In this thesis the influence of microstructure on fracture toughness is investigated for two different medium/high strength Al-alloys for aerospace application. In weldable AA6156 (Al-Mg-Si-Cu) alloy sheet, the quench sensitivity in toughness is assessed via enhanced Kahn tear tests. Toughness was seen to be reduced for both 60°C water quenched and air cooled materials cf. 20°C water quench material. Fractography via scanning electron microscopy (SEM) and synchrotron radiation computed tomography (SRCT), as well as Differential Scanning Calorimetry (DSC) and Transmission Electron Microscopy (TEM) studies, have clarified the mechanisms of the quench sensitivity with respect to toughness. Both the coverage of grain boundary decoration and precipitate free zone (PFZ) width increase with reduced quench rates. The failure morphology of the air cooled material appears consistent with classical intergranular ductile failure. Coarse voiding and shear decohesion was prevalent in 20°C water quenched material (depending on local triaxiality), whilst the 60°C water quenched material showed a mixture of transgranular and intergranular fracture modes. The experimental toughness trends are compared to models in the literature and a simple new model is suggested. Fracture toughness anisotropy of AA2139 (Al-Cu-Mg), a candidate alloy for age forming, in T351 and T8 conditions has been investigated via mechanical testing of smooth and notched specimens of different geometries, loaded in the rolling direction (L) or in the transverse direction (T). Fracture mechanisms are again investigated via SEM and SRCT. Fracture toughness is seen to be anisotropic for both heat treatment conditions tested, but is substantially reduced for the T8 condition compared to the T351. Contributions to failure behaviour have been identified with: (i) anisotropic initial void shape and growth, (ii) plastic behaviour, including isotropic/kinematic hardening and plastic anisotropy, and (iii) nucleation at a 2nd population of 2nd phase particles leading to coalescence via narrow crack regions. SRCT analysis of arrested cracks revealed alignment of voids in the crack during propagation in the rolling direction, resulting in shorter intervoid ligaments than for crack propagation in the transverse direction. Coalescence through shear decohesion in the crack initiation and propagation region was found indicating the necessity to investigate and account for this mechanism. A model based in part on the Gurson-Tvergaard- Needleman approach is constructed to describe and predict deformation behaviour, crack propagation and, in particular, toughness anisotropy. Model parameters are fitted using microstructural data and data on deformation and crack propagation for a range of small test samples. The model accounts for the material features found in the experimental study and its transferability has been shown by simulating tests of large M(T) samples showing strong fracture toughness anisotropy. A parametric study shows that nucleation of small voids at different strains for different loading directions is crucial for a correct model of toughness anisotropy; the combined effects of kinematic hardening and void growth anisotropy can not fully describe fracture toughness anisotropy.

In the current, environmentally-aware, climate aircraft designers are under increasing pressure to produce fuel efficient vehicles. Weight reduction is an important method for increasing fuel efficiency. Fibre reinforced polymer (FRP) composites are known to offer weight savings over traditional metallic components, due to their excellent stiffness and strength to weight ratios. However, the major limiting factor for the use of aerospace quality composites is the manufacturing cost. The costs incurred in the conventional process of prepreg cured in an autoclave are well documented. The research in this thesis is concerned with reducing the cost of manufacturing aircraft standard carbon fibre composite sandwich panels, whilst maintaining mechanical performance. The overall aim of the EngD is to provide a unified approach for assessing the performance of carbon fibre sandwich secondary structure that are manufactured using several different techniques. Cost and performance criteria are defined so that an optimal panel can be produced. The work has been motivated by the industrial sponsor, GE Aviation Systems. Five combinations of raw material and processing techniques, manufacturing options (MOs) were considered in incremental steps from the baseline of unidirectional prepreg cured in an autoclave to the noncrimp fabric (NCF) infiltrated using resin film infusion (RFI) and cured in a conventional oven. For cost and performance analysis a generic panel has been designed that is representative of secondary wing structure on commercial passenger aircraft. The cost was estimated by monitoring the manufacture of generic panels using each MO, whilst the performance was measured by both mechanical characterisation tests and by full scale tests on a custom designed rig. The rig applies a pressure load using a water cushion and allows optical access to the surface of the panel enabling the use of optical techniques, i.e. thermoelastic stress analysis (TSA) and digital image correlation (DIC). Feasibility tests on TSA and DIC demonstrated their use on the materials considered in this thesis, and were used to validate finite element (FE) models. The RFI out-of-autoclave process was found to reduce generic panel manufacture time by almost 30%, and the material cost was reduced by almost 40%. The mechanical characterisation tests suggested the ‘new’ process could produce laminates with a similar fibre volume fraction to that of the original process and similar in and out-of-plane mechanical properties. The in-plane stiffness was slightly reduced by 7 %, but the strength showed an increase of 12%. Full scale tests on the generic panels using point out-of-plane deflection measurements and full field TSA demonstrated the panel produced using the ‘new’ process has adequate performance. Moreover the full-field tests indicated an improvement in performance. Further work is required to optimise the design of the panel for weight, in particular the weight of the raw material, and investigating methods for modelling the NCF for certification.

Noise generated by aircraft landing gears is a major contributor to the overall airframe noise of a commercial aircraft during landing approach. Because of the complex geometry of landing gears, the prediction of landing gear noise has been very difficult and currently relies on empirical tools, which have limited reliability and flexibility on the applications of unconventional gear architectures. The aim of this research is to develop an efficient and accurate numerical method to investigate the generation and far field radiation of the landing gear noise. In this thesis a hybrid approach is developed that combines near field flow computations with an integral radiation model to enable the far field signal to be evaluated without the need to directly resolve the propagation of the acoustic waves. The recent advances in the CAA methods are implemented with high-order finite difference compact schemes and a characteristics-based multi-block interface treatment. Aerodynamic noise from a generic two-wheel landing gear model, provided by Airbus LAGOON (landing gear noise database for CAA validation) program, is predicted by using the hybrid approach and compared with the LAGOON database. The unsteady flow field is computed by using a compressible Navier-Stokes solver based on high-order finite difference schemes. The calculated time history of surface data is used in a FW-H solver to predict the far field noise levels. Both aerodynamic and aeroacoustic results are compared with wind tunnel measurements in good agreement. Individual contributions from three components, i.e. wheels, axle and strut of the landing gear model are also investigated to identify the major noise source component. It is found that strong flow-body interaction noise is generated by the flow separated from tire rim impinging on the axle. Based on the same landing gear model, the comparison study using conventional CFD solver FLUENT is performed with a second-order Navier-Stokes finite volume solver to compute the unsteady near field flow and the built-in FW-H solver to calculate the far field sound propagation. The comparison suggests that although conventional CFD method can obtain good timeaveraged aerodynamic results, its ability of predicting sound radiation is limited by the inherent low-order numerical discretizations. The aerodynamic noise from the isolated undercarriage wheel with detailed hub configuration is also investigated using FLUENT. The asymmetric phenomenon in the mean flow is discovered in the wake region of the wheel, which contributes to a positive lift force for the wheel. It is predicted that the isolated wheel radiates relatively strong noise to the sides with several strong tonal noise.

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The de-icing of aircraft wings by the injection of fluid through a slot in the leading edge of the wing is analysed. A review of current de-icing methods is presented and the semi-infinite slot-injection equation derived, which is a singular partial integro-differential equation. The Stefan condition is used to close the system. A discretisation of the equation is presented and the subsequent numerical results are analysed. The model is then revised to account for the retraction of the ice layer away from the slot. An asymptotic result for the thin ice layers is also presented. The problem of describing the motion of a thin, flexible membrane fixed at both ends (a 'sail') is then considered. The steady sail is analysed for the case of an inextensible sail and previous work on this topic is extended by using a discretisation of the singular integro-differential equation that is pertinent to the later analysis of the unsteady sail. An asymptotic expression for the eigenvalues of the system, defined as the values of the tension parameter for which the sail generates zero lift, is also presented. The problem is then extended to that of an extensible sail and numerical results are presented for both the sail with excess length and the membrane without slack. The case where the angle of incidence of the sail to the free stream is a prescribed function of time is then analysed. Previous work on this subject is extended to include the extensible sail and numerical results are presented. A linear stability analysis is then undertaken for both the extensible and elastic sails; the resulting quadratic eigenvalue problem is solved numerically and is in agreement with the numerical experiments. The trailing edge of the membrane is now permitted to move freely and thus the motion of a 'flag' is analysed. The inclusion of bending stiffness is found to be crucial to the stability properties of the flag. The steady equation of motion is numerically approximated for both a hinged flag and a flag that is clamped at the leading edge. The unsteady flag equation is then discretised and numerical results are presented. A linear stability analysis is performed, the conclusions of which are consistent with the numerical approximations of the unsteady flag equation.

An analysis of Earth deformation, earthquakes and tides has been undertaken using Earth tide and interferometric SAR data from ESA satellites. The ability of Earth tides to trigger earthquakes has been investigated by measuring the statistical relationship between earthquake occurrence and Earth tides. Analysing Earth tides data with earthquakes occurrence using Shuster’s test has shown that there is a correlation between Earth tides and earthquake occurrence. The significance of this correlation has been examined as a function of location, earthquake depth and magnitude. It has been found that the correlation is especially significant for low magnitude earthquakes. The two regions in California examined show similar correlations but significant differences in the phase angle of the correlation. Possible reasons for these differences and the role of ocean load tides are discussed in this thesis. SAR data are used to detect the Earth displacement related to two recent earthquakes in the Californian area. InSAR technique has been applied successfully to the Bam earthquake in Iran in 2003. The results for Earth displacements in the Parkfield area are less satisfactory but a number of interference fringes over a wide area due to a particular earthquake have been observed.

Over 120 cases of on-orbit breakups have now been recorded. Many more undetected events are believed to have occurred. Each time an object breaks up, whether by explosion or collision, a cloud of debris is formed. The overall objective of the PhD is to examine the interaction between the debris clouds produced by on-orbit fragmentation events and specific space systems. A breakup event will give rise to concentrations of debris which, for some time after the event, will have spatial densities considerably higher than the background flux. Thus, a detailed knowledge of the extent to which the cloud will grow over a given time period, and an accurate assessment of the risk of collision for a spacecraft passing through it, may prove to be important in mission planning and satellite shielding design. The SDS (Space Debris Simulation) software suite has been developed to carry out the analysis presented in this thesis and now represents the state-of-theart in debris cloud modelling. The integrated structure of the developed software enables a wide variety of analyses to be conducted and simulations of both historic and potential future orbital fragmentation events to be performed. Program BREAKUP uses a combination of empirical and analytical models to simulate catastrophic and non-catastrophic collisions, and also variable intensity explosive fragmentations. Included in BREAKUP is a novel parametric model for producing and controlling non-isotropic fragment spreads. TRAJECTORY acts as a test-bed for orbit propagation techniques, providing the facility for convenient and direct method comparison. EVOLUTION enables the complex dynamics of debris cloud growth to be visualised and in particular the effects of propagation method to be examined. Program TARGET employs a novel implementation of the method of probabilistic continuum dynamics to perform collision hazard assessments for spacecraft which encounter debris clouds. Among the additional new developments included in TARGET are the consideration of atmospheric drag, a direct interface with a non-isotropic cloud model, the use of a cellular target spacecraft representation and impact energy-related damage assessment algorithm, and a built-in satellite constellation analysis facility. A number of case studies are presented to illustrate the modelling capabilities of the SDS software suite, including the simulation of several historic fragmentation events and the debris cloud collision risks to ENVISAT-1 and the Iridium™ satellite constellation. The results produced by the models are validated by comparisons with other simulation software and, wherever possible, with actual breakup event, debris impact and spacecraft, orbit, data.

Application prospects in the automotive industry have led to increasing studies on Al-Mg- Cu-Si alloys. In this thesis, nine Al-(1-3)Mg-(0-0.4)Cu-0.15Si-0.25Mn (in wt%) alloys with potential applications in packaging and automotive industries have been investigated. By means of mechanical testing, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), several mechanisms was identified that influence the final strength of cold rolled alloys during ageing: solid solution, work hardening, recovery and precipitation. Microstructure analyses revealed the formation of undissolved particles consuming the small Si addition, which influences age hardening behaviour of the alloys. Tensile testing was performed to evaluate the strength and work hardening. The integrated experimental results showed that for cold worked samples, b² (Mg2Si) contributes to age hardening of Cu-free alloys, whilst both b² and S (Al2CuMg) contribute to that of Cu-containing alloys. According to the experimental findings, a yield strength model has been developed to elucidate the relation between processing and the final strength. It consists of three main components: i) dissolution of intermetallic phase Mg2Si; ii) precipitation of two strengthening phases b² and S; iii) strengthening contributions from solution strengthening, dislocation strengthening and precipitation hardening due to the strengthening phases. The model was calibrated and tested using separate tensile data and was applied to predict the yield strength evolution of cold worked samples during ageing. An accuracy of 8.6 MPa (about 4% of the total range of strengths) has been achieved. Based on the analysis of the relation of work hardening with cold work, composition and ageing time, three primary findings were obtained: i) cold worked samples usually have the lowest work hardening rate (WHR); ii) WHR increases after 30-minute ageing due to recovery and iii) WHR increases with decreasing level of cold work and increasing Mg and Cu contents. Work hardening models based on the Kocks-Mecking (KM) model and the Kocks-Mecking-Estrin (KME) model have been utilized to explain the main trends. The modelling results showed that the KM model is able to predict the work hardening behaviour of cold worked samples reasonably well. However, the KME model is insufficient to fully describe that of cold-worked-and-aged samples.

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No description available.