Institute of Lightweight StructuresNew structural concepts – structural simulation and design optimization methods –experimental structural and material investigations

n A highlight in 2015 was the finalization of the laboratory model of an advanced large, precise and also structurally deployable space antenna reflector, which is now under rigorous testing in the institute’s labs and at ESA. Its concept has been derived and investigated in recent years from design considerations, large scale mechanical-thermal-electrical simula- tions as well as special material developments and investigations.

For the scaled demonstrator of the Sagitta advanced unmanned aerial vehicle, the fiber composite structure developed in cooperation with DLRhas been built and delivered for further

testing to the project coordinator Airbus. It will be used as a flying testbed for several advanced aerospace techno- logies contributed by different GermanUniversities and DLR.

Univ. Prof. Dr.-Ing. Horst Baier

The Sagitta Unmanned Aerial Vehicle

The advanced Sagitta unmanned aerial vehicle (UAV) is a research and demonst- rator project under the lead of Airbus DS with contributions from several German universities and DLR. This project combi- nes different research studies specifically relevant for such UAV together withthe full aircraft development process. Its carbon fiber structure developed in cooperation of LLB together with DLRhas been delivered to the project

Contact

www.llb.mw.tum.de info@llb.mw.tum.de Phone +49.89.289.16103

coordinator Airbus, where further advanced technical contributions from several Universities and from DLR are integrated for flight testing and demons- trations. Because of the relatively high flight speed and advanced communication and control methods, this testing will be also quite challenging.

Bottom view onto Sagitta during hardware integration

Top view into Sagitta withouts its upper CFRP skin(CAD drawing)

In addition to the demonstrator and flight model structure, investigations have been carried out by LLB on shape morphing rear wing parts of this delta wing aircraft. In-flight aerodynamics and agility shall be further improved, while noise and radar visibility shall be reduced. Like in other cases of morphing structures, certain highly flexible degrees of freedom are generated to allow significant shape changes while at the same time beingable to take significant flight loads. Derived shape morphing parts achieve their deformability in certain degrees of freedom like in shear,

while all the other degrees of freedom like those for bending are sufficiently stiff to take high loads and also to satisfy e.g. aero-elastic require- ments.

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Part of Sagitta’s morphing rearwing

Scaled morphing demonstration part generated by an additive manufacturing process

Large Space Antenna Reflectors

Generic space antenna reflector

Large space antenna reflectors are to be used for communication, radio astronomy and earth observation. Due to their size, they have to be stowed for launch and precisely deployed in orbit, where then only little deviations from the ideal surface shape are allowed even under extreme temperature variations. After years of investigations mainly funded by ESA,LLB established a large laboratory model which has been accepted by ESA for further rigorous tests. Emphasis is put on thermo-elastic behavior at higher and very low temperatures (-170º C).

Laboratory test model in gravity compensation device

Shape Morphing Structures

While in operation, (aerospace) structures more or less keep their shape. This then means that their shape designed for certain relevant but more or less fixed‘design points’ loses its efficiency at other conditions. This for example holds for aircraft components and structures in their flow fields, or for those in electromagnetic fields for space antennas with radiated electromagnetic beams patterns to be modified. These are a quite challeng-ing tasks, because on the one side high loads or high shape precision have to be taken or maintained, while on the other

A view on a generic shape morphing inlet of an aircraft propulsion system actuated by varying pneumatic pressure

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side the significant shape changes call for special mechanical flexibility also in order to limit the required actuation effort. Pro- per concepts are identified via a synthesis between design and material concepts together with sophisticated simulation and experimental steps. For example, special material models have to be combinedwith structural models allowing for large geometrical deformations.

LLB is working in different potential application fields for shape morphing structures:n Shape morphing rear aircraft wings or

wing trailing edges, which shall reduce aerodynamic drag, noise and radar visibility, like for Sagitta mentioned above

n Shape morphing inlets of aircraftpropulsion systems, which adapt their geometric shape to different oper- ational conditions for takeoff, landing and cruise. An increase of propulsion efficiency and noise reduction shallbe achieved (see project MorphElle described below).

n Shape morphing space antenna reflec-tors, where due to (drastic) change in the shape of the reflecting surface the

radiated electromagnetic wave pattern can be adapted to changing communi- cation needs between different regions of the earth.

In the EU project MorphElle on shape morphing inlets of aircraft propulsion systems, different European university institutes, Bauhaus Luftfahrt (BHL) and an industrial advisory committee have investigated different concepts.

The basic goals for this are the further improvement of efficiency by an even more uniform airflow inside the inlet at different flight conditions, where at thesame time radiated noise is to be reduced. Derived from system requirements established by BHL and air flow and noise simulations carried out by KTH Stock- holm, LLB together with University of Bristol established, simulated and tested design concepts in combination with new types of materials. An important key is the development of shape morphing skins. They consist of special metal wire meshes imbedded into an elastomer, thus allow-ing significant pneumatically actuated

shape changes via its low in-plane shear stiffness. Different multi-scale simulation and homogenization steps allowed model- based optimization iteration loops to achieve the required behavior. Different materials and structural propertieshave been determined by optimization techniques such that under proper pneumatic actuation the inlet goal shapes determined from KTH are achieved in the best possible way.Representative sections of shape mor- phing inlets have been tested at University of Bristol for preliminary concept vali- dation and simulation-test correlations.

Structural sections morphing under pneumatic actuators in a test rig at University of Bristol

Multi-scale modeling steps from micro- over meso- to the structural macro-level

High Performance MathematicalDesign Optimization MethodsStructural design optimization techniques based on mathematical optimization algorithms have been researched and extended in two directions:

n Computationally expensive system models like in highly nonlinear dynamic systems are treated by appropriate simulation model reduction techniques. For optimization, such reduced models still have to keep their design or optimization parameters e.g. related to topology, geometry of materials. This leads to

parametrized reduced order models, or P-ROMsn The scope of applications has been

further broadened by inclusion of verbal or qualitative knowledge, which is transferred to numerical models via soft-computing techniques based on

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Interaction of structural mechanics, manufacturing effort and uncertainties in design optimization

fuzzy methods. These models then are added to the structural simulation and optimization models to establish comprehensive overall models.

Structuring the solution process for combined structu- ral and manufacturing effort optimization

to the arrangement and type of their stiffeners needed for example to prevent instability of the thin outer shell. Conven- tional structures with orthogonal arrange- ment of circular and longitudinal stiffeners can be realized with even lower massesif stiffeners are arranged in ‘geodesic’topologies. This arrangement derived from mathematical topology optimization tells that a higher percentage of longitudinal stiffeners in the top and bottom part of the fuselage should be used, while a more +/-45 degrees arrangement at the side parts is preferable.

Standard arrangement of stiffen- ers in a fuselage shell

Geodesic stiffener arrangement derived by topology optimization

The latter leads to integrated structural mechanics and manufacturing effort models of components and structures for the mathematically based designoptimization process. For consideration ofqualitative knowledge e.g. on manufactu- ring effort, ‘what if’-questions for assumed parameter variations and their conse- quences on the effort are asked to specia- lists. From their qualitative replies like‘increases a lot’ or ‘decreases slightly’, numerical relationships between design optimization parameters and relevant output quantities like for manufacturing effort are established. These then get part of the overall optimization model together e.g. with FEM for the structural mechanics aspects.In most of these cases, parallel computing on multi-processor computers plays an important role to keep computation timeor so called turn-around time within limits. For example, aircraft fuselage shell struc- tures have been investigated with respect

Relative manufacturing effort (ME) before and after combined structural-ME optimization

Such design optimization activities were also investigated within the Spitzencluster MAI Design to properly cover the interac- tion of manufacturing effort and structural behavior under different operational conditions. Car body frames consisting of A-pillars and roof frame parts were usedas reference examples for C-fiber braidingand C-fiber lamination manufacturing processes. Again, mechanical simulation models covered the behavior from micro- to macroscale, while the manufacturing effort as function of different design para- meters has been

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derived from interviews of specialists and transferred

to numerical models via fuzzy methods. Optimization

algorithms modified geometric cross-

Modeling steps for C-fiber braided automotive frame section section and fiber arrangement

parameters

such that the manufacturing effort (production time) e.g. for the braiding process could be reduced by around 50% with only marginal mass penalty. Compari-son of results from frames manufactures

by braiding and lamination process then also allow the selection of proper manufacturing methods based on an even more rational trade-off between structuralproperties and manufacturing effort.

Hybrid Material Structures

Since a basic approach in lightweight structures is to identify and put proper materials to proper components and posi- tions within a structure, several activities have been carried out in this area also in order to identify and reduce possible of mismatches of the different materials. This not only relates to their interfaces, but may become also relevant on components and structural level.For example, many thermal cycles incarbon fiber reinforced materials and parts may lead to micro-cracks in the polymer, especially so for cycling amplitudes in the range of +100°C down to -150°C which is typical for many structures in space. Due to a softening effect of such micro-cracks, the C-fiber gets even more dominant for the overall properties of the composite. This then results into a further decrease ofthe coefficient of thermal expansion, whichcould change from say -0.2 E-6 / °K down to -1 E-6 / °K. Though still very small in absolute terms, this large relative change might become relevant for structural thermo-elastic deformations and related loss of geometric precision in orbit.Steel fiber reinforced aluminum parts have

Simulated and tested behavior of steel ropes in aluminum base material relevant for crash properties

been investigated in cooperation withInstitutes from KIT and TU Dortmund inthe SFB/TR10 on advanced manufacturing processes for such materials. Since such parts are mainly aimed for automotive applications, investigations on crash absorption have been finalized end of this year. These investigations covered related materials and components simulationsand experimental investigations. Results showed the benefits of such reinforce- ments, which allow to control the crash behavior.

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Research Focusn Adaptive and shape morphing

structuresn Hybrid material structuresn Large space structuresn Model-based design optimization

methods

Competencen Adaptive structures and smart materialsn Design optimization methodsn Structural mechanics and design

conceptsn Mechanical and environmental testing

Infrastructuren Computer cluster with 250 processorsn CAD and several FEM toolsn Dynamic simulation toolsn Design optimization toolsn Workshop for metal and fiber compo-

site parts

n Mechanical and environmental test facilities incl. cryogenic temperatures

n Extensive measurement systemsn Non-destructive materials and

parts inspection

Coursesn Leichtbaun Luft- und Raumfahrtstrukturenn Multidisciplinary Design Optimizationn Adaptive Strukturenn Faserverbundwerkstoffen Membranstrukturenn Betriebsfestigkeitn Multifunctional Polymer Partsn Vibro-Akustik und Lärmn Testmethoden im Flugzeugbau und

Leichtbau

ManagementProf. Horst Baier, Director

Adjunct Professors Prof. Dr. Pierre Mertiny Prof. Dr. Rudolf Schwarz

Administrative StaffAmely Schwörer

Research ScientistsDipl.-Ing. Johannes AchleitnerDr. Valeria AntonelliDipl.-Ing. Luiz da Rocha-Schmidt Dr. Leri Datashvili, senior research scientistDipl.-Ing. Stephan EndlerDipl.-Ing. Matthias FriemelLali GigineishviliAndreas Hermanutz, M.Sc. Dipl.-Ing. Peter KremplTao Luo, M.Sc.Nikoloz Maghaldadze, M.Sc. Dipl.-Ing. Martin MahlDipl.-Ing. Alexander Morasch Dipl.-Ing. Gunar Reinicke Dipl.-Ing. Bernhard Sauerer Dipl.-Ing. Markus Schatz Liang Si, M.Sc.Dipl.-Ing. Holger StaackDipl.-Ing. Felix StroscherTanut Ungwattanapanit, M.Sc. Erich Wehrle, M.Sc.Dipl.-Ing. Rainer WehrleBin Wei, M.Sc.Dipl.-Ing. Matthias Weinzierl

Technical StaffManfred BauerDipl.-Ing. Karl-Ludwig KrämerBernhard Lerch Christian Mörlein Dirk Steglich Josip Stokic

Publications 2015

n Antonelli, V.; Weinzierl, M.; Baier, H.: Feasibility

study of a sandwich chopper disc for a time of flight (TOF) spectrometer. ICCM 20 – 20th International Conference on Composite Materials, Copenhagen,2015

n Antonelli, V.; Weinzierl, M.; Baier, H.: CFRP chopper discs: state of the art and long term perspective. DENIM 2015 – 4th Design and Engineering of Neutron Instruments Meeting, Budapest, 2015

n Baier, H.: Lightweight design of fiber composite

structure with emphasis on vibration behavior. International Conference on Dynamics of Composite Structures, Arles (Frankreich), 2015

n Baier, H.; Wehrle, R.; Ungwattanapanit, T.: Definition

alternativer Rumpfbauweisen von Verkehrsflugzeu- gen über modellbasierte Entwurfsoptimierung. DGLR Jahrestagung, Rostock, 2015

n Da Rocha-Schmidt, Luiz; Hermanutz, A.; Baier, H.:

A Morphing Lip Concept for Shape Variable AircraftEngine Nacelles. DGLR Jahrestagung, Rostock,2015

n Hermanutz, A.; Da Rocha-Schmidt, L.; Baier, H.: Technology Investigation of Morphing Inlet Lip Concepts for Flight Propulsion Nacelles. EUCASS, Krakau, 2015

n Häußler, M.; Schatz, M.; Baier, H.; Mertiny, P.:Optimization of polymer composite pipe under consi-deration of hybridization. ASME Pressure Vessels and Piping Conference, Boston, Massachusetts, USA, 2015

n Mahl, M.; Friemel, M.; Capobianco, L.; Haridas, A.;

Baier, H.: Influences of Adhesive Properties on Strain Measurement Result of Rayleigh Backscattering Based Fibre Optic Sensors. IWSMH (International Workshop on Structural Health Monitoring), Stanford, USA, 2015

n Morasch, A.; Reeb, A.; Baier, H.; Weidenmann, K. A.;

Schulze, V.: Characterization of debonding strength in steel-wire-reinforced aluminum and its influenceon material fracture. Engineering Fracture Mechanics(141), 2015, pp. 242-259

n Ozdemir, N. G.; Scarpa, F.; Craciun, M.; Remillat, C.; Lira, C.; Jagessur, Y.; da Rocha-Schmidt, L.: Morphing nacelle inlet lip with pneumatic actuators and a flexible nano composite sandwich panel. Smart Materials and Structures Vol. 24, Nr. 12, 2015

n Reinicke, Gunar: Aktive Schwingungsdämpfung in Satellitenbauteilen bei verschiedenen Anregungs- spektren – Simulation und experimentelle Verifika- tion. Dissertation TU München, 2015

n Schatz, M.; Baier, H.: Integration von Fertigungsauf-wänden in die Entwurfsoptimierung von kurz- bis endlosfaserverstärkten Strukturen. NAFEMS Conference on Optimization and Robust Design, Wiesbaden, 2015

n Schatz, M.; Baier, H.: Effizierte Strukturoptimie-rung von Flechtstrukturen mit Hilfe entkoppelter Mehrskalenhomogenisierung und Berücksichtigung von Fertigungsrestriktionen. Landshuter Leichtbau Colloqium, 2015

n Schatz, M.; Baier, H.: Optimization of laminatedstructures considering manufacturing efforts. World Congress of Structural and Multidisciplinary Optimisation (WCSMO), Sydney, 2015

n Si, L.; Baier, H.: Real-Time Impact VisualizationInspection of Aerospace Composite Structures with distributed Sensor, Sensors, 2015, pp. 16536-16556

n Staack, H.; Seibert, D.; Baier, H.: Application orientedfailure modeling and characterization for polymers in automotive pedestrian protection. International Conference on Computational Plasticity (COMPLAS). Fundamentals and Application, Barcelona, 2015

n Wehrle, Erich Josef: Design optimization oflightweight space-frame structures considering crashworthiness and parameter uncertainty, Dissertation TU München, 2015

n Weinzierl, M.; Baier, H.; Krämer, L.; Antonelli, V.:Design and certification of the chopper disks for theNEAT II TOF spectrometer: A lesson learned. DENIM2015 – 4th Design and Engineering of NeutronInstruments Meeting [9, 2015, Budapest], 2015

n Weinzierl, M.; Schatz, M.; Antonelli, V.; Baier, H.: Structural Design Optimization of CFRP Chopper Disks. International Conference on Composite Structures, Lisbon, 2015

n Zhang, Yang: Efficient Procedures for StructuralOptimization with Integer and Mixed-Integer DesignVariables. Dissertation TU München, 2015