COMPONENT FATIGUE TEST FACILITIES FOR FULL-SCALE LP STEAM ... · The need for large steam turbines...

Proceedings of the 1st Global Power and Propulsion ForumGPPF 2017

Jan 16-18, 2017, Zurich, Switzerlandwww.pps.global

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COMPONENT FATIGUE TEST FACILITIES FOR FULL-SCALE LP STEAMTURBINE END STAGE BLADES

Shilun ShengSiemens AG Power and Gas Division

[email protected]. 100, 45478 Muelheim, Germany

Johan FleglerSiemens AG Power and Gas Division


Balazs Janos BecsSiemens AG Power and Gas Division


Michael DankertSiemens AG Power and Gas Division


ABSTRACTIn connection with the energy transition, the demand for

large steam turbines for power plant applications and theirefficiency has significantly increased over the last years. Dueto an increased injection of renewable energy sources,operational flexibility of power plants is getting more andmore important. Robust low-pressure (LP) end stage bladesare one of the key components and a crucial success factor tomeet current and future market requirements and to masterchallenges associated.

In operation, LP end stage blades are exposed to highand complex mechanical loads. Against the background ofincreasing operational flexibility of power plants associatedwith a higher number of load cycles, reliable and verifieddesign methods are essential for designing LP end stageblades and their structural integrity assessment. Supportingthis purpose, several fatigue test facilities for full-scale LPend stage blades were established at Siemens recently.Besides the determination of component fatigue strength andthe validation of the design procedure, the tests serve as partof up-front validation, minimizing risk for first timeimplementation of newly developed as well as nextgeneration blades, and demonstrating operational robustnessof the existing fleet.

This paper describes the development and setup offatigue test facilities for LP steam turbine end stage blades atSiemens and the successful execution of component testswith focus on High-Cycle Fatigue (HCF) regime. Results forblades of different sizes, surface conditions and materialshave been evaluated. In addition, crack growth and thresholdbehaviour of blades has been investigated in detail. Based onthe test results, validation of the corresponding designmethods has been performed.

INTRODUCTIONThe need for large steam turbines for fossil power plant

applications significantly increased over the last years andwill increase further in the future. In the past, fossil-poweredplants have usually been operated as base load power plants(full load over a long period). Enhancements were mainlydriven by power and efficiency increase under full load,reliability, and a growing number of operating hours.

Connected to the energy transition, associated withincreased and fluctuating injection of renewable energy,operational flexibility of power plants is becoming more andmore important. Additional challenges arise from theincreasingly flexible operation of fossil power plants. Themain challenges are high efficiency under part load, highernumber of load cycles, higher load gradients, and shorterstart-up times. Therefore, reliable and robust turbines have tobe designed. As one of the key components, LP steamturbine end stage blades are the crucial success factor to meetcurrent and future market as well as customer requirements,and to master challenges associated to this.

The design of turbine blades is complex. In operation,end stage blades of LP turbines are often exposed to high andcomplex mechanical load regimes. These loads lead to highstresses, often resulting in fatigue lifetime consumption.Manufacturers of large turbines for power plant applicationsusually cannot test their turbines and correspondingcomponents in test facilities in advance. In addition, designfaults could lead to failure of the components in operationand to substantial commercial damages. Because of thesereasons, appropriate, reliable, and verified design methodsand guidelines are essential for the development and designof LP turbine end stage blades. Their lifetime prediction andstructural integrity assessment, enabling reliable operation ofturbine blades over the entire life cycle of the power plant, is

2Copyright © 2017 by Siemens AG

attached to these design methods and guidelines. Inparticular, this applies for describing precisely fatiguephenomena, i.e. Low-Cycle Fatigue (LCF) and HCF, andcomponent (including the connection between blade root androtor groove) behaviour under operating loads.

For large turbine blades, especially for LP steam turbineend stage blades, lengths up to 1800 mm and mass up to 330kg result in huge centrifugal forces up to 15 MN per blade.Efficiency increase and power plant optimization yield tolarger turbine outlet areas associated with larger LP turbineend stage blades. In addition, high mass flow and pressureyield to high steam bending forces.

Cyclic stresses mainly originating from variablecentrifugal forces during start-up and shutdown have to beassessed against LCF criteria. The criteria for airfoilvibration induced by changing steam-driven bending forcesare HCF/ endurance limit criteria.

Figure 1 shows the typical loading conditions of an LPend stage blade in real operation schematically.

Figure 1: Typical loading conditions of LP blades

In order to develop and set up testing environmentssuitable for LP end stage blades, the following existing testfacilities have been analysed.

In the past, efforts have been made within theframework of FVV (German Research Association forCombustion Engines) to investigate the fatigue behaviourusing small-scale T-shaped specimens [1]. The T-specimen,see Figure 2a, represents the notch radius of a blade root.Under axial and bending loads, a similar local stress gradientas in a real blade root is ensured. However, the blade-to-rotorconnection cannot be examined with this setup (Figure 2b).

Another biaxial test facility is reported in [2]. Here,combined-cycle-fatigue (CCF) behaviour of gas turbineblades in high temperature environment has been analysedusing specifically designed specimens with a transitionradius representing the critical location for crack initiation ofa gas turbine blade. Excitation for HCF load was achievedusing a shaker. With this test setup, the connection betweenblade root and rotor groove cannot be considered either.

The main challenge of setting up suitable test facilities isthe appropriate application of huge centrifugal forces incombination with bending loads on full-scale components tosimulate real operating conditions.

Figure 2: a) Testing principle b) Testing facility forsmall-scale T-specimens [1]

This paper deals with the development and setup ofseveral component fatigue test facilities at Siemens and thesuccessful execution of component tests with full-scale LPsteam turbine end stage blades with main focus on HighCycle Fatigue (HCF) regime. Results have been evaluatedand validation of the corresponding design methods has beenperformed.

COMPONENT TEST FACILITIESSiemens has developed and established several test

facilities for full-scale LP end stage blades. Focus is on HCFand LCF strength under operational loading as well as onblade-to-rotor connection.

First HCF test results are reported in [3].

HCF component test facilityThe initial purpose of the HCF component test facility

was to determine the endurance limit of full-scalefreestanding LP end stage blades under real operationconditions. Thereby, the requirements for the test facilitywere considered omitting centrifugal load. In order to ensuresimilar conditions compared to those in real operation, thecorrect dynamic stress distribution and amplitude level in theblade root have to be met. This can be achieved by theexcitation of blade eigenmodes at their eigenfrequenciescombined with an even more important proper clamping ofthe blade root. Clamping of the root is achieved by applyinga constant pressure on the fir-tree root.

An extension of the test rig comprises an automatedclamping pressure variation, allowing simulating the effect ofstart-up and shutdown cycling on the upper bearing flank ofthe blade root.

Development and setup of the HCF test facilityThe most crucial part of the component test facility is

the clamping device. The clamping device consists of aseismic block with a prismatic pocket where the two-piecegroove block is assembled in. The blade is clamped betweenthe rotor groove blocks. A hydraulic cylinder captures thepressing force, and the cylinder force is distributed via athrust plate. Therefore, the groove blocks are lifted into theseismic block and, caused by the bevelled bearing surfaces of


the blocks, shifted laterally inwards. Figure 3 depicts the testsetup principle.

Figure 3: Clamping device

The interchangeable groove blocks achieve testing ofeach root shape and size from Siemens’ portfolio of streamturbine blades.

The latest modification to the test facility is an extensionregarding the clamping pressure. Now, automated loadingand unloading cycles can be achieved within meaningfulpressure limits. The load cycle characteristics consist of aloading ramp, holding time, a relief ramp, and anotherholding time at low-pressure load. The frequency of the loadcycles can be adjusted to meet testing needs, e.g. to keep theentire test duration in acceptable limits.

Since the setup, consisting of several individual parts,bears difficulties regarding a proper blade assembly, highestmanufacturing accuracies have to be kept. Themanufacturing procedure, along with the quality assurancemeasures, are described in [3] in closer detail.

A lateral shaker is used to oscillate the blade at itseigenfrequency. The shaker is connected to the blade airfoilvia a thrust rod and a clamping frame enclosing the airfoil. ALASER sensing device, monitoring the airfoil motion, isused for vibration amplitude controlling. During one test, theamplitude level is kept constant. If the shaker powerincreases during testing, a change of the eigenfrequency ofthe blade is likely, indicating a crack. The entire setup isdepicted in Figure 4.

Figure 4: HCF component test facility

Calibration and instrumentationAs presented in [3], proper clamping of the blade root is

indicated by a uniform contact pattern on the bearing flanks,verified by the copper foil imprint technique. An additional3D measurement is used to prove a proper test setup. Afurther indication of proper clamping is the coincidence ofthe blade eigenfrequency in the test facility with the real,freestanding, blade eigenfrequency. The eigenfrequency ismeasured for various cylinder pressures to acquire asaturation curve and to confirm the appropriate staticloading.

For pure HCF testing, strain gauges were applied in theupper notch above the contact surface. At the predicted cracklocation, a strain gauge chain was used. Here, the higheststress gradient can be found. The location of strain gauges isdepicted in [3]. Data acquisition is conducted dynamically,i.e. only the alternating load (peak-to-peak) is recorded.

Figure 5: Location of strain gauges at suction side

For combined LCF pre-damage and subsequent HCFtesting, the static strain level has to be determined as well.Therefore, the connection of several of the strain gauges ofthe HCF instrumentation was changed into a 3-wireconnection, and additional strain gauges for static strainacquisition were applied between the upper and middle fir-tree root teeth. The extended instrumentation is depicted inFigure 5. The purpose of pre-damage testing is to investigateits influence on the HCF strength.

In order to verify operational capability of the test rig,the blade airfoil is excited and the foil vibration and strainamplitudes are recorded. This data is compared to thecalculated figures. This calibration step is performed forevery single blade. It can generally be stated that there is agood agreement of measured data and finite element (FE)results, proving a proper test setup.

The clamping pressure applied during pre-damagetesting series is captured by static strain gaugemeasurements. During commissioning, it was observed thatthe static strain in the blade root was introduced properlywhile increasing the pressure (start-up simulation). However,during the relief phase, the strain in the root did not dropdown as assumed. This behaviour could be addressed to thecontact surfaces of the groove blocks and the seismic table.Here, despite of lubricating the contact faces, frictional


forces led to a sticking effect, resulting in a persistingresidual load. To eliminate this effect, the lubricant in thecontact areas of the groove blocks and the seismic table wastaken out and a PTFE interlayer was applied instead. Theefficacy of this method is depicted in Figure 6, below. It canclearly be seen that during relief phases (upper part, dashedlines), the strain drops to a constant level around -10 % forthe PTFE-separated test rig parts (solid red line) while thereis a significant residual strain in the lubricant-separated case(starting at approximately -55 % and increasing to -90 %,solid black line). Note that the lubricant was applied to testrig parts only, i.e. not at the interface of the root to theclamping device at all.

Figure 6: Influence of lubrication on the reliefbehaviour

LCF test facility for full-scale bladesTo determine the LCF strength of full-scale LP end stage

blades, Siemens developed another test facility. Here, theblade root can be charged with a load similar to centrifugalforces under operation. One greater challenge was the properclamping of the blades. Enabling the transmission of LCFforces up to 5 MN, the airfoil geometry was modified. Asolid clamping device attached to the blade airfoil and amassive groove part enveloping the blade root were used tointroduce the force. All parts were manufactured highlyaccurate to meet the quality requirements needed for properclamping. The testing setup is shown in Figure 7.

Figure 7: LCF component test facility

To acquire crack initiation and crack growth, straingauges were applied in the upper root notch both at pressureand at suction side of the root. Providing an additionalcontrol mechanism, further strain gauges were installed onthe blade airfoil. To calibrate the test rig, the strainamplitudes were recorded and compared to the calculateddata. A near-perfect fit was achieved for the locations ofstress maxima and strain levels at the location of the straingauges.

Test facility for blade-to-rotor connectionsIn order to extent testing to the interface of the root,

including the groove, a third test rig was established. Again,at the regions of interest, similar loading conditions as underoperation had to be kept.

For this testing program, a uniaxial testing concept waschosen. The Materials Testing Institute of the University ofStuttgart (MPA Stuttgart) was selected as outside partner.

A 3D model of the blade part and groove block isdepicted in Figure 8 along with a photograph of the testpieces in the LCF testing machine at MPA Stuttgart.

Figure 8: 3D model and test facility for blade-to-rotor connection investigations

The blade part was designed to ensure a proper load ofthe blade-to-rotor connection. Again, the manufacturingquality was essential to obtain reliable testing data.

Because of the high manufacturing accuracy and thethoughtful design, a similar strain distribution compared toreal blades in operation was achieved for the tested bladeroots and rotor grooves. One example is shown in Figure 9.Here, the very good agreement between operation and testfacility is obvious for the lower notch of rotor groove atpressure side.

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Figure 9: Strain distribution in the test facilitycompared to real operation

Strain data was acquired using strain gauges in thenotches of the groove and the blade part as well as on themounting shanks. The data was used for calibration purposesand for operating the test rig.

COMPONENT TEST RESULTS

HCF Testing ResultsThe HCF component tests were carried out at the first

eigenfrequency (mode 1) of the tested blades at specifieddeflection amplitudes up to 108 cycles or to cycles of crackinitiation. Reliable indicators for crack initiation were achange of the blade eigenfrequency along with an increase ofshaker power [3]. Furthermore, the occurrence of a crack canbe detected by changes of the acquired strain levels.

At the end of each test, the cracked blades wereexamined regarding the crack initiation locations and thecrack depths.

The observed crack locations correlated very good withthe predicted position, see Figure 10. Hence, the generaldesign calculation methods could be positively confirmed.

In the following sections, investigation results fromdifferent blade types of Siemens’ portfolio of LP end stageblades are exemplary shown. Its root and airfoil shapes aswell as size characterize a blade type.

Figure 10: Crack initiation location of tested bladecompared to prediction

The investigation of the influence of different bladematerials on the component endurance limits was conductedwith blades of type I with the same surface treatment (shot-peened).

The results depicted in Figure 11 clearly show anincreased endurance limit for material II with the higherstrength properties. The determined increase matches withthe expected endurance limit change derived from materialproperties (shown as stress-cycle- (S-N-) curves). Exceedingthe results shown in [3], the testing scope for material I wasextended to higher amplitude levels, i.e. to the low cyclefatigue regime. It also applies for these amplitudes that thecorrelation with the S-N-curve is very good.

Figure 11: Influence of the material on theendurance limits

With blade type II, the influence of the surface treatmenton the endurance limit was examined. In [3], it could clearlybe shown that shot-peening increases the endurance limit incomparison to milled-only surfaces. Here, the influence ofspinning of shot-peened blade roots on the HCF strength isinvestigated closer. Based on the testing results, there is nosignificant difference between non-spun and spun blades, seeFigure 12. The endurance limits and fatigue strengths are inthe same scatter band. Hence, spinning of blades, which is aninherent part of the manufacturing process, does notinfluence the blade root itself.

Figure 12: Influence of spinning of shot-peenedblades on the fatigue behavior

Another testing aspect was fracture mechanics. Afteridentifying crack initiation, threshold and crack propagationwere investigated. As stated before, strain gauge data andshaker power were analyzed to notice crack initiation. Non-destructive testing (NDT) utilizing the phased arrayultrasonic testing method (PA-UT) was applied to prove


crack initiation and to monitor crack propagation withoutdisassembling the blade. However, at certain intervals, theblade was removed for standard dye penetrant testing (PT) toverify crack initiation or crack propagation indicated by thePA-UT technique.

The heat-tinting method was used to colorize the cracksurface. This method enables the determination of crack sizeand shape and the investigations regarding crack initiation,threshold and crack propagation.

Finally, the blade root was cut from the entire blade andbroken up along the crack surface for an exact determinationof crack size and shape. Figure 13 shows one example forthis examination.

Figure 13: Crack surface analyses a) Fracturesurface b) Crack initiation area

The last stage of crack propagation can clearly be seenin Figure 13a. Maximum crack depth is approximately31 mm, and the crack length is approximately 150 mm as itwas determined by PT testing.

Details of the fracture surface relating to crack initiationare shown in Figure 13b. Maximum crack depth for thethreshold test is 2.2 mm and 7 mm and the total length of theinitiated crack is approximately 60 mm.

The crack sizes have been predicted by fracturemechanical analysis. The measured and predicted crackpropagation in crack depth direction are in good agreement,see Figure 14.

Figure 14: Comparison of measured and predictedcrack depths

LCF Component Test ResultsThe LCF tests for full-scale LP end stage blades were

carried out with a constant loading rate of three cycles perminute. Crack initiation was predicted by means of measuredstrain changes and the potential drop method. At the end ofeach test, the cracked blade was examined regarding thecrack initiation location and crack size.

As shown in Figure 15, the crack initiation locationscorrelate well with the prediction by means of FEcalculations. The primary crack was initiated in the uppernotch at suction side (note that the crack in the second notchwas initiated later during other tests).

Figure 15: Crack initiation location of a tested bladecompared to prediction

All tests were performed for one blade type with threedifferent surface conditions to determine the LCF strengthand to investigate the influence of surface treatment in therange of LCF numbers relevant for blade design. As shownin Figure 16, shot peening and rolling significantly improvesthe LCF life, and the determined LCF lifetime lies above theLCF curve of standard smooth specimens. The definition ofLCF lifetime relates to crack initiation detected by strainchange correlating very well with typical crack initiationdefinitions.

Figure 16: Comparison of LCF numbers

Blade-to-rotor Connection Test ResultsA modern lifetime prediction method of blade roots and

rotor grooves requires improved understanding of blade-to-rotor-connections under real operational conditions. Siemenshas conducted a series of full-scale blade-to-rotor connectiontests. These tests were performed with the same loading rateas for the full-scale blades, i.e. three cycles per minute.

Testing revealed a significant drop of strains in the lowernotch of the grooves at the very beginning of the test. Thisbehaviour can be addressed to friction effects correlating

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Fatigue curve, smooth specimenFull-scale blades, milled onlyFull-scale blades, milled and shot-peenedFull-scale blade, milled and rolled


with increase of friction coefficient. At a certain number ofcycles a constant strain level and ultimate value of frictionrespectively is reached. The phenomenon observed correlatesvery well with published literature, e.g. reported in [4].

The investigation was conducted on a full-scale endstage blade-to-rotor connection with different surfaceconditions of the blade vs. the groove, i.e. milled vs. milledand shot-peened vs. milled. The testing results areexemplarily shown in Figure 17 and Figure 18.

Figure 17: Measured strains under LCF loading

The strains gained from the surface conditioncombination milled vs. milled decreases faster compared tothe combination shot-peened vs. milled, Figure 17. However,at a higher number of cycles, the measured strains convergeachieving finally a steady state value.

Based on the measured strains and FE-modelling,friction coefficients were derived for the tested full-scaleblade-to-rotor connections. As shown in Figure 18, thefriction coefficients of both surface combinations increasedat the beginning and reached a similar ultimate value offriction in the steady state after approximately 1,700 cycles.At the beginning, the friction coefficient of the surfacecombination milled vs. milled was higher compared to thesurface combination shot-peened vs. milled.

Figure 18: Friction coefficients under LCF loading

VALIDATION OF THE HCF CALCULATION METHODThe HCF blade tests have also been utilized to validate

the HCF calculation method. The procedure of this validationis exemplary described for blade type I with material I in [3].

Here, the good match between the calculation and thetesting results of blade type II prove the design methodfurther.

The test rig was modelled using the FE analysis softwareAbaqus 6.11.2. The FE model of the test setup was preparedwith contact between the blade root and the groove. Thesidewalls of the groove blocks are slidable on their matingsurface in the seismic table. A static load corresponding to

the cylinder pressure acting from below was applied to holdand load the blade statically. Then, a modal analysis wasperformed as a linear perturbation step. 1st-mode results areused for the evaluation.

The measured strains and blade deflection werecompared with the FE results. The measured strains along thegauge chain and blade deflection were used for theevaluation.

An example of the good match between measuredstrains and the strains predicted by an FE analysis is depictedbelow (Figure 19) for several blades. Here, strains are scaledto the same amplitude level. The green dots represent the FE-calculated strains while the lines show the measured strainsalong the strain gauge chain.

Figure 19: Comparison of measurement and FEresults along the strain gauge chain

Based on the good match between calculation andmeasurement, the maximum strains, at locations that cannotdirectly be captured by strain gauges, could be estimated.The strains were then transformed into stresses using the FEanalysis result.

The resulting dynamic notch stresses of each individualblade are plotted into a component Wöhler diagram(normalized airfoil amplitude vs. load cycles). Here, theresults for blade type I and for blade type II are coincidentlyshown (see Figure 20).

Figure 20: Validation of design Wöhler curves

For both blade type I and blade type II, it can be seenthat the fatigue strengths lie above their corresponding design

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Design curve type IBlades type IDesign curve type IIBlades type II


limit curves. This proves the design method and displays thatthere is sufficient safety margin against HCF failures.

OUTLOOKThe testing facilities and methods presented in this paper

get along with idealizations. Hence, the next consequent stepis to extend the testing method as appropriate.

A research project within the framework of AG-Turbo,partly funded by German government, has been initiated incooperation with MPA Stuttgart in 2013 [5]. Current focus ofthe project is HCF for blades under consideration ofcentrifugal load.

SUMMARY AND CONCLUSIONLP end stage blades are exposed to complex mechanical

load, resulting in stresses mainly due to blade vibration andhigh centrifugal forces. For lifetime and endurancepredictions, it is necessary to understand the componentbehaviour under real operation loading including blade-to-rotor connection. Especially in cases where experimental lifetime prediction is technically not possible or economicallynot justifiable, appropriate, reliable and verified designmethods and guide lines are essential for development anddesign of LP turbine end stage blades. This belongs to theirlifetime prediction and structural integrity assessment as wellenabling reliable operation of turbine blades over the entirelifetime of the power plant. Design faults could lead tofailure of the components in operation and to substantialcommercial damages. Although much effort has been madeto test specimens, real component behaviour cannot beinvestigated sufficiently. Tests on full-scale blades arebecoming more and more important.

To investigate full-scale LP end stage blades includingblade to rotor connection under real operating conditions,several test facilities have been developed, setup andestablished at Siemens. Significant calibration effort has beenperformed to verify their operational capability. HCF andLCF strength (fatigue and endurance limits) as well asthreshold and fatigue crack propagation under real operatingloads have been successfully determined for different LP endstage blade types. The impact of different materials andsurface treatments on fatigue and endurance limit has beeninvestigated. The positive effect of compressive residualstresses on LCF and HCF strength has been proven. Basedon the results, all calculation methods applied have beensuccessfully validated.

In order to extend the investigations on full-scale LP endstage blades, a test facility allowing for combined HCFloading, centrifugal force and LCF loading is underdevelopment.

NOMENCLATURE/ ABBREVIATIONAdyn Dynamical deflection of airfoilCCF Combined-cycle fatigueFB Bending forceFC Centrifugal forceFE Finite-ElementHCF High-cycle fatigue

LCF Low-cycle fatigueLP Low-pressureNDT Non-destructive testPA-UT Phased array ultrasonic testing, NDTPT Penetrant testing, NDTPTFE PolytetrafluoroethyleneSG Strain gaugeS-N Stress-cycle

PERMISSION FOR USEThe content of this paper is copyrighted by Siemens AG

and is licensed to GPPS for publication and distribution only.Any inquiries regarding permission to use the content of thispaper, in whole or in part, for any purpose must be addressedto Siemens AG directly.

REFERENCES[1] MPA Stuttgart: HCF-LCF-Bauteilbeanspruchung„Untersuchung des langzeitigen Werkstoffverhaltens beibauteilrelevanten Belastungskombinationen aus nieder-zyklischer Zugschwell- und hochzyklischerWechselverformung unter Berücksichtigung derKerbwirkung“, FKM Forschungsheft 277, 2003[2] Weser, S., Gampe, U., Raddatz, M., Parchem, R. andLukas, P.: Advanced experimental and analyticalinvestigations on combined cycle fatigue (CCF) ofconventional cast and single-crystal gas turbine blades,Proceedings of ASME Turbo Expo, GT2015-43234, June 15-19, 2015, Montreal, Canada[3] Sheng, S., Flegler, J., Becs, J.-B. and Dankert, M.:HCF component tests on full-scale low pressure steamturbine end stage blades, ASME Turbo Expo, GT2016-56988, June 13-17, 2016, Seoul, South Korea[4] Leidich, E. and Hauschild, S.: RechnerischeFestigkeitsbewertung zur Auslegung vonreibdauerbeanspruchten Maschinenbauteilen, FVVInformationstagung, 2016, Bad Neuenahr[5] AG-Turbo Project COOREFELX 4.1.2:Endstufenschaufeln für hoch-flexible Fahrweise und hoheStartzahlen, 2013

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