Residual Stress Effects on Fatigue Phenomena in Aerospace Structures

Experimental and numerical analysis performed and reported in this PhD thesis contribute to better understanding on how voluntarily and non-voluntaritly introduced residual stresses can influence fatigue related phenomena in aerospace structures, having its focus on one technology in particular - Laser Shock Peening. The thesis itself is divided in three parts, as follows: In its first part, the work of this PhD thesis deals with the measurement and modeling of residual stresses in aeronautical structures (Chapter 2), where sectioning method of residual stress measurement has been investigated in more detail. Chapter 3 of the first part of the thesis brings experimental results of residual stress measurements and fatigue tests on integral stiffened panels. Last three chapters focus on the numerical prediction of residual stress influence on fatigue related problems, such as crack propagation (Chapter 4), and on the prediction of other residual stress related phenomena, such as out of plane deformations (Chapter 5) and the presence of residual stresses in bonded structures (Chapter 6). The second part of this PhD thesis (Chapters 7-13) has been carried out in collaboration with Research and Technology department of Airbus Germany, backing up the efforts of industry to better understand how a novel technology such as Laser Shock Peening (LSP) can contribute to contrasting of fatigue related problems in metallic aeronautical structures. Chapter 7 gives a short description of types of lasers used for LSP applications with a short description of laser functioning. Chapter 8 gives a detailed overview of Laser Shock Peening technology, its development and current applications, where other possible applications are suggested. Chapters 9 and 10 bring experimental results of residual stress measurements and fatigue life tests of thick aluminium plates and thin aluminium sheets, for materials and geometries used in real aeronautical structural components. Chapters 11, 12 and 13 introduce the Finite Element Analysis of LSP, where numerical models for materials subjected to LSP are illustrated, together with obtained FEM results for specimens investigated experimentally. Numerical analyses presented in this PhD thesis contribute to better understanding how different parameters of Laser Shock Peening influence the obtained residual stress distributions and, as a consequence, fatigue behaviour of Laser Shock Peened structural components. In the last chapter, the most important conclusions deriving from this work are presented together with suggested future work. The last part of the thesis is composed of appendices. The first appendix gives a list of publications of the author of this PhD thesis that have served as a reference point for writing this work. Second appendix gives the material properties used for FEM modelling presented in the first part of the thesis. Third appendix is reserved for literature overview of most important works dealing with Laser Shock Peening, dividing them in three groups: historic development of the LSP, the most important works regarding LSP as a method for introducing residual stresses in structures for improvement of fatigue properties and finally, FEM analyses of LSP.