubber thickness affects the maximum pressure and contact length linearly. A trade- off between compaction force and rubber thickness can be made based on the force limit of the equipment and pressure requirements.

Future wind turbines require larger rotor blade radius, leading to an increase of rotor swept area. The larger rotor swept area results in more energy production of the wind turbine. Further increase is however limited as a result of the structural weight, which induces a large bending moment and could result in buckling of the rotor blade. An innovative design solution resulting in a lower weight blade design could be acquired by introducing a grid stiffened structure as a substitution of the conventional sandwich structure. Design and analysis showed an increase in specific stiffness of the grid stiffened structure for higher loading conditions. In an attempt to validate the obtained results, one sandwich and one orthogrid panel has been manufactured and tested.

Over the past decade, wind turbine towers have grown taller and wider to support high capacity turbines. It may, therefore, be prudent to investigate materials alternative to steel to mitigate an increase in tower mass, cost, and complexities in transportation & manufacturing associated with the steel towers. The current research focuses on the preliminary design of economically feasible composite wind turbine towers.Some of the project objectives involve setting up the design tool for tubular and lattice tower made up of Glass Fiber Reinforced Plastic (GFRP) and Carbon Fiber Reinforced Plastic (CFRP) material, preliminary design of possible joining techniques, setting up the cost model for various manufacturing approaches and assembly techniques and comparison of composite towers to steel tower design regarding mass and cost. The design tool will incorporate all the essential load cases, structural and geometric constraints and will help to analyze the composite towers with various hub heights and for 2.1 MW and 5 MW turbinecapacities.Loads and constraints are estimated through the literature study. Preliminary design of the tubular tower has been carried out using an analytical approach. For the lattice towers, a Finite Element Method (FEM) approach using Matlab was the suitable method to perform the analysis. A minimum first natural frequency constraint of 0.27Hz and 0.22Hz have been incorporated during the design for 2.1MW and 5MW turbine capacity towers respectively. For the joint design, bolted and adhesive joints are considered and the failure modes associatedwith these joints have been incorporated into the design to get an estimate of the joint mass and cost. The cost modeling of composite structures was done using the parametricequations that fit the Process Cost Analysis Database (PCAD) cost model in the region of interest. These parametric equations are usually functions of the surface area of the structure, perimeter, the number of plies and the complexity of the part. The final part involves comparison of the GFRP and CFRP towers with steel towers and comments on the feasibility of the composite towers.CFRP tubular and lattice towers show a mass reduction of up to 60% for lower hub height towers (<=100m), but this advantage decreases with height. Due to the high material cost, the CFRP towers are at least 3-4 times costlier than steel towers. For the GFRP material system, up to 35% mass reduction was found in the tubular design with lower hub height (<=100m). 5MW-100m hub height GFRP tubular tower showed the highest mass advantage (35%) and was the closest in cost to the steel design with the GFRP tower being 4% costlier. The GFRP lattice towers showed a similar trend with up to 35% but were at least 27% costlier than steel tower due to the high material and joint costs.The only region where composite currently shows any promises of mass and cost feasibility is for smaller hub heights (<=100m) and on tubular towers. The major existing difficulties with steel towers are for heights in the range of 125-150m, and in this region, the composite tower design does not show mass or cost advantages. Based on the results of the various trade studies and optimized designs, it was concluded that the composites do not hold a definitive promise as an alternative material for wind turbine towers over steel. With the current technology and understanding of the tower designs, the mass advantage promised by composite towers is not enough for composites to be deemed a viable option that can thrive in a competitive market for renewable energy. Thus a few scopes for future research are provided that can help in strengthening the understanding of the composite tower design

For several years T-bolts have been a popular choice for joints in the field of wind energy, specifically for connecting the blade roots to the hub of the wind turbine. Their use is mandated by the geometry of the joint and they perform very well under pure axial loading. However recent analyses have shown significant bending stresses in the T-bolts preventing their use to their full capacity. These bending stresses are unavoidable due to the presence of a slew bearing between the blade root and the hub. The bending stresses generated while loading the blade root, causes the blade root-bearing joint to gradually open, causing excessive loading of the T-bolt above a certain load.
It is hypothesized that modifying the blade root design to reduce the effects of local bending can open up the possibility of reducing its mass and cost. To test this hypothesis, the blade root is initially studied and the stress ratio is identified as an appropriate joint performance parameter. The performance of the joint is boosted by increasing the pretension of the bolt. After an initial phase of over designing the joint to reduce the constraint stresses, the joint optimization is carried out using the Sequential Quadratic Programming algorithm. The optimization culminates with the mass reducing by roughly 110Kg and the material cost reducing by approximately 13% per blade root. The number of bolts reduces from 88 to 52. Thus, a simpler design is achieved, that promises simpler and cheaper manufacturability, higher reliability and lesser sites for crack nucleation in the laminates. The current design strategy at Suzlon is to employ a greater number of T-bolts with thinner shanks. Curiosity in the field of cost optimization that initiated from within Suzlon has proved that there exists a different design strategy that holds great promise for delivering structurally equivalent if not better designs with improved cost, mass and reliability.

The automotive industry has developed an interest in manufacturing structural parts from continuous fibre-reinforced polymers because legislation is becoming more strict on the permissible CO₂ emission of newly produced vehicles. Additional requirements on the level of recyclability has raised the question if composite materials with a thermoplastic matrix in particular can be utilised in the body structure. From the perspective of cost sensitivity, the conventional automotive manufacturing chain has to remain as it is and requires the body structure to pass through the automotive paint shop and subjects all structural elements to significant hygrothermal loading. The objective of this research is to analyse the deformation behaviour of continuous fibre-reinforced thermoplastics subjected to hygrothermal loading characteristic for the automotive painting process.

Studying the theory behind viscoelastic problems revealed that numerical solutions to the integral form of the linear viscoelastic constitutive equations often pose a problem regarding computational memory usage because of the importance of strain history. Utilising a recursive formulation of the constitutive equations eliminated this problem and identified the required material parameters for the numerical model. Thermal expansion, hygroscopic shrinkage, polymer-chemical effects, and stress relaxation were the four phenomena that governed the deformation. Measurements with a dilatometer, a micrometer, and an analytical scale yielded quantitative results about hygrothermal expansion. Thermogravimetric analysis provided information on the moisture diffusion and dynamic mechanical analysis quantified the stress relaxation behaviour. Quasi-static tensile tests confirmed the linearity of the viscoelasticity and digital image correlation supplied the major Poisson’s ratio.

A finite element model has been developed that implements hygrothermal expansion and takes into account orthotropic linear viscoelastic behaviour by means of a material user subroutine. Adopting a sequential uncoupled simulation approach allowed the prediction of heat transfer, moisture diffusion, and stress distribution. A semi-numerical simulation approach enabled the calculation of the expansion of symmetric balanced laminates through classical laminate theory whilst taking into account the time- and temperature-dependency of the mechanical properties computed with a micromechanical model created for special orthotropic laminae. Sensitivity studies justified the usage of one-dimensional heat transfer- and moisture diffusion simulations. Moreover, mesh- and time step convergence studies revealed the sensitivity of the simulation to these parameters.

Dilatometer experiments with dry- and moisture saturated multi-directional specimens confirmed the correct calculation of hygrothermal expansion. Increasing in complexity, measuring the out-of-plane deformation of a clamped unidirectional tensile specimen subjected to a temperature profile that resembles the most severe thermal loading found in the automotive painting process confirmed a satisfactory agreement between the numerical results and the experimental data. Geometric compensation for the thermal expansion of the fixture and choosing graphite as construction material kept the thermal expansion of the fixture to a minimum. Components with the geometry of the roof bow that is currently in series production for the BMW 7 Series were manufactured by a thermoforming method to allow validation of the simulation with complex geometry. A proper agreement between the predicted deformation behaviour by the semi-numerical simulation and the optical measurements of the dry- and moisture saturated roof bows with various multi-directional lay-ups validated the functioning of the developed simulation.

A novel continuum damage mechanics model for 2D woven fabrics has been developed and implemented in a VUMAT subroutine for Abaqus/Explicit. The model takes into account the shear non-linearity, the toughening mechanisms associated to tensile failure, and the influence of shear stresses in the initiation and propagation of tensile damage.

The non-linear shear behavior is reproduced by means of a Ramberg-Osgood equation. Permanent deformation, and the degradation of the secant shear modulus associated to the accumulation of matrix damage have been coupled to the formulation of this unidimensional plasticity law. The physical reference required to define completely the shear constitutive response proposed can be obtained from the experimental tensile test of ±45 off-axis coupons. Failure initiation in tension is identified using Hashin's quadratic failure criterion, which accounts for the interaction of shear stresses in the promotion of tensile failure. A bilinear softening relation was selected to represent the combination of fibre breakage and toughening mechanisms characteristic of the intralaminar tensile fracture of these materials. Effort was placed on the development of a procedure for calibrating the softening relation associated to this failure mode. Specifically, this work addresses the applicability of linking the definition of a bilinear tensile damage law in homogenized continuum damage mechanics models for woven composites to the shape of a crack growth resistance curve measured with compact tension tests.

The constitutive model was validated including it in a set of finite element models of unnotched and open hole coupons with different multistacking sequences, under quasi-static tensile loading conditions, and comparing the results obtained by the simulation of the coupons against an experimental benchmark. A good correlation was achieved between the ultimate strength predicted with finite element analysis and the experimental data.

Deployable structures using tape-springs are a promising option to minimize the volume occupied by satellites at launch and reduce launching costs. The attractive characteristics of tape-springs made of ultra-thin woven composites are their high specific stiffness and bi-stable behavior. However, the most important shortcoming is the complexity of their mechanical analysis. Textile composites are formed by the weaving of bundles of fibers creating a very complex microstructure, and tape-springs present a highly nonlinear mechanical behavior subjected to multi-axial loadings. Therefore, conventional mechanical models developed for unidirectional lamina no longer apply since they do not take into account stress gradient effects through the representative volume element (RVE). In this work, a multi-scale approach is validated to predict the failure initiation of tape-springs. Micromechanical models are used to predict analytically the stiffness properties of the fiber bundles, while FE mesoscale models describe the woven structure and provides stiffness and strength properties for computations on macroscale. A case study of hybrid laminates composed of woven and unidirectional layers is analyzed for its application on tape-springs. Manufacturing defects related to ply waviness of the UD layer have been detected and taken into account to estimate the reduction of the overall stiffness properties. Finally, a dedicated failure criteria based on the force and moment resultants have been used to predict failure initiation of the tape-springs under multi-axial loading conditions. A good correlation was found between the finite element analysis and experimental observations.