Design and numerical modeling of a pressurized airframe bulkhead joint

The structural loading on a conceptual lap joint in the empennage of a civil aircraft has been investigated. The lap joint interfaces the end-pressure part-hemispherical bulkhead to the cylindrical fuselage. The pressure bulkhead is made of carbon fiber reinforced plastic materials. The aim of the study is to present numerical results of the induced structural loading from the fuselage positive internal pressure differential and the localized high stress intensity field at the lap joint location. A methodology for the appropriate numerical approach to analyze the domed pressure bulkhead is presented. The results of the numerical investigation showed that the laminate loading levels calculated by the use of either initial sizing analytical formulas for pressurized domes or by the use of equilibrium nodal loading from finite element models of low fidelity compared to refined finite element analysis can be significantly underestimated. Some of the implications on carbon fiber reinforced plastic structural sizing at the specified location are developed

Μοντελοποίηση ινωδών σύνθετων υλικών και προσδιορισμός των ελαστο-στατικών σταθερών τους με υπολογιστικές μεθόδους.

90 σ.Εθνικό Μετσόβιο Πολυτεχνείο--Μεταπτυχιακή Εργασία. Διεπιστημονικό-Διατμηματικό Πρόγραμμα Μεταπτυχιακών Σπουδών (Δ.Π.Μ.Σ.) “Υπολογιστική Μηχανική”Η ακόλουθη εργασία έχει συνταχθεί στα πλαίσια διερεύνησης της καταλληλότητας προς χρήση της μεθόδου των πεπερασμένων στοιχείων για την μοντελοποίηση και στατική ανάλυση μικρο-μηχανικού μοντέλου από σύνθετα υλικά. Το πρόβλημά μας είναι ο υπολογισμός ελαστο-στατικών μηχανικών ιδιοτήτων ενός ινώδους σύνθετου υλικού μακροσκοπικά, το οποίο απαρτίζεται από ισότροπες και ομογενείς φάσεις με διαφορετικές επιμέρους μηχανικές ιδιότητες. Εν μέσω των παραπάνω θεωρήσεων, προκύπτει και το πρόβλημα της μοντελοποίησης της λεγόμενης ενδιάμεσης φάσης, η οποία έχει μεταβαλλόμενες μηχανικές ιδιότητες βάσει μιας χωρικής συντεταγμένης. Οι μακροσκοπικές μηχανικές ιδιότητες του συνθέτου υλικού θεωρούμενου σαν ένα σώμα που θα προσπαθήσουμε να προσεγγίσουμε με την μέθοδο των πεπερασμένων στοιχείων, είναι: - Το διάμηκες μέτρο ελαστικότητας ΕL (longidudinal modulus of elasticity) - Το εγκάρσιο μέτρο ελαστικότητας ΕT (transverse modulus of elasticity) - Το λόγο Poisson νLT, ο οποίος ορίζεται ως ο λόγος του αντιθέτου της εγκάρσιας τροπής προς την διαμήκη - Το λόγο Poisson νTT, ο οποίος συσχετίζει τις εγκάρσιες τροπές διαφορετικών διευθύνσεων Θα προσπαθήσουμε να διερευνήσουμε την καταλληλότητα της χρήσης των πεπερασμένων στοιχείων συγκρίνοντας: - Αποτελέσματα από αναλυτικές λύσεις στοιχειωδών προβλημάτων με αυτές που προκύπτουν από μοντελοποίηση με πεπερασμένα στοιχεία - Τα αποτελέσματα των αναλυτικών λύσεων και πειραματικών αποτελεσμάτων των δημοσιεύσεων [1] και [2], με αυτά των πεπερασμένων στοιχείων Στο πρώτο κεφάλαιο της παρούσας μελέτης, διατυπώνεται το ελαστο στατικό πρόβλημα προς επίλυση. Γιαννόπουλος Ιωάννης Σελίδα 6 από 90 Στο δεύτερο κεφάλαιο, μελετάται η καταλληλότητα διαφόρων ειδών πεπερασμένων στοιχείων για την επίλυση του προβλήματός μας. Στο τρίτο κεφάλαιο, επιλύονται στοιχειώδεις γεωμετρίες αναλυτικά και συγκρίνονται τα αποτελέσματα τους με αυτά των πεπερασμένων στοιχείων. Στο τέταρτο κεφάλαιο αναλύεται η προσεγγιστική αριθμητική μοντελοποίηση της ενδιάμεσης φάσης του σύνθετου υλικού. Στο πέμπτο κεφάλαιο επιλύεται αριθμητικά το σύνθετο μοντέλο και τα αποτελέσματα της ανάλυσης ως προς το διάμηκες και εγκάρσιο μέτρο ελαστικότητας και λόγου του Poisson, συγκρίνονται με προ υπάρχουσες αναλυτικές λύσεις και πειραματικά αποτελέσματα. Στο έκτο κεφάλαιο βρίσκονται τα συμπεράσματα της μελέτης αυτής καθώς και προτάσεις για περαιτέρω έρευνα.Rresults of the application of finite elements for modeling static analysis of micromechanical composite materials. The result of investigation is the calculation of macroscopic elasto-static mechanical properties of a composite material which comprises of two main isotropic and homogeneous phases, being the fiber and the matrix whilst knowing the properties of the individual constituents. The third phase that is taken into account, the so called mesophase, is the third phase of our composite and its properties vary according to a spatial coordinate via an assumed known analytic function. The macroscopic elastic properties of the composite material under investigation are: - The longidudinal modulus of elasticity, ΕL - The transverse modulus of elasticity, ΕT - The Poisson’s ratio νLT, which is the ratio of the opposite of the transverse to the longitudinal strain - The Poisson’s ratio νTT, which relates the transverse strains of different directions We shall validate our finite element solution by comparing it to: - Analytical solutions from elementary problems - Results from analytical solution already published and relevant test results i.e. [1] & [2] In the first chapter of this study, the elasto static problem to be solved is posed In the second chapter, different element formulations are investigated, that could be employed for our problem. Γιαννόπουλος Ιωάννης Σελίδα 8 από 90 In the third chapter, elementary problems are solved and compared to already known closed form analytical solutions In the fourth chapter, the idealization of the mesophase is explained and the way to be implemented into the FE model. In the fifth chapter, the complete model is solved and various results are drawn with respect to the elasto static properties of the composite material, versus analytic and test results. In the sixth chapter, the study is concluded and future investigations are proposed.Ιωάννης Κ. Γιαννόπουλο

Micromechanical modelling and interfacial strength prediction of multidirectional laminated fibre reinforced polymers

Delamination initiation and propagation is a common failure mode in laminated composites that must be considered when assessing damage in composite structures. Delamination usually propagates at the interface between laminas. Current approved testing procedures address the inter-laminar strength in fracture modes I and II for interfaces of unidirectional laminas oriented in the same direction. The aim of this study was to investigate the interlaminar fracture initiation strength in multi-layer lamina interfaces by the use of micromechanical numerical analysis. Representative volumetric elements with randomly distributed fibres and the ability of numerically modelling fibre-matrix interfacial debonding were generated with different ply interfacial orientations. Failure initiation and damage sequences were captured and the global stresses where failure initiated were determined for the studied configurations. Insights on the variations in the strength observed due to the different lamina orientations were provided

Preliminary analysis method for FRP laminate impact damage size prediction

Low velocity impact damage on carbon reinforced polymer laminate composites has been identified as a key threat to airframe structural integrity since it reduces the strength under compressive loading. Airworthiness certification specifications dictate that the airframe structural components up to the full scale subassemblies have to adhere to the strength and fatigue airworthiness requirements imposed whilst being damaged. The study presented herein combines a set of numerical tools for generating an approach to numerically quantify the damage size after low velocity impact on FRP laminates

Hemodynamic causes of deterioration in acute ischemic stroke

SummaryNeurological deterioration can occur in 13–38% of patients with acute ischemic stroke due to hemodynamic and non-hemodynamic causes. Several non-hemodynamic mechanisms can lead to ischemic lesion extension and subsequent neurological worsening, including infections, cerebral edema, hemorrhagic conversion of infarction and metabolic disorders. The most common hemodynamic causes related to infarct expansion, leading to neurologic deterioration in the setting of acute cerebral ischemia are the following: (i) cardiac complications, (ii) arterial reocclusion, (iii) intracranial arterial steal phenomenon, and (iv) cerebral microembolization. The present review aims to address the underlying mechanisms and potential clinical implications of the hemodynamic causes of neurological deterioration in patients with acute cerebral ischemia. The contribution of neurosonology in detection of changes in cerebral hemodynamics in real-time are also going to be discussed. Finally, potential treatment strategies for specific causes of hemodynamic deterioration in acute ischemic stroke patients are reported

Sensitivity of composite scarf joints to manufacturing deviation and disbond under tensile load

Scarf joints are an effective method of bonding thick composite laminates for applications such as the repair of composite aircraft structures. However, concerns remain about their damage tolerance characteristics. Typically composite scarf repairs to aircraft structures require use of hand tools or rudimentary jigs. If the scarf is incorrectly prepared, this may cause a profile deviation to the joint, affecting the bond line stresses and in turn, reducing the residual strength of the joint or repair. The subject of this work examined the sensitivity of composite scarf joints to machining profile deviation and artificial disbond, when subject to static tensile load. Tensile test specimens were prepared with two different configurations of scarf for representing an undercut or imprecise scarf typical of a machining error. In addition, sensitivity of the scarf joints in the presence of an artificial disbond was also tested. Results indicated that for the specimens tested, the scarf is relatively insensitive to minor profile deviation, but highly sensitive to an artificial disbond. Experimental results were also compared with finite element analysis

The Airbus A320 family fan cowl door safety modification: a human factors scenario analysis

Purpose The Airbus A320 family engine fan cowl doors (FCDs) safety issue is known to the industry for almost 18 years; however, it has not been addressed adequately by the aircraft manufacturer and the various operators and regulating authorities. The purpose of this paper is to examine in a systematic way the possible operational and safety implications of a new modification on the engine FCDs. Design/methodology/approach An array of error-prone scenarios is presented and analysed under the prism of human factors in a non-exhaustive qualitative scenario analysis. Findings All examined scenarios are considered more or less probable. A number of accident prevention solutions are proposed for each of the scenario examined, in view of the acceptance and implementation of this modification by operators. Research limitations/implications As these scenarios are neither exhaustive nor have been tested/validated in actual aircraft maintenance practice, the further analysis is necessary. A substantial follow-up survey should take place, which should include a wider array of scenarios. This would allow obtaining the necessary data for a quantitative (statistical) analysis. Practical implications This case study identifies issues in relation to this modification, introduced by Airbus and the European Aviation Safety Agency (EASA), which may prove problematic from the point of view of safety effectiveness and disruption of operations. Originality/value This case study examines a long-standing aviation safety issue and the implications of a solution proposed by the aircraft manufacturer and adopted by EASA. This can be useful in increasing the awareness around these issues and highlight the importance of a human-centric and scenario-based design of engineering modifications towards minimising error in aircraft technical operations

Uplink NOMA for UAV-Aided Maritime Internet-of-Things

Maritime activities are vital for economic growth, being further accelerated by various emerging maritime Internet of Things (IoT) use cases, including smart ports, autonomous navigation, and ocean monitoring systems. However, broadband, low-delay, and reliable wireless connectivity to the ever-increasing number of vessels, buoys, platforms and sensors in maritime communication networks (MCNs) has not yet been achieved. Towards this end, the integration of unmanned aerial vehicles (UAVs) in MCNs provides an aerial dimension to current deployments, relying on shore-based base stations (BSs) with limited coverage and satellite links with high latency. In this work, a maritime IoT topology is examined where direct uplink communication with a shore BS cannot be established due to excessive pathloss. In this context, we employ multiple UAVs for end-to-end connectivity, simultaneously receiving data from the maritime IoT nodes, following the non-orthogonal multiple access (NOMA) paradigm. In contrast to other UAV-aided NOMA schemes in maritime settings, dynamic decoding ordering at the UAVs is used to improve the performance of successive interference cancellation (SIC), considering the rate requirements and the channel state information (CSI) of each maritime node towards the UAVs. Moreover, the UAVs are equipped with buffers to store data and provide increased degrees of freedom in opportunistic UAV selection. Simulations reveal that the proposed opportunistic UAV-aided NOMA improves the average sum-rate of NOMA-based maritime IoT communication, leveraging the dynamic decoding ordering and caching capabilities of the UAVs

Do You Need Instructions Again? Predicting Wayfinding Instruction Demand

The demand for instructions during wayfinding, defined as the frequency of requesting instructions for each decision point, can be considered as an important indicator of the internal cognitive processes during wayfinding. This demand can be a consequence of the mental state of feeling lost, being uncertain, mind wandering, having difficulty following the route, etc. Therefore, it can be of great importance for theoretical cognitive studies on human perception of the environment. From an application perspective, this demand can be used as a measure of the effectiveness of the navigation assistance system. It is therefore worthwhile to be able to predict this demand and also to know what factors trigger it. This paper takes a step in this direction by reporting a successful prediction of instruction demand (accuracy of 78.4%) in a real-world wayfinding experiment with 45 participants, and interpreting the environmental, user, instructional, and gaze-related features that caused it

The parameterized complexity of some geometric problems in unbounded dimension

We study the parameterized complexity of the following fundamental geometric problems with respect to the dimension $d$: i) Given $n$ points in \Rd, compute their minimum enclosing cylinder. ii) Given two $n$-point sets in \Rd, decide whether they can be separated by two hyperplanes. iii) Given a system of $n$ linear inequalities with $d$ variables, find a maximum-size feasible subsystem. We show that (the decision versions of) all these problems are W[1]-hard when parameterized by the dimension $d$. %and hence not solvable in ${O}(f(d)n^c)$ time, for any computable function $f$ and constant $c$ %(unless FPT=W[1]). Our reductions also give a $n^{\Omega(d)}$-time lower bound (under the Exponential Time Hypothesis)