MEASURING FATIGUE LOADS OF BOLTS IN RING FLANGE CONNECTIONS

Dr.-Ing. M. Seidel, Univ.-Prof. Dr.-Ing. P. SchaumannUniversity of Hannover, Institute for Steel Construction

Appelstr. 9A, D-30167 Hannoverhttp://www.stahlbau.uni-hannover.de

ABSTRACT: This paper presents experimental and numerical investigations of ring flange connections in steel tow-ers of wind turbines. The experimental program covers laboratory tests as well as tests in operating wind turbines. Anoverview for both parts of the experiments and an explanation of the chosen instrumentation is given. The laboratorytests serve to verify a three-dimensional finite-element-model. For practical purposes, an overview of simplified cal-culations models that allow to omit FE-calculations is also given. Results from another FE-model of the total ringflange give insight into effects that are neglected when only one segment of the ring flange is investigated.

Keywords: Wind Turbines (HAWT)-Towers, Fatigue, Numerical methods

1 INTRODUCTION

Towers for wind turbines are usually tubular steeltowers, that are manufactured in two or three parts,depending on their total height. They are connected onthe construction site with bolted ring flanges.

Ring flangeSegment

Stresses in shell

Z

Z

Fig. 1: Segment model

The calculation of stresses in the bolt for fatigue as-sessment and the as-sessment of ultimatestrength are performedwith one segment of thetotal flange. The seg-ment is loaded with atension force that sumsup the stresses in theshell. This approach iscalled „segment model“(Fig. 1).

The stresses in the boltdepend nonlinearly onthe tension force as theconnection has pre-loaded bolts. A typicalgraph showing thenonlinear correlation ofexternal load and ten-sion force in the bolt isshown in Fig. 2. Thebehaviour is similar forthe bending moment inthe bolt.

The complex nonlinear

behaviour of this eccentrically loaded connection andhigh dynamic loads of wind turbines with more then109 load cycles in 20 years demand for safe and eco-nomic design methods. Experimental investigations onflange segments in the laboratory and in operating windturbines have been performed to calibrate the results ofsimplified calculation models against experimentalvalues. Additionally, a 3D finite element model hasbeen used to extend the range of investigated parame-ters.

2 EXPERIMENTAL INVESTIGATIONS

The purpose of the experimental investigations was toobtain normal and bending stresses in the bolt in de-pendence of the external loads. The measurementequipment used is described below.

Measured values in the laboratory included strains inthe bolt, deformation of the flange segment with adisplacement transducer, strains in the flange and out-put data of the testing machine. In the operating windturbines, strains in the bolt and in the tower shell and

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Range 1:Approx. linear curve, stresses between flanges are reduced while contact zone is closed

Range 2:Sucessive opening of the flanges

Range 3:Open connection with slope depending on loads and geometry.

Range 4:Plastification of bolt and/or flange until failure of the connection

ZU F / γRange of service loads

Fig. 2: Nonlinear curve of bolt force vs. external force

output values from the wind turbine (wind speed, ratedpower, rotating frequency and yaw angle) were re-corded.

2.1 Measuring method to obtain stresses in the bolt

The tests were performed with bolt fittings according toDIN 6914. Strain gauges were applied on the bolt shankin order to obtain the axial force and the bending mo-ment in the bolt. The shank diameter was reduced by3mm to avoid contact between the strain gauges and theflange. The cables were run through a sloped hole,extending from the reduced area to the bolt head. Alter-native paths e.g. through a notch in the flanges, was notpossible because of the installation procedure of thetower. The influence of the reduced stiffness is rela-tively small. It can be directly considered when per-forming comparative calculations for the laboratorytests. In the flange in the operating wind turbine, thestresses in the bolt are proportional to the axial andbending flexibilities of machined and unmachinedbolts.

Fig. 3: Bolt M30 with strain gauges

The configuration of the strain gauges was chosen tomatch these criteria:

1. Unequivocal determination of the bolt’s stress statewas required. Thus three test points were arrangedaround the circumference of the bolt to describe thecomplete stress plane.

2. The measurement should not be influenced bytemperature effects. Thus full bridge strain gaugeswere used.

3. Full bridge strain gauges have the additional ad-vantage that measurement of axial components re-sulting from tension force and bending moment andshear components resulting from the torsional mo-ment do not influence each other.

The measurement of the torsional moment allows thecalculation of the friction coefficient of the bolt thread.This is useful for checkingthe plausibility of themeasured preloads.

A complete measuring boltis shown in Fig. 3. Thecopper rings were used tocenter the bolt in the hole.

The chosen instrumenta-tion of the bolt is veryexpensive, because of highcost of the application ofthe eight strain gauges andtheir cost of approximately150 Euro. These costs arejustified because of themeaningful informationthat is obtained.

2.2 Laboratory tests

The test setup in the laboratory is shown in Fig. 5. Theflange segment is loaded with up to 530 kN tensileforce in ultimate load tests by the structural actuator.The data provided by the test system (proof load andpiston stroke) and the strain-gauge signals are recordedwith a Spider-8 universal digital multichannel system.

Fig. 5: Test with flange segment in the laboratory

A total of four flange segments with different dimen-sions were tested in the laboratory (Fig. 4). Two flangeswere machined out of original flanges used in largewind turbines, the other two flanges were machinedfrom the first ones by reducing the thickness. The thinflanges 3 and 4 were used to cover a wide range ofparameters. The original flanges 1 and 2 were „thick“flanges, flanges 3 and 4 were rather thin.

The analysis of test data was performed with a specialsoftware that calculates axial force, bending momentand direction of the bending moment. The software hasbeen developed at the Institute for Steel Construction.

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M36-215c=90mm

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M30-95c=117mm

Fig. 4: Test flanges

2.3 Tests in operating wind turbines

Tests in operating wind turbines were performed as acomparison to the laboratory tests. To determinestresses in the tower, strain gauges were applied belowthe flange on the inner and outer side of the shell. Fourtest points were arranged around the circumference, soa check of the measured values was possible by com-parison of opposite test points. In contrast to otherinvestigations, e.g. load spectra measurements, onlinereduction of the data was not possible. Correspondingvalues of all channels were required, thus time serieswere recorded. An example is shown in Fig. 8. Thechart shows the external tension force that was calcu-lated from the strains in the shell and the correspondingbolt force. It is obvious that external force and boltforce correlate, but the variation of bolt force is muchsmaller than the variation of the external force due tothe positive effect of the preloaded bolts.

To reduce the amount of recorded data, a special con-trol of saving frequency was applied. The saving ratewas dynamically adjusted from 1/60 to 10 Hz depend-ing on the wind speed. Thus a good resolution wasguaranteed in times of high loads without saving use-less data in times of low loads. Nevertheless, around 10GB of data were recorded within eight months.

Additionally the flatness of the flange was measured togain an impression of the deviation from the targetgeometry. The thickness of the coating was measured toevaluate the influence on loss of preload because ofsettlement effects.

3 NUMERICAL INVESTIGATIONS

As the investigation of flange connections can not beexclusively performed by experiments, a 3D-FE-modelwas build to compare the numerical results with theexperiments and to extend the range of parameters thatare investigated. The tests serve to calibrate the model.A wide range of numerical calculations can then beused to evaluate simplified calculation methods.

3.1 FE-Modelling

Different 3D-models of a flange segment and of thetotal flange were used. A detailed description of themodels can be found in [1]. A summary with focus onmodelling of the bolt was published in [3].

Two FE models have been used: A 3D model of onesegment (Fig. 6) and an refined model of the totalflange. As the use of brick elements in the total modelproduces too many elements, so called super elementswere implemented. With this calculation technique, apart of the model is condensed into the stiffness matrixbefore the main calculation. This matrix element iscalled a superelement. The superelement is part of thetotal model just like any other element. As it does onlyallow linear properties, it can not be used if non-lineareffects, e.g. plasticity or geometric non-linearity, arepresent. When calculating fatigue loads, plasticity is notan issue and geometric non-linearity is negligible. Theadvantage is that a relatively fine mesh can be used withreasonable computing time.

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Tension force in tower wall

Bolt force

Z [kN] FS [kN]

Fig. 8: Example for a time series of calculated load perflange segment (left ordinate) and bolt force (right ord.)

Fig. 7 shows the comparison of test result and FE-calculation for test flange 1 (dimension see Fig. 4). Theagreement is good, similar results were obtained for theother test flanges.

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Fig. 6: Test flange 1 and FE-model

Fig. 7: Comparison of test result and FE-calculation for test flange 1

3.2 Simplified calculation models

Even though finite element calculations are not un-common for the assessment of wind turbines, simplifiedmethods are useful for predesign and comparativestudies. In Germany, proposals from Petersen [2] andSchmidt/Neuper [4] have been used to assess flanges. Anew proposal from Seidel [1] allows for a better ap-proximation of the non-linear relationship betweenexternal loads and bolt force. Additionally, the bendingmoment in the bolt is obtained. Fig. 9 shows a compari-son of the three calculation methods with the FE-resultsfor test flange 1. The older models attain a safe ap-proximation of bolt force and slope of the curve for lowtension force, but for high tension forces the slope canbe underestimated. This can be important for loadcycles with a high mean value. The approach fromSeidel [1] attains a safe approximation for both boltforce and slope of the curve for all tension forces. Aslight underestimation of the bending moment occursfor high tension forces, but this is compensated by theoverestimation for the axial force.

The amount of bending stresses that is superimposed tothe axial stresses is considerable. In this case, the addi-tional stresses ∆σN and ∆σM are in the same order ofmagnitude. Thus it is not justifiable to neglect thebending stresses for fatigue assessment. According toEurocode 3-2 [5] the assessment can be performedusing detail category 50* against the sum of axial andbending stresses.

3.3 Effects of the total system

An FE model of the total ring flange was used to evalu-ate the influence of effects that are neglected with thesegment model. The following results were obtained:

1. The difference between the segment model and thetotal flange model is small for typical flanges ofwind turbines within the range of service loads. Thedifference becomes greater for high loads that areclose to the limit load.

2. The deformation of each segment is dominated bythe total system. Thus a local change in stiffness,e.g. by a low preload of one bolt, does not influencethe additional stresses.

3. A constant gap on the inner side of the flange isadvantageous for the carrying behaviour and resultsin lower additional stresses in the bolt.

4 SUMMARY AND PERSPECTIVE

The stresses in bolts of eccentrically loaded, preloadedflanges depend nonlinearly on the external loads. Accu-rate knowledge of axial and bending stresses in thebolts is necessary for fatigue assessment. An expensivebut useful experimental setup for laboratory and fieldtests has been presented for the investigation of thebolt’s stress state. A comparison of experimental andnumerical values has shown that a good agreement canbe achieved. As an addition to numerical methods,simplified methods have been investigated to show theaccuracy of results. As axial and bending stresses occurin the same order of magnitude, only models that allowfor the calculation of both should be used.

In the complete system, effects that are not covered bythe segment model can also be important. A FE modelof the total flange has shown that the calculation ofstresses for fatigue assessment can be performed withthe segment model because the differences are negligi-ble for the investigated range of loads.

5 REFERENCES

[1] Seidel, M.: Zur Bemessung geschraubter Ring-flanschverbindungen von Windenergieanlagen.Schriftenreihe des Instituts für Stahlbau (Heft 20),Shaker 2001.

[2] Petersen, C.: Stahlbau, 3. Auflage Braunschweig:Wiesbaden: Vieweg 1997.

[3] Schaumann, P.; Kleineidam, P.; Seidel, M.: ZurFE-Modellierung von Schraubenverbindungen.Stahlbau 70 (2001), S. 73-84.

[4] Schmidt, H., Neuper, M.: Zum elastostatischenTragverhalten exzentrisch gezogener L-Stöße mitvorgespannten Schrauben. Stahlbau 66 (1997), S.163–168.

[5] ENV 1993-2: Eurocode 3, Part 2: Steel Bridges.1997.

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Fig. 9: Comparison of FE-results and simplified calculation methods