Πρόβλεψη δημιουργίας διάδοσης και συνένωσης ρωγμών σε αεροπορικά δομικά στοιχεία με πολλαπλή βλάβη

Abstract

Structural integrity degradation of aeronautical structures due to fatigue, as well as its assessment, represents a major problem of aeronautics. This problem, combined to the fact that numerous aircrafts that have already exceeded their initial design operational life are still in service, led the international scientific community and the aeronautical industry to the in-depth study of aging aircraft. The main problem of an aging aircraft structure is widespread fatigue damage at various areas of the structure, that increase the probability of catastrophic failures. Riveted joints of fuselage or wing panels are areas of high stress concentration and corrosion, therefore, sites susceptible to multiple cracks development. The state of multiple interacting cracks existence at the same structural element is called Multiple Site Damage (MSD) and leads to residual significant strength reduction and structural integrity degradation of the structure. Study and handling of multiple site damage problem is an important research field, as it involves a number of different interacting phenomena, such as crack initiation, propagation and link-up and final structural failure under MSD conditions, as well as the effect of environmental factors like corrosion on the material behaviour. The aim of the present work is the development of an integrated methodology for the prediction of MSD evolution at aeronautical structural elements, in order to make feasible the assessment of their structural integrity throughout their service life. The technological issues that are faced in the frame of the present work by the development of innovative methodologies are: • Stress analysis and stress intensity factors calculation of structures under MSD condition. • Crack initiation and propagation prediction. • Prediction of crack link-up and residual strength. For the stress analysis and stress intensity factor calculation of un-stiffened of stiffened structural elements suitable super-elements are developed for the simulation of crucial structural elements. The super-element methodology is based on finite element sub-structuring technique and, in the present work, is chosen in order to obtain a significant reduction of computation cost with compared to the classical finite element method, so that handling of such complex problems becomes feasible. Fatigue crack initiation at a structural element is a problem exhibiting a strongly stochastic character. A crack initiation prediction methodology is developed according to which the probability of crack initiation at a site of the structure depends on the local stress state combined to a stochastic (random) variation of the fatigue strength at every site susceptible to crack initiation. For the methodology application statistical data from simple fatigue tests are used. For the crack link-up and the residual strength estimation of a multiple cracked structural element an energy-based link-up criterion is developed. The proposed criterion is based on the strain energy variation during the failure of the ligament of two adjacent cracks. Finally, the initiation, propagation and link-up models are combined and implemented in an integrated methodology that can be used for the assessment of multiple site damage evolution. The effectiveness of the methodology is validated using suitable fatigue tests of multiple-site damaged panels.