ICCS20, International Conference on Composite Structures 4-7 September 2017 CNAM, Paris, France Bolted joints with radial pretension for thickwalled composites structures Lutz Beyland, Composite Design Engineer, German Aerospace Center (DLR) Institute of Composite Structures and Adaptive Systems Elisabeth Ens RWTH Aachen DLR.de Folie 2 > ICCS20 > Beyland > 05.09.2017 Outline State of the art bolted joints in thick walled laminates Concept of bolted joint with radial pretension Analytical approach FE-Calculation Bearing tests Conclusion and Outlook DLR.de Slide 3

> ICCS20 > Beyland > 05.09.2017 Bolted joints for thick laminates - state of the art Fields of application for many decades multi-stage rockets for transportation of satellites connection of aircraft wing and fuselage root connection of rotor blades of wind turbines Payload adapter Ariane 5 [DIEM] Failure modes Tension Shear Out T-Bolt connection for rotor blades [HAU] Cleavage Bearing DLR.de Slide 4 > ICCS20 > Beyland > 05.09.2017 Problems of state of the art bolted joints Big Clearance

P Joint stiffness and wear depend on bolt-hole clearance Avoiding big bolt-hole clearance leads to High effort for precise drilling of borehole Need for reaming / drilling of both joint parts in one process Low Stiffness, high wear P Small / No Clearance High Stiffness, medium wear P P Radial Pretension Very high stiffness, low wear P

P DLR.de Slide 5 > ICCS20 > Beyland > 05.09.2017 Inner ring Concept of radial pretension 1 - Clearance 2 - No Clearance Outer ring Outer ring Inner ring 3 - Radial Pretension Plate with bore hole Bolt Inner ring

Outer ring Clamping bush [MDLER] Clamping bush overcomes clearance and creates a radial pretension between bolt and bore hole Radial pretension by axial tightening of inner and outer ring Outer ring of clamping bush is slatted >> Diameter increase of 1 mm possible DLR.de Slide 6 > ICCS20 > Beyland > 05.09.2017 Research topics on bolted joint with radial pretension Analytic model to describe stress state around bore hole Bearing tests Static and fatigue strength FE model to validate mathematical model and for detailed analysis DLR.de Slide 7

> ICCS20 > Beyland > 05.09.2017 Analytic approach Superposition Pretension [Timoshenko] Bolt load [Zhang] DLR.de Slide 8 > ICCS20 > Beyland > 05.09.2017

2D FE model - setup Disk Bolt FE-Software: Ansys Workbench 18.1 Fixed support Thermal expansion Bolt load Symmetry Boundary conditions Symmetry at midplane Fixed support of disks peripheral surface Bolt load as distributed force on bolts peripheral surface Radial pretension by thermal expansion of bolt Frictionless contact between bolt and hole Material Linear-elastic model Disk: GFRP, quasi-isotropic stiffness Bolt: Steel DLR.de Slide 9

> ICCS20 > Beyland > 05.09.2017 2D FE model - results Use case Radial stress Parameter Variable Value Unit Radius bore hole a 32,5 mm Outer radius disk b

800 mm Youngs Modulus disk E 27 GPa Poisson's ratio disk 0,343 - Pretension pi 270 MPa

Radial expansion u0 0,5 mm p 28 kN/mm (caused by pretension) Bolt load DLR.de Slide 10 > ICCS20 > Beyland > 05.09.2017 Comparison of analytical and FE model = Stresses at bore hole (circumferential path) rad maximum difference: 2 MPa

tan maximum difference: 5 MPa Conclusion: Analytical model is accurate for stresses at bolt hole = Circumferential path = DLR.de Slide 11 > ICCS20 > Beyland > 05.09.2017 Comparison of analytical and FE model = Stresses at bore hole (radial path) rad maximum difference: 16 MPa (~7%) tan maximum difference: 20 MPa (~17%) Conclusion: Analytical model is sufficient for stresses at whole disk = Radial path

= DLR.de Slide 12 > ICCS20 > Beyland > 05.09.2017 Effect of friction on stress distribution 2D-FE Contact status Near Friction strongly influences stress distribution Coefficient of friction between bolt and hole is not well known Literature [Cuntze] suggests = 0,1 0,3 >>> = 0,1 chosen = = Circumferential path = =0

Sliding Sticking = 0,1 = 0,5 DLR.de Slide 13 > ICCS20 > Beyland > 05.09.2017 Effect of pretension on stress distribution 2D-FE Szenario Pretension pi [MPa] Expansion u0 [mm] High pretension

270 0,5 Medium pretension 135 0,25 No pretension 0 0 Amount of pretension significantly influences both radial and tangential stress High pretension avoids clearance under bolt load >> no tangential stress peak at Medium pretension results in minimum overall tangential and radial stresses = = Circumferential path =

DLR.de Folie 14 > ICCS20 > Beyland > 05.09.2017 Bearing test setup Special 3-Point bearing test setup Laminate thickness t = 1020 mm Maximum static load: F = 500 kN Maximum fatigue load: -50 kN < F < +200 kN Specimen GFRP [+45 / -45 / 0 / 90]5s Bolt d = 40 mm Bearing failure Bore hole for fixation

DLR.de Folie 15 > ICCS20 > Beyland > 05.09.2017 Clearance fit vs. radial pretension Clearance fit Clearance = 0,1 mm Clamping bush for radial pretension Radial expansion u0 = 0,40,8 mm DLR.de Folie 16 > ICCS20 > Beyland > 05.09.2017 Results in bearing strength Bearing strength F bolt load at failure d hole diameter t laminate thickness Static bearing strength with radial pretension -15% compared to clearance fit Fatigue bearing strength with radial pretension +40% compared to clearance fit High pretension results in slightly higher fatigue strength

DLR.de Slide 17 > ICCS20 > Beyland > 05.09.2017 Conclusion and Outlook Conclusion Analytic model for bolted joints with radial pretension is found and validated by FEcalculation (neglecting friction and cleavage) Friction has a big impact especially on tangential stresses Radial pretension results in smooth distribution of stresses around the bore hole Bearing tests with radial pretension show an increase in fatigue strength, but a decrease in static strength Outlook Detailed understanding of bearing failure with radial pretension Prediction of failure load from FE-calculation >>> failure criteria for bearing Bearing tests with different pretensions >> evaluation of optimum pretension Bearing tests with combined radial and axial pretension DLR.de Slide 18

> ICCS20 > Beyland > 05.09.2017 MANY THANKS FOR YOUR ATTENTION ! DO YOU HAVE ANY QUESTIONS ? Lutz Beyland, Composite Design Engineer, German Aerospace Center (DLR), Institute of Composite Structures and Adaptive Systems Lilienthalplatz 7, 38108 Braunschweig, Germany [email protected] DLR.de Slide 19 > ICCS20 > Beyland > 05.09.2017 References [DIEM] Diem, H.: Tragfhigkeit von Bolzenverbindungen in dickwandigen Faserverbundstrukturen; Dissertation; TU Mnchen; 19.2.2007 [HAU] Hau, E.: Windkraftanlagen: Grundlagen, Technik, Einsatz, Wirtschaftlichkeit, Springer Berlin Heidelberg, ISBN 978-3-540-72150-5 [MDLER] Mdler GmbH: Spannstze COM-AS Bohrung 19 bis 100mm; http://www.maedler.de/product/1643/1621/spannsaetze-com-as-bohrung-19-bis-100mm; accessed on 30.8.2017 [TIMOSHENKO] Timoshenko, S.: Theory of Elasticity; McGraw- Hill; 1934 [ZHANG] Zhang, K.-D.; Ueng, C. E.: Stresses Around a Pin-loaded Hole in Orthotropic Plates; Journal of Composite Materials 18 (1984), No. 5, p.

432-446 [CUNTZE] CUNTZE, R.; Das Versagensmoduskonzept, ein praktikables Auslegungswerkzeug bei neuen Leichtbauwerkstoffen; Handout, Dresdner Leichtbausymposium, 7.-9. June 2001 DLR.de Folie 20 > ICCS20 > Beyland > 05.09.2017 Anhang DLR.de Folie 21 > ICCS20 > Beyland > 05.09.2017 Lsungsraum fr segmentierte Rotorbltter Megawind T-bolts DEBRA-25 JOULE III Bolting in longitudinal

direction Enercon E126 Metallic inserts JOULE III Detachable Connection principle Bolting in transversal direction Bolting of pieces with a large overlap NonDetachable Welding of thermoplasts Bonding of

thermosets Gamesa Innoblade Form-fit Indeol / CENER Force-fit Connection tubes JOULE III Bolting of shear web Single lap Multi lap Single lap Multi lap Modular Wind Energy DLR.de Folie 22

> ICCS20 > Beyland > 05.09.2017 Wo teilen? Bolting Bonding Transportation Spar cap loads Secondary loads Big extra mass = extra cost Extra mass = dynamic loads Little space 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8

1 DLR.de Folie 23 > ICCS20 > Beyland > 05.09.2017 Konzept Stegverschraubung y Tip-Stege x Tip-Gurt Wurzel-Gurt Wurzel-Stege Detail Z Schnitt A-A B z x

Detail Z y x WurzelGurte ca. 5m Stegbuchse Wurzelsegment Schraube mit Mutter (ca. M36) Wurzel-Steg A Stegbuchse Wurzelsegment Tip Gurte A Stegbuchse Tipsegment Stegbuchse Tipsegment

Tip-Steg B Schnitt B-B z y DLR.de Folie 24 > ICCS20 > Beyland > 05.09.2017 CAD-Modell Stegverschraubung in Nordex-Rotorblatt NR58.5 Tipsegment tipseitige Verschraubung Endkantengurt Hauptholm Verbindungsbereich X Z Y

Wurzelsegment wurzelseitige Verschraubung DLR.de Folie 25 > ICCS20 > Beyland > 05.09.2017 Segmentiertes Rotorblatt Testpyramide (10-2016) Design Konzeptentwicklung Vorauslegung CAD-Modellierung Coupon-Tests Lochleibungstests Biegebalken Bauteil-Tests Verschraubte Holme Full Scal e Test DLR.de Folie 26

> ICCS20 > Beyland > 05.09.2017 Lochleibungstests BiLo Klebstoff Spielpassung Spannbuchse >> verklebte Probe ohne Flansch >> gesteckte Probe >> verspannte Probe DLR.de Folie 27 > ICCS20 > Beyland > 05.09.2017 Rax-Buchse Funktionsdemonstrator + Patentanmeldung

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