Additive Manufacturing of 17-4PH Alloy: Tailoring the Printing Orientation for Enhanced Aerospace Application Performance

Additive manufacturing (AM) is one of the fastest-growing markets of our time. During its journey in the past 30 years, its key to success has been that it can easily produce extremely complex shapes and is not limited by tooling problems when a change in geometry is desired. This flexibility leads to possible solutions for creating lightweight structural elements while keeping the mechanical properties at a stable reserve factor value. In the aerospace industry, several kinds of structural elements for fuselage and wing parts are made from different kinds of steel alloys, such as 17-4PH stainless steel, which are usually milled from a block material made using conventional processing (CP) methods. However, these approaches are limited when a relatively small element must withstand greater forces that can occur during flight. AM can bridge this problem with a new perspective, mainly using thin walls and complex shapes while maintaining the ideal sizes. The downside of the elements made using AM is that the quality of the final product is highly dependent on the build/printing orientation, an issue extensively studied and addressed by researchers in the field. During flight, some components may experience forces that predominantly act in a single direction. With this in mind, we created samples with the desired orientation to maximize material properties in a specific direction. The goal of this study was to demonstrate that an additively manufactured part, produced using laser powder bed fusion (LPBF), with a desired build orientation has exceptional properties compared to parts produced via conventional methods. To assess the impact of the build orientation on the LPBF parts’ properties, one-dimensional tensile and dynamic fracture toughness tests were deployed

Assessment of the Impact of Material Selection on Aviation Sustainability, from a Circular Economy Perspective

Climate change and global warming pose great sustainability challenges to the aviation industry. Alternatives to petroleum-based fuels (hydrogen, natural gas, etc.) have emerged as promising aviation fuels for future aircraft. The present study aimed to contribute to the understanding of the impact of material selection on aviation sustainability, accounting for the type of fuel implemented and circular economy aspects. In this context, a decision support tool was introduced to aid decision-makers and relevant stakeholders to identify and select the best-performing materials that meet their defined needs and preferences, expressed through a finite set of conflicting criteria associated with ecological, economic, and circularity aspects. The proposed tool integrates life-cycle-based metrics extending to both ecological and economical dimensions and a proposed circular economy indicator (CEI) focused on the material/component level and linked to its quality characteristics, which also accounts for the quality degradation of materials which have undergone one or more recycling loops. The tool is coupled with a multi-criteria decision analysis (MCDA) methodology in order to reduce subjectivity when determining the importance of each of the considered criteria

Πειραματική μελέτη της επίδρασης ατελειών, μη ανιχνεύσιμων με τεχνικές μη καταστροφικού ελέγχου, στη δυσθραυστότητα αεροπορικών συνδέσεων συνθέτων υλικών με κόλλα

Adhesive bonding is a desirable joining technique as compared to conventional mechanical fastening, especially for aircraft components made from composite or polymeric material, as it presents some significant advantages including uniform distribution of the load, attractive strength to weight ratio and design flexibility. However, the application of this technology is limited to the assembly of less critical aircraft structures due to limited strength of bonded joints, the requirement to redesign the joints so that they are subjected to shear, difficulties in inspecting bondline quality, sensitivity of bondline integrity to environmental attack as well as the condition of the adherend surface prior to bonding which may result in the formation of weak bonds at the interface. Particularly for the case of aeronautical applications, the parameters which may act as contaminants, thus leading to weak bonds, have not yet been fully identified and their effects on the integrity of the bondline have not been quantified. The present work aims to contribute on identifying these contaminating factors and quantify the effect of weak bonds on the mode I fracture toughness of adhesively bonded composite joints. The investigation focuses on contaminating factors and resulting defects which are not detectable by means of conventional non destructive testing techniques, such as ultrasonic inspection or radiography. To accomplish this, an extensive experimental investigation has been conducted. The contaminating factors have been classified with regard to their origin, i.e. whether they are related to the manufacturing process of the parts to be joined or to the in-service conditions of the joined structure. Five different parameters have been considered, namely pre-bond moisture, release agent, thermal degradation, a hydraulic fluid and curing temperature. The above factors were found to generally degrade the fracture toughness of the bonded joints; that was also confirmed by the statistical ANOVA analysis that followed. Where feasible, identification and quantification of the defects by means of advanced/hybrid NDT techniques was carried out and a correlation between the results obtained from these techniques and the mechanical testing results for every investigated contamination scenario was made. To support and understand the experimental results, an effort was made to relate the obtained results to the underlying physics of the contamination process. Finally, a representative aircraft service-related scenario concerning the synergistic influence of the quality-reduction factors studied herein and an environment of a specific relative humidity and temperature, which is likely to happen during aircraft service, was considered. The specific environmental conditions examined herein were found to enhance the fracture toughness rather than degrading it.The results of this study will contribute to i) the identification of a number of parameters, related either to the manufacturing process or to the in-service conditions of the aircraft structure expected to cause contamination ii) the quantification of the degradation of mode I fracture toughness of the composite bonded joints, iii) evaluation of the suitability of advanced/hybrid NDT techniques for the detection of weak bonds caused by the factors considered herein and assist on their further development, iv) understanding of the underlying mechanisms resulting to the observed degradation of the mechanical behavior, v) motivating the development of numerical models capable of predicting the mechanical behavior of bonded joints under realistic aircraft-related conditions, and finally vi) emphasizing the need for the establishment of standards targeted at realistic aeronautics conditions under which an adhesively bonded joint is loaded.Μέχρι σήμερα, η συναρμολόγηση κύριων αεροπορικών δομών από σύνθετα υλικά γίνεται μέσω μηχανικών συνδέσεων. Η συγκεκριμένη κατασκευαστική μεθοδολογία οδηγεί μεν σε ασφαλείς συνδέσεις, οδηγεί όμως συγχρόνως και στην αύξηση του βάρους της κατασκευής, που προκύπτει από την υπερδιαστασιολόγηση των συνδεόμενων μερών και τη χρήση μεγάλου αριθμού ήλων, καθώς και σε συγκεντρώσεις τάσεων στις οπές, κτλ. Μια ενδιαφέρουσα και πολλά υποσχόμενη εναλλακτική μέθοδος για την σύνδεση αεροπορικών δομών είναι η σύνδεση με κόλλα. Για το λόγο αυτό, η ανάπτυξη της τεχνολογίας συνδέσεων σύνθετων υλικών με κόλλες αποτελεί ένα τομέα στην αεροναυπηγική όπου διεξάγεται εκτεταμένη έρευνα. Στα πλεονεκτήματα των συνδέσεων με κόλλα έναντι των μηχανικών συνδέσεων περιλαμβάνονται η εξοικονόμηση βάρους και κόστους λόγω της αποφυγής της υπερδιαστασιολόγησης και η πιο ομοιόμορφη κατανομή τάσεων κατά μήκος της σύνδεσης. Παρ’όλα αυτά, η εφαρμογή αυτής της τεχνολογίας περιορίζεται στην σύνδεση δευτερευουσών αεροπορικών δομών όπου πιθανή αστοχία της σύνδεσης δεν θέτει σε κίνδυνο την δομική ακεραιότητα της αεροπορικής δομής και κατ’επέκταση του αεροσκάφους. Οι λόγοι της περιορισμένης χρήσης συνδέσεων με κόλλα σε κύριες αεροπορικές δομές είναι πολλοί και περιλαμβάνουν την μειωμένη αντοχή των συνδέσεων τέτοιου τύπου και την απαίτηση επανασχεδιασμού ώστε το φορτίο ανάμεσα στα συνδεόμενα μέρη να μεταφέρεται με διατμητικές δυνάμεις, την εξάρτηση της διεπιφανειακής αντοχής από την κατάσταση (φυσικοχημικές ιδιότητες) της επιφάνειας του σύνθετου υλικού πριν την σύνδεση (π.χ. λόγω έκθεσης του σύνθετου υλικού σε επιθετικό για το υλικό περιβάλλον), την ανεπάρκεια των τεχνικών μη καταστροφικού ελέγχου (ΜΚΕ) να ανιχνεύσουν ορισμένους τύπους ατελειών που παρατηρούνται στις συνδέσεις και οδηγούν σε μειωμένες μηχανικές ιδιότητες, κτλ. Στην παρούσα εργασία μελετήθηκε η επίδραση μιας σειράς παραμέτρων οι οποίες υποβαθμίζουν την ποιότητα της σύνδεσης δομικών μερών του αεροσκάφους, οδηγώντας πολλές φορές σε ατέλειες μη ανιχνεύσιμες από τις συμβατικές τεχνικές ΜΚΕ. Οι παράμετροι αυτές είναι: προϋπάρχουσα υγρασία στο σύνθετο υλικό, αποκολλητική ουσία, προϋπάρχον αεροναυπηγικό λάδι, θερμική υποβάθμιση του σύνθετου υλικού πριν την κόλληση και θερμοκρασία πολυμερισμού. Η δημιουργία ατελειών από τις παραπάνω παραμέτρους μπορεί να προκύψει λόγω προσβολής της επιφάνειας κατά την κατασκευή των δομικών μερών, κατά την σύνδεσή τους σε μια μεγαλύτερη δομή, κατά την λειτουργία της δομής αυτής ή κατά την επισκευή της. Ας σημειωθεί ότι μέχρι τώρα δεν έχει αναγνωριστεί το σύνολο των παραμέτρων αυτών, ενώ και οι συνέπειες των παραμέτρων που έχουν αναγνωριστεί δεν έχουν μελετηθεί επαρκώς; επιπρόσθετα, οι σχετικές μελέτες δεν είναι προσανατολισμένες σε αεροναυπηγικές εφαρμογές. Ως συμβολή για την αντιμετώπιση του παραπάνω προβλήματος, ταυτοποιήθηκε και ταξινομήθηκε μια σειρά παραμέτρων που δημιουργούν βλάβες μη ανιχνεύσιμες από συμβατικές τεχνικές ΜΚΕ ενώ στη συνέχεια πραγματοποιήθηκε συστηματική μελέτη και ποσοτικοποίηση της επίδρασής της κάθε μίας από τις παραμέτρους αυτές στην μηχανική συμπεριφορά της σύνδεσης, ιδιαίτερα κάτω από συνθήκες που αφορούν σε αεροναυπηγικές εφαρμογές. Αρχικά έγινε ποσοτικοποίηση των ατελειών μέσω αναλυτικών εργαστηριακών μεθόδων (XRF, IR, XPS). Ακολούθησε μη καταστροφικός έλεγχος των συνδέσεων μέσω υπερήχων (C-scan) και ακτίνων Χ ο οποίος επιβεβαίωσε ότι οι παράμετροι που εξετάστηκαν οδηγούν σε ατέλειες μη ανιχνεύσιμες από συμβατικές τεχνικές ΜΚΕ. Η αντοχή των συνδέσεων μετρήθηκε μέσω πειραμάτων δυσθραυστότητας τύπου Ι (mode I) και ακολούθησε στατιστική επεξεργασία των αποτελεσμάτων με τη μέθοδο ANOVA προκειμένου να ανιχνευθούν οι πιθανές διαφορές μεταξύ των διαφορετικών ποσοτικοποιημένων ατελειών για κάθε μία παράμετρο που εξετάστηκε. Οι παράμετροι που εξετάστηκαν στην παρούσα διατριβή βρέθηκαν να υποβαθμίζουν γενικά την αντοχή των συνδέσεων, κάτι το οποίο επιβεβαιώθηκε και από τα αποτελέσματα της στατιστικής επεξεργασίας με τη μέθοδο ANOVA. Εν συνέχεια, έγινε αναγνώριση του τύπου αστοχίας που προκαλεί κάθε μία παράμετρος και περιγραφή των φυσικών μηχανισμών που οδήγησαν στην υποβάθμιση της ποιότητας της σύνδεσης. Επιπρόσθετα, έγινε μια προσπάθεια συσχέτισης των πειραματικών αποτελεσμάτων με αποτελέσματα προηγμένων/υβριδικών τεχνικών ΜΚΕ οι οποίες είναι προσαρμοσμένες στην ανίχνευση βλαβών μη ανιχνεύσιμων με συμβατικές μη καταστροφικές τεχνικές, όπως αυτές που προκαλούν οι παράμετροι που εξετάστηκαν στην παρούσα διατριβή; τα συμπεράσματα που προέκυψαν από αυτή την προσπάθεια είναι αρκετά ενθαρρυντικά. Τέλος, εξετάστηκε η επίδραση των παραμέτρων που μελετήθηκαν σε συνδυασμό με ένα αντιπροσωπευτικό περιβάλλον υγρασίας και θερμοκρασίας που αφορά πραγματικές συνθήκες λειτουργίας αεροναυπηγικών δομών. Το συγκεκριμένο αντιπροσωπευτικό περιβάλλον βρέθηκε να οδηγεί σε αύξηση της αντοχής των συνδέσεων παρά σε υποβάθμισή της.Η κύρια συμβολή και πρωτοτυπία της διατριβής εντοπίζεται στην ταυτοποίηση μιας σειράς παραγόντων που δημιουργούν ατέλειες μη ανιχνεύσιμες από συμβατικές τεχνικές ΜΚΕ και οι οποίοι οδηγούν σε υποβάθμιση της αντοχής των συνδέσεων με κόλλα, καθώς και στον προσδιορισμό της επίδρασης των παραπάνω ατελειών στην δυσθραυστότητα των συνδέσεων.Επιμέρους πρωτότυπα στοιχεία της διατριβής κρίνονται τα ακόλουθα: Ταξινόμηση των παραμέτρων (που αφορούν είτε την παραγωγή και την σύνδεση δομικών στοιχείων είτε την επισκευή είτε τις συνθήκες λειτουργίας των συνδέσεων) οι οποίες δημιουργούν ατέλειες μη ανιχνεύσιμες από συμβατικές τεχνικές ΜΚΕ, καθώς και συστηματική μελέτη της επίδρασής τους στην δυσθραυστότητα τύπου Ι της σύνδεσης, ιδιαίτερα για συνθήκες οι οποίες είναι ρεαλιστικές για αεροναυπηγικές δομές.Συμβολή στην κατανόηση και περιγραφή των φυσικών μηχανισμών που οδηγούν στην υποβάθμιση και αστοχία των συνδέσεων με κόλλα λόγω της ύπαρξης των παραπάνω ατελειών.Διαπίστωση της μη επάρκειας των υπαρχουσών προδιαγραφών για τον έλεγχο της δυσθραυστότητας συνδέσεων αεροναυπηγικών δομών με κόλλα

Assessment of the Impact of Material Selection on Aviation Sustainability, from a Circular Economy Perspective

Climate change and global warming pose great sustainability challenges to the aviation industry. Alternatives to petroleum-based fuels (hydrogen, natural gas, etc.) have emerged as promising aviation fuels for future aircraft. The present study aimed to contribute to the understanding of the impact of material selection on aviation sustainability, accounting for the type of fuel implemented and circular economy aspects. In this context, a decision support tool was introduced to aid decision-makers and relevant stakeholders to identify and select the best-performing materials that meet their defined needs and preferences, expressed through a finite set of conflicting criteria associated with ecological, economic, and circularity aspects. The proposed tool integrates life-cycle-based metrics extending to both ecological and economical dimensions and a proposed circular economy indicator (CEI) focused on the material/component level and linked to its quality characteristics, which also accounts for the quality degradation of materials which have undergone one or more recycling loops. The tool is coupled with a multi-criteria decision analysis (MCDA) methodology in order to reduce subjectivity when determining the importance of each of the considered criteria

Sensitivity Analysis of a Hybrid MCDM Model for Sustainability Assessment—An Example from the Aviation Industry

When it comes to achieving sustainability and circular economy objectives, multi-criteria decision-making (MCDM) tools can be of aid in supporting decision-makers to reach a satisfying solution, especially when conflicting criteria are present. In a previous work of the authors, a hybrid MCDM tool was introduced to support the selection of sustainable materials in aviation. The reliability of an MCDM tool depends decisively on its robustness. Hence, in the present work, the robustness of the aforementioned tool has been assessed by conducting an extensive sensitivity analysis. To this end, the extent to which the results are affected by the normalization method involved in the proposed MCDM tool is examined. In addition, the sensitivity of the final output to the weights’ variation as well as to the data values variation has been investigated towards monitoring the stability of the tool in terms of the final ranking obtained. In order to carry out the analysis, a case study from the aviation industry has been considered. In the current study, carbon fiber reinforced plastics (CFRP) components, both virgin and recycled, are assessed and compared with regard to their sustainability by accounting for metrics linked to their whole lifecycle. The latter assessment also accounts for the impact of the fuel type utilized during the use phase of the components. The results show that the proposed tool provides an effective and robust method for the evaluation of the sustainability of aircraft components. Moreover, the present work can provide answers to questions raised concerning the adequacy of the CFRP recycled parts performance and their expected contribution towards sustainability and circular economy goals in aviation

A Novel Process to Produce Ti Parts from Powder Metallurgy with Advanced Properties for Aeronautical Applications

Titanium and its alloys have excellent corrosion resistance, heat, and fatigue tolerance, and their strength-to-weight ratio is one of the highest among metals. This combination of properties makes them ideal for aerospace applications; however, high manufacturing costs hinder their widespread use compared to other metals such as aluminum alloys and steels. Powder metallurgy (PM) is a greener and more cost and energy-efficient method for the production of near-net-shape parts compared to traditional ingot metallurgy, especially for titanium parts. In addition, it allows us to synthesize special microstructures, which result in outstanding mechanical properties without the need for alloying elements. The most commonly used Ti alloy is the Ti6Al4V grade 5. This workhorse alloy ensures outstanding mechanical properties, demonstrating a strength which is at least twice that of commercially pure titanium (CP-Ti) grade 2 and comparable to the strength of hardened stainless steels. In the present research, different mixtures of both milled and unmilled Cp-Ti grade 2 powder were utilized using the PM method, aiming to synthesize samples with high mechanical properties comparable to those of high-strength alloys such as Ti6Al4V. The results showed that the fine nanoparticles significantly enhanced the strength of the material, while in several cases the material exceeded the values of the Ti6Al4V alloy. The produced sample exhibited a maximum compressive yield strength (1492 MPa), contained 10 wt.% of fine (milled) particles (average particle size: 3 μm) and was sintered at 900 °C for one hour

A holistic End-of-Life (EoL) Index for the quantitative impact assessment of CFRP waste recycling techniques

In the present study, a holistic End-of-Life (EoL) Index is introduced to serve as a decision support tool for choosing the optimal recycling process among a number of alternative recycling techniques of CFRP waste. For the choice of the optimal recycling process, quality of the recycled fibers as well as cost and environmental impact of the recycling methods under consideration, are accounted for. Quality is interpreted as the reusability potential of the recycled fibers; that is quantified through the equivalent volume fraction of recycled fibers that balances the mechanical properties of a composite composed of a certain volume fraction of virgin fibers. The proposed Index is offering an estimated balanced score, quantifying a trade-off between the reusability potential of the recycled fibers as well as the cost and the environmental impact of the recycling methods considered

On using network RAM as a non‐volatile buffer

Summarization: File systems and databases usually make several synchronous disk write accesses in order to make sure that the disk always has a consistent view of their data, so that it can be recovered in the case of a system crash. Since synchronous disk operations are slow, some systems choose to employ asynchronous disk write operations that improve performance at the cost of low reliability: in case of a system crash all data that have not yet been written to disk are lost. In this paper we describe a software‐based Non‐Volatile RAM system that achieves the high performance of asynchronous write operations without sacrificing the reliability of synchronous write operations. Our system takes a set of volatile main memories residing in independent workstations and transforms it into a non‐volatile memory buffer – much like RAIDS do with magnetic disks. It then uses this non‐volatile buffer as an intermediate storage space in order to acknowledge synchronous write operations before actually writing the data to magnetic disk, but after writing the data to (intermediate) stable storage. We demonstrate the performance advantages of our system using both simulation and experimental evaluation.Presented on: Cluster Computin

On using reliable network RAM in networks of workstations

Summarization: File systems and databases usually make several synchronous disk write accesses in order to make sure that the disk always has a consistent view of their data, and that data can be recovered in the case of a system crash. Since synchronous disk operations are slow, some systems choose to employ asynchronous disk write operations, at the cost of low reliability: in case of a system crash all data that have not yet been written to disk are lost.In this paper we describe a software-based approach into using the network memory in a workstation cluster as a layer of Non-Volatile memory (NVRAM). Our approach takes a set of volatile main memories residing in independent workstations and transforms it into a fault-tolerant memory - much like RAIDS do with magnetic disks. This layer of NVRAM allows us to create systems that combine the reliability of synchronous disk accesses with the cost of asynchronous disk accesses. We demonstrate the applicability of our approach by integrating it into existing database systems, and by developing novel systems from the ground up.We use experimental evaluation using well-known database benchmarks and detailed simulation to characterize the performance of our systems. Our experiments suggest that our approach may improve performance by as much as two orders of magnitude.Presented on: Parallel and Distributed Computing and Network