Papers by Ioanna Dakanali
H αποκατάσταση της μονολιθικότητας των δομικών μελών μαρμάρινων μνημείων είναι ένα πρόβλημα το οπ... more H αποκατάσταση της μονολιθικότητας των δομικών μελών μαρμάρινων μνημείων είναι ένα πρόβλημα το οποίο απαιτεί τον συγκερασμό πολλών αντικρουόμενων απόψεων (αρχαιολόγων, αρχιτεκτόνων, πολιτικών μηχανικών, χημικών κ.α.). Το βασικό πρόβλημα είναι ότι στις περισσότερες περιπτώσεις τα μάρμαρα είναι έντονα κατακερματισμένα. Για να αποκατασταθεί η στατική επάρκεια και η δομική αρτιότητα των μνημείων προτάθηκε, πριν μερικές δεκαετίες, μία πρωτοποριακή μέθοδος από τους επιστήμονες οι οποίοι εργάζονταν στα αναστηλωτικά έργα της Ακρόπολης των Αθηνών. Σύμφωνα με αυτή την τεχνική τα μαρμάρινα θραύσματα συγκολλούνται με λευκό τσιμέντο Portland και ενισχύονται με κοχλιοτομημένες ράβδους τιτανίου (Korres et al., 1989). Σκοπός της παρούσας εργασίας είναι να εξετάσει κατά πόσο μπορούν τεχνικές μη καταστροφικού ελέγχου να παρακολουθήσουν τη συμπεριφορά των αποκαταστημένων μνημείων και να εντοπίσουν έγκαιρα σημεία που χρήζουν ιδιαίτερης προσοχής. Κατά το σχεδιασμό των συνδέσεων για την αποκατάσταση της μονολιθικότητας των επιστυλίων βασικό μέλημα είναι να μην αστοχήσει ούτε το μάρμαρο αλλά ούτε και το τιτάνιο. Αν όμως υπάρξει αστοχία αυτή θα πρέπει ιδεατά να είναι η σχετική ολίσθηση της ράβδου ως προς το μάρμαρο (εξόλκευση οπλισμού). Υλοποιήθηκε σειρά πειραμάτων εξόλκευσης σε μαρμάρινους όγκους διαφορετικών διαστάσεων εφαρμόζοντας την τεχνική των Ακουστικών Εκπομπών και των ηλεκτρικών σημάτων (PSC). Τα αποτελέσματα των τεχνικών συσχετίστηκαν με τα αντίστοιχα των κλασικών μηχανικών τεχνικών ώστε να διαπιστωθεί αν είναι εφικτό να εντοπιστούν φαινόμενα από το εσωτερικό του δοκιμίου τα οποία οδηγούν στην αστοχία πολύ πριν αυτό γίνει μακροσκοπικά αντιληπτό. Έγιναν εναλλακτικοί σχεδιασμοί των πειραμάτων εξόλκευσης με στόχο να επιτευχθεί η εξάλειψη των παρασιτικών σημάτων που καταγράφουν οι τεχνικές και οφείλονται στις συνοριακές συνθήκες στήριξης του δοκιμίου αλλά και γενικότερα στην πειραματική διάταξη. Τα αποτελέσματα ήταν ιδιαίτερα ενθαρρυντικά δίνοντας καθαρά σήματα για τη συμπεριφορά της σύνδεσης μαρμάρου - τσιμεντοκονίας - ράβδου τιτανίου αλλά και για το πότε επίκειται η έναρξη της εξόλκευσης της ράβδου. Δεδομένου όμως ότι οι παραπάνω δοκιμές αφορούν σύνθετα συμπλέγματα (κατασκευές όπου συνδυάζονται 3 διαφορετικά υλικά) είναι σαφέστατα πιο πολύπλοκο να διαχωριστούν και να ταυτοποιηθούν οι πηγές των σημάτων. Συνεπώς, κρίθηκε αναγκαίο οι βασικές τεχνικές μη καταστροφικού ελέγχου να εφαρμοστούν κατ’ αρχήν σε απλά τυποποιημένα δοκίμια από μεμονωμένα υλικά που χρησιμοποιούνται στην αποκατάσταση των μνημείων. Έγιναν πειράματα εφελκυσμού σε δύο κατηγορίες δοκιμίων μορφής «Double Edge Notched Τensile (DENT)» από μάρμαρο Διονύσου με συμμετρικές εγκοπές ως προς τον κατακόρυφο άξονα του δοκιμίου. Η θραύση του δοκιμίου καταγράφηκε με τη βοήθεια κάμερας με υψηλή ταχύτητα καταγραφής (high speed) ώστε να εντοπιστεί με βεβαιότητα από ποια εγκοπή ξεκίνησε η ρωγμή και πως αυτή διαδόθηκε. Η τεχνική συσχέτισης ψηφιακής εικόνας (Digital Image Correlation DIC) κατέγραψε τη συνολική μηχανική συμπεριφορά του δοκιμίου και συσχετίστηκε επιτυχώς με τις καταγραφές απλών μηχανικών τεχνικών (clip gauges). Τέλος οι τεχνικές των Ακουστικών εκπομπών (Acoustic Emission AE) και των ηλεκτρικών σημάτων (Pressure Stimulated Currents PSC) έδωσαν ενδιαφέροντα αποτελέσματα πλήρως συσχετιζόμενα με τα αντίστοιχα των υπόλοιπων τεχνικών. Επίσης έγιναν δοκιμές κάμψης 3 σημείων σε δοκίμια από τσιμεντοκονία και κονίαμα σε αναλογίες ίδιες με εκείνες που χρησιμοποιούνται στα έργα αποκατάστασης των μνημείων της Ακρόπολης των Αθηνών. Εφαρμόστηκαν οι τεχνικές των Ακουστικών Εκπομπών AE αλλά και των ηλεκτρικών σημάτων PSC, από τις οποίες προέκυψαν ενδιαφέροντα αποτελέσματα τόσο για το χαρακτηρισμό των σημάτων κατά τη διάρκεια των πειραμάτων όσον και για τον εντοπισμό πιθανών ενδείξεων ότι το υλικό ευρίσκεται κοντά στη αστοχία (όσον το δυνατόν ενωρίτερον σε σχέση με το χρονικό σημείο της τελικής θραύσης). Τέλος, πραγματοποιήθηκαν δοκιμές μονοαξονικής θλίψης και κάμψης τριών σημείων σε δοκίμια από μαργαϊκό ασβεστόλιθο (Αλφάς) εφαρμόζοντας τις τεχνικές των ακουστικών και ηλεκτρικών σημάτων. Το πέτρωμα αλφάς εξορύσσεται σε λατομεία κοντά στο χωριό Αλφά της επαρχίας Μυλοποτάμου (Ρέθυμνο Κρήτης), είναι υπόλευκο και έχει χρησιμοποιηθεί στην δόμηση σειράς μνημειακών κατασκευών στην Κρήτη. Τα πειράματα έδωσαν ικανοποιητικά αποτελέσματα.Βασικός στόχος των παραπάνω πειραμάτων είναι η ταυτοποίηση των σημάτων και η συσχέτιση με τις πηγές από τις οποίες αυτά προέρχονται, σε δοκίμια απλά και ελεγχόμενα ώστε να μπορούν να εφαρμοστούν οι τεχνικές και να ερμηνευτούν τα αποτελέσματα τους σε πειράματα με πλέον σύνθετα και πλέον περίπλοκης γεωμετρίας δοκίμια. Συγκεκριμένα, πραγματοποιήθηκε πείραμα κάμψης 10 σημείων σε προσομοίωμα ενός θραυσμένου επιστυλίου με κεκλιμένο επίπεδο ρήγματος. Για τη συγκόλληση των θραυσμάτων χρησιμοποιήθηκαν 6 ράβδοι τιτανίου και πάστα λευκού τσιμέντου σύμφωνα με την τεχνική που ακολουθείται στα έργα αναστήλωσης και αποκατάστασης των μνημείων της Ακρόπολης. Στο πείραμα…
The present study describes the first stage of an ongoing research project aiming to investigate ... more The present study describes the first stage of an ongoing research project aiming to investigate the failure mechanisms activated when reinforcing bars are pulled-out from the body of the reinforced element. Attention is given to the progressive failure of the interfaces between the constituent elements of the marble-cement paste-titanium (MCT) complex, which appears in case marble structural members of classic stone monuments are restored according to the pioneering technique used on the Acropolis of Athens worksite. Experience from already implemented projects indicates that, when restored structural members are subjected to tension or bending, the failure of these interfaces leads to debonding and finally to catastrophic pull-out of the reinforcing bar from the body of the marble volume. In this direction, a series of pull-out experiments were implemented, with specimens made of Dionysos marble, in the form of prisms of various dimensions with threaded titanium bars inserted into predrilled holes which were then filled with suitable cement paste. The results obtained from these experiments, by employing both conventional and innovative sensing techniques, were used for the calibration of a Finite Element model, which will be used for the thorough investigation of the parameters affecting the overall response of the restored element. The commercial package ABAQUS was used for the implementation of the numerical analysis. The surface contact features of the package were properly exploited in the direction of comparatively exploring and evaluating of possible modeling approaches for simulating the actual interfaces of the marble-cement-titanium complex. A reliable performance law, allowing for the incorporation of damage initiation and evolution criteria, was determined permitting satisfactory modeling of the pull-out behavior at the interface between marble and cement paste. Critical parameters, material properties and modeling choices that have a significant impact on the final outcome were considered. The numerical model finally designed approaches in a very satisfactory manner the experimentally obtained load-displacement curve, providing an easy-to-use, flexible and reliable tool for further study of the pull-out phenomenon.
Procedia structural integrity, 2017
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Frattura ed Integrità Strutturale
Acoustic Emission (AE) is the technique most widely used nowadays for Structural Health Monitorin... more Acoustic Emission (AE) is the technique most widely used nowadays for Structural Health Monitoring (SHM). Application of this technique for continuous SHM of restored elements of stone monuments is a challenging task. The co-existence of different materials creates interfaces rendering “identification” of the signals recorded very complicated. To overcome this difficulty one should have a clear overview of the nature of AE signals recorded when each one of the constituent materials is loaded mechanically. In this direction, an attempt is here described to enlighten the signals recorded, in case a series of structural materials (natural and artificial), extensively used for restoration projects of classic monuments in Greece, are subjected to 3-point bending. It is hoped that obtaining a clear understanding of the nature of AE signals recorded during these elementary tests will provide a valuable tool permitting “identification” and “classification” of signals emitted in case of stru...
The present study describes the first stage of an ongoing research project aiming to investigate ... more The present study describes the first stage of an ongoing research project aiming to investigate the failure mechanisms activated when reinforcing bars are pulled-out from the body of the reinforced element. Attention is given to the progressive failure of the interfaces between the constituent elements of the marble-cement paste-titanium (MCT) complex, which appears in case marble structural members of classic stone monuments are restored according to the pioneering technique used on the Acropolis of Athens worksite. Experience from already implemented projects indicates that, when restored structural members are subjected to tension or bending, the failure of these interfaces leads to debonding and finally to catastrophic pull-out of the reinforcing bar from the body of the marble volume. In this direction, a series of pull-out experiments were implemented, with specimens made of Dionysos marble, in the form of prisms of various dimensions with threaded titanium bars inserted into...
Springer Proceedings in Materials, 2019
Masonry structures are complex systems that require detailed knowledge and information in order t... more Masonry structures are complex systems that require detailed knowledge and information in order to determine their behavior under seismic stimulation. Therefore, an appropriate modeling is a necessity for their reliable design. Moreover, the use of consolidates is a common general preservation action, but no extensive research has been made on historic masonries. In the present paper, an innovative approach was applied in the historic building «Medikon Mansion» in Mani, for the realistic depiction of its behavior under seismic loading. This was done by creating a Finite Element Model which is behavior after the application of three different consolidation proposals of inorganic (HAP) and alkoxysilanes (ES and TEOS) materials was investigated. The FEM was created with the aim to assess appropriate retrofit innovations and materials according to criteria of compatibility and performance. This is an inclusive methodology to approach historic buildings and thus laboratory tests were performed on samples from the local quarry, before the creation of the FEM, in order to create a dynamic analysis of the building in contrast with the most static researches. The analysis showed that the TEOS alkoxysilane proved to be the most effective consolidate. Moreover, the probability of failure due to tensile strength was reduced, but not to a satisfactory degree, in contradiction to the notable reduction that was noticed to the fundamental period (up to 8%) and displacements (up to 30%) of the mansion.
H αποκατάσταση της μονολιθικότητας των δομικών μελών μαρμάρινων μνημείων είναι ένα πρόβλημα το οπ... more H αποκατάσταση της μονολιθικότητας των δομικών μελών μαρμάρινων μνημείων είναι ένα πρόβλημα το οποίο απαιτεί τον συγκερασμό πολλών αντικρουόμενων απόψεων (αρχαιολόγων, αρχιτεκτόνων, πολιτικών μηχανικών, χημικών κ.α.). Το βασικό πρόβλημα είναι ότι στις περισσότερες περιπτώσεις τα μάρμαρα είναι έντονα κατακερματισμένα. Για να αποκατασταθεί η στατική επάρκεια και η δομική αρτιότητα των μνημείων προτάθηκε, πριν μερικές δεκαετίες, μία πρωτοποριακή μέθοδος από τους επιστήμονες οι οποίοι εργάζονταν στα αναστηλωτικά έργα της Ακρόπολης των Αθηνών. Σύμφωνα με αυτή την τεχνική τα μαρμάρινα θραύσματα συγκολλούνται με λευκό τσιμέντο Portland και ενισχύονται με κοχλιοτομημένες ράβδους τιτανίου (Korres et al., 1989). Σκοπός της παρούσας εργασίας είναι να εξετάσει κατά πόσο μπορούν τεχνικές μη καταστροφικού ελέγχου να παρακολουθήσουν τη συμπεριφορά των αποκαταστημένων μνημείων και να εντοπίσουν έγκαιρα σημεία που χρήζουν ιδιαίτερης προσοχής. Κατά το σχεδιασμό των συνδέσεων για την αποκατάσταση της ...
Procedia Structural Integrity, 2016
Journal of Theoretical and Applied Mechanics, 2018
An experimental protocol, including the combined application of both innovative and traditional s... more An experimental protocol, including the combined application of both innovative and traditional sensing techniques, is described aiming to explore the mechanical response of marble and also to check the possibilities of detecting precursor phenomena designating upcoming catastrophic fracture. The protocol consisted of three-point bending tests with notched prismatic beams made of Dionysos marble, the material extensively used for restoration of the Acropolis of Athens monuments. The sensing system improvised included techniques relying on completely different physical foundations, which permit simultaneous detection and recording of the Pressure Stimulated Currents, Acoustic Emissions, three dimensional displacement fields and Notch Mouth Opening Displacements. Analysis of the results revealed interesting features of the mechanical response of Dionysos marble and indicated, also, that classical Continuum Fracture Mechanics fails to describe accurately the response of marble, at least in the presence of notches. In addition, strong correlations between the Pressure Stimulated Currents, the rate of acoustic hits and the rate of change of the opening of the pre-existing notch have been enlightened. Moreover, the onset of catastrophic crack propagation appears following distinguishable changes of the Pressure Stimulated Currents recorded. Therefore (and taking into account the very small size of the respective sensors as well as the simple complementary equipment needed), it is concluded that the specific technique could be considered as a simple and reliable tool for an alternative approach to the in-situ Structural Health Monitoring of classical stone monuments.
Construction and Building Materials, 2018
Direct tension, bending and Brazilian tests provide different tensile strength. The axial stress-... more Direct tension, bending and Brazilian tests provide different tensile strength. The axial stress-strain curve of Dionysos marble strongly deviates from linearity. Critical Notch Opening Displacement appears inappropriate as fracture criterion. Both mode-I and II phenomena appear during direct tension of marble DENT specimens. The AE technique permits crack classification both in simple and complex structures.
Frattura ed Integrità Strutturale, 2019
Fatigue & Fracture of Engineering Materials & Structures, 2017
The potentialities of the pressure stimulated currents technique to be used as a continuous struc... more The potentialities of the pressure stimulated currents technique to be used as a continuous structural health monitoring tool in applications related to stone monuments are assessed experimentally. The data gathered by this technique from experiments with notched marble plates submitted to direct tension are considered in juxtaposition to the respective ones provided by the acoustic emissions technique, a well-founded and widely used sensing tool. The time evolution of the pressure stimulated currents produced provides clearly distinguishable fingerprints of upcoming fracture well before visible cracking is macroscopically observed. These fingerprints are in good qualitative agreement with similar ones provided by the acoustic emissions, and therefore, they can be considered as safe prefailure indicators designating entrance to a "critical stage." Thus, the use of the pressure stimulated currents technique as an alternative structural health monitoring system appears quite promising, taking also into account the low cost and the small size of the respective sensors.
Frattura ed Integrità Strutturale, 2017
The efficiency of two modern sensing techniques, namely the "Acoustic Emissions" and the "Pressur... more The efficiency of two modern sensing techniques, namely the "Acoustic Emissions" and the "Pressure Stimulated Currents" ones, when they are used as Continuous Structural Health Monitoring tools, is assessed experimentally. The protocol includes multi-point bending of an accurate copy of a fractured marble epistyle of the Parthenon's Temple on the Acropolis of Athens, under a scale of 1:3. The integrity of the epistyle is restored with three pairs of bolted titanium bars, according to the pioneering technique developed by the scientists of the "Committee for the Conservation of the Acropolis Monuments". The data provided by the above techniques are considered in juxtaposition to each other and also in comparison to data provided by the "Digital Image Correlation" technique. It is concluded that, at least from a qualitative point of view, the data of all three techniques are in good mutual agreement. Combined exploitation of the various sets of experimental data enlightens interesting aspects concerning the succession of failure mechanisms activated during the loading procedure, revealing the critical role of the internal interfaces characterizing the restored epistyle. Moreover it is definitely indicated that both the "Acoustic Emissions" and the "Pressure Stimulated Currents" techniques provide clear signs of upcoming failure well before macroscopically visible damages are detected at the external surface of the specimen.
Procedia Structural Integrity, 2017
During their operation, modern aircraft engine components are subjected to increasingly demanding... more During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Procedia Structural Integrity, 2017
Procedia Structural Integrity, 2017
Latin American Journal of Solids and Structures, 2016
This work discusses the experimental results of Acoustic Emission (AE) recordings during repetiti... more This work discusses the experimental results of Acoustic Emission (AE) recordings during repetitive loading/unloading loops of cement mortar beams subjected to three point bending. Six repetitive loading cycles were conducted at a gradually higher load level until the failure of the specimens. The experimental results clearly show the existence and dominance of the Kaiser effect during each loading loop. Regarding the AE data, alternative analysis was conducted using the improved b-value, and the cumulative energy behaviour. Both quantities considered, show qualitative and quantitative characteristics that could be used as pre-failure indicators. In addition, a novel statistical physics analysis involving the AE interevent times was conducted by calculating the cumulative probability function P(>δτ) that follows a q-exponential equation. The entropic index q and the relaxation parameter βq of this equation show systematic changes during the various stages of the failure process. The last cycle led to a q value equal to 1.42, implying the upcoming fracture which is in good agreement with previous results obtained from a wide range of fractured materials.
Procedia Structural Integrity, 2016
The present work aims at investigating the mechanisms activated during the extraction of titanium... more The present work aims at investigating the mechanisms activated during the extraction of titanium bars from a marble volume (pull-out phenomenon), used for rejoining fragmented marble structural members at the monuments of the Acropolis of Athens. The restoration protocol includes the insertion of threaded titanium bars into pre-drilled holes in the body of the structural members. The adhesion between marble and bars is achieved by an initially liquid cementitious material. There are two main aspects on which this project focuses. Firstly, the weak link of the marble-cement-titanium complex, the marble-cement interface, is inaccessible for traditional sensing techniques. In this context, the Acoustic Emission technique is employed in order to detect failure and damages at the interior of the complex. Secondly, the experimental specimen and procedure should simulate the same stress conditions in which all three phases within the marble-cement-titanium complex are under, while the structural member is in service. For this purpose, two modified tests are proposed aiming at efficiently simulating the bars' behavior as they are sliding through a marble epistyle. The new tests achieve promising results with regards to keeping the examined surface between bars and marble free from the parasitic stresses caused by the experimental setup .
The present study describes the first stage of an ongoing research project aiming to investigate ... more The present study describes the first stage of an ongoing research project aiming to investigate the failure mechanisms activated when reinforcing bars are pulled-out from the body of the reinforced element. Attention is given to the progressive failure of the interfaces between the constituent elements of the marble-cement paste-titanium (MCT) complex, which appears in case marble structural members of classic stone monuments are restored according to the pioneering technique used on the Acropolis of Athens worksite. Experience from already implemented projects indicates that, when restored structural members are subjected to tension or bending, the failure of these interfaces leads to debonding and finally to catastrophic pull-out of the reinforcing bar from the body of the marble volume. In this direction, a series of pull-out experiments were implemented, with specimens made of Dionysos marble, in the form of prisms of various dimensions with threaded titanium bars inserted into predrilled holes which were then filled with suitable cement paste. The results obtained from these experiments, by employing both conventional and innovative sensing techniques , were used for the calibration of a Finite Element model, which will be used for the thorough investigation of the parameters affecting the overall response of the restored element. The commercial package ABAQUS was used for the implementation of the numerical analysis. The surface contact features of the package were properly exploited in the direction of comparatively exploring and evaluating of possible modeling approaches for simulating the actual interfaces of the marble-cement-titanium complex. A reliable performance law, allowing for the incorporation of damage initiation and evolution criteria, was determined permitting satisfactory modeling of the pull-out behavior at the interface between marble and cement paste. Critical parameters, material properties and modeling choices that have a significant impact on the final outcome were considered. The numerical model finally designed approaches in a very satisfactory manner the experimentally obtained load-displacement curve, providing an easy-to-use, flexible and reliable tool for further study of the pull-out phenomenon.
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Papers by Ioanna Dakanali