Objective 2: Conduct Experimental Activities on Performance of Sensor‐Equipped Composite Elements

In this report we present the basic principles of our exploratory research for embedding sensors and energy-harvesting devices in sandwich composite structures. A range of piezopolymer sensors where used, varying in their dimensions, their capacitance and the electrode coating material. Sensors were tested in the surface-attached and embedded formats on composite sandwich laminates. Specimens were tested under three-point bending configuration to measure both the sensor voltages and the generated laminate strains. The electromechanical sensitivity performance with respect to frequency and strain amplitude were the key performance parameters investigated. Primary conclusions, of a practical nature, were extracted concerning the appropriate type of sensor for embedding applications as well as the parameters affecting the sensitivity and efficiency of the devices to perform as energy both strain gauges and energy-harvesting devices.JRC.G.5-European laboratory for structural assessmen

Analytical models for smart piezoelectic materials and structures encompassing self health monitoring capabilities

Σκοπός της παρούσας διδακτορικής διατριβής είναι η ανάπτυξη προτύπων πεπερασμένων στοιχείων ικανών να προσομοιώσουν την μορφική και δυναμική απόκριση ευφυών δοκών με διαστρωματική αποκόλληση. Έμφαση δίνεται στη χρήση πιεζοηλεκτρικών διεγερτών και αισθητήρων. Στη πρώτη ενότητα πραγματοποιούνται πειραματικές μετρήσεις σε δοκούς με αποκόλληση χρησιμοποιώντας διάφορες μεθοδολογίες διέγερσης και δειγματοληψίας, οι οποίες περιλαμβάνουν πιεζοηλεκτρικά στοιχεία (actuators,sensors) εκτός των κλασικών μεθόδων (shaker, accelerometer). Από τις πειραματικές μετρήσεις διαπιστώνονται οι δυνατότητες χρήσης πιεζοηλεκτρικών στοιχείων ενώ ανιχνεύεται και ποσοτικοποιείται η επίδραση της ύπαρξης και έκτασης αποκόλλησης στα μορφικά χαρακτηριστικά των δοκών. Στη συνέχεια διατυπώνονται μοντέλα μεσομηχανικής αποσκοπώντας στη προσομοίωση της ηλεκτρομηχανικής συμπεριφοράς ευφυών δοκών με αποκόλληση. Τα πρότυπα αυτά είναι συζευγμένα ηλεκτρομηχανικά και με τη χρήση κατάλληλου πεπερασμένου στοιχείου δοκού οδηγούν στο ηλεκτρομηχανικό σύστημα το οποίο χρησιμοποιείται για να προβλέψει τη στατική και μορφική απόκριση δοκών με διαστρωματική αποκόλληση. Στην επόμενη ενότητα τα πρότυπα μεσομηχανικής καθώς και το πεπερασμένο στοιχείο επεκτείνονται, έτσι ώστε να επιβάλλονται με φυσικό τρόπο συνθήκες επαφής και περιορισμού της οριζόντιας κίνησης (τριβής) μεταξύ των δύο μερών της αποκόλλησης. Το συζευγμένο μη?γραμμικό ηλεκτρομηχανικό σύστημα που προκύπτει χρησιμοποιείται για να προσδιορίσει τη μεταβατική απόκριση δοκών με αποκόλληση και επαληθεύεται με πειραματικές μετρήσεις. Τέλος παρουσιάζεται ένα δισδιάστατο πρότυπο πεπερασμένο στοιχείο ευφυούς δοκού με αποκόλληση. Στο πρότυπο αυτό τα μοντέλα μεσομηχανικής επεκτείνονται περιλαμβάνοντας μεταβλητό πεδίο επίπεδων και εγκάρσιων αξονικών μετατοπίσεων, αποτελώντας τις δύο διαστάσεις του μοντέλου ώστε να προβλέψει τη στατική, μορφική και δυναμική απόκριση δοκών μεγάλου πάχους με αποκόλληση σε υψηλές συχνότητες. Χρησιμοποιείται για να προσομοιάσει συμμετρικές κατά το πάχος ιδιομορφές και την κυματική διάδοση υψηλών συχνοτήτων ευφυών δοκών με αποκόλληση. Τέλος τα μοντέλα εφαρμόζονται για τον περαιτέρω προσδιορισμό της βλάβης μέσα από την απόκριση χρόνου?συχνότητας

The conceptual design of a safety system: For the 5MW Deepwind wind turbine

This research work proposes the initial design considerations of the safety system of the Deepwind offshore floating vertical axis wind turbine. Deepwind is a wind turbine model and prototype development project under the umbrella of the 7th European Framework Programme. The safety system is one of the aspects of this project and it is dealt within this Thesis work. Safety is one of the most important features that modern wind turbines should include. Statistics and industrial experience have indicated to the regulation organizations (e.g. IEC,DNV) to add safety systems in the complex electromechanical system of a wind turbine. The most crucial safety feature is the over-speeding control. Usually the controller functions alleviate this problem but an additional safety level is more than necessary to avoid irreparable incidents. This safety feature has been developed within the scope of this Thesis. Therefore, the major task is the definition and design of the safety system functions. This task was conducted in the context of the Conceptual Design method. Several possibilities were investigated. This search led to systems using the aerodynamic and hydrodynamic principles of operation. Many aspects were taken into account concerning the functionality and compatibility of these safety systems. These aspects were addressed from literature review and generation of engineering models in MATLAB. Finally, through multi-criteria analysis, which is one of the tools of Conceptual Design, all the systems were compared and a solution was formulated with the initial design configurations for further development. The proposal of this Master Thesis is to sink the wind turbine system inside the sea, by adding seawater into the spar buoy in an effective time response. Consequently, the blades of the wind turbine hit the seawater and thus create enough drag forces to reduce effectively the rotational speed. The particular wind turbine characteristics make this solution the most promising, as presented through the whole process. This report proposes the initial design characteristics but also suggests further steps on the design process of the safety system.Aerospace EngineeringSustainable Energy Technolog

Nonlinear wave structural health monitoring method using an active nonlinear piezoceramic sensor for matrix cracking detection in composites

A structural health monitoring methodology, based on nonlinear wave modulation spectroscopy, is presented and aims to detect matrix cracks in composites. Experiments were conducted on cross-ply carbon/epoxy strips containing matrix cracks, induced via three-point bending. Damage in all tested specimens was categorized according to the acoustic emission hits recorded during loading. The nonlinear ultrasonics methodology is applied via an active nonlinear acoustoultrasonic piezoelectric sensor, enabling low-cost and wide-frequency operational bandwidth. This active sensor configuration involves two piezoceramic wafer actuators, one excited with a low- and the other with a high-frequency signal, and a piezoceramic sensor, all permanently bonded on the tested structure. The sensitivity of the nonlinear active sensor response at specific high carrier frequencies is depicted and damage indices are proposed. The experimental results illustrate the effectiveness of the nonlinear ultrasonic wave mixing method in matrix crack detection, as well as the potential of the new active sensor for permanent structural health monitoring.JRC.G.4-European laboratory for structural assessmen

Evaluation of the physical and fatigue performance of embedded piezopolymer sensor systems in sandwich composite laminates

It has been claimed that embedding piezo-ceramic devices as structural diagnostic systems in advanced composite structures may introduce mechanical impedance miss-matches that favour the formation of intralaminar defects; this and other factors such as cost and their high strain sensitivity has motivated the use of thin-film piezo-polymer sensors. In this paper, we examine the performance of sandwich composite panels fitted with embedded piezopolymer sensors. Our experimental campaign examines both how such thin-film sensors perform within a structure and the converse effect; how the inclusion of sensor films affects structural performance. Strain-controlled tests on sandwich panels subjected to three-point bending under wide-ranging static and dynamic strains lead us to conclude that embedding thin piezopolymer films has no marked reduction on the tensile strength for a wide range of strain loading paths and magnitudes, and that the resilience of the embedded sensor is itself satisfactory, even up to the point of structural failure. Comparing base-line data obtained from standard surface-mounted sensors and foil gauges, we note that whereas it is possible to match experimental and theoretical strain sensitivities, key properties, especially the pronounced orthotropic electromechanical factor of such films must be duly considered before an effective calibration can take place.JRC.G.4-European laboratory for structural assessmen

Investigating efficiency of non-linear wave modulation spectroscopy for early detection of critical buckling damages in sandwich composite panels

This work presents experimental investigations into the potential of using, piezo-actuated, ultrasound waves for the detection of damage or incipient failure in fibre-reinforced sandwich panels under compressive axial load. Two methodologies are applied based on (i) frequency modulation spectroscopy and (ii) single-mode resonance shifting. Sensitivity of the techniques is studied during the loading till the failure of the specimens and also post failure. Representative measurements for both approaches are presented and discussed herein.JRC.G.4-European laboratory for structural assessmen

Evaluation of the sensitivity and fatigue performance of embedded piezopolymer sensor systems in sandwich composite laminates

It has been claimed that embedding piezo-ceramic devices as structural diagnostic systems in advanced composite structures may introduce mechanical impedance miss-matches that favour the formation of intralaminar defects; this and other factors such as cost and their high strain sensitivity has motivated the use of thin-film piezo-polymer sensors. In this paper, we examine the performance of sandwich composite panels fitted with embedded piezopolymer sensors. Our experimental campaign examines both how such thin-film sensors perform within a structure and the converse effect; how the inclusion of sensor films affects structural performance. Strain-controlled tests on sandwich panels subjected to three-point bending under wide-ranging static and dynamic strains lead us to conclude that embedding thin piezopolymer films has no marked reduction on the tensile strength for a wide range of strain loading paths and magnitudes, and that the resilience of the embedded sensor is itself satisfactory, even up to the point of structural failure. Comparing base-line data obtained from standard surface-mounted sensors and foil gauges, we note that whereas it is possible to match experimental and theoretical strain sensitivities, key properties, especially the pronounced orthotropic electromechanical factor of such films must be duly considered before an effective calibration can take place.JRC.G.4-European laboratory for structural assessmen