Μέθοδοι φασματικών πεπερασμένων στοιχείων στο πεδίο του χρόνου για την ανάλυση κυματικής διάδοσης σε σύνθετες πολύστρωτες πλάκες με ενεργούς πιεζοηλεκτρικούς αισθητήρες

The aim of the current dissertation is the development of novel laminate mechanics and finite element methods capable of predicting the guided wave propagation in laminated composite strips and plates, excited by physically modeled piezoelectric actuators. The present thesis is subdivided into three main parts; the first one addresses the development of high order and layerwise laminate theories which can effectively model the propagation of guided waves in laminated or/and sandwich composite strips and plates; the second part describes the formulation of novel time domain spectral finite elements, while the third part describes the adopted experimental procedures and the laboratory tests that were performed in order to validate the developed time domain spectral finite element models.The developed laminate mechanics theories presented in the first part, were inspired by the Rayleigh-Lamb wave solution, which assumes the axial and the transverse displacements as a summation of sines and cosines. Consequently, high-order approximations and additional degrees of freedom were considered in the kinematic assumptions of both axial and transverse displacements, enabling the prediction of symmetric and anti-symmetric type guided waves. Thereafter, while proving that polynomials up to the third degree were adequate for the simulation of fundamental and modes, a new high order laminate theory was proposed using for the through thickness interpolation functions third order cubic Hermite splines. The newly developed theory for strips and plates, further facilitates the approximation of the displacements and their respective rotations at the interfaces of the strip or the plate. In addition, enables the efficient layerwise expansion for multiple applications, such as the modeling of thick sandwich laminates, the prediction of higher order guided wave modes and the physical representation of piezoelectric actuators and sensors. The second part describes the combination of the aforementioned laminate theories into a time domain spectral finite element formulation. The time domain spectral finite elements have as a basis the high order polynomial Lagrange shape functions which ensure efficient spatial approximation of very small wavelengths in the length of the strip or in the plane of the plate. More importantly, the integration points are collocated with the element’s nodes using the Gauss-Lobbato-Legendre quadrature, which gives rise to multi-node strip and plate elements with diagonal consistent mass matrices. Thus, a substantial boost of the computational speed of a central-difference explicit time integration scheme is presented while at the same time, improved accuracy compared to the explicit finite element using lumped mass matrices is observed.At the third part, the TFSDEs were evaluated and validated against well-established semi-analytical methods, commercial FE algorithms and experimental results. The outcome of the evaluation proved that the numerical tool developed within the framework of the present thesis improves the already stated works at the scientific sector of wave propagation in composite laminates. Improvements were observed both in the prediction accuracy and the required analysis time. Finally, a layerwise explicit integration scheme was presented for the coupled electromechanical system.Ο σκοπός της παρούσας διδακτορικής διατριβής είναι η ανάπτυξη καινοτόμων θεωριών διάτμησης για την ανάλυση διαστρωματώσεων από σύνθετα υλικά, καθώς και μεθόδων πεπερασμένων στοιχείων ικανών να προβλέπουν την κυματική διάδοση στο εσωτερικό δοκών και πλακών από σύνθετα υλικά, η διέγερση των οποίων έχει προέλθει από την φυσική παρουσία ενεργών πιεζοηλεκτρικών αισθητηρίων. Η παρούσα διατριβή μπορεί να υποκατηγοριοποιηθεί σε τρεις επιμέρους τομείς. Ο πρώτος από τους οποίους έχει ως θέμα την ανάπτυξη θεωριών ανώτερης τάξης, μέσω των οποίων μπορεί να προσομοιωθεί επιτυχώς η κυματική διάδοση σε πλάκες κατασκευασμένες από σύνθετα υλικά, όπως επίσης σε πλάκες με έντονα ανομοιογενής στρώσεις τύπου “sandwich”. Το δεύτερο κομμάτι της διατριβής παρουσιάζει τη μεθοδολογία για την ανάπτυξη φασματικών πεπερασμένων στοιχείων στο πεδίο του χρόνου, ενώ το τρίτο και τελευταίο κομμάτι ασχολείται με τα πειράματα που πραγματοποιήθηκαν καθ’ όλην τη διάρκεια της διδακτορικής διατριβής, με απώτερο σκοπό την επιβεβαίωση των καινοτόμων στοιχείων της διατριβής μέσω της ταυτοποίησης των αποτελεσμάτων.Οι κινηματικές υποθέσεις που αναπτύχθηκαν με σκοπό την προσομοίωση της κυματικής διάδοσης σε σύνθετες κατασκευές, εμπνεύστηκαν από τη λύση της εξίσωσης διάδοσης κύματος των Rayleigh-Lamb, οποία προϋποθέτει ότι οι μετατοπίσεις ισούνται με το άθροισμα ημιτόνων και συνημίτονων. Στη συνέχεια με τη βοήθεια του αναπτύγματος Taylor εισήχθησαν μη-γραμμικοί όροι στην αξονική και εγκάρσια μετατόπιση, μια καινοτομία που ευνοεί την έγκυρη πρόβλεψη των συμμετρικών και αντισυμμετρικών τύπου κυμάτων. Στη συνέχεια αφού οι όροι τρίτης τάξης αποδείχθηκαν αρκετοί για την πρόβλεψη της κυματικής διάδοσης σε πλάκες και δοκούς από σύνθετα υλικά, προτάθηκε μια καινούργια θεωρία ανώτερης τάξης, στην οποία χρησιμοποιούνται ως συναρτήσεις ενδοπαρεμβολής Ερμιτιανά πολυώνυμα τρίτης τάξης. Η συγκεκριμένη θεωρία προϋποθέτει ως βαθμούς ελευθερίας τις μετατοπίσεις και αντίστοιχες περιστροφές τους, των πάνω και κάτω διακριτών επιφανειών της σύνθετης στρώσης. Μια μοναδική ιδιότητα η οποία επιτρέπει την επέκταση από θεωρία μονής στρώσης σε θεωρία διακριτών στρωμάτων, δίνοντας τη δυνατότητα προσομοίωσης πολύπλοκων διαστρωματώσεων, καθώς και την ένωση τομέων της κατασκευής με διαφορετικό αριθμό διακριτών στρωμάτων. Ένα χαρακτηριστικό παράδειγμα είναι η παρουσία ενός πιεζοηλεκτρικού διεγέρτη περιορισμένο σε μια διακριτή επιφάνεια της κατασκευής, κατά συνέπεια με δεύτερη διακριτή στρώση πρέπει να εισαχθεί μόνο σε αυτή τη διακριτή επιφάνεια.Στο δεύτερο μέρος της παρούσας διατριβής παρουσιάζεται η ενοποίηση των προαναφερθέντων θεριών με την ανάπτυξη φασματικών πεπερασμένων στοιχείων στο πεδίο του χρόνου. Τα χρονικά φασματικά πεπερασμένα στοιχεία έχουν ως βάση πολυωνυμικές συναρτήσεις μορφής ανώτερης τάξης τύπου Lagrange. Η ανώτερη τάξη των συναρτήσεων μορφής επιτρέπει τη δημιουργία πολύκομβων φασματικών πεπερασμένων στοιχείων, δίνοντας την ευχέρεια προσομοίωσης υψίσυχνων σημάτων με πολύ μικρό μήκος κύματος με τη χρήση πολύ λίγων φασματικών πεπερασμένων στοιχείων. Επιπλέον οι κόμβοι των φασματικών πεπερασμένων στοιχείων συμπίπτουν με τα σημεία ολοκλήρωσης των Gauss-Lobbato-Legendre, βοηθώντας στη σύνθεση συνεπών διαγώνιων μητρώων μάζας, επιτρέποντας την γρήγορη λύση του ηλεκτρομηχανικού συστήματος με τη μέθοδο κεντρικών διαφορών άμεσης ολοκλήρωσης.Τελειώνοντας, το τρίτο επιμέρους κομμάτι της διατριβής ασχολείται με τα δοκίμια πολύστρωτων δοκών και πλακών (από ίνες άνθρακα σε εποξειδική ρητίνη) που κατασκευάστηκαν στη παρούσα διατριβή, και με τα πειράματα που διεξήχθησαν σε αυτά με σκοπό την πειραματική επαλήθευση και πιστοποίηση των αριθμητικών αποτελεσμάτων

Input files to run finite element analysis simulations

The input files provided can be used to run representative finite element analysis (FEA) simulations using commercial software ABAQUS/Standard, as part of the in silico work of this study.</p

A Road Map to the Structural Optimization of a Type-C Composite UAV

The present paper investigates the design process and the dimensioning of a tailless type-C composite sandwich unmanned aerial vehicle (UAV). The objective is to investigate an innovative aircraft configuration which exceeds the standard approach of ribs and spars and replaces them with a sandwich structure for future unmanned aerial systems. The necessity of carbon fiber-reinforced materials arose due to the weight constraint of a Class C UAV, i.e., the whole vehicle must be under 25 kg, which limits the mass of the structure to 9 kg. The structural design of composite structures differs from the one of traditional isotropic structures. The number of holes should be limited, as drilling down the composite aerostructure would conclude to the generation of delaminations. In addition, the joints between sections with different thicknesses could lead to stress concentrations and disbands. Therefore, the present report is crucial for the continuance of the present project as it has contributed both to the structural design and assessment of the UAV. This work focusses on the computation of loads, the process of structural sizing through a multi-disciplinary optimization approach, and the simulation-based structural proof. Particular attention is paid to the specifically developed semi-analytical method for predicting the aero-elastic load. Based on the detailed finite element model of the global structure, the applicability of the minimum number of bolts as a major structural joining variant is proven. The design process from single components to the assembly of the overall aircraft results in the realization of the demonstrator structure

Experimental and simulation data used to produce the manuscript figures

This Microsoft Excel file tabulates all the in vivo and in silico results that were produced in this study.</p

Image3_The influence of internal pressure and neuromuscular agents on C. elegans biomechanics: an empirical and multi-compartmental in silico modelling study.pdf

The function of a specific tissue and its biomechanics are interdependent, with pathologies or ageing often being intertwined with structural decline. The biomechanics of Caenorhabditis elegans, a model organism widely used in pharmacological and ageing research, has been established as biomarker for healthy ageing. However, the properties of the constituent tissues, and their contribution to the overall mechanical characteristics of the organism, remain relatively unknown. In this study we investigated the biomechanics of healthy C. elegans cuticle, muscle tissue, and pseudocoelom using a combination of indentation experiments and in silico modelling. We performed stiffness measurements using an atomic force microscope. To approximate the nematode’s cylindrical body we used a novel three-compartment nonlinear finite element model, enabling us to analyse of how changes in the elasticity of individual compartments affect the bulk stiffness. We then fine-tuned the parameters of the model to match the simulation force-indentation output to the experimental data. To test the finite element model, we modified distinct compartments experimentally. Our in silico results, in agreement with previous studies, suggest that hyperosmotic shock reduces stiffness by decreasing the internal pressure. Unexpectedly, treatment with the neuromuscular agent aldicarb, traditionally associated with muscle contraction, reduced stiffness by decreasing the internal pressure. Furthermore, our finite element model can offer insights into how drugs, mutations, or processes such as ageing target individual tissues.</p

DataSheet1_The influence of internal pressure and neuromuscular agents on C. elegans biomechanics: an empirical and multi-compartmental in silico modelling study.PDF

The function of a specific tissue and its biomechanics are interdependent, with pathologies or ageing often being intertwined with structural decline. The biomechanics of Caenorhabditis elegans, a model organism widely used in pharmacological and ageing research, has been established as biomarker for healthy ageing. However, the properties of the constituent tissues, and their contribution to the overall mechanical characteristics of the organism, remain relatively unknown. In this study we investigated the biomechanics of healthy C. elegans cuticle, muscle tissue, and pseudocoelom using a combination of indentation experiments and in silico modelling. We performed stiffness measurements using an atomic force microscope. To approximate the nematode’s cylindrical body we used a novel three-compartment nonlinear finite element model, enabling us to analyse of how changes in the elasticity of individual compartments affect the bulk stiffness. We then fine-tuned the parameters of the model to match the simulation force-indentation output to the experimental data. To test the finite element model, we modified distinct compartments experimentally. Our in silico results, in agreement with previous studies, suggest that hyperosmotic shock reduces stiffness by decreasing the internal pressure. Unexpectedly, treatment with the neuromuscular agent aldicarb, traditionally associated with muscle contraction, reduced stiffness by decreasing the internal pressure. Furthermore, our finite element model can offer insights into how drugs, mutations, or processes such as ageing target individual tissues.</p

Image5_The influence of internal pressure and neuromuscular agents on C. elegans biomechanics: an empirical and multi-compartmental in silico modelling study.pdf

The function of a specific tissue and its biomechanics are interdependent, with pathologies or ageing often being intertwined with structural decline. The biomechanics of Caenorhabditis elegans, a model organism widely used in pharmacological and ageing research, has been established as biomarker for healthy ageing. However, the properties of the constituent tissues, and their contribution to the overall mechanical characteristics of the organism, remain relatively unknown. In this study we investigated the biomechanics of healthy C. elegans cuticle, muscle tissue, and pseudocoelom using a combination of indentation experiments and in silico modelling. We performed stiffness measurements using an atomic force microscope. To approximate the nematode’s cylindrical body we used a novel three-compartment nonlinear finite element model, enabling us to analyse of how changes in the elasticity of individual compartments affect the bulk stiffness. We then fine-tuned the parameters of the model to match the simulation force-indentation output to the experimental data. To test the finite element model, we modified distinct compartments experimentally. Our in silico results, in agreement with previous studies, suggest that hyperosmotic shock reduces stiffness by decreasing the internal pressure. Unexpectedly, treatment with the neuromuscular agent aldicarb, traditionally associated with muscle contraction, reduced stiffness by decreasing the internal pressure. Furthermore, our finite element model can offer insights into how drugs, mutations, or processes such as ageing target individual tissues.</p

The influence of internal pressure and neuromuscular agents on C. elegans biomechanics: an empirical and multi-compartmental in silico modelling study

The function of a specific tissue and its biomechanics are interdependent, with pathologies or ageing often being intertwined with structural decline. The biomechanics of Caenorhabditis elegans, a model organism widely used in pharmacological and ageing research, has been established as biomarker for healthy ageing. However, the properties of the constituent tissues, and their contribution to the overall mechanical characteristics of the organism, remain relatively unknown. In this study we investigated the biomechanics of healthy C. elegans cuticle, muscle tissue, and pseudocoelom using a combination of indentation experiments and in silico modelling. We performed stiffness measurements using an atomic force microscope. To approximate the nematode’s cylindrical body we used a novel three-compartment nonlinear finite element model, enabling us to analyse of how changes in the elasticity of individual compartments affect the bulk stiffness. We then fine-tuned the parameters of the model to match the simulation force-indentation output to the experimental data. To test the finite element model, we modified distinct compartments experimentally. Our in silico results, in agreement with previous studies, suggest that hyperosmotic shock reduces stiffness by decreasing the internal pressure. Unexpectedly, treatment with the neuromuscular agent aldicarb, traditionally associated with muscle contraction, reduced stiffness by decreasing the internal pressure. Furthermore, our finite element model can offer insights into how drugs, mutations, or processes such as ageing target individual tissues