As a consequence of operational eﬃciency because of rising energy costs, future transport systems need to be mission-adaptive. Especially in aircraft design the limits of lightweight construction, reduced aerodynamic drag and optimized propulsion are pushed further and further. The ﬁrst two aspects can be addressed by using a morphing leading edge. Great economic advantages can be expected as a result of gapless surfaces which feature longer areas of laminar ﬂow. Instead of focusing on the kinematics, which are already published in a great number of varieties, this paper emphasizes as major challenge, the qualiﬁcation of a multi-material layup which meets the compromise of needed stiﬀness, ﬂexibility and essential functions to match the ﬂight worthiness requirements, such as erosion shielding, impact safety, lighting protection and de-icing. It is the aim to develop an gapless leading edge device and to prepare the path for higher technology readiness levels resulting in an airborne application. During several national and European projects the DLR developed a gapless smart droop nose concept, which functionality was successfully demonstrated using a two-dimensional 5 m in span prototype in low speed (up to 50 m/s) wind tunnel tests. The basic structure is made of commercially available and certiﬁed glass-ﬁber reinforced plastics (GFRP, Hexcel Hexply 913). This paper presents 4-point bending tests to characterize the composite with its integrated functions. The integrity and aging/fatigue issues of diﬀerent material combinations are analyzed by experiments. It can be demonstrated that only by adding functional layers the mentioned requirements such as erosion-shielding or de-icing can be satisﬁed. The total thickness of the composite skin increases by more than 100 % when required functions are integrated as additional layers. This fact has a tremendous impact on the maximum strain of the outer surface if it features a complete monolithic build-up. Based on experimental results a numerical model can be set up for further structural optimizaton of the multi-functional laminate.
State of the art smart materials such as piezo ceramics or electroactive polymers cannot feature both, mechanical stiﬀness and high active strain. Moreover, properties like low density, high mechanical stiﬀness and high strain at the same time driven by low energy play an increasingly important role for their future application. Carbon nanotubes (CNT), show this behavior. Their active behavior was observed 1999 the ﬁrst time using paper-like mats made of CNT. Therefore the CNT-papers are electrical charged within an electrolyte thus forming a double- layer. The measured deﬂection of CNT material is based on the interaction between the charged high surface area formed by carbon nanotubes and ions provided by the electrolyte. Although CNT-papers have been extensively analyzed as well at the macro-scale as nano-scale there is still no generally accepted theory for the actuation mechanism. This paper focuses on investigations of the actuation mechanisms of CNT-papers in comparison to vertically aligned CNT-arrays. One reason of divergent results found in literature might be attributed to diﬀerent types of CNT samples. While CNT-papers represent architectures of short CNTs which need to bridge each other to form the dimensions of the sample, the continuous CNTs of the array feature a length of almost 3 mm, along which the experiments are carried out. Both sample types are tested within an actuated tensile test set-up under diﬀerent conditions. While the CNT-papers are tested in water-based electrolytes with comparably small redox-windows the hydrophobic CNT-arrays are tested in ionic liquids with comparatively larger redox-ranges. Furthermore an in-situ micro tensile test within an SEM is carried out to prove the optimized orientation of the MWCNTs as result of external load. It was found that the performance of CNT-papers strongly depends on the test conditions. However, the CNT-arrays are almost unaﬀected by the conditions showing active response at negative and positive voltages. A micro alignment as result of tensile stress can be proven. A comparison of both results point out that the actuation mechanism strongly depends on the weakest bonds of the architectures: Van-der-Waals-bonds vs. covalent C-bonds.
In the fields of smart materials there is still a demand for a material featuring high modulus, low density and large strain. Carbon materials catch enormous scientific attention not only since carbon fibers were used for highperformance composite structures. But more and more the scientific attention moves from the macroscale to the nanoscale. This paper focuses with a adaptive point of view on one of the carbon allotropes: carbon nanotubes (CNTs). Beside excellent electromechanical properties another interesting feature was first mentioned 1999 - the active behavior of paper-like mats (bucky-papers) made of CNTs. CNT-papers are electrically activated using a double-layer interaction of ions provided by an electrolyte and the charged, high specific surface area of the paper formed by carbon nanotubes. Until now the detailed mechanism behind the strain/force generation of CNT-based architectures is unknown. A clarification of this principle reveals the potential of carbon tubes to be or not to be a resilient smart material in order to use their strong covalent carbon bonds instead of weak van der Waals force as tube-linking. This paper presents further investigations about the composition of CNT-papers and their performance in contrast to vertical aligned CNT-arrays using an actuated tensile test set-up. For better comparison the experiments of both specimen-types are carried out in dry, wet and wet/charged conditions. Especially in the case of CNT-arrays it is essential to preload the specimens because the curly CNT-structure superimposes the vertical orientation. While the CNT-paper is tested in an aqueous solution of one molar sodium chloride, the hydrophobic character of CNT-arrays requires an ionic liquid (IL) as electrolyte. It is found that the mechanical properties of CNT-papers drop significantly by wetting and can be controlled by charging what indicates an electrostatic dominated effect. In contrast the CNT-arrays show identical results regardless of the test conditions and furthermore an active, reversible behavior of tube elongation by charging. These results indicate strongly a quantum mechanical effect as reason of a CNT-array actuation.
Former research on morphing droop-nose applications revealed great economical and social ecological advantages in terms of providing gapless surfaces for long areas of laminar flow. Furthermore a droop-nose for laminar flow applications provides a low noise exposing high-lift system at the leading-edge. Various kinematic concepts for the active deployment of such devices are already published but the major challenge is still an open issue: a skin material which meets the compromise of needed stiffness and flexibility. Moreover additional functions have to be added to keep up with standard systems. As a result of several national and European projects the DLR developed a gapless 3D smart droop-nose concept, which was successfully analyzed in a low speed wind tunnel test under relevant loads to prove the functionality and efficiency. The main structure of this concept is made of commercial available glass fiber reinforced plastics (GRFP). This paper presents elementary tests to characterize material lay-ups and their integrity by applying different loads under extreme thermal conditions using aged specimens. On the one hand the presented work is focused on the integrity of material-interfaces and on the other hand the efficiency and feasibility of embedded functions. It can be concluded that different preparations, different adhesives and used materials have their significant influence to the interface stability and mechanical property of the whole lay-up. Especially the laminate design can be optimized due to the e. g. mechanical exploitation of the added systems beyond their main function in order to reduce structural mass.
Since carbon nanotubes (CNTs) have been discovered in 1991, worldwide scientific research reveals excellent properties. Most of the found properties refer to almost defect-free, single-walled carbon nanotubes (SWCNTs) with nano-scale dimensions. However, scientists try to incorporate CNTs into applications to transfer their features in order to push the specific performance. Typically the results are comparably lower than expected because of the varying quality of used CNTs. This paper presents results of research using CNTs as actuators. In contrast to published paper which analyzed architectures of entangled CNTs as active components, like papers or yarns, to measure their bulk-strain this paper focuses on scattered, highly aligned CNTs. This approach promises to clarify the effect of actuation, whether it is a quantum-mechanically, or rather an combined electrostatic volume-transfer effect. Two experimental set-ups are presented. The first experiment is carried out using highly aligned multi-walled CNTs (MWCNT-arrays). Their orientation is investigated intensively in comparison to other analyzed CNT-arrays. Furthermore their substrate consists of electrical conductive carbon. The CNT-array is optically analyzed along the longitudinal geometry of the vertically aligned MWCNTs. The interfaces of the set-up, which may influence the measurement, have been analyzed in order to avoid second-order effects like thermal swelling or chemical degradation. The results reveal comparable high deflections starting at an activation-voltage of ±1.75V. The ionic liquid is tested within a voltage-range of ±2V due to time-staple performance. The second presented results are found by using Raman-spectroscopy to analyze single SWCNTs. This paper presents the first results of the challenging test campaign to analyze single-walled nanotubes within an electrolyte during charging. A shifting of Raman-peaks according to the wavenumber can be directly attributed to a geometry-change. Thus, the two presented experiments uses aligned CNTs a driving actuation mechanism of single CNTs may be identified.
The various excellent properties of carbon nanotubes (CNTs) are in the focus of researchers since years. Moreover
architectures, built of CNTs, show active behavior in terms of deflections. Therefore they have to be set up like
a capacitor within an electric field and covered by a matrix of free movable ions. The mechanism behind this
phenomenon is still discussed obsessively rather to be an quantum-mechanical elongation of the C-bonds or to be
caused by electrostatic repulsion of charged agglomerated CNTs. Formally investigated paper-like architectures,
known as Bucky-papers, consist of randomly oriented CNTs. This paper presents several experimental approaches
and results documenting doubts about the ability to clarify the active mechanism by investigating the electro-mechanical
properties of those paper-like architectures. In contrast a novel test set-up for analyzing specimen,
providing highly vertically aligned CNTs, is presented. This high resolution test set-up is designed to analyze
CNT-specimen in thickness-direction optically. The vertically aligned CNT-architectures, also called CNT-arrays,
consist of multi-walled CNTs (MWCNTs). The MWCNT-arrays are highly hydrophobic and can only
be moistened by polar liquids like ionic liquids (ILs). The latest results of the electro-mechanical system as
well as further challenges dealing with ILs and different kinds of CNT-arrays are presented. The presented
measurement method allows an even more precise investigation of the electro-mechanical behavior of a single
MWCNT and the strain-mechanism simultaneously. Furthermore this configuration points out an efficient mode
of future CNT-based actuators.
Natural laminar flow is one of the challenging aims of the current aerospace research. Main reasons for the
aerodynamic transition from laminar into turbulent flow focusing on the airfoil-structure is the aerodynamic
shape and the surface roughness. The Institute of Composite Structures and Adaptive Systems at the German
Aerospace Center in Braunschweig works on the optimization of the aerodynamic-loaded structure of future
aircrafts in order to increase their efficiency. Providing wing structures suited for natural laminar flow is a step
towards this goal. Regarding natural laminar flow, the structural design of the leading edge of a wing is of special
interest. An approach for a gap-less leading edge was developed to provide a gap- and step-less high quality
surface suited for natural laminar flow and to reduce slat noise. In a national project the first generation of the
3D full scale demonstrator was successfully tested in 2010. The prototype consists of several new technologies,
opening up the issue of matching the long and challenging list of airworthiness requirements simultaneously.
Therefore the developed composite structure was intensively tested for further modifications according to meet
requirements for abrasion, impact and deicing basically. The former presented structure consists completely
of glass-fiber-prepreg (GFRP-prepreg). New functions required the addition of a new material-mix, which has
to fit into the manufacturing-chain of the composite structure. In addition the hybrid composites have to
withstand high loadings, high bending-induced strains (1%) and environmentally influenced aging. Moreover
hot-wet cycling tests are carried out for the basic GFRP-structure in order to simulate the long term behavior of
the material under extrem conditions. The presented paper shows results of four-points-bending-tests of the most
critical section of the morphing leading edge device. Different composite-hybrids are built up and processed. An
experimental based trend towards an optimized material design will be shown.
Excellent properties like low density, high mechanical stiffness as well as an outstanding thermal and electrical
conductivity make researchers focusing on carbon nanotubes (CNTs) since years. Beside that it is found that
structures made of CNTs can be actuated when they are set up like a capacitor. Usually two dimensional (2D)
CNT-papers with randomly oriented CNTs, called Bucky-papers, are used. They are charged and divided by
an electrically insulating but ionic conductible electrolyte. Experiments demonstrate low voltages for actuation
(±1V). Although the mechanism of CNT-actuation is still an open issue theoretical studies suggest a charge and
ion induced lengthening of the C-bonds, which predict theoretical strains up to 1%. These characteristics make
CNTs a potential candidate for lightweight and powerful actuators of future adaptive aerospace applications.
The presented work gives an overview of possible CNT-actuator configurations. Comprehensive analysis
tools for 2D mats of randomly oriented CNTs have been developed to guarantee a consistent data base for the
comparison of different CNT-configurations. It is focused on the electro-mechanical properties with respect to
the processing and configuration of CNT-actuators. For a more efficient use of the mechanical advantages of
the CNT-geometry a new aligning manufacturing approach is presented, to get highly oriented 2D CNT-papers.
Their properties are compared with randomly oriented CNT-papers. Finally a new test set-up will be introduced,
which enables deflection measurements directly on the top of vertically aligned CNTs (CNT-arrays). The buildup
and necessary prework are shown, as well as results of the first experiments. The method of measuring along
the axis of aligned CNTs qualifies this set-up to get a deeper understanding about the actuation mechanism
of CNTs. Vertically aligned CNTs promise to be a more efficient actuator configuration because of their high
stiffness in direction of actuation.
At the Institute of Composite Structures and Adaptive Systems (FA, Prof. Wiedemann) of the DLR the structure of a
flexible and gapless wing leading edge has been developed for testing in large scale structure-system ground tests. The
absence of gaps in a flexible wing leading edge allows for a significant noise reduction and provides an additional key
technology for realizing wings with a fully natural laminar flow. In the years 2009 and 2010 the work in the project
SmartLED within the 4<sup>th</sup> German Aviation Research Program (LuFo) was focused on the preparation and realization of
the first ground test of the in the project developed overall system. The overall smart droop nose concept arose from the
cooperation of Airbus and EADS, whereas the DLR Institute FA dealt with the structural design, the test of the material
systems, the simulation of the overall system, and the development of manufacturing technologies for the composite
structures to be employed in the planned tests. The detailed presentation of this work forms the content of this paper
which has been made possible through the application of the process chain for composite structures established at the
Institute FA of the DLR.
Future adaptable applications require electro-mechanical actuators with a high weight-related energy. Among
modern multi-functional materials carbon nanotubes (CNTs) have some special characteristics which give them
the potential to solve this demand. On the one hand raw CNTs have excellent mechanical properties like their
low density (1330kg/m<sup>3</sup>) and very high estimated stiffness of about 1TPa. On the other hand CNTs have the
ability under presence of ions, wired like a capacitor and activated by a charge injection to perform a dimensionchange
(length of C-C bondings). Calculations and experiments present achievable active strains of 1% at low
voltage of ±1V what qualifies CNT-based materials for leightweight powerful actuators.
In this paper the former work done with actuators using CNT-containing mats and Nafion as solid electrolyte
is evaluated by analyzing the two main-components in more detail. On the one hand the CNT-based modelmaterial
SWCNT-mats called Bucky-paper (BP) and on the other hand ion donating electrolytes in liquid-phase
like a NaCl-solution and its solid equivalent Nafion as thin-foils are tested. Additional methods of fabrication,
preparation and characterization of the CNT-powder and the manufactured BPs containing randomly oriented
single-walled carbon nanotubes (SWCNTs) are presented which provide a deeper system-understanding. Both
materials (BPs and Nafion-foils) are intensively investigated in different deflection-test-rigs due to their structural
assembly. This paper presents a method for electro-mechanical measurements of BPs in an in-plain test set-up
which avoids sensing secondary effects like thermal expansion or mass-transport and confirm that BP-deflection
should only be a capacity-driven effect. Nafion as solid electrolyte will be tested in an out-of-plane facility
to measure its possible actuation within the lamellar-direction. With this approach the dependencies of each
component and their individual characters on the deflection can be estimated. The active response can be
referred to the internal structure of both components as well as of the whole structural assembly.
The results give a certain direction to a BP-optimization referring to active strain, density, structural integrity
and conductibility. In addition to these facts the active character of BPs using CNTs of different suppliers and
Nafion is analyzed. These investigations are of particular importance for detection of global dependencies and
using both materials in a hybrid-assembly like solid actuators which are needed for structural applications.
Actuators based on carbon nanotubes (CNT) have the potential to generate high forces at very low voltages. The
density of the raw material is just 1330 kg/m3, which makes them well applicable for lightweight applications.
Moreover, active strains of up to 1% can be achieved - due to the CNTs dimensional changes on charge injection.
Therefore the nanotubes have to be arranged and electrically wired like electrodes of a capacitor. In previous
works the system's response of the Nanotubes comprising a liquid electrolyte was studied in detail. The major
challenge is to repeat such experiments with solid electrolytes, which is a prerequisite for structural integration.
In this paper a method is proposed which makes sure the expansion is not based on thermal expansion. This
is done by analysing the electrical system response. As thermal expansion is dominated by ohmic resistance the
CNT based actuators show a strong capacitive behavior. This behavior is referable to the constitution of the
electrochemical double layer around the nanotubes, which causes the tubes to expand. Also a novel test setup is
described, which guarantees that the displacement which is measured will not be caused by bending of a bimorph
but due to expansion of a single layer of nanotubes. This paper also presents experimental results demonstrating
both, the method of electrical characterization of CNT based actuators with implemented solid electrolytes
and the novel test setup which is used to measure the needed data. The actuators which were characterized
are hybrids of CNT and the solid electrolyte NAFION which is supplying the ions needed to constitute the
electrochemical double layer. The manufacturing, processing of these actuators and also some first experimental
results are shown. Unfortunately, the results are not as clear as those for liquid electrolytes, which depend on
the hybrid character of the analyzed devices. In the liquid electrolyte based case the CNTs are the only source
of stiffness, whereas in the solid electrolyte case electrodes and electrolyte contribute to the overall stiffness and
damping as well. Since the introduction of solid electrolytes is a major stumbling block in the development of
such actuators, this work is of particular importance.