A computationally efficient analysis capability for the geometrically non-linear response of compressively loaded prismatic plate structures was developed. Both a "full" finite strip solution procedure and a "reduced" solution procedure were implemented in a FORTRAN 90 computer code, and comparisons were made with results available in the technical literature. Both the full and reduced solution procedures were demonstrated to provide accurate results for displacement and strain quantities through moderately large post-buckling loads. The full method is a non-linear finite strip analysis of the semi-analytical, multi-term type. Individual finite strips are modeled as balanced and symmetric laminated composite materials which are assumed to behave orthotropically in bending, and the structure is loaded in uniaxial or biaxial compression. The loaded ends of the structure are assumed to be simply supported, and geometric shape imperfections may be modeled. The reduced solution method makes use of a reduced basis technique in conjunction with the full finite strip analysis. Here, the potentially large set of non-linear algebraic equations produced by the finite strip method are replaced by a small set of system equations. In the present implementation, the basis vectors consist of successive derivatives of the non-linear solution vector with respect to a loading parameter. Depending on the nature of the problem, the reduced solution procedure is capable of computational savings of up to 60%+ compared to the full finite strip method. The reduced method is most effective in reducing the computational cost of the full method when the most significant portion of the cost of the full method is factorization of the assembled system matrices. The robustness and efficiency of the reduced solution procedure was found to be sensitive to the user specified error norm which is used during the reduced solution procedure to determine when to generate new sets of basis vectors. In parallel with this effort, a new method for performing global/local design optimization of large complex structures (such as aircraft wings or fuselages) was developed. A simple and flexible interface between the global and local design levels was constructed using response surface methodology. The interface is constructed so as to minimize the changes required in either the global design code or the local design codes(s). Proper coupling is maintained between the global and local design levels via a "weight constraint" and the transfer of global stiffness information to the local level. The method was verified using a simple isotropic global wing model and the local panel design code PASCO. / Ph. D.

Ultra-lightweight thermoformed stiffened structures are emerging as a viable option for spacecraft applications due to their advantage over inflatable structures. Although pressurization may be used for deployment, constant pressure is not required to maintain stiffness. However, thermoformed stiffening features are often locally nonlinear in their behavior under loading. This thesis has three aspects: 1) to understand stiffness properties of a thermoformed stiffened ultra-lightweight panel, 2) to develop finite element models using a phased-verification approach and 3) to verify panel response to dynamic loading. This thesis demonstrates that conventional static and dynamic testing principles can be applied to test ultra-lightweight thermoformed stiffened structures. Another contribution of this thesis is by evaluating the stiffness properties of different stiffener configurations. Finally, the procedure used in this thesis could be adapted in the study of similar ultra-lightweight thermoformed stiffened spacecraft structures.

An efficient beam-column approach, using an improved step-by-step numerical method, is developed in the current research for studying the ultimate strength problems of stiffened panels with two load cases: 1) under longitudinal compression, and 2) under transverse compression. Chapter 2 presents an improved step-by-step numerical integration procedure based on (Chen and Liu, 1987) to calculate the ultimate strength of a beam-column under axial compression, end moments, lateral loads, and combined loads. A special procedure for three-span beam-columns is also developed with a special attention to usability for stiffened panels. A software package, ULTBEAM, is developed as an implementation of this method. The comparison of ULTBEAM with the commercial finite element package ABAQUS shows very good agreement. The improved beam-column method is first applied for the ultimate strength analysis of stiffened panel under longitudinal compression. The fine mesh elasto-plastic finite element ultimate strength analyses are carried out with 107 three-bay stiffened panels, covering a wide range of panel length, plate thickness, and stiffener sizes and proportions. The FE results show that the three-bay simply supported model is sufficiently general to apply to any panel with three or more bays. The FE results are then used to obtain a simple formula that corrects the beam-column result and gives good agreement for panel ultimate strength for all of the 107 panels. The formula is extremely simple, involving only one parameter: the product λΠorth2. Chapter 4 compares the predictions of the new beam-column formula and the orthotropic-based methods with the FE solutions for all 107 panels. It shows that the orthotropic plate theory cannot model the "crossover" panels adequately, whereas the beam-column method can predict the ultimate strength well for all of the 107 panels, including the "crossover" panels. The beam-column method is then applied for the ultimate strength analysis of stiffened panel under transverse compression, with or without pressure. The method is based on a further extension of the nonlinear beam-column theory presented in Chapter 2, and application of it to a continuous plate strip model to calculate the ultimate strength of subpanels. This method is evaluated by comparing the results with those obtained using ABAQUS, for several typical ship panels under various pressures. / Ph. D.

Design of thermoelastic structures can be highly counterintuitive due to design-dependent loading and impact of geometric nonlinearity on the structural response. Thermal loading generates in-plane stresses in a restrained panel, but the presence of geometric nonlinearity creates an extension-bending coupling that results in considerable transverse displacement and variation in stiffness characteristics, and these affects are enhanced in post-bucking regimes. Herein a methodology for structural sizing of thermally stressed post-buckled stiffened panels is proposed and applied for optimization of the blade and hat stiffeners using a gradient-based optimizer. The stiffened panels are subjected to uniform thermal loading and optimized for minimum mass while satisfying stress and stability constraints. The stress constraints are used to avoid yielding of the structure, whereas the stability constraints are used to ensure static stability. Corrugation of the hat stiffeners is also studied through variation of its magnitude and position. A continuation solver has been validated to tackle the highly nonlinear nature of the thermoelastic problem, and formulations for the stability constraints have been derived and imposed to satisfy the static stability of the structure. The study confirms that geometric nonlinearity is an important aspect of sizing optimization and is needed for an accurate modeling of the structural behavior. The results also show that modeling of geometric nonlinearity adds extra complexity to the thermoelastic problem and requires a path-tracking solver. Finally, this work supports that corrugation enhances the stability features of the panel but requires a blending function to reduce stresses at the panel boundaries.

Manufacturing considerations have been incorporated into the design optimization of a blade-stiffened composite panel. For the manufacturing analysis, a one-dimensional resin film infusion model is developed to compute the infiltration time of the resin into a fabric preform of the panel. Results are presented showing the effects of structurally important design variables, such as cross-sectional geometry and material properties, on the manufacturing performance of the panel. In addition, the effects of manufacturing process variables, such as pressure and temperature, on the structural performance are studied. The structural problem is formulated to minimize the panel mass subject to buckling constraints. A simplified buckling analysis model for the panel is used to compute the critical buckling loads. The objective of the manufacturing problem is to minimize the resin infiltration time. Optimum panel designs for the manufacturing and structures problems alone, as well as for the combined problem, are generated using a genetic algorithm. These results indicate a strong connection between the structures and manufacturing design variables and trade-offs between the responses, illustrating that a multidisciplinary approach to the problem is essential to incorporating manufacturing into the preliminary design stage. / Master of Science

Unitized aircraft structures have the potential to be more efficient than current aircraft structures. The Electron Beam Freeform Fabrication (EBF3) process can be used to manufacture unitized aircraft structures. The structural efficiency of blade stiffened panels made with EBF3 was compared to panels made by integrally machining from thick plate. The panels were tested under two load cases in a combined compression-shear load test fixture. One load case tested the panels' responses to a higher compressive load than the shear load. The second load case tested the panels' responses to an equal compressive and shear load. Finite element analysis was performed to compare with the experimental results. The EBF3 panels failed at a 18.5% lower buckling load than the machined panels when loaded mostly in compression but at an almost two times higher buckling load than the machined panels when the shear matched the compressive load. The finite element analysis was in good agreement with the experimental results prior to buckling. The results demonstrate that the EBF3 process has the capabilities of manufacturing stiffened panels that behave similarly to machined panels prior to buckling. Once the EBF3 panels buckled, the buckled shape of the EBF3 panels was different from the machined panels, generally buckling in the opposite direction of what was observed with the machined panels. This was also expected based on the finite element analysis. The different post-buckling response between the two manufacturing techniques was attributed to the residual stress and associated distortion induced during the EBF3 manufacturing process. / Master of Science

Stiffeners attached to composite panels and shells may significantly increase the overall buckling load of the resultant stiffened structure. Initially, an extensive literature review was conducted over the past ten years of published work wherein research was conducted on grid stiffened composite structures and stiffened panels, due to their applications in weight sensitive structures. Failure modes identified in the literature had been addressed and divided into a few categories including: buckling of the skin between stiffeners, stiffener crippling and overall buckling. Different methods have been used to predict those failures. These different methods can be divided into two main categories, the smeared stiffener method and the discrete stiffener method. Both of these methods were used and compared in this thesis. First, a buckling analysis was conducted for the case of a grid stiffened composite pressure vessel. Second, a buckling analysis was conducted under the compressive load on the composite stiffened panels for the case of one, two and three longitudinal stiffeners and then, using different parameters, stiffened panels under combined compressive and shear load for the case of one longitudinal centric stiffener and one longitudinal eccentric stiffener, two stiffeners and three stiffeners. / Master of Science / Aircraft in flight is subjected to different loads due to maneuvers and gust, external forces cause internal loads, which depends on the location of the panel in the aircraft, those internal loads, may result in the buckling of the panel. There is an imminent need for structural efficiency, strong and lightweight material. Stiffened composite panels is a promising technology capable of addressing those needs. Composite stiffened panels have many advantages including but not limited to, small manufacturing cost, high stability, great energy absorption, superior damage tolerance etc. The main failure modes for stiffened composite panels is buckling. Buckling failure modes could be of a global nature, local skin buckling or stiffener/rib crippling, predicting those failure is of high practical importance and a predominant design criterion. An extensive literature review on buckling of stiffened composite panels was conducted in this thesis. Buckling analysis as well as a parametric study of grid stiffened composite cylindrical shell for a pressure vessel was conducted, an analytical solution was derived and verified using ABAQUS, a Finite Element Software. Buckling analysis as well as a parametric study of stiffened panels with longitudinal stiffeners, under different structural situations, was also conducted and results verified.

No description available.

The structural optimization of a cantilever aircraft wing with curvilinear spars and ribs and stiffeners is described. The design concept of reinforcing the wing structure using curvilinear stiffening members has been explored due to the development of novel manufacturing technologies like electron-beam-free-form-fabrication (EBF3). For the optimization of a complex wing, a common strategy is to divide the optimization procedure into two subsystems: the global wing optimization which optimizes the geometry of spars, ribs and wing skins; and the local panel optimization which optimizes the design variables of local panels bordered by spars and ribs. The stiffeners are placed on the local panels to increase the stiffness and buckling resistance. The panel thickness, size and shape of stiffeners are optimized to minimize the structural weight. The geometry of spars and ribs greatly influences the design of stiffened panels. During the local panel optimization, the stress information is taken from the global model as a displacement boundary condition on the panel edges using the so-called "Global-Local Approach". The aircraft design is characterized by multiple disciplines: structures, aeroelasticity and buckling. Particle swarm optimization is used in the integration of global/local optimization to optimize the SpaRibs. The interaction between the global wing optimization and the local panel optimization is usually computationally expensive. A parallel computing technology has been developed in Python programming to reduce the CPU time. The license cycle-check method and memory self-adjustment method are two approaches that have been applied in the parallel framework in order to optimize the use of the resources by reducing the license and memory limitations and making the code robust. The integrated global-local optimization approach has been applied to subsonic NASA common research model (CRM) wing, which proves the methodology's application scaling with medium fidelity FEM analysis. Both the global wing design variables and local panel design variables are optimized to minimize the wing weight at an acceptable computational cost. The structural weight of the wing has been, therefore, reduced by 40% and the parallel implementation allowed a reduction in the CPU time by 89%. The aforementioned Global-Local Approach is investigated and applied to a composite panel with crack at its center. Because of composite laminates' heterogeneity, an accurate analysis of these requires very high time and storage space. In the presence of structural discontinuities like cracks, delaminations, cutouts etc., the computational complexity increases significantly. A possible alternative to reduce the computational complexity is the global-local analysis which involves an approximate analysis of the whole structure followed by a detailed analysis of a significantly smaller region of interest. We investigate here the performance of the global-local scheme based on the finite element method by comparing it to the traditional finite element method. To do so, we conduct a 2D structural analysis of a composite square plate, with a thin rectangular notch at its center, subjected to a uniform transverse pressure, using the commercial software ABAQUS. We show that the presence of the thin notch affects only the local response of the structure and that the size of the affected area depends on the notch length. We investigate also the effect of the notch shape on the response of the structure. Stiffeners attached to composite panels may significantly increase the overall buckling load of the resultant stiffened structure. Buckling analysis of a composite panel with attached longitudinal stiffeners under compressive loads is performed using Ritz method with trigonometric functions. Results are then compared to those from ABAQUS FEA for different shell elements. The case of composite panel with one, two, and three stiffeners is investigated. The effect of the distance between the stiffeners on the buckling load is also studied. The variation of the buckling load and buckling modes with the stiffeners' height is investigated. It is shown that there is an optimum value of stiffeners' height beyond which the structural response of the stiffened panel is not improved and the buckling load does not increase. Furthermore, there exist different critical values of stiffener's height at which the buckling mode of the structure changes. Next, buckling analysis of a composite panel with two straight stiffeners and a crack at the center is performed. Finally, buckling analysis of a composite panel with curvilinear stiffeners and a crack at the center is also conducted. ABAQUS is used for these two examples and results show that panels with a larger crack have a reduced buckling load. It is shown also that the buckling load decreases slightly when using higher order 2D shell FEM elements. A damage tolerance framework, EBF3PanelOpt, has been developed to design and analyze curvilinearly stiffened panels. The framework is written with the scripting language PYTHON and it interacts with the commercial software MSC. Patran (for geometry and mesh creation), MSC. Nastran (for finite element analysis), and MSC. Marc (for damage tolerance analysis). The crack location is set to the location of the maximum value of the major principal stress while its orientation is set normal to the major principal axis direction. The effective stress intensity factor is calculated using the Virtual Crack Closure Technique and compared to the fracture toughness of the material in order to decide whether the crack will expand or not. The ratio of these two quantities is used as a constraint, along with the buckling factor, Kreisselmeier and Steinhauser criteria, and crippling factor. The EBF3PanelOpt framework is integrated within a two-step Particle Swarm Optimization in order to minimize the weight of the panel while satisfying the aforementioned constraints and using all the shape and thickness parameters as design variables. The result of the PSO is used then as an initial guess for the Gradient Based Optimization using only the thickness parameters as design variables. The GBO is applied using the commercial software VisualDOC. / Ph. D.

O σχεδιασμός και η ανάπτυξη μιας σύγχρονης αεροναυπηγικής κατασκευής περιλαμβάνει ως επιμέρους φάσεις (μεταξύ άλλων) τον αρχικό και τον προκαταρκτικό σχεδιασμό. Οι φάσεις αυτές έχουν ιδιαίτερη σημασία διότι εκεί δίνεται η αρχική μορφή και οι διαστάσεις της κατασκευής. Είναι γεγονός ότι η συμβατική σχεδίαση των βασικών δομικών στοιχείων των αεροσκαφών έχει φτάσει σε πολύ υψηλό επίπεδο βελτιστοποίησης που επιδέχεται πλέον μόνο μικρά περιθώρια περαιτέρω βελτίωσης. Οι σύγχρονες όμως απαιτήσεις των ελαφρών κατασκευών, όπως δραστική μείωση του βάρους, αύξηση του ωφέλιμου φορτίου κτλ. ωθεί τις αεροναυπηγικές βιομηχανίες στη δημιουργία δομών που ξεφεύγουν από τις παραδοσιακές (μη-συμβατικές δομές). Παράλληλα με τα παραπάνω γίνεται προσπάθεια για μερική αντικατάσταση μεταλλικών υλικών από σύνθετα υλικά στις πρωτεύουσες δομές αεροναυπηγικών κατασκευών. Για να σχεδιαστούν και να εξελιχθούν μη-συμβατικές αεροναυπηγικές δομές χωρίς να καταφύγει κάποιος σε εκτενείς πειραματικές δοκιμές, η σύγχρονη τάση είναι η ανάπτυξη και ο συνδυασμός προτύπων συμπεριφοράς στη λογική της εξομοίωσης των πειραματικών δοκιμών. Η εξομοίωση αυτή επιτυγχάνεται με τη βοήθεια ηλεκτρονικών υπολογιστών και κατάλληλων μεθόδων βασισμένων στη θεωρία των πινάκων (Πεπερασμένα Στοιχεία, Συνοριακά Στοιχεία κλπ.). Στη φάση του αρχικού και προκαταρκτικού σχεδιασμού η εφαρμογή των μεθοδολογιών προσομοίωσης δεν είναι πάντοτε εύκολη και απλή, λόγω των πολλαπλών αλλαγών στη γεωμετρία, το υλικό και τις κατασκευαστικές λεπτομέρειες που πραγματοποιούνται στη δομή κατά την επαναληπτική διαδικασία του σχεδιασμού. Επομένως, η αποκλειστική χρήση αριθμητικών μεθόδων ανάλυσης καθίσταται αναποτελεσματική από άποψη χρονικών απαιτήσεων, αν δεν συνοδεύεται από αναλυτικές ή ημιαναλυτικές προσεγγίσεις επιμέρους προβλημάτων του σχεδιασμού. Βασικό μέρος του προκαταρκτικού σχεδιασμού μιας πτέρυγας μη συμβατικής δομής αποτελεί η αποφυγή της αστοχίας του άνω τμήματός της, διότι οι λεπτότοιχες ενισχυμένες με δοκούς πλάκες που χρησιμοποιούνται στην κατασκευή υφίστανται λυγισμό λόγω των θλιπτικών φορτίσεων που κυρίως παραλαμβάνουν. Η διαστασιολόγηση των σύνθετων πλακών που φέρουν δοκούς ενίσχυσης στις κατασκευές αυτές απαιτούν συνήθως πλήθος επαναληπτικών υπολογισμών για διαφορετικές γεωμετρίες, φορτίσεις, οριακές συνθήκες κλπ. Η εξέταση της κάθε περίπτωσης μεμονωμένα με τη χρήση αριθμητικών μεθόδων καθιστά την επίλυση ολόκληρης της κατασκευής εξαιρετικά χρονοβόρα. Για το λόγο αυτό, στη φάση της αρχικής θεωρητικής μελέτης και της αρχικής διαστασιολόγησης η χρησιμοποίηση αναλυτικών μεθόδων για την εύρεση του κρίσιμου φορτίου λυγισμού πλακών με δοκούς ενίσχυσης οδηγεί στην εξοικονόμηση υπολογιστικού κόστους. Επομένως, η ανάπτυξη αναλυτικών ή ημιαναλυτικών μεθόδων προσδιορισμού των φορτίων λυγισμού ενισχυμένων με δοκούς συνθέτων πλακών και κελυφών θεωρείται πολύ σημαντική. Για τον σκοπό αυτό, στο πλαίσιο αυτής της διατριβής, αναπτύσσονται αναλυτικές και ημιαναλυτικές λύσεις για το λυγισμό πολύστρωτων πλακών ενισχυμένων με ενισχυτικές διαμήκεις δοκούς, οι οποίες ενσωματώνονται σαν κριτήρια στη μέθοδο διαστασιολόγησης της δομής. Η μεθοδολογία συμπληρώνεται με πλήθος άλλων κατάλληλων κριτηρίων για τον έλεγχο της αντοχής των δομικών στοιχείων της πτέρυγας καθώς και με κριτήρια για την επαναδιαστασιολόγηση των στοιχείων κατά την επαναληπτική διαδικασία της βελτιστοποίησης. Με τη μεθοδολογία που αναπτύσσεται διερευνούνται διατάξεις δομής πτερύγων από σύνθετα υλικά με πολυάριθμες κύριες δοκούς. Πιο συγκεκριμένα, αναπτύσσονται αναλυτικές/ημιαναλυτικές λύσεις ολικού και τοπικού λυγισμού πλακών που φέρουν δοκούς ενίσχυσης. Όσον αφορά τον ολικό λυγισμό αναπτύσσεται μια μεθοδολογία που βασίζεται στη μαθηματική μετατροπή μιας πλάκας που φέρει δοκούς ενίσχυσης σε μια ισοδύναμη ομογενή πλάκα. Η αναπτυχθείσα μεθοδολογία ομογενοποίησης των ενισχυμένων πλακών εμφανίζει σημαντικά πλεονεκτήματα σε σύγκριση με τις αντίστοιχες ήδη υπάρχουσες. Παράλληλα, η ενεργειακή μέθοδος Rayleigh-Ritz εφαρμόζεται για τη λύση προβλημάτων λυγισμού μερικώς ανισότροπων πλακών με ενισχυτικές δοκούς από σύνθετα υλικά, λαμβάνοντας διακριτά υπόψη τις ενισχυτικές δοκούς. Όσον αφορά το πρόβλημα του τοπικού λυγισμού, αναπτύσσεται μια νέα μεθοδολογία για την εύρεση των κρίσιμων φορτίων τοπικού λυγισμού λεπτότοιχης πλάκας με χρήση ενεργειακών μεθόδων. Το μαθηματικό μοντέλο που χρησιμοποιείται για την περίπτωση του τοπικού λυγισμού της επικάλυψης είναι η απομόνωση του τμήματος της επικάλυψης μεταξύ δυο ενισχυτικών δοκών και η αντικατάσταση της δυσκαμψίας της υπόλοιπης πλάκας με ελατήρια μεταβλητής δυσκαμψίας. Η μεθοδολογία αυτή επεκτείνεται και στον προσδιορισμό της μεταλυγισμικής συμπεριφοράς μιας πλάκας ενισχυμένης με διαμήκεις δοκούς. Οι παραπάνω μεθοδολογίες υπολογισμού του κρίσιμου φορτίου λυγισμού που αναπτύσσονται, στα πλαίσια αυτής της διατριβής, εφαρμόζονται στη διαστασιολόγηση πτέρυγας μη συμβατικής δομής από σύνθετα υλικά με πολυάριθμες κύριες δοκούς, σε αντίθεση με τις συμβατικές πτέρυγες (με δύο κύριες δοκούς). Η ανάλυση τάσεων της πτέρυγας πραγματοποιείται με τη βοήθεια της μεθόδου των πεπερασμένων στοιχείων. Η τελική διαστασιολόγηση επιτυγχάνεται με επαναληπτική διαδικασία βελτιστοποίησης βασισμένη σε αναλυτικές και ημιαναλυτικές σχέσεις. Με τον τρόπο αυτό, συγκρίνεται λεπτομερώς η συμβατική δομή πτέρυγας με 2 κύριες δοκούς και οι αντίστοιχες πτέρυγες με 4, 5 και 6 κύριες δοκούς. Για την περαιτέρω βελτιστοποίηση της συμπεριφοράς της πτέρυγας, διερευνάται η επίδραση που έχει η αλλαγή των μηχανικών ιδιοτήτων του υλικού και των επιτρεπόμενων ορίων παραμόρφωσης στη δυνατότητα ελαχιστοποίησης της μάζας της πτέρυγας. Υπολογίστηκε ότι κάτω από συγκεκριμένες συνθήκες η χρήση της μη συμβατικής πτέρυγας μπορεί να οδηγήσει σε μείωση μάζας μέχρι και 12%. / The design and development of a modern aerospace structure consists of many design stages. The most important stages are the conceptual and the preliminary where the initial sizing of the structure is obtained. It is known that the conventional design of the aircraft’s main structural members has reached a high optimization level, where margins for further improvement are small. The current demands of the lightweight structures such as weight reduction, payload increase etc. have led the aerospace industries develop unconventional structures and partially substitute the metallic materials of the primary structures with composites. The current trend of designing and evolving unconventional aerospace structures, without performing extended experimental tests, leads to the development of behavior models. The simulation of the experimental tests (through the behavior models) is achieved using high performance computers and numerical methods (Finite Element Method, Boundary Element Method etc). To apply simulation methods during the conceptual and preliminary stage is not an easy task. Most of the difficulties are the numerous geometrical, material parameters and the structural details that alter during the iterative process of the design. So, the exclusive usage of numerical analysis methods becomes very time consuming, if it is not accompanied by analytical or semi analytical methods of the sub-problems of the design. Part of the preliminary design of an unconventional wing structure is to prevent upper skin from failure. The stiffened panels that comprise the upper skin of the wing suffer from buckling due to the applied compressive loads. The sizing of the composite stiffened panels usually requires numerous of iterative calculations for various geometries, loading and boundary conditions etc. The examination of each case separately, with the use of numerical methods, results to time consuming analyses of the entire structure. Therefore, the development of appropriate analytical or semi analytical methods for estimating stiffened panels’ critical buckling load is of great importance. For this purpose, in the present thesis, analytical and semi analytical methodologies are developed for estimating the critical buckling load of stiffened panels. The developed methodologies are incorporated as design criteria in the sizing routine of the entire structure. The sizing routine comprises additional sizing criteria for checking the strength of wing’s structural members at each phase of the iterative process. Applying the developed sizing routine in various wing configurations made of composite materials, multispar wing designs are studied. Specifically, analytical and semi analytical methods for global and local buckling problems of stiffened panels are developed. The methodology of global buckling problems is based on the mathematical conversion of a stiffened panel to an equivalent homogeneous panel. The developed method of homogenization of stiffened panels appears to have significant advantages over the already existed homogenization methods. Additionally, the energy method Rayleigh-Ritz is applied for solving global buckling problems of stiffened panels with partial anisotropy considering discrete stiffeners. Regarding local buckling problems of stiffened panels, a new methodology is developed for estimating the critical local buckling load with the use of energy methods. The approach considers the stiffened panel segment located between two stiffeners, while the remaining panel is replaced by equivalent transverse and rotational springs of varying stiffness, which act as elastic edge supports. The buckling analysis of the segment provides an accurate and conservative prediction of the panel local buckling behavior. Consequently, the developed methodology is extended in the prediction of post-buckling response of stiffened panels where skin has undergone local buckling. The developed methodologies for calculating the critical buckling load are applied for sizing the wing members of an unconventional wing (multispar configuration) from composite materials. An efficient methodology based on fast Finite Element (FE) stress analysis combined to analytically formulated design criteria is presented for the initial sizing of a large scale composite component. A detailed comparison between optimized designs of conventional (2-spar) and three alternative wing configurations which comprise 4-, 5-, and 6-spars for the wing construction is performed. In order to understand the effect of different material properties, as well as the variation of maximum strain level allowed in the total wing mass, parametric analyses are performed for all wing configurations considered. It arises that under certain conditions the multispar configuration demonstrates significant advantages over the conventional design. This would lead to a mass reduction of 12%.