Measurement of LSP-induced residual stresses and fatigue life on thin-walled aluminium alloys specimens

This thesis work encloses activities carried out in the Laser Center of the Polytechnic University of Madrid and the laboratories of the University of Bologna in Forlì. This thesis focuses on the superficial mechanical treatment for metallic materials called Laser Shock Peening (LSP). This process is a surface enhancement treatment which induces a significant layer of beneficial compressive residual stresses underneath the surface of metal components in order to improve the detrimental effects of the crack growth behavior rate in it. The innovation aspect of this work is the LSP application to specimens with extremely low thickness. In particular, after a bibliographic study and comparison with the main treatments used for the same purposes, this work analyzes the physics of the operation of a laser, its interaction with the surface of the material and the generation of the surface residual stresses which are fundamentals to obtain the LSP benefits. In particular this thesis work regards the application of this treatment to some Al2024-T351 specimens with low thickness. Among the improvements that can be obtained performing this operation, the most important in the aeronautic field is the fatigue life improvement of the treated components. As demonstrated in this work, a well-done LSP treatment can slow down the progress of the defects in the material that could lead to sudden failure of the structure. A part of this thesis is the simulation of this phenomenon using the program AFGROW, with which have been analyzed different geometric configurations of the treatment, verifying which was better for large panels of typical aeronautical interest. The core of the LSP process are the residual stresses that are induced on the material by the interaction with the laser light, these can be simulated with the finite elements but it is essential to verify and measure them experimentally. In the thesis are introduced the main methods for the detection of those stresses, they can be mechanical or by diffraction. In particular, will be described the principles and the detailed realization method of the Hole Drilling measure and an introduction of the X-ray Diffraction; then will be presented the results I obtained with both techniques. In addition to these two measurement techniques will also be introduced Neutron Diffraction method. The last part refers to the experimental tests of the fatigue life of the specimens, with a detailed description of the apparatus and the procedure used from the initial specimen preparation to the fatigue test with the press. Then the obtained results are exposed and discussed.

Analisi idrogeologica dell'area carsica dei Monti Alburni (Salerno, Campania)

The Alburni Massif is the most important karstic area in southern Italy and It contains about 250 caves. Most of these caves are located on the plateau, between 1500 m a.s.l. and 700 m a.s.l., and only a few reach the underground streams that feed the springs and the deep aquifer. The main springs are Grotta di Pertosa-Auletta (CP1) and Auso spring (CP31), both located at 280 m a.s.l., the first on the south-eastern margin whereas the second on south-west margin, and the springs present in Castelcivita area, the Castelcivita-Ausino system (CP2) and Mulino di Castelcivita spring (CP865), located at 60 m a.s.l.. Some other secondary springs are present too. We have monitored Pertosa-Auletta’s spring with a multiparameter logger. This logger has registered data from November 2014 to December 2015 regarding water level, electric conductivity and temperature. The hydrodynamic monitoring has been supported by a sampling campaign in order to obtain chemical water analyses. The work was done from August 2014 to December 2015, not only at Pertosa but also at all the other main springs, and in some caves. It was possible to clarify the behavior of Pertosa-Auletta’s spring, almost exclusively fed by full charge conduits, only marginally affected by seasonal rains. Pertosa-Auletta showed a characteristic Mg/Ca ratio and Mg2+ enrichment, as demonstrated by its saturation index that always showed a dolomite saturation. All other spring have characteristic waters from a chemical point of view. In particular, it highlights the great balance between the components dissolved in the waters of Mulino’ spring opposed to the variability of the nearby Castelcivita-Ausino spring. Regarding the Auso spring the variable behavior in terms of discharge and chemistry is confirmed, greatly influenced by rainfall and, during drought periods, by full charge conduits. Rare element concentrations were also analyzed and allowed to characterize further the different waters. Based on all these data an updated hydrogeological map of the Alburni massif has been drawn, that defines in greater detail the hydrogeological complexes on the basis of lithologies, and therefore of their chemical characteristics.