Dimitrios E Vlachos

Abstract:

Abstract: Aircraft structures are designed to withstand further to dynamic mechanical loadings thermal loads too. In the event of an external fire (while the aircraft is on the ground) the fuselage structure has to withstand and delay fire penetration. Prolonged burnthrough time is the design target. In the current work, a progressive fire-degradation material model is developed that links decomposition kinetics with the thermophysical properties of polymer composite materials. The material model is then implemented in a FE model to simulate the response of the flat panels under fire burnthrough conditions. Experimental investigation is performed in accordance to the ISO2685:1998 (E) Standard. �© 2011 Springer Science+Business Media B.V.

Abstract: The aircraft fuselage is a complex structure with relatively low margins of safety, operating in a highly demanding dynamic environment. In the present work, the loading scenario of an on board explosion is investigated. The main scope is to assess the damage induced on an operating fuselage by an explosive charge. Finite element models of a typical commercial fuselage were generated for two material configurations, aluminium and GLARE. The simulation was performed in three stages; initialization phase, where the flight loads are applied on the structure, the blast phase and the final phase where the flight loads are applied on the damaged structure. Simulations were performed for different charge locations for both material configurations. The extent and location of damage allowed the generation of a vulnerability index.

Abstract: GLARE is a combination of metal and composite materials (known as fibre metal laminates) that was developed as an alternative for aluminium in aerospace applications due to its superior fatigue characteristics and impact resistance. Since GLARE is increasingly being used for aircraft structural components, an interesting scenario is the blast loading of a GLARE structural panel due to an onboard explosion. In the present work, finite element models were generated for various charge sizes for a typical GLARE configuration used for fuselage skin, and the numerical results were compared against experimental results. The numerical models were generated in a way that each ply was modelled separately, and a tie-break contact algorithm was implemented in order to capture debonding between the various layers. Additionally, an elastic plastic material model was implemented for the aluminium layers, and a composite material model with damage parameters that introduces non-linearities into the material model was considered for the composite layers. �© Institute of Materials, Minerals and Mining 2011.

Abstract: The present work deals with the development of anisotropic damage in alumina/alumina continuous fiber ceramic composites (CFCCs). The composites were isothermally exposed to a corrosive/high temperature environment at 1100 degrees C, which simulates the working conditions of a gas turbine. Stiffness matrix components and strength were experimentally defined as a function of exposure duration by means of ultrasonic stiffness measurements and quasi-static tensile tests. In order to determine the stiffness matrix components, a new ultrasonic stiffness characterisation technique was employed. According to this method, the through transmission phase velocities are measured using a custom built immersion set-up. The experimental data are subsequently used in order to solve the inverse scattering problem and reconstruct the stiffness matrix of the composite at successive thermal exposure levels. The stiffness matrix of the composite was assumed to be orthotropic. Damage functions were formulated to describe the high temperature/corrosive exposure effect on the stiffness matrix of the composite. Finally, quasi-static tensile tests were used to assess the stiffness reduction of the composite and compare the values to those acquired non-destructively. The effect of exposure time on the strength of the composite was determined in the same way. (c) 2005 Elsevier Ltd. All rights reserved.

Abstract: The design and construction is described of a vehicle bridge made of glass-reinforced polyester pultruded box beams. The bridge has a simply supported span 11.6 m long and 4 m wide. It has been designed as a Class 30 (300 kN load capacity) according to DIN 1072 and represents a single traffic lane. The composite bridge consists of a 3-D truss structure made of thick-wall fibre-reinforced plastic longitudinal box elements of hollow square cross-section. The bridge design proposed allows for fast construction as it consists of pre-fabricated, ready to assemble elements. The total of the composite bridge does not exceed 135 kN.

Abstract: The monitoring of the elastic properties of Al2O3/Al2O3 composites during the exposure at high temperature environment that simulates the working conditions of a gas turbine has been performed non-destructively using ultrasonics. The applied methodology is based on velocity measurements of the elastic waves that propagate in an orthotropic medium. These were estimated experimentally using a custom pulser-receiver setup which allows control of the angle of the incident pulse on the sample, while the latter is immersed in a water bath. The derivation of the elastic constants in order to reproduce the stiffness matrix of the composite is an inverse wave propagation problem described by the Christoffel equation. The damage initiation and propagation as depicted by the deterioration of the moduli of the material was described using deterministic and stochastic approaches. Finally, the damage accumulation process was simulated as a Markov process.

Abstract: Continuous Fibre Ceramic (Matrix) Composites (CFCCs) have found during the last decade numerousof industrial applications in a variety of technological areas, where structural components are subjected to high temperature combined with significant mechanical loading. The present work deals with the application of an innovative design methodology for the development of an industrial gas turbine combustor chamber made of oxide/oxide composite materials. Oxide/oxide composites offer high-temperature structural stability without the need of any kind of oxidation protection and thus permit the increase of working temperature of the gas turbine, increasing the efficiency of the system and decreasing NOx emissions. Since, oxide/oxide composites degrade their structural properties as a function of the operating temperature (for temperature higher than 10000 degreesC) and the exposure time, an incremental approach has been introduced as a structural design methodology for the combustion chamber, where each increment represents a thermal exposure stage. The data set required for the application of the present design approach, was obtained through an extensive material characterization program based on the measurement of the anisotropic properties of oxide/oxide composites using ultrasonic techniques.

Abstract: The energy absorption during impact provided by a motorcycle safety helmet is always of critical importance in order to protect the rider against head injury during an accident. In the present study, a parametric analysis has been performed in order to investigate the effect of the composite shell stiffness and the damage development during impact, on the dynamic response of a composite motorcycle safety helmet. This kind of parametric analysis may be used as a tool during helmet design for minimising testing needs. The LS-DYNA3D explicit hydrodynamic finite element code was used to analyse a detailed model of the helmet-headform system (composite shell/foam liner/metallic headform) and to simulate its dynamic response during impact. A significant part of the work was focused on the modelling of the mechanical behaviour of the composite materials, including damage and delamination development. The dynamic response of the different helmet-headform systems was judged in terms of the maximum acceleration monitored at the centre of gravity of the headform and the maximum value of head injury criterion. It was shown that composite shell systems exhibiting lower shear performance provide additional energy absorbing mechanisms and result to better crashworthiness helmet behaviour. (C) 2002 Elsevier Science Ltd. All rights reserved.

Abstract: The validity of indentation tests for the characterization of the mechanical properties of coatings relies greatly on the indentation depth. Deep indentation concludes to unreliable results due to the substrate effect on the measured properties. At shallow depths the size effect can also be an important error factor. The purpose of the present study is the determination of the critical ratio of coating thickness to indentation depth, up to which the substrate properties have a negligible effect on the force versus indentation depth curve and thus on determined mechanical properties of the coating. The analysis required, was conducted using finite element method. A spherical (ball) indenter was used and a three dimensional model of the indenter/coating-substrate system was applied. The effect of the coating to substrate Yield strength ratio, on the critical coating thickness to indentation depth ratio, was investigated for three different coating to substrate Young's modulus ratios. The results of this work provide considerable insight for the determination of the confidence indentation depth during micro-indentation for layered systems with different properties.

Abstract: ��κε�έ� �ο�έ� ��ή�ξαν ε�ι��ημονικέ� ανακαλ��ει� �ο� �ε�ίμεναν �ην δημιο��γία νέ�ν ή �ην βελ�ί��η γν����ν �λικ�ν, ��ο�ο� β�ο�ν ε�α�μογή. Σε άλλε� �ε�ι����ει� �άλι, καινο�ομίε� ��ην �ε�νολογία ��ν �λικ�ν ��ή�ξαν �ο κίνη��ο για �ην ανά���ξη νέ�ν ε�α�μογ�ν και ε�ε��έ�ε�ν. �ίναι ��άγμα�ι εν�����ιακ� �ο ���α α�� �α ��γ��ονα �ε�νολογικά ε�ι�ε�γμα�α ο�είλο�ν �ην ��α�ξή �ο�� ��ην, ���αία �ολλέ� �ο�έ�, ανακάλ��η εν�� ����ο�ο�ιακο� �λικο�, μια� νέα� μεθ�δο� ε�εξε�γα�ία�, ή άγν����ν ιδιο�ή��ν �ε γν��ιμα �λικά. Σήμε�α ο �ομέα� ��ν �λικ�ν α�ο�ελεί ανεξά��η�ο �εδίο ε�ι��ημονική� έ�ε�να� με έν�ονη δ�α��η�ι��η�α. Πα�άλληλα ε�ι�ελεί�αι �ημαν�ικ� έ�γο με ����ο �ην αξιο�οίη�η α�� �ην �ε�νολογία, ��ν ε�ε�νη�ικ�ν α�ο�ελε�μά��ν �η� ε�ι��ήμη� ��ν �λικ�ν. �ία ά�ο�η �η� δ�α��η�ι��η�α� α��ή� είναι η ει�αγ�γή �ε �ει�αμα�ικ� ε�ί�εδο, νέ�ν �λικ�ν �ε γν���έ� ε�α�μογέ�. �ν και α��� α�ο�ελεί ένα �ημαν�ικ� βήμα, �����ο η �ιοθέ�η�η εν�� �λικο� α�� �ην βιομη�ανία με �ην εί�οδ� �ο� ��ην �α�αγ�γή, εξα��ά�αι α�� �ολλο�� �α�άγον�ε�. �να� α�� �ο�� �ιο καθο�ι��ικο��, ειδικά ��ο α�ο�ά ε�α�μογέ� ��ηλή� �ε�νολογία�, είναι η δ�να���η�α ��ο�α�μογή� ��ν διαδικα�ι�ν ��εδια�μο� ��ι� ��νθήκε� �ο� διαμο���νον�αι α�� �α ιδιαί�ε�α �α�ακ�η�ι��ικά �η� ��μ�ε�ι�ο�ά� �ο� �λικο�, �ε ��νδ�α�μ� με �ι� α�αι�ή�ει� �η� ε�α�μογή�. Σε α��� �ο γενικ� �λαί�ιο εν�ά�ε�αι η �α�ο��α ε�γα�ία, η ο�οία έ�ει �� αν�ικείμενο �ην ανά���ξη μεθοδολογία� ��εδια�μο�, κα�άλληλη� για ε�α�μογή �ε ��οι�εία α�� Σ�νθε�α �ε�αμικά Υλικά Σ�νε��ν �ν�ν (Continuous Fiber Ceramic Composites � CFCC�s) κα�α�ονο�μενα α�� θε�μομη�ανικά �ο��ία. Τα Σ�Υ Σ�νε��ν �ν�ν α�ο�ελο�ν εξελιγμένη μο��ή ��ν Π�οηγμέν�ν �ε�αμικ�ν Υλικ�ν, �α ο�οία είναι ί��� �α �λέον ανθεκ�ικά �λικά �ε �ε�ιβάλλον�α ��ηλ�ν θε�μοκ�α�ι�ν �ο� έ�ο�ν ανα����θεί έ�� �ήμε�α. Τα Σ�Υ είναι αν�ικείμενο ��νε�ο�� έ�ε�να� και ανά���ξη� και αναμένε�αι να ε�ι�έ�ο�ν �ημαν�ική ���οδο �ε �ολλο�� �ομεί�. �ία α�� �ι� �ολλέ� ε�α�μογέ� για �ι� ο�οίε� θε��ο�ν�αι ελκ���ικά, είναι οι ���οβιλοκινη�ή�ε� αε�ο�ημά��ν και οι ���οβιλομη�ανέ� �α�αγ�γή� ενέ�γεια�. � αν�ικα�ά��α�η ��ν με�αλλικ�ν κ�αμμά��ν α�� Σ�Υ ��α «θε�μά» μέ�η ��ν ���οβιλομη�αν�ν εκ�ιμά�αι ��ι είναι δ�να�� να ε�ι�έ�ει α�ξη�η �ο� βαθμο� α��δο�η� έ�� και 20% με �α�����ονη μεί��η ��ν εκ�εμ��μεν�ν ����ν. Σ�ην �ε�ιο�ή α��ή ε��ιάζε�αι η �α�ο��ία�η �η� ��ο�ειν�μενη� μεθοδολογία� και ��γκεκ�ιμένα ��ον ��εδια�μ� και �ην ανάλ��η εν�� ������ο� θαλάμο� κα��η� ���οβιλομη�ανή� κα�α�κε�α�μένο� α�� Σ�Υ. � μεθοδολογία α��ή ε�ι�ει�εί να καλ��ει �ι� ιδιαί�ε�ε� α�αι�ή�ει� �η� μον�ελο�οίη�η� ��ν Σ�Υ και �η� ανάλ��η� ��οι�εί�ν κα�α�κε��ν α�� Σ�Υ, �ο� ��οκ���ο�ν α�� �α �α�ακ�η�ι��ικά �η� ��μ�ε�ι�ο�ά� �ο�� ��ι� ��νθήκε� λει�ο��γία� ��ν ���οβιλομη�αν�ν. Ω� βα�ικ� ��οι�είο �η� μεθοδολογία� ��εδια�μο� ��μ�ε�ιλαμβάνε�αι ��ην �α�ο��α ε�γα�ία και η λε��ομε�ή� �ε�ιγ�α�ή �ο� �ει�αμα�ικο� �α�ακ�η�ι�μο� εν�� ���ο� Σ�Υ με μή��α και ενί����η οξειδί�ν. Σ�ην διαδικα�ία α��ή ει�άγε�αι και μια ��������η μέθοδο� �ο�ο�ικο� �α�ακ�η�ι�μο� �ο� ��η�ίζε�αι �ε μη-κα�α���ο�ικέ� δοκιμέ� με ��ή�η ��ε�ή��ν. � διαδικα�ία ε�α�μογή� �η� μεθ�δο�, ��ο α�ο�ά �ο �ει�αμα�ικ� και �ο ��ολογι��ικ� μέ�ο� �η� ανα����θηκε εξ�ολοκλή�ο� ��α �λαί�ια �η� �α�ο��α� ε�γα�ία� και ε�ι��έ�ει �ην ανί�νε��η και �ον �ο�ο�ικ� ��ο�διο�ι�μ� ανι����ο�η� βλάβη� �ε Σ�Υ. � ε�εξε�γα�ία ��ν α�ο�ελε�μά��ν �η� διαδικα�ία� �α�ακ�η�ι�μο�, οδηγεί ��ην διαμ�����η μαθημα�ικο� μον�έλο� �ο� �ε�ιγ�ά�ει �ην μη�ανική ��μ�ε�ι�ο�ά �ο� �λικο� κά�� α�� δεδομένε� ��νθήκε� λει�ο��γία�. Σ�γκεκ�ιμένα, ��ο α�ο�ά �α Σ�Υ με μή��α και ενί����η α�� οξείδιο �ο� αλο�μινίο� ��α ο�οία ε��ιάζε�αι η ε�γα�ία, δια�ι���θηκε ��οβάθμι�η ��ν ελα��ικ�ν ιδιο�ή��ν και �η� αν�ο�ή� λ�γ� �α�αμονή� �ο� �λικο� �ε �ε�ιβάλλον ��ηλή� θε�μοκ�α�ία�. � ��οβάθμι�η α��ή ��ο�διο�ί��ηκε �ο�ο�ικά �αν ��νά��η�η �η� θε�μοκ�α�ία� και �ο� ���νο� έκθε�η�. � μεθοδολογία ��εδια�μο� ��η�ίζε�αι �ε μεγάλο βαθμ� ��ην ανάλ��η και ��ο�ομοί��η �η� λει�ο��γία� ��οι�εί�ν κα�α�κε��ν με �ην μέθοδο ��ν Πε�ε�α�μέν�ν Σ�οι�εί�ν. Σημαν�ικ� μέ�ο� �η� �α�ο��α� ε�γα�ία� α�ο�ά �ην ανά���ξη ��ολογι��ικο� κ�δικα ο ο�οίο� ε�ι��γ�άνει �ην ��ο�α�μογή �ο� ��������ο� μον�έλο� �λικο� �ε εμ�ο�ικ� λογι�μικ� ανάλ��η� με ΠΣ. ��ι����θε�α ο ��ολογι��ικ�� κ�δικα� ε�α�μ�ζει μον�ελο�οίη�η ��οοδε��ική� α��ο�ία� �ο� �ε�ιλαμβάνει ��εί� ξε���ι��ο�� μη�ανι�μο�� δομική� α��ο�ία�. Τέλο�, ο κ�δικα� α���� ��η�ιμο�οιεί�αι ��ην ��ο�ομοί��η �η� λει�ο��γία� εν�� ���ικο� θαλάμο� κα��η� ���οβιλομη�ανή� �α�αγ�γή� ενέ�γεια� �ε ��νθήκε� ��αθε�ή� κα�ά��α�η� (steady state), κα�α�κε�α�μένο� α�� ����μα�ο�οιημένο Σ�Υ ��νε��ν ιν�ν και �α�ο��ιάζον�αι �α α�ο�ελέ�μα�α �η� ανάλ��η�.