Failure of metallic structures operating under shock loading is a common occurrence in engineering applications. It is difficult to estimate the response of complicated systems analytically, due to structure&rsquo / s dynamic characteristics and varying loadings. Therefore, experimental, numerical or a combination of both methods are used for evaluations. The experimental analysis of the shocks due to firing is done for 12.7mm Gatling gun and 25mm cannon. During the tests, the Gatling gun and the cannon are located on military Stabilized Machine Gun Platform and Stabilized Cannon Platform respectively. For the firing tests, ICP (integrated circuit piezoelectric) accelerometers are attached to obtain the loading history for corresponding points. Shock Response Spectrum (SRS) analysis (nCode Glypworks) is done to define the equivalent shock profiles created on test pieces and the mount of 25mm cannon by means of the gun and the cannon firing. Transient shock analysis of the test pieces and the mount are done by applying the obtained shock profiles on the parts in a finite element model (ANSYS). Furthermore, experimental stress analysis due to shock loading is performed for two different types of material and different thicknesses of the test pieces. The input data for the analysis is obtained through measurements from strain rosette precisely located at the critical location of the test pieces. As a result of the thesis, a proposal is tried to be introduced where strain energy density theory is applied to predict the shock failure at military structures.

A novel and efficient methodology is developed for computer simulations of realistic two-dimensional (2D) and three-dimensional (3D) microstructures. The simulations incorporate realistic 2D and 3D complex morphologies/shapes, spatial patterns, anisotropy, volume fractions, and size distributions of the microstructural features statistically similar to those in the corresponding real microstructures. The methodology permits simulations of sufficiently large 2D as well as 3D microstructural windows that incorporate short-range (on the order of particle/feature size) as well as long-range (hundred times the particle/feature size) microstructural heterogeneities and spatial patterns at high resolution. The utility of the technique has been successfully demonstrated through its application to the 2D microstructures of the constituent particles in wrought Al-alloys, the 3D microstructure of discontinuously reinforced Al-alloy (DRA) composites containing SiC particles that have complex 3D shapes/morphologies and spatial clustering, and 3D microstructure of boron modified Ti-6Al-4V composites containing fine TiB whiskers and coarse primary TiB particles. The simulation parameters are correlated with the materials processing parameters (such as composition, particle size ratio, extrusion ratio, extrusion temperature, etc.), which enables the simulations of rational virtual 3D microstructures for the parametric studies on microstructure-properties relationships. The simulated microstructures have been implemented in the 3D finite-elements (FE)-based framework for simulations of micro-mechanical response and stress-strain curves. Finally, a new unbiased and assumption free dual-scale virtual cycloids probe for estimating surface area of 3D objects constructed by 2D serial section images is also presented.

Extrapolation methods for the solution of partial differential equations are commonly based on the existence of error expansions for the approximate solution. Implicit extrapolation, in the contrast, is based on applying extrapolation indirectly, by using it on quantities like the residual. In the context of multigrid methods, a special technique of this type is known as \034 -extrapolation. For finite element systems this algorithm can be shown to be equivalent to higher order finite elements. The analysis is local and does not use global expansions, so that the implicit extrapolation technique may be used on unstructured meshes and in cases where the solution fails to be globally smooth. Furthermore, the natural multilevel structure can be used to construct efficient multigrid and multilevel preconditioning techniques. The effectivity of the method is demonstrated for heat conduction problems and problems from elasticity theory.

In several practical applications, hot-finished steel pipe that exhibits Lüders bands is bent to strains of 2-3%. Lüders banding is a material instability that leads to inhomogeneous plastic deformation in the range of 1-4%. This work investigates the influence of Lüders banding on the inelastic response and stability of tubes under rotation controlled pure bending. It starts with the results of an experimental study involving tubes of several diameter-to-thickness ratios in the range of 33.2 to 14.7 and Lüders strains of 1.8% to 2.7%. In all cases, the initial elastic regime terminates at a local moment maximum and the local nucleation of narrow angled Lüders bands of higher strain on the tension and compression sides of the tube. As the rotation continues, the bands multiply and spread axially causing the affected zone to bend to a higher curvature while the rest of the tube is still at the curvature corresponding to the initial moment maximum. With further rotation of the ends, the higher curvature zone(s) gradually spreads while the moment remains essentially unchanged. For relatively low D/t tubes and/or short Lüders strains, the whole tube eventually is deformed to the higher curvature entering the usual hardening regime. Subsequently it continues to deform uniformly until the usual limit moment instability is reached. For high D/t tubes and/or materials with longer Lüders strains, the propagation of the larger curvature is interrupted by collapse when a critical length is Lüders deformed leaving behind part of the structure essentially undeformed. The higher the D/t and/or the longer the Lüders strain is, the shorter the critical length. This class of problems is analyzed using 3D finite elements while the material is modeled as an elastic-plastic solid with an “up-down-up” response over the extent of the Lüders strain, followed by hardening. The analysis reproduces the main features of the mechanical behavior provided the unstable part of the response is suitably calibrated. The uniform curvature elastic regime terminates with the nucleation of localized banded deformation. The bands appear in pockets on the most deformed sites of the tube and propagate into the hitherto intact part of the structure while the moment remains essentially unchanged. The Lüders-deformed section has a higher curvature, ovalizes more than the rest of the tube, and develops wrinkles with a characteristic wavelength. For every tube D/t there exists a threshold of Lüders strain separating the two types of behavior. This bounding value of Lüders strain was studied parametrically. / text

Term of remote-lab is certain types of laboratories which practical experiments are directedfrom a separate area by remote controller devices. This study is part of developing andupgrading advanced vibration remote laboratory. In the new remote lab, users have theability to measure the dynamic characteristics of the test object similar to the current existingremote lab. But in addition to current existing remote lab, they are capable to modifydynamic properties of the test object remotely by attaching vibration test instruments; such asa block of mass, spring-mass or non-linear spring. Doing several accurate experimental testsremotely on the test object are the toughest issues we faced as designers. In creating anddeveloping of this remote-lab, number of different approaches was adopted for producingwell-defined tests. Also, instead of implementing routine devices and techniques for regularvibration laboratories, the new prototypes were designed by finite elements method (FEM)and LABVIEW. For instance, the desirable test object, the attachment mechanism, usefulapplications, and proper software for managing via internet were prepared.

This thesis presents a comprehensive study of the compressive response of hexagonal honeycomb panels from the initial elastic regime to a fully crushed state. Expanded aluminum alloy honeycomb panels with a cell size of 0.375 in (9.53 mm), a relative density of 0.026, and a height of 0.625 in (15.9 mm) are laterally compressed quasi statically between rigid platens under displacement control. The cells buckle elastically and collapse at a higher stress due to inelastic action. Deformation then first localizes at mid-height and the cells crush by progressive formation of folds; associated with each fold family is a stress undulation. The response densifies when the whole panel height is consumed by folds. The buckling, collapse, and crushing events are simulated numerically using finite element models involving periodic domains of a single or several characteristic cells. The models idealize the microstructure as hexagonal, with double walls in one direction. The nonlinear behavior is initiated by elastic buckling while inelastic collapse that leads to the localization observed in the experiments occurs at a significantly higher load. The collapse stress is found to be mildly sensitive to various problem imperfections. For the particular honeycomb studied, the collapse stress is 67% higher than the buckling stress. It was also shown that all aspects of the compressive behavior can be reproduced numerically using periodic domains with a fine mesh capable of capturing the complexity of the folds. The calculated buckling stress is reduced when considering periodic square domains as the compatibility of the buckles between neighboring cells tends to make the structure more compliant. The mode consisting of three half waves is observed in every simulation but its amplitude is seen to be accented at the center of the domains. The calculated crushing response is shown to better resemble measured ones when a 4x4 cell domain is used, which is smoother and reproduces decays in the amplitude of load peaks. However, the average crushing stress can be captured with engineering accuracy even from a single cell domain. / text

Σκοπός της παρούσας Διπλωματικής Εργασίας είναι η μελέτη της εξέλιξης της νόσου της οστεοπόρωσης στη σπονδυλική στήλη με τη χρήση της μεθόδου των πεπερασμένων στοιχείων. Συγκεκριμένα, μοντελοποιείται η εφαρμογή θλιπτικού φορτίου στον δωδέκατο θωρακικό σπόνδυλο ασθενούς με οστεοπόρωση σε δύο διαδοχικές καταστάσεις – πριν και ένα χρόνο μετά την χορήγηση φαρμακευτικής αγωγής. Επιπλέον, κατά τη μελέτη, δοκιμάστηκαν διαφορετικές αποδόσεις μηχανικών ιδιοτήτων (μέτρο ελαστικότητας και βαθμός ανισοτροπίας) στο οστό, αρχικά στο μοντέλο ενός υγιούς σπονδύλου και στη συνέχεια στο εν λόγω μοντέλο του οστεοπορωτικού. Όλα τα μοντέλα κατασκευάστηκαν με την ίδια μεθοδολογία. Η σύγκριση τόσο των δύο καταστάσεων της νόσου, όσο και των διαφορετικών αποδόσεων των μηχανικών ιδιοτήτων, γίνεται βάσει των ισοδύναμων Τάσεων και Παραμορφώσεων κατά von Mises και των Μετατοπίσεων κατά τον οβελιαίο άξονα z. Οι υπό εξέταση μέγιστες τιμές των προαναφερθέντων μεγεθών που προκύπτουν από την ανάλυση πεπερασμένων στοιχείων, παρουσιάζουν μείωση στη δεύτερη κατάσταση. Επιπλέον, τα αποτελέσματα της διαφορετικής απόδοσης μηχανικών ιδιοτήτων, μαρτυρούν ότι ο βαθμός ανισοτροπίας και ο τρόπος συσχέτισης του μέτρου ελαστικότητας με την οστική πυκνότητα, επηρεάζουν το αποτέλεσμα της μηχανικής ανάλυσης. Παρόλα αυτά, η συγκριτική αξιολόγηση των δύο διαδοχικών καταστάσεων του σπονδύλου δεν φαίνεται να επηρεάζεται από τον τρόπο ορισμού των μηχανικών ιδιοτήτων. Γενικά, η αλλαγή που παρατηρείται στις αξονικές τομογραφίες της δεύτερης κατάστασης σε σχέση με την πρώτη σε συνδυασμό με τη μεταβολή των υπό εξέταση μεγεθών, ιδιαίτερα της ισοδύναμης Παραμόρφωσης κατά von Mises, οδηγούν στο συμπέρασμα ότι στη δεύτερη κατάσταση ο υπό εξέταση σπόνδυλος καταπονείται λιγότερο που σημαίνει ότι η ακολουθούμενη φαρμακευτική αγωγή έχει θετικά αποτελέσματα. / The purpose of this master thesis is to perform a computational (finite element, FEM) study on the progress of the spinal osteoporosis, between two sequential examinations consisting of CT scanning. For this purpose, two finite element models of the twelfth thoracic vertebra of a patient with osteoporosis were created, one for every examination – before and one year after the prescription of medication – and compression loading was simulated. Moreover, a material sensitivity study has taken place, first on a FE model of a healthy vertebra and then on the two models of the osteoporotic one. All FE models were created with the same methodology. For the comparison of the two sequential examinations, as well as of the different material assignment, three mechanical quantities were used: equivalent von Mises Stress, equivalent von Mises Strain and Displacement on the sagittal axis (z). From the output of FE analysis, it seems that the maximum values of the beforementioned quantities decrease in the second model of the osteoporotic vertebra. Also, it becomes clear that the degree of anisotropy and the correlation of bone mineral density vs. Young’s modulus, significantly affect the output of FE analysis. However, the comparison of the two osteoporotic models is not affected by the material assignment. As a conclusion, there is a remarkable change in the CT scans of the second examination combined with a reduction of the strains in the vertebra - coming as a result of the FE analysis - that shows the positive influence of the prescribed medication.

Το αντικείμενο της παρούσας εργασίας είναι η θεωρητική μελέτη κυμάτων ιονισμού που παράγονται κατά τη δημιουργία ηλεκτρικών εκκενώσεων ψυχρού πλάσματος. Μια πολύ χαρακτηριστική περίπτωση κύματος ιονισμού είναι ο streamer. Προκειμένου να μελετήσουμε τη συμπεριφορά του, αναπτύξαμε ένα δισδιάστατο αριθμητικό μοντέλο. Σε αυτό το υδροδυναμικό μοντέλο, επιλύθηκαν οι εξισώσεις συνεχείας για τα φορτισμένα σωματίδια της εκκένωσης καθώς και η εξίσωση Poisson για τον υπολογισμό του ηλεκτρικού πεδίου. Εξαιτίας των έντονων βαθμώσεων που εμφανίζονται κατά τη διάδοση του streamer χρησιμοποιήθηκαν εξειδικευμένες αριθμητικές τεχνικές. Το μοντέλο αναπτύχθηκε στα πλαίσια των πεπερασμένων στοιχείων χρησιμοποιώντας διορθωτές ροών για τη ακριβή περιγραφή των όρων μεταφοράς των υπερβολικών διαφορικών εξισώσεων συνέχειας. Το μοντέλο μας επιπλέον συγκρίθηκε και με ένα άλλο μοντέλο βασιζόμενο στους πεπερασμένους όγκους. Η συμφωνία που βρέθηκε μεταξύ των δυο μοντέλων ήταν πολύ καλή. Ακολούθως, με τη χρήση του προαναφερθέντος μοντέλου, μελετήσαμε το φαινόμενο του streamer, σε σχετικά μικρά (~1cm) μη ομογενή διάκενα. Διεξήχθησαν εξομοιώσεις τόσο στον αέρα όσο και στο άζωτο σε ατμοσφαιρική πίεση. Εξετάσθηκε η επίδραση της ακτίνας καμπυλότητας της ακίδας, η εφαρμοζόμενης τάσης και ο τύπος του αερίου στο κατά πόσο επηρεάζουν τη μορφολογία του streamer καθώς και τις ιδιότητες του κύματος ιονισμού γενικότερα. Τέλος μελετήθηκε και η αλληλεπίδραση του streamer με το υλικό της καθόδου. / The subject of this work is the theoretical study of the ionization waves that are produced and propagate at cold plasmas gas discharges. One of the most characteristic cases of ionization waves is the streamer and in this work it is investigated thoroughly. In order to elucidate the several physical mechanisms that related to the aforementioned phenomenon a two dimensional hydrodynamic numerical model is implemented. On this model the system of continuity equations for the charged species of the discharge (electrons, ions) coupled with the Poisson equation for the calculation of the electric field is solved numerically. Due to the shock character of the streamer special numerical techniques are adopted. The model is developed on the context of finite elements (FE) by using the flux corrected transport (FCT) algorithm for the accurately description of the transport part of continuity equations. Extended numerical tests are performed to validate correct implementation of FE-FCT. Moreover, our streamer model is also compared with an independent one, based on finite volumes (FV) framework, giving very similar results. By using the numerical model, streamer discharges at relative small, non uniform gaps (point to plane electrodes configuration) are studied. Simulations are performed in atmospheric pressure air and nitrogen. It is examined the influence of the radius of curvature of the tip, the level of applied voltage and the type of the gas, on the morphology of the streamer as well as on the properties of the ionization wave. Special attention is also drawn on the interaction of the streamer with the cathode electrode surface.

The advent of modern computer technologies provided a powerful tool in numerical simulations. One of the most frequently used method for the discretization of the physical domain is Finite element Method (FEM). One of the main problems in a finite element analysis is the adequacy of the finite element mesh. Since the quality of the finite element solution directly depends on the quality of meshes, the additional process to improve the quality of meshes is necessary for reliable finite element approximation. In order to perform quality-assessed numerical simulation, the adaptive finite element strategies have been developed. These strategies integrate the finite element analysis with error estimation and fully automatic mesh modification, user interaction with this process is limited by initial geometry data and possible error tolerance definition. The finite element solution, obtained during adaptive finite element strategy process, approximates quite good different engineering structures. Despite many works in this area the problem of the adequate finite element mesh is not fully solved and additional developments are needed in order to improve adaptive mesh refinement strategy process. The aim and tasks of the work – to obtain methodology for quality assessed discretization to finite elements for complex geometry engineering structures by adaptive finite element strategies. To realize this purpose the following scientific tasks have been dealt: to develop an automatic... [to full text]

The advent of modern computer technologies provided a powerful tool in numerical simulations. One of the most frequently used method for the discretization of the physical domain is Finite element Method (FEM). One of the main problems in a finite element analysis is the adequacy of the finite element mesh. Since the quality of the finite element solution directly depends on the quality of meshes, the additional process to improve the quality of meshes is necessary for reliable finite element approximation. In order to perform quality-assessed numerical simulation, the adaptive finite element strategies have been developed. These strategies integrate the finite element analysis with error estimation and fully automatic mesh modification, user interaction with this process is limited by initial geometry data and possible error tolerance definition. The finite element solution, obtained during adaptive finite element strategy process, approximates quite good different engineering structures. Despite many works in this area the problem of the adequate finite element mesh is not fully solved and additional developments are needed in order to improve adaptive mesh refinement strategy process. The aim and tasks of the work – to obtain methodology for quality assessed discretization to finite elements for complex geometry engineering structures by adaptive finite element strategies. To realize this purpose the following scientific tasks have been dealt: to develop an automatic... [to full text]