There are numerous benefits associated with replacing hydraulic actuators with electrical counterparts as part of an all electric landing gear including reduced consumption of non-propulsive engine power, reduced weight, reduced cost and the elimination of hydraulic systems. The development of health monitoring systems to support the introduction of electrical actuation systems into landing gears will aid in guaranteeing reliability and to optimise landing gear maintenance activities. One of the difficulties with designing health monitoring for industrial integration involves the large number of subject areas involved, ranging from architectural design, software and signal processing design, hardware selection and business modelling. The reason that many health monitoring systems never reach full development maturity is that there is a failure in realising a holistic design process. The purpose of this thesis and the overall contribution which has been made is to bring together a combined understanding of landing gear design, health monitoring and the business environment for aircraft maintenance in order for a holistic design process for landing gear health monitoring to be realised.

Le transport aérien des marchandises et de personnes est devenu un des piliers de notre société mondialisée. Néanmoins, la croissance du secteur aéronautique a accentué les problèmes liés aux hauts niveaux sonores émis par les aéronefs. Dans le cadre de la réduction du bruit produit par les trains d’atterrissage en phases d’approche et d’atterrissage, cette thèse a pour but de proposer une méthodologie numérique précise et efficace permettant de prédire un tel bruit. Elle est basée sur l’utilisation du code Navier-Stokes CEDRE développé à l’ONERA mettant en œuvre des maillages non-structurés de haute qualité ainsi qu’un modèle de turbulence de type Zonal Detached Eddy Simulation. Cette démarche a été validée grâce au cas LAGOON, notamment à partir de deux simulations réalisées durant cette thèse, avec l’obtention de résultats très précis (moins de 1dB d’écart avec la mesure expérimentale des niveaux de bruit intégrés) avec un coût de calcul très raisonnable. Par la suite, cette méthodologie a été appliquée à un cas plus complexe, le PDCC, représentatif d’un train d’atterrissage réel. Une nouvelle fois, les résultats obtenus notamment les niveaux de bruit sont très précis, comme c’était le cas pour LAGOON. Ces bons résultats sont obtenus pour des choix différents : pour le PDCC des lois de paroi ont été utilisées en plusieurs endroits de la géométrie, dans le but de réduire encore plus le coût de calcul, tandis que dans le cas LAGOON, les couches limites ont été résolues à l’aide d’un maillage dédié. Finalement, la nature des sources acoustiques présentes dans le train LAGOON a été aussi étudiée. Pour cela, une simulation numérique d’un écoulement se développant autour d’une roue isolée du train a été réalisée afin d’identifier les mécanismes physiques responsables de la réponse tonale de la cavité, observée lors de la campagne expérimentale sur le train à 2 roues. Cette étude peut être aussi considérée comme celle d’une cavité installée soumise à un écoulement rasant non-uniforme. Les résultats obtenus sont comparés avec succès à ceux issus de la littérature du bruit de cavité. / In a globalized world, air transportation of goods and persons has become an important pillar of our societies. However, the growth of this sector has raised an important issue, the high noise associated. In the framework of reducing the noise emitted by aircraft at approach and landing, and more precisely the noise emitted by the landing gear, this thesis aims to provide an accurate and efficient numerical methodology enabling to predict such noise. It is based on the use of ONERA’s in house Navier-Stokes code CEDRE coupled with the use of high quality unstructured meshes and a Zonal Detached Eddy Simulation turbulence model. This procedure has been developed thanks to the LAGOON test case, of which two simulations were performed during this thesis, obtaining very accurate results (less than 1dB of error in the OASPL results compared to the experimental measurements) at a moderated cost. In the following, it was applied to a more complex case, the PDCC, representative of a realistic landing gear. Once again, the results obtained were very accurate as was the case for LAGOON, even if an important difference existed between the two cases: for the PDCC, wall functions were used for several geometrical parts, aiming to decrease even more the cost of the simulation while all the boundary layers developing over the LAGOON case were completely solved with a devoted grid. Finally, the nature of the acoustic sources present in the LAGOON landing gear was also addressed. At this end, a simulation was carried out over an isolated wheel, aiming to identify the mechanisms underlying the tonal response of these cavities observed during the LAGOON experimental campaign. This study can be considered as the study of an installed cavity submitted to a non uniform grazing flow, and its analysis relied on comparisons with the results obtained on common cavities from literature.

<p>The properties and microstructure of Ti-5Al-5V-5Mo-3Cr were characterized under various stress states after the following heat treatments: 1) annealing above the β transus, followed by cooling at various rates and ageing for different times; 2) solution heat treatment in the α-β range, fan-cooling and ageing for various temperatures and times.</p> <p>Heat treatment above the ptransus temperature causes complete recrystallization of the as-forged microstructure. The as-cooled microstructure consists of equiaxed β grains with an average grain size of 200μm. Water quenching from above the β transus results in precipitation of a dispersion of nano-sized ω phase; while the fan-cooled microstructure contains nano-sized ω and α precipitates. Ageing of the fan-cooled microstructure at 790°C or 600°C precipitates sub micron acicular α throughout the β grains. The tensile properties of this condition could not be determined using standard tensile specimens due to brittle failure at the grips.</p> <p>Controlled cooling from above the β transus to the ageing temperature at slower rates produces a coarser α+β microstructure. Acicular α laths are produced with cooling rates of 1°C/min, while lamellar α develops at cooling rates of 3.4°C/min. The β annealed and fan-cooled condition is characterized by relatively low strength (~850MPa) and low ductility (~6% elong.).The fracture mode is by intensely localized slip and the creation of transgranular cracks. Localization of slip is attributed to shearing of the nano-scale ω precipitates by dislocations. A linear relationship between the grain size, d⁻¹/² , and the yield and fracture stresses was established, as described by the Hall-Petch relation. With controlled cooling, the strength and ductility improve by precipitation of lamellar α within the β matrix. Improvements in ductility and strength are achieved by reducing the slip length.</p> <p>Solution heat treatment below the β transus and fan-cooling results in complete dissolution of the as-forged acicular α phase. Solutionizing at 50°C below the ptransus yields a volume fraction of 16.5% primary α in a matrix of retained β. Low angle grain boundaries and globular primary α, each measuring 2-4μm average in diameter, are uniformly distributed throughout the retained β matrix. The tensile strength in this condition is relatively low, i.e. (900MPa) and the ductility relatively high (~16% elong.). With ageing in the 500°C to 600°C temperature range, precipitation of α within the retained β begins within 5 minutes of the start of ageing. Precipitating is heterogeneously nucleated at dislocations and grain boundaries. The yield and ultimate tensile strengths reach values of roughly 1200 and 1300MPa, respectively, and remain relatively constant for up 48 hours ageing</p> <p> The fracture stresses for the solution treated condition and for material subsequently aged at 500°C and 600°C are quite similar in magnitude. This similarity is due to the fact that the fracture mechanism, which controls the fracture stress, is the same for all these conditions. The fracture mechanism for all the solution treated conditions begins with shear decohesion of the primary α/β interfaces.</p> <p>For each condition, the damage mechanisms and final fracture modes were evaluated and rationalized on the basis of microstructural features. The yield and fracture stresses for the various conditions were calculated and plotted on a two-principal stress axis coordinate system, thus creating the failure envelope for Ti-5553. For the β annealed and fan cooled and for the α-β solution heat treated and aged conditions the yield and fracture envelopes are two concentric ellipses in good agreement with the shear strain energy (van Mises) model for failure.</p> <p>The fracture toughness and stress corrosion cracking behaviour for the STA condition were evaluated and compared against other β titanium alloys.</p> / Thesis / Doctor of Philosophy (PhD)