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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Fatigue And Fracture Analysis Of Helicopter Fuselage Structures

Ozcan, Riza 01 February 2013 (has links) (PDF)
In this study a methodology is developed for the fatigue and fracture analysis of helicopter fuselage structures, which are considered as the stiffened panels. The damage tolerance behavior of the stiffened panels multiaxially loaded is investigated by implementing virtual crack closure technique (VCCT). Validation of VCCT is done through comparison between numerical analysis and the studies from literature, which consists of stiffened panels uniaxially loaded and the panel with an inclined crack. A program based on Fortran programming language is developed to automate the crack growth analysis under mixed mode conditions. The program integrates the prediction of the change in crack propagation direction by maximum circumferential stress criterion and the computation of energy release rate by VCCT. It allows reducing the computation time for damage tolerance evaluation for mixed mode cases through finite element analysis and runs the procedure file of MSC.Marc/Mentat for numerical analysis and the program generated by Patran Command Language (PCL) of MSC.Patran for remeshing. The developed code is verified by comparing the crack growth trajectories obtained by numerical analysis with the experimental studies from literature. A submodeling technique is utilized to analyze a particular fuselage portion of helicopter tail boom. Effects of different skin/stringer configurations of the helicopter fuselage structure on stress intensity factor are studied by means of the developed program. Fatigue crack growth analysis is performed by using stress intensity factors obtained from numerical analysis and fatigue propagation models proposed in literature.
2

Propagation de coupure en fatigue sur composites tissés – Etude expérimentale et modélisation / Fatigue Crack Growth in woven composites – Experimental study and numerical modeling

Rouault, Thomas 18 June 2013 (has links)
Les pales d’hélicoptère sont des structures composites soumises à un chargement cyclique multiaxial, et leur criticité impose de porter une attention particulière à la tolérance aux dommages. Leur revêtement peut potentiellement présenter des criques suite à certains évènements (impact, défaut, foudre). Ces travaux se focalisent sur un matériau de revêtement donné (tissu de verre) et concernent l’étude de la propagation de coupure (crique) sous chargement cyclique. Les sollicitations de service ont amené à considérer la traction et le cisaillement plan. Une étude expérimentale a été menée afin d’étudier les modes d’endommagement du matériau et sa résistance à la propagation de coupure pour différentes sollicitations (en traction et en cisaillement) et pour les drapages les plus courants. Elle a permis de dégager les mécanismes d’endommagement mis en jeu, et a fourni un ensemble important de propriétés matériau et de données quantitatives de vitesse de propagation. Elle a par ailleurs guidé vers une modélisation par éléments finis adaptée à l’architecture du matériau, et la manière dont il se dégrade en fatigue. Ce modèle repose sur un maillage à l’échelle de la mèche, et la prédiction de la propagation est obtenue par l’utilisation d’une courbe de fatigue S-N. La simulation a été évaluée par comparaison des faciès de rupture, des vitesses de propagation et de l’étendue des zones d’endommagement avec les essais réalisés sur éprouvettes. / Helicopter blades consist of composite structures which have to sustain multi-axial cyclic loading. Because of their criticality, damage tolerance has to be considered carefully. Their skin is subjected to environmental events like impact, flaw, lightning which can cause through-thethickness cracks. The present work focuses on one given skin material (woven glass fabric) and concerns the study of the through-the-thickness crack growth under cyclic loading. In-flight loading lead to consider tension and shear. An experimental study has been carried out to study damage in the material and its crackgrowth resistance under different loadings (tension and shear) and for usual stacking sequences. It highlighted damage mechanisms and provided an important set of material data and crack growth speeds. Besides, this led to a finite element approach adapted to the woven fabric architecture, anddamage feature under fatigue loading. This modeling is based on a bundle scale mesh, a semidiscrete damage modeling and an S-N curve to predict fiber failure. Numerical simulations of crack growth tests were carried out, and results were compared with experiments in terms of crack direction, crack growth speed, and size of damaged area.

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