AbstractsEngineering

The damage tolerance approach has been used in the design of light structures in the transport sector since the early 1970’s. It refers to the ability of a structure to withstand damage and therefore understanding and predicting the behavior of a damaged structure is the basis for successful implementation of this design philosophy. In aeronautics industry, airframes are primarily made of riveted Al-alloy panels, with a skin and stringer configuration. The requirement for reducing manufacturing and operational costs has lead aerospace industries to look for new, attractive ways to substitute conventional riveting techniques in airframes. Towards this direction, advanced welding technologies such as friction stir and laser beam welding are a promising solution. In welded structures, damage tolerance assessment is a difficult problem due to the variation of material characteristics in the weld region. The simultaneous presence of a modified due to welding microstructure and weld residual stresses influences the behavior of a propagating crack. For this reason, simulation tools for assessment of crack growth in a welded joint, demand a careful examination of the mechanisms controlling crack propagation. In the present thesis, the effects of weld microstructure and weld residual stresses on fatigue crack propagation in a 2024 T3 friction stir weld have been assessed experimentally and analytically. To investigate the effect of weld microstructure on fatigue crack growth, the microstructure in the heat affected zone (HAZ) was simulated in parent material using a special heat treatment method. Fatigue crack growth rate was experimentally determined in the HAZ simulated material and in the friction stir weld (FSW) by means of fatigue crack growth tests. In the analytical part, a model was developed to predict mode I fatigue crack growth perpendicular to the friction stir weld line. The model takes into account the independent influences of weld microstructure and residual stresses on fatigue crack propagation. Comparison of analytical results to experimental data showed that the model is suitable for predicting the rate of a growing crack in a friction stir weld under mode I loading.From the present investigation, additional knowledge is extracted concerning the basic mechanisms that contribute to fatigue crack propagation in a friction stir weld region. Furthermore, an analytical tool is proposed for the evaluation of the remaining fatigue life of a cracked friction stir welded aluminum panel. Στην αεροναυπηγική, οι συνδέσεις των δομικών μερών του αεροσκάφους πραγματοποιούνται κυρίως με χρήση ήλων. Τα τελευταία χρόνια η αυξημένη απαίτηση για μείωση του λειτουργικού κόστους και του κόστους παραγωγής, οδήγησε στην αναζήτηση νέων τεχνολογικών λύσεων με στόχο την αντικατάσταση των ηλωτών συνδέσεων και τη δημιουργία ενιαίων δομικών στοιχείων. Μία από τις εναλλακτικές λύσεις προς αυτή την κατεύθυνση είναι η εφαρμογή προηγμένων τεχνολογιών συγκόλλησης σε δομικά μέρη τουαεροσκάφους σε συνδυασμό με την χρήση κραμάτων…