Corrosion damage accumulation represents a major threat for the structuralintegrity of metallic aircraft structures and moreover has a strong effect on the loadbearing capacity of aging aircraft structures. Corrosion damage is evaluated bymeans of metallographic features such as pitting density, depth and shape of pits,onset of exfoliation, etc. For the case of static loading, corrosion damage is usuallyaccounted through reducing the metal thickness by the depth of corrosion attackand then calculating the corresponding stress increase. For the case of fatigue,corrosion pits are considered as possible onsets for fatigue cracks.The aim of the present PhD thesis is to contribute to establish a link betweenthe metallographic features of corrosion damage and the degradation of themechanical properties of a corroded material. Towards this objective, amethodology is developed which allows the numerical simulation of the tensilebehavior of the corroded material based on the metallographic features of thecorrosion damage. The present work is divided in two parts: a) the experimentalinvestigation and b) the numerical analysis.The experimental part includes an extensive metallographic investigation ofthe occurring corrosion damage. Moreover, tensile tests were performed on the precorrodedmaterial which was exposed to the corrosive solution for several exposureperiods. Finally, an examination of the fracture surfaces for the identification of thephysical mechanisms of the damage has also been conducted. The main conclusionextracted from the metallographic procedure is that corrosion damage evolves frompitting to exfoliation progressively. The tensile tests performed on the pre corrodedmaterial revealed a moderate reduction concerning the tensile strength but asignificant degradation of the tensile ductility even after short exposure periods.The examination of the fracture surfaces revealed the presence of quasi-cleavagezones beneath the depth of corrosion attack. The formation of these zones has been attributed by previous investigations to hydrogen diffusion and trapping into thecorroded material during the corrosion process.The simulation procedure involves the development of a multi scale finiteelement model. The corrosion damage has been accounted for by introducing 3DRepresentative Unit Cells (RUCs) developed in the micro scale, with geometricalcharacteristics obtained by the metallographic analysis data of the corrodedmaterial. The degradation of the Representative Unit Cell’s mechanical propertiesdue to the presence of the damage has been recorded. A 3D Finite Element model ofa tensile specimen has been developed. This model has been used to simulate thetensile behavior of the corroded material, by including elements with degradedproperties extracted from the RUC analysis. For the different exposure times RUCswith different geometrical characteristics were used so as to account for theevolving corrosion damage. The simulation results correlate well with therespective tensile behavior of the alloy obtained by the mechanical tests. As far astensile ductility is concerned a significant deviation was observed, due to the factthat the finite element model does not account for the embrittlement of the materialdue to hydrogen absorption.The developed methodology represents a step towards the establishment ofa link between the metallographic features of the corrosion damage and the residualmechanical properties of the material, and thus the more reliable estimation of theresidual strength of the corroded aircraft structures.
Experimental and numerical investigation of the effect of holes on the springback in cold roll forming of UHSS
