This master thesis deals with optimalization of stiffened integral wing panel of L-610 aircraft to its fatique life. Analysis has been made using FEM, AFGROW software and PYTHON language.

Diplomová práce se zabývá návrhem metodiky výpočtu závislého šíření trhlin ve více konstrukčních částech s využitím programu AFGROW. Prezentována metodika, která také zahrnuje stanovení zbytkové pevnosti, byla aplikována v damage tolerance analýzách vybraných míst na konstrukci křídla L 410 NG.

Master’s thesis deals with study of fatigue crack growth retardation models. This document presents description, evaluation, verification and finally application of retardation models demonstrated on the damage tolerant structure. Analysis was provided on the L 410 NG aircraft in the location of the rear spar lower flange joint. Output of this thesis is comparison of the damage tolerance analysis with the current method using in the Aircraft Industry a. s., i.e. linear model without using load interaction models. The comparisons are fatigue crack growth curves, threshold and intervals of inspection program. AFGROW software was used for verification and application of retardation models.

This Ph.D. thesis deals with methodology for determination of residual fatigue lifetime of railway axles based on damage tolerance approach. This approach accepts an existence of potential defect, which could lead to fatigue failure of whole axle. The behavior of crack in railway axle is described by approaches of linear elastic fracture mechanics. There are plenty of factors, which more or less influence determined residual fatigue lifetime. The aim of this thesis is to quantify effects of these factors. The first part of Ph.D. thesis represents overview of studied problems relating to fatigue damage of railway axles. This part is focused on parameters, which influence fatigue crack growth in railway axle materials. The second part of thesis shows procedure for determination of residual fatigue lifetime, which was developed at the Institute of Physics of Materials of the Academy of Sciences of the Czech Republic. The main aim of this thesis was to improve current procedure for more precise estimation of residual fatigue lifetime. Significant part of this work is determination of significance of studied factors, which influencing calculated residual fatigue lifetime of railway axles (e.g. effect of threshold value, load spectrum, retardation effects, residual stress, axle geometry, material of axle etc.). The procedures described and results obtained can be also used for determination of residual fatigue lifetime of general mechanical component (not only railway axles). Therefore, results obtained in this Ph.D. thesis can be used e.g. for assessment of regular inspection intervals of cyclically loaded general mechanical parts.

This Ph.D. thesis deals with methodology for determination of residual fatigue lifetime of railway axles based on damage tolerance approach. This approach accepts an existence of potential defect, which could lead to fatigue failure of whole axle. The behavior of crack in railway axle is described by approaches of linear elastic fracture mechanics. There are plenty of factors, which more or less influence determined residual fatigue lifetime. The aim of this thesis is to quantify effects of these factors. The first part of Ph.D. thesis represents overview of studied problems relating to fatigue damage of railway axles. This part is focused on parameters, which influence fatigue crack growth in railway axle materials. The second part of thesis shows procedure for determination of residual fatigue lifetime, which was developed at the Institute of Physics of Materials of the Academy of Sciences of the Czech Republic. The main aim of this thesis was to improve current procedure for more precise estimation of residual fatigue lifetime. Significant part of this work is determination of significance of studied factors, which influencing calculated residual fatigue lifetime of railway axles (e.g. effect of threshold value, load spectrum, retardation effects, residual stress, axle geometry, material of axle etc.). The procedures described and results obtained can be also used for determination of residual fatigue lifetime of general mechanical component (not only railway axles). Therefore, results obtained in this Ph.D. thesis can be used e.g. for assessment of regular inspection intervals of cyclically loaded general mechanical parts.

The main aim of this research was to investigate the behaviour of adhesive joints exposed to repeated low-velocity impact i.e. impact fatigue (IF), and to compare this loading regime with standard fatigue (SF), i.e. non-impacting, constant amplitude, sinusoidal loading conditions. Two types of lap joint configuration using rubber toughened modified epoxy adhesives were used and exposed to various loading conditions in order to determine the fatigue behaviour of the joints for each load conditions. The fatigue life was investigated using bonded aluminium alloy (7075-T6) single lap joint (SLJ) specimens, where it was seen that IF is an extremely damaging load regime compared to SF. Different trends were visible in force-life plots for these two types of loading. In SF a gradual decrease in the fatigue life with increasing load was observed, whereas, in IF a significant decrease in life was seen at relatively modest levels of maximum force after relatively few cycles. Comparisons of the fatigue life show a considerably earlier failure in IF than in SF for comparable levels of force and energy. Additionally, it was demonstrated that the maximum force per cycle, loading time, stiffness and strength decreased as a result of damage generated in the sample during IF.

Commercial composite aerospace structure is required to be designed and managed under the damage tolerant principle. Airworthiness is maintained through a process of regulated inspections and if required maintenance. Currently inspections use visual and assisted visual (non-destructive inspection - NDI) techniques. Damage tolerant operation is therefore reliant on inspectability. Unlike metal structure composite and adhesively bonded structure may show few if any recognisable indicators prior to rapid failure, either visually or using NDI. Although stringent manufacturing processes are demanded to best ensure components are fit for service strategies such as reducing stresses by oversizing components or in the case of bonded features additional mechanical fasteners may be included to allow operation with this potential structural uncertainty. Structural Heath Monitoring (SHM) uses data from in-situ sensors to assess the condition of the structure. If via SHM any uncertainty associated with difficult to inspect components could be eliminated less reliance would be required of additional structure or features allowing lighter and more efficient structure to be viable with no impact on current airworthiness demands. Despite much previous research no SHM system is in use with in-service composite or bonded aerospace components. When operating a structure under Damage-tolerance operational requirements damage must be positively identified to allow repairs to be made whist ensuring appropriate airworthiness demands are maintained. Such demands must also be met by structure inspected using SHM. Unlike previous studies this research combines the process of structural design and in-situ monitoring to address the issues identified. Termed SHM enabled design this approach allows the implementation of monitoring technology and the potential for benefits including the reduced reliance on inefficient additional structure to be viably included in actual structure ... [cont.].

DNA-damage tolerance (DDT) in yeast is composed of two parallel pathways and mediated by sequential ubiquitination of proliferating cell nuclear antigen (PCNA). While monoubiquitination of PCNA promotes translesion synthesis (TLS), which is dependent on low fidelity polymerase ζ (Pol ζ) composed of a catalytic subunit Rev3 and a regulatory subunit Rev7, polyubiquitination of PCNA by Mms2-Ubc13-Rad5 promotes error-free lesion bypass. Inactivation of these two pathways results in a synergistic effect on DNA-damage responses; however, this two-branch DDT model has not been reported in any multicellular organisms. In order to examine whether Arabidopsis thaliana possesses a two-branch DDT system, rad5a rev3 double mutant plants were created and compared with the corresponding single mutants. Arabidopsis rad5a and rev3 mutations are indeed synergistic with respect to growth inhibition induced by replication-blocking lesions, suggesting that AtRAD5a and AtREV3 are required for error-free and TLS branches of DDT, respectively. Unexpectedly this study reveals three modes of genetic interactions in response to different types of DNA damage, indicating that plant RAD5 and REV3are also involved in DNA damage responses independent of DDT. By comparing with yeast cells, it is apparent that plant TLS is a more frequently utilized means of lesion bypass than error-free DDT. In addition, it was also observed that treatments with the DNA damaging agent methylmethanesulfonate increased the nuclear ploidy level in the double mutant plants.

Offshore wind power production is an attractive clean energy option, but the difficulty of access can lead to expensive and rare opportunities for maintenance. Smart loads management (controls) are investigated for their potential to increase the fatigue life of damaged offshore wind turbine rotor blades. This study will consider two commonly encountered damage types for wind turbine blades, the trailing edge disbond (bond line failure) and shear web disbond, and show how 3D finite element modeling can be used to quantify the effect of operations and control strategies designed to extend the fatigue life of damaged blades. Modern wind turbine blades are advanced composite structures, and blade optimization problems can be complex with many structural design variables and a wide variety of aeroelastic design requirements. The multi-level design method is an aeroelastic structural design technique for beam-like structures in which the general design problem is divided into a 1D beam optimization and a 2D section optimization. As a demonstration of aeroelastic design, the multi-level design method is demonstrated for the internal structural design of a modern composite rotor blade. Aeroelastic design involves optimization of system geometry features as well as internal features, and this is demonstrated in the design of a flying wing aircraft. Control methods such as feedback control also have the capability alleviate aeroelastic design requirements and this is also demonstrated in the flying wing aircraft example. In the case of damaged wind turbine blades, load mitigation control strategies have the potential to mitigate the effects of damage, and allow partial operation to avoid shutdown. The load mitigation strategies will be demonstrated for a representative state-of-the-art wind turbine (126m rotor diameter). An economic incentive will be provided for the proposed operations strategies, in terms of weighing the cost and risk of implementation against the benefits of increased revenue due to operation of damaged turbines. The industry trend in wind turbine design is moving towards very large blades, causing the basic design criterion to change as aeroelastic effects become more important. An ongoing 100 m blade (205 m rotor diameter) design effort intends to investigate these design challenges. As a part of that effort, this thesis will investigate damage tolerant design strategies to ensure next-generation blades are more reliable.

Αντικείμενο της διατριβής είναι η ανάπτυξη νέας γενιάς ινωδών σύνθετων υλικών με αυξημένη ανοχή στη βλάβη, βελτιωμένα θραυστομηχανικά και μηχανικά χαρακτηριστικά. Αυτό θα επιτευχθεί με ενίσχυση (ντοπάρισμα) της πολυμερούς μήτρας που περιβάλλει την ινώδη ενίσχυση του σύνθετου υλικού με μικρή ποσότητα –μικρότερη ή ίση του 1% κατά βάρος- νανοσωληνίσκων άνθρακα ομοιόμορφα κατανεμημένων στη μήτρα. Οι νανοσωληνίσκοι άνθρακα προσφέρουν μια εξαιρετικά μεγάλη διεπιφάνεια μήτρας-ενίσχυσης η οποία και οδηγεί στη συνέχεια το φαινόμενο της θραύσης απαιτώντας την κατανάλωση μεγάλων ποσοτήτων ενέργειας για την αστοχία της διεπιφάνειας και τη διάδοση της βλάβης. Το κύριο κομμάτι της διατριβής αφορούσε τη μελέτη της επίδρασης των ΝΣΑ σε σύνθετα υλικά. Το πρώτο μέλημα ήταν εάν μπορούσαν να κατασκευαστούν νανοσύνθετα με τις κλασικές μεθόδους κατασκευής σύνθετων υλικών κάτι που πραγματοποιήθηκε με επιτυχία τόσο για τα σύνθετα με ενισχυτική φάση τις ίνες άνθρακα σε δύο διαφορετικές αλληλουχίες στρώσεων όσο και για τις ίνες κέβλαρ. Καταλήγοντας συμπεραίνουμε πως η κατασκευή νανοσύνθετων υλικών είναι εφικτή με τις ήδη υπάρχουσες κατασκευαστικές μεθόδους. Τα νανοσύνθετα έχουν να επιδείξουν ανώτερες μηχανικές ιδιότητες σε σχέση με τα απλά σύνθετα ιδιαίτερα στα φαινόμενα της θραύσης και της κόπωσης κάτι το ιδιαίτερα σημαντικό για την εφαρμογή τους σε δομικά τμήματα κατασκευών από σύνθετα υλικά. Επιπλέον είναι δυνατό να αποτελέσουν τη βάση για τα σύνθετα υλικά της επόμενης γενιάς μέσω των πολύπλευρων ιδιοτήτων που μπορούν να προσδώσουν (multifunctionality) όπως επισκόπηση της αναπτυσσόμενης βλάβης σε πραγματικό χρόνο μέσω της μέτρησης της ηλεκτρικής αγωγιμότητας, βελτιστοποίηση των ιδιοτήτων απόκρισης και αύξηση της θερμικής αγωγιμότητας. / The aim of the current PhD thesis was realised during 2004-2008 at University of Patras, Grece, in the Dept. of Mechanical & Aeronautical Engineering at the Applied Mechanics Laboratory under the supervision of Prof. V. Kostopoulos was to investigate and optimise the reinforcing effect of Multi Wall Carbon Nanotubes (MWCNTs) on a typical aerospace carbon reinforced epoxy composite. The main conclusions/findings of this PhD thesis are summarized below: • The dispersion protocol of MWCNTs onto a typical aerospace resin using a mechanical device was established. • Different techniques were investigated such as to improve the dispersion of the CNTs into the matrix material. • The reinforcing effect of MWCNTs onto a typical aerospace resin was investigated and optimized at 0.5 w.t. % using a high shear mixing device i.e. torus mill. • The CNT-enhanced aerospace resin was successfully used for the manufacturing of two CFRPs lay-ups (UD and Quasi). The nano-reinforced panels exhibited a higher tensile modulus, a 60% increased on fracture toughness, increased fatigue life and even though no significant increase was noted on the low velocity impact properties the effective compressive modulus after fatigue and the fatigue life after impact was also enhanced. The aforementioned reinforcement is attributed to the extra energy that the CNTs anticipate in order to be broken or pulled-out of the matrix material • The CNT-enhanced aerospace resin was successfully used for the manufacturing of Kevlar reinforced composite panels. The nano-reinforced panels exhibited a higher tensile modulus and a significant increase on the fracture toughness properties. • MWCNTs were successfully grafted on typical aerospace carbon fibres in order to manufacture a reinforcement incorporating two scales i.e. both micro and nano.