aramid-fiber reinforced elastomers as future …€¦ · a nominal load range 5–200 n, at the...

ARAMID-FIBER REINFORCED ELASTOMERS AS FUTUREAIRCRAFT ACTUATORS—A PROOF-OF-CONCEPT

INVESTIGATION

Mikko Kanerva∗ , Minna Poikelispää∗ , Paulo Antunes∗∗∗Tampere University of Technology, Laboratory of Materials Science,

P.O. Box 589, FIN-33101 Tampere, Finland∗∗I3N & Aveiro Physics Department and Instituto de Telecomunicações,

Campus Universitário de Santiago, P-3810-193 Aveiro, Portugal

Keywords: Actuator, Elastomer, Composite, Conceptual design, Finite element analysis

Abstract

In this investigation, we focus on elastomer ac-tuators with aramid-fiber reinforcement. Prop-er reinforcement of elastomer actuators improvesthe efficiency of the actuator and is used to adjustanisotropy mainly controlling actuators’ bendingresponse. Experimental and finite element simu-lation results are presented in the report.

1 Introduction

The revolutionary era of various kinds of un-manned aerial vehicles (UAVs) has begun andthe performance limits of traditional aircraft arebeing exceeded. For example, due to the lackof pilot onboard, the flight performance in termsof accelerations can be extended far beyond hu-man strength. Also, depending on the autho-rized flight zone, the redundancy as well asthe reliability of the structures and systems canbe re-considered. For the UAVs smallest insize, many new technologies have been devel-oped and applied on the system level. To makeUAVs lightweight, heavy-duty systems such ashydraulics are typically omitted.

Soft actuators have been used rather exten-sively within robotics, where different modernautonomous systems must be adaptable. Theadaptability means that different robot arms mustnot harm or damage surfaces that they might hit.

For example, the McKibben actuators are alreadywell-known and have several commercialized ap-plications. The most potential soft actuators arefibre-reinforced to control the material behaviourof the actuator body. [1] [2]

For aeronautics, fibre-reinforced elastomeractuators could be the future option as alightweight actuator with exceptional dampingcharacteristics. We have recently studied the de-sign of reinforcement of different elastomer ac-tuators to maximize the force and displacementoutput in the bending mode [3]. These featuresof a tubular actuator are affected by the elastomerselection (i.e., hardness, Young’s modulus, andcompressibility), reinforcement type, reinforce-ment design (i.e., location and size of axial s-tiffener, number of fiber turns over the tube perlength, number or fiber layers), and the appliedpressure.

Due to the multiplicity of the factors affect-ing the actuator performance, numerical simula-tions are necessary to be carried out. Elastomer-s present the classical challenges of defining thematerial model for a hyper-elastic, incompress-ible media to be computed by a finite elementcode. Already producing high-quality test datafor Poisson’s ratio and force-strain curves is chal-lenging.

In this study, we present a survey of the d-ifferent actuator technologies available for flying

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MIKKO KANERVA , MINNA POIKELISPÄÄ , PAULO ANTUNES

vehicles of any size. A summary of the litera-ture research and an experimental work carriedat Tampere University of Technology are givenin the first part. In the second part, we focus onthe experimental procedures for acquiring datafor numerical material models for finite elementmethods (FEM). In the third part, we focus on thesimulation of different reinforced elastomer actu-ators to screen the capabilities of actuators basedon natural rubber and aramid-fiber.

2 Current actuator technologies

2.1 Traditional actuators

Traditional actuators for flap, aileron or rudderactuation in aircraft are carbon steel and high-strength steel based heavy-duty structures withservo hydraulic power input. These traditionalactuators typically have a significant force out-put compared to their mass (weight efficiency).For example, mechanical input type actuatorsweigh 2.3...27 kg covering a rated force range of4.4...178 kN [4], which leads to a weight efficien-cy range of 1900...6600 N/kg.

An electro-mechanical servo actuator is a typ-ical solution for UAVs. For the tiniest UAVs, ormodel planes, the actuators are extremely sim-ple and small. The lightest actuators weigh on-ly ≈1 g and can exert a 0.054 N-actuation force(presuming a 4.7 mm moment arm) [5], givingapproximately 0.05 N/g (50 N/kg) weight effi-ciency. Both translative and rotational electro-mechanical actuation exist.

2.2 Soft actuators

Soft actuators cover rather a wide range of tech-nologies. Typically they are understood as shapememory alloy (SMA) based systems, electroac-tive polymers, and pneumatic actuators. A vastamount of research has been subjected to SMAs.For aeronautical applications, SMAs incur prob-lems such as slow actuation speed, hysteresis,and coupling with the thermoelastic environment(of aircraft). Electroactive polymers (EAPs) di-vide into ionic EAPs and electronic EAPs. A-gain, ionic EAPs perform very slow actuation and

incur hysteresis. Electronic EAPs are based ondielectric elastomers and have true potential inairborne vehicles. Actually, foldable electronicEAPs have been constructed as microair vehicleactuators and can create forces up to 0.102 Nand weigh 14.4 g—giving weight efficiency of7.1 N/kg [6].

Pneumatic soft actuators include air bellows,pneumatic muscles, and fully elastomeric multi-cell actuators. Air bellows are very simple intheir structure, can be used for high-frequencydynamic actuation, and they can be relativelylight-weight [7]. The downside of air bellows isthat their force output is directly proportional tothe actuation distance—the further the bellow ac-tuates, the less is the actuation force [8].

McKibben actuators have been used fordecades and commercial pneumatic actuators areavailable. The weigh efficiency of typical McK-ibben actuators is of the order of that of air bel-lows and they have merely been developed foruniaxial actuation. Specialized soft pneumaticactuators have been developed, e.g. for twistingmotion [10]. Pneunets’ actuators represent themulti-cell actuators. Pneunets actuators by SoftRobotics Toolkit are advanced soft actuators asregards the geometry and the respective actua-tion force and movement [9]. Complex sets ofchannels and multi-material cells makes it possi-ble to create systems that mimic nature and areoptimized concerning speed, force and weight.

3 Experiments

3.1 Materials

Pneumatic actuators are a potential concept foractuators of flying vehicles and they can be exten-sively modified. Therefore, we chose to study theapplicability of reinforced pneumatic elastomeractuators in detail. We targeted to bending actu-ators, which can create moments and work as ac-tuators without additional mechanisms in theory.Two different elastomeric materials were select-ed for the experiments:

• natural rubber (NR);

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ARAMID-FIBER REINFORCED ELASTOMERS AS FUTURE AIRCRAFT ACTUATORS—APROOF-OF-CONCEPT INVESTIGATION

• polydimethylosiloxane (PDMS) in ST-EZ-60-001 tube form (provided by Etra& Silikon-Technik Siltec GmbH).

Natural rubber, NR, was selected because itscomposition and final form can be easily adjust-ed. NR compound was mixed in-house by adding45 phr N-234 carbon black as well as other ingre-dients, such as curatives (sulphur, N-Cyclohexyl-2-Benzothiazolesulfenamide, CBS), zinc oxide,stearic acid, antioxidants (N-(1,3-dimethylbutyl)-N’-phenyl-1,4-Benzenediamine, 2,2,4-trimethyl-1,2-dihydroquinoline) and plasticizer (distillatearomatic extract oil). The mixing was per-formed by using a GK 1.5 E intermeshing mixer(Krupp Elastomertechnik). CBS and sulphurwere mixed by using an open two-roll mill.

3.2 Specimen preparation

To create anisotropic elastomer actuators andcontrol the bending upon loading, reinforcemen-t was applied axially on one side of each tubu-lar actuator body. Three different stiffener type-s were considered: polyamide (PA) fabric, pres-sure sensitive tape, and aramid fibre (TechnoraT-240, Teijin). For all actuators, radial expansionof the actuator body was controlled by manual-ly bundle-winding aramid fibre around the elas-tomer tube. This hoop reinforcement was ad-justed by varying the revolutions of fibre bun-dle per unit length of the tube. Since NR wascompounded in-house, the tube form had to beformed in a laboratory. The process of makingthe tubular body of an actuator is presented inFig. 1. In Fig. 1, a tube (length 170 mm, ra-dius 8 mm, inner radius 5 mm) is formed bywrapping a 1.5 mm-thick layer of rubber sheetaround a metal rod (diameter 5 mm). The flowof rubber during vulcanization was prevented bywrapping a thin silicone sheet tightly around thetube. The other end of the tube was sealed bya 15 mm-long cap prepared from of the sameNR compound. The tube was cured in a heat-ing press at 150°C for 20 minutes. The end-s of the commercial tubes (length 170 mm, ra-dius 8 mm, inner radius 5 mm) were sealed by a1.5 mm-long cap fabricated from acrylic foam

Fig. 1 a)–e) Preparation of fibre-reinforced NRactuator specimens.

film tape (VHB4910, 3M).For the fitting process of a numerical mate-

rial model for elastomer, tensile test specimenswere prepared. Tensile specimens were cut fromNR sheet with a nominal thickness of 1 mm in-to a planar, rectangular shape of 110 × 20 mmand standard dog-bone shaped specimens. Dueto the hyperelastic nature of elastomers, also bi-axial tests were needed for the material model fit-ting. For biaxial tests, specimens were cut from aNR sheet with a nominal thickness of 1 mm intoa planar, rectangular shape of 88 × 88 mm.

3.3 Mechanical testing

Standard tensile tests (dog-bone specimens) wererun according to ISO 37 standard. FiveNR specimens were tested using a tensile

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tester (Messphysik) with a 1 kN load cell and500 mm/min displacement rate. Additionally,tensile testing (wide specimen) of NR was per-formed using a tensile tester (Messphysik) witha 1 kN cell. These tests were displacement-controlled, at a constant rate of 10 mm/min,and performed at ambient conditions (30% RH,26°C); a low displacement rate was used tofit lower strain-range tests with (DIC) measure-ments in practice as well as the finite elementscheme.

Biaxial testing of NR was performed usinga biaxial stretcher (KARO IV, Brückner) witha nominal load range 5–200 N, at the premisesof VTT Technical Research Centre of Finland.Biaxial tests were displacement-controlled, at a3/3 stretching ratio, in two subsequent load stepswith a dwell time (hold) between load ramps, andat ambient conditions (42% RH, 20.5°C).

The actuation testing of the fibre-reinforcedactuators were performed by connecting the ac-tuators to an air pressure source with pressurecontrol, measurement (10 N load cell, LTS-1KA, Kyowa) and analysis software (LabVIEW2012). The pressure was applied in ≈50 kPasteps (0...500 kPa) to account for any dynamiceffects. The load cell was attached to the sealedend and above the actuator specimen.

3.4 Digital image correlation

Strain and displacement fields over the tensile(wide) and biaxial NR specimens were acquiredusing a two-camera setup and digital imagecorrelation (DIC) analysis software (LaVision).A random speckle pattern was made by usingspray paint on one side of tensile and biaxialspecimens. Data was synchronized with the test-ing machines’ data by observed first signal peak.

3.5 Optical fibre strain sensing

The actuator internal deformation was traced byusing embedded optical fibres with four FiberBragg Grating (FBG) sensors multiplexed in eachfibre. The signal processing was performedvia a W3/1050 Series Fiber Bragg Grating In-terrogator (Smart Fibers). A wavelength range

of 1510–1590 nm was applied and the interroga-tor was operated using a Remote Interface W3WDM (version 1.04). The fibers with the FBGswere individually tailored by Instituto de Teleco-municações (Aveiro, Portugal). Per reinforcedactuator, an optical fibre with FBGs was fixedalong with the axial aramid fibre reinforcementbundle, under the wound reinforcement layer.

4 Modelling and simulation routines

4.1 Finite element models

Finite element (FE) simulations were performedto (1) fit a hyper-elastic material model and (2)to survey the potential performance of fibre-reinforced elastomeric actuators. All the mod-elling and implicit solutions were carried out us-ing Abaqus

®2017 standard (Simulia). The ten-

sile test specimen (full model, planar dimensions110 mm × 20 mm, thickness 0.95 mm, Fig. 2)and the bi-axial specimen (quarter model, planardimensions 44 mm × 44 mm, thickness 1.1 mm,Fig. 3) were modelled to validate a material mod-el for NR. The load was applied as enforced dis-placement and boundary conditions were givenusing symmetry and zero displacements at edges.A reinforced actuator was modelled as a systemof hyperelastic body and a reinforcement layercovering the body, as shown in Fig. 3. The axialreinforcement was modelled using a partitionedsection in the body. All the FE models weremeshed using linear C3D8H brick elements (hy-brid formulation), see mesh densities in Figs. 2–4.

In order to fit a proper material model for theelastomer body, Abaqus’ property fitting tool wasused to define parameters for three different hy-perelastic models using the standard tensile testdata (see Section 3.3). The three material modelcandidates were second order Ogden, Yeoh, andNeo-Hookean hyperelastic models [11]. The va-lidity of each model was first demonstrated bysimulating the tensile test (wide specimen) andsubsequently by simulating the biaxial test. Forall the hyperelastic material models, NR was thereference material. To compare the simulation

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Fig. 2 FE model, element mesh, and boundaryconditions of the tensile test specimen.

results with the DIC based strains, data was col-lected over nodal regions that corresponded to theanalysis of the DIC measurements (five points onwide tensile specimen, 60 mm × 60 mm regionon biaxial specimen).

4.2 Actuator performance survey

The performance survey of the reinforced elas-tomer actuator concept was carried using theprocess-integration and optimization tool Isight(Simulia). The aim was to understand the crit-icality of stiffness in different directions in thewound reinforcement. Therefore, the reinforce-ment layer was modelled by using engineer-ing constants (nine constants in total) of whichfive were designated as variables (ERR, Eθθ,EZZ , Poisson’s ratio νZθ). Additionally, the im-portance of stiffness in the axial reinforcement(EZZ,axial) was included as variable. Latin Hy-percube Sampling method was used for defining

Fig. 3 FE model, element mesh, and boundaryconditions of the biaxial test specimen.

the computation case matrix. Here, 50 cases weresimulated to estimate the effect of each materi-al parameter over its value range. Finally, theessence of each material parameter was definedusing the (linear) Correlation Factor. The targetresponse was the translation (U2) of the actua-tor free end at the completion of each simulationcase. The applied value ranges were as follows:

• ERR : [70 GPa...192 GPa]

• Eθθ : [70 GPa...192 GPa]

Fig. 4 FE model of the reinforced actuator.

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• EZZ : [0.5 GPa...1.1 GPa]

• νZθ : [0.01...0.33]

• EZZ,axial : [10 GPa...192 GPa]

5 Results and analysis

The ISO-37 tensile test results are shown inFig. 5. Variation in stiffness between the speci-mens was low but ultimate strength was scattereddue to some air cavities in NR. The results wereused to determine the following material modelparameters:

• Ogden: µ1 = 101832.293; α1 = 3.2073128;µ2 = 1681595.86; α2 = -6.5337968; D1 =4.546361 ×10−8; D2 = 0,

• Yeoh: C10 = 553538.2; C20 = 63425;C3 = -896; D1 = 7.32389 ×10−8; D2 = 0,D3 = 0,

• Neo-Hookean: C10 = 975779.66; D1 =4.15468186 ×10−8.

Fig. 5 The standard (dog-bone) tensile test re-sults for NR.

6 Force output results from experiments

The effect of the axial reinforcement type on theforce output of a reinforced PDMS actuator is

Fig. 6 The effect of axial reinforcement on forceoutput (PDMS actuator body). Applied pressure500 kPa.

shown in Fig. 6. The results indicate, rather in-terestingly, that the reinforcement type is not ex-tremely crucial for high force output.

In turn, the effect of the wound hoop rein-forcement in terms of the revolutions of bun-dle wound is clear—the more reinforcement iswound around the tubular elastomer body of anactuator, the higher the capability of the actuatorto actuate.

Fig. 7 The effect of wound reinforcement onforce output (revolutions of aramid fibre onPDMS body). Applied pressure 500 kPa.

The actuation results in Figs. 6 and 7 showedthat the configuration of the wound reinforce-ment is an essential design item to make a strongelastomer actuator—even more than the axial

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Fig. 8 The effect of tube wall thickness on forceoutput (NR actuator body).

reinforcement underneath the wound reinforce-ment layer. In addition to the fibrous reinforce-ment, it is anticipated that the geometry of thetubular body influences the force output. InFig. 8, it can be seen that the wall thickness ofthe body affects the force output. As is intuitive-ly clear, a thinner wall will allow the actuator toexert higher forces per given pressure load. Natu-rally, the wall thickness must be considered withrespect to the tube size (outer diameter). Largertube creates higher absolute force output.

The mass of an elastomeric actuator is mainlydetermined by the dimensions of the elastomer-ic body. For example, the PDMS actuators withwound reinforcement (results in Fig. 7) weighed8.84 g (n=30), 8.75 g (n=60), 8.63 g (n=120),8.75 g (n=240). The force output lead to weightefficiency values of 28.3, N/kg, 35.4 N/kg,54.5 N/kg, and 62.9 N/kg, respectively.

6.1 Hyperelastic material model validation

The simulated force-displacement curves for thewide tensile test specimen are shown in Fig. 9. Itcan be seen that all the three hyperelastic mod-els follow well the experimental reference forlow displacement range (say 0–10 mm). For alarger displacement range, the simulated behav-ior begins to show the character of the standardtests (Fig. 5). To model a reinforced actuator,

Fig. 9 The simulated tensile test (wide specimen).

model validity over the lower end of displace-ment (strain) range is important (the test with awide specimen was run only till 10% strain). TheOgden and Yeoh models produce similar force-displacement curves but the Neo-Hookean be-havior is different meaning that the tangent mod-ulus decreases as the displacement (strain) in-creases. Based on the tensile test simulation, theNeo-Hookean model tends to fit the tensile ex-periments of this study best.

The simulation results of the biaxial test areshown in Figs. 10 and 11. The simulation andtest results are shown separately for the test ma-chine X and Y directions (namely MDX andTDX directions). It can been that the simulatedstrains (mean value over a central region of 60 m-m × 60 mm in the test specimen and 30 mm ×30 mm region in quarter FE model) follow rather

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Fig. 10 The simulated biaxial test (MDX direction).

well the experimental results (DIC data from bi-axial tests). When comparing the different mod-els, both Yeoh and Neo-Hookean models produceoverly loose stretching over the central region ofthe specimen. In other words, the second orderOgden model concentrates stretching at the testmachine gripping region, where the load condi-tion is three-dimensional (pressure of the grip-ping mechanism was included in the FE modelat the specimen edge). Since the strain accumu-lation predicted by the Ogden model correspond-ed better the experiment, the fitted Ogden modelwas selected for the actuator simulation.

6.2 Surveying of essential material proper-ties for actuation performance

The performance of the modelled elastomer ac-tuator was defined as the capability to bend (up-

Fig. 11 The simulated biaxial test (TDX direction).

wards, in U2 direction). For an unconnected e-lastomer actuator, the bending performance cor-responds to the force output as long as the de-formation is stable as a function pressure. Theresults of the automatized mapping are shown inFig. 12. Based on the results, it can be seen thatthe stiffness of the wound reinforcement in thetube’s axial direction is of primary importance.Additionally, axial reinforcement underneath thewound reinforcement layer plays a part while theeffect of stiffness in the radial and tangential di-rections is negligible. These results are supportedby the experimental results presented in Figs. 6and 7.

The simulated deformation shape of the rein-forced actuator is shown in Fig. 13 demonstrat-ing a correct kind of actuator bending. The FEactuator in this study applied a continuum mod-

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el for the wound reinforcement, which is too astiff compared to the wound aramid fibre bun-dle in reality. Therefore, the axial deformation issmall and the bending results in slight compres-sive stress state on the actuator surface above theaxial reinforcement. When compared to exper-imental results (FBG strains upon an actuationtest are all positive, see Fig. 14), the real actua-tor is essentially free to deform (stretches) axiallybecause the wound reinforcement layer does notcreate a stiffening effect in the axial direction.

Fig. 12 The automated simulation routine results.

6.3 Conclusions

This study includes three different parts: (1) a lit-erature survey of the efficiency of various types

Fig. 13 Simulated behavior of the fibre-reinforced elastomer actuator.

Fig. 14 Load-strain results using FBG sensorsembedded into the actuator.

of actuators in airborne vehicles, (2) an exper-imental work entity to study elastomeric fibre-reinforced actuators, and (3) a numerical sim-ulation entity to survey the essential materialparameters when developing an efficient elas-tomer actuator. Based on the results, the weight-normalized force output of soft actuators is nowa-days low and further research is needed to profitthe advantage of soft actuators in aircraft applica-tions. Our experimental results show that a tubu-lar elastomer actuator can be made anisotropicto produce moments. The bending actuation re-quires efficient reinforcement in the axial direc-tion both for controlling the local stiffness alongthe long axis of the actuator and to reinforce theelastic walls against a high internal pressure. Theweight efficiency of the laboratory scale actuatorswas 28–63 N/kg, which is in the order of weightefficiency of electro-mechanical servo actuatorsin model planes and UAVs. Based on the FE sim-ulations in this study, the wound reinforcementconcept, a low helical angle, i.e. low axial stiff-ness, is of primary importance to produce exten-sive bending actuation.

6.4 Acknowledgements

This study was funded by a grant fromAcademy of Finland (project ActiveFit). The au-thors want to acknowledge researcher Olli Orell(Tampere University of Technology, Finland) for

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DIC measurements, Satu Pasanen for biaxial test-ing (VTT, Finland), and researcher Jarno Joki-nen (Tampere University of Technology, Finland)for support in the simulation work. ProfessorJyrki Vuorinen and Professor Reza Ghabcheloo(Tampere University of Technology, Finland) areacknowledged for their project-organizationalactions.

References

[1] Polygerinos P, Wang Z and Overvelde JTB.Modeling of soft fiber-reinforced bending actu-ators. IEEE Transactions on Robotics, Vol. 31,No. 3, pp 778–789, 2015.

[2] Galloway KC, Polygerinos P, Walsh CJ andWood RJ. Mechanically programmable bend ra-dius for fiber-reinforced soft actuators. 16th In-ternational Conference on Advanced Robotic-s 2013 (Proceedings), Montevideo, Uruguay,November 25-29, pp 1-6, 2013.

[3] Kanerva M, Poikelispää M and Vuorinen J.Continuum simulation of hyper-elastic elas-tomer actuators: the effect of material mod-els. Molecular order and mobility in polymersystems - 9th International IUPAC Symposium(Book of Abstracts), St.Petersburg, Russia, June19-23, O-91, pp 115–116, 2017.

[4] Technical specifications. Aerospace Actuators.Parker, http://ph.parker.com/us/, 2018 (cited).

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[6] Shintake J, Rosset S, Schubert BE, FloreanoD, Shea HR. A Foldable Antagonistic Actua-tor. IEEE/ASME Transactions on Mechatronics,Vol. 20, No. 5, pp 1997–2008, 2015.

[7] Technical data sheet. Actuators, Air bellows.IMI Norgren, http://pages/norgren.com/, 2018(cited).

[8] Technical data sheet. Fluidic Muscle. Festo,https://www.festo.com/, 2017/10).

[9] Design information. PneuNets Bend-ing Actuators. Soft Robotics Toolkit,http://softroboticstoolkit.com/, 2018 (cited).

[10] Gorissen B, Chishiro T, Shimomura S, Rey-naerts D, and De M. et al. Flexible pneumatictwisting actuators and their application to tilting

micromirrors. Sensors Actuators A Phys, Vol.216, pp 426–431, 2014.

[11] Ali A, Hosseini M, Sahari BB. A Review of con-stitutive models for rubber-like materials. Amer-ican Journal of Engineering and Applied Sci-ences, Vol. 3, No. 1, pp 232–239, 2010.

7 Contact Author Email Address

mailto: [email protected]

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aramid-fiber reinforced elastomers as future …€¦ · a nominal load range 5–200 n, at the...

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