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

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

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

Liquid hydrogen storage tank loading generation for civil aircraft damage tolerance analysis

The study presented is a preliminary approach and a proposal to the derivation of a loading spectrum for fatigue and damage tolerance analysis for civil aviation Liquid Hydrogen storage tanks. It is anticipated for the first generation of LH2 storage tanks for aviation to utilize metallic lightweight materials. Existing solutions are either too structurally heavy or with a short life span, both constraints making them unsuitable for aircraft vehicles were less mass and longevity is of paramount importance. The objective of the work was to provide suggestions for the generation of representative loading spectra for storage tank fatigue and damage tolerance preliminary design analysis and sizing

Effects of bolt torque tightening on the strength and fatigue life of airframe FRP laminate bolted joints

The experimental study presented herein, investigated the effects of bolt torque tightening on the strength and fatigue design of bolted AS7/8552 fibre reinforced polymer laminates. Damage initiation and final failure manifestation on the joints was investigated and presented using optical microscopy. Subsequent experimental result analysis explored the application domain of bolted joints within the airframe design sector, bound by the current airworthiness certification requirements and expected airframe design life. The reasons for the static strength of the joint laminates or the fatigue failure of the bolt being the main design drivers for the tested joints were highlighted. The study concluded with comments and suggestions on the application of bolt torque tightening in relation to the strength, fatigue life and damage tolerance characteristics of joints on similar fibre reinforced polymer laminate composite material systems

Ballistic impact and virtual testing of woven FRP laminates

The aim of the work was to investigate the numerical simulations correlation with the experimental behaviour of steel ball high velocity impact onto a 2 × 2 twill woven carbon composite laminate. The experimental set up consisted of a pressurised gas-gun able to shot steel ball projectiles onto two different composite plate layup configurations of plates made of the same composite material fabric. Subsequently, the experiments were replicated using the LSDYNA explicit finite element analysis software package. Progressive failure numerical models of two different fidelity levels were constructed. The higher fidelity model was simulating each of the plys of the composite panels separately, tied together using cohesive zone modelling properties. The lower fidelity model consisted of a single layer plate with artificial integration points for each ply. The simulation results came out to be in satisfactory agreement with the experimental ones. While the delamination extent was moderately under predicted by the higher fidelity model, the general behaviour was complying with the experimental results. The lower fidelity model was consistent in representing the damage of the panel during the impact and better predicted the impactor residual velocities due to the better matching of the pane stiffness. Despite the competency of the higher fidelity model to capture the damage of the laminate in a more detailed level, the computational cost was 80% higher than the lower fidelity case, which rendered that model impractical against the lower fidelity one, to use in larger models representing more substantial or more complex structures