The aim of this study is to analyse the influence of ply drop-off regions and related parameters on the VARTM cure-induced residual stress generation and related distortions within thick thermoset composites. These parameters include the resin pocket material properties and geometry, ply orientations and cure cycle parameters. By means of a coupled thermal-mechanical cure model it is observed that during the initial part of the cure process, both the chemical shrinkage and thermal expansion of the resin pocket evolve in counteracting manner. The resin pocket geometry and orientation of the surrounding plies influence the out-of-plane direct tensile stress within these plies whenever either the aspect ratio of the resin pocket or the in-plane stiffness of these plies is increased. The orientation of the terminated ply proved to be of influence as well, where higher in-plane shrinkage and thermal contraction of this ply increases the out-of-plane stress around the drop-off region. Implementing multiple drop-off regions introduce stress interactions caused by the different ply orientations used. These regions influence the local out-of-plane stress levels around the drop-off regions, where the stagger distance between these drop-offs generally lowers this effect. As result of this study, existing drop-off design guidelines are reconsidered and new guidelines are proposed for implementing ply drop-off regions within composite laminates.

As collisions between aircraft and birds happen many thousands of times every year, all aspects of such impact events need to be taken into account in the process of designing an aircraft. One aspect that poses a risk is caused by the vibrations induced during bird impact. These vibrations are propagated through the aircraft structure by stress waves. Severe vibrations can negatively affect aircraft components and electrical systems located near the point of impact. While a vast amount of knowledge and research on the modelling of direct bird impact damage is already available, a gap in understanding exists regarding such vibrations. The purpose of this thesis is to help fill this research gap. Therefore, this thesis tries to answer whether the shock effect caused by the stress waves generated during a bird impact on an aluminium alloy plate can be accurately predicted using state-of-the-art numerical modelling techniques. Two aspects are investigated. Firstly, it is evaluated whether the stress waves that propagate the impact loads through the structure can be identified in the model. Secondly, it is analysed how well the severity of the induced vibrations can be modelled. This is done by utilising five bird impact tests, which were performed by Airbus. In each of these tests, a bird impacts an aluminium alloy test plate, which represents a simplified aircraft structure. A dummy mass, attached to these test structures, represents an electrical system influenced by the vibratory response of the plate. The displacement of the plate is recorded using digital image correlation (DIC). Accelerometers at several locations of the test structure are used to record the vibrations. The author created a dynamic explicit finite element model of the test structures to analyse how well current modelling techniques can determine these stress waves and vibrations. The structure in this model employs mostly conventional shell elements. For the bird, a smooth particle hydrodynamic (SPH) model is used. Several types of stress waves are analysed. The main stress wave types evaluated are the in-plane longitudinal and the out-of-plane Rayleigh wave. An attempt is made to identify these waves in both the numerical model and the physical test. It is found that the DIC data of the impact test shows too much disturbance to evaluate these waves. In the numerical model, both an in-plane and an out-of-plane stress wave is seen. Their recorded velocities are seen to approach their theoretical value as the mesh is refined. The propagation velocity of the out-of-plane stress wave is found to depend on the structure of the mesh. When propagating diagonally to the mesh, a 17% lower wave propagation velocity is found compared to a wave travelling tangentially to the mesh. No research was available in which the vibrations of an aircraft structure due to bird impact are analysed. Therefore, the shock response spectrum (SRS) technique, as applied in the space industry to evaluate the severity and damaging potential of vibrations, is utilised. This is the first known research in which an SRS is applied to a bird impact problem. In this research, the SRS is used to compare vibrations from the tests and the numerical model. Such an SRS is generated from the acceleration-time signal of a vibration. It is found that the trend of the SRSs obtained from the numerical model approximates the SRSs from the bird impact test closely. However, due to the fickle pattern of the SRSs, a maximum discrepancy of 5 to 10 dB between model and test is common.

A composite wind turbine blade is partially thick-walled to comply with high strength criteria. When the manufacturer’s recommended cure cycle (MRCC), which is designed for thin-walled components, is used for thick-walled components it can lead to high levels of residual stress. This experimental research investigates different cure cycles for thick-walled glass-fibre epoxy composites made by vacuum infusion with the objective of minimising residual stresses and cycle time. It is found that a two-dwell cure cycle with a low second dwell temperature will significantly decreases the exothermic peak temperature, increases the maximum shear stress, decreases the acoustic emission activity and reduces time compared to the single dwell MRCC.

As the demand on new lightweight materials increases due to economic and environmental reasons, fiber reinforced plastics seem an interesting solution. In order to maximize their potential, ways have to be found to introduce damage tolerant designs. To this end, in the present exercise, the damage caused by cyclic loading is investigated.
Cyclic loading causes loss of stiffness and strength of composite structures. In the present work a progressive damage analysis model has been built. Damage is tracked by a macroscopic failure criterion. Failure occurring at levels below the macroscopic level are captured by a degradation theory which requires S-N curves as input. An attempt is made to use S-N curves derived from static tests only. Stress solutions are obtained using a finite element package. In order to bring the macroscopic damage, degradation and stress solution parts together, a cycle jumping algorithm has been developed which minimizes the amount of FEM stress solutions required and the required amount of function evaluations.
In its current form, the model is able to properly predict the trend of stiffness and strength behavior. Showing that a model like this has potential. For the proper predictions of mag- nitudes, additional failure modes should be modeled and the usage of S-N curves obtained from large specimens for applying on smaller geometries (elements) should be reconsidered.