Fatigue Testing at 1000Hz Testing Frequency

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Procedia Structural Structural IntegrityIntegrity Procedia1300(2018) (2016)676–679 000–000

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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity

Fatigue Testing at 1000Hz Testing Frequency Fatigue Testing at 1000Hz Testing Frequency

XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paço de Arcos, Portugal

Markus Berchtold*, Ingbert Klopfer Markus Berchtold*, Ingbert Klopfer

Russenberger Prüfmaschinen AG Switzerland Thermo-mechanicalRUMUL modeling of a high pressure turbine blade of an RUMUL Russenberger Prüfmaschinen AG Switzerland airplane gas turbine engine Abstract

Abstract a b c P. Brandão , V. machine, Infantewith , A.M. Deus * of 1000Hz. The dynamic load of In 2014 RUMUL could present a new resonant fatigue testing a testing frequency In 2014 aRUMUL could present a new resonant fatigue testing machine, with a to testing frequency of 1000Hz. dynamic of maximum 50kN peak-peak is produced with an electromagnetic system. Similar established resonant systemsThe which run onload testing Department of Mechanical Engineering, Superior Técnico, de Lisboa, Av. Roviscosystems 1049-001 Lisboa, maximum 50kN is produced withstatic anInstituto electromagnetic Similar tobyestablished resonant which runload on testing frequencies frompeak-peak about 40 up to 250H. The portion of the system. load isUniversidade provided a mechanical spindle,Pais, the 1, maximum of the Portugal frequencies about up to 250H. The static portion of the load is provided by that a mechanical spindle, maximum loadcan of the b system is +/-from 50kN. Any40 ratio can be selected. Flat and round specimen types are normally usedthe inPais, fatigue testing be IDMEC, Department ofload Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco 1, 1049-001 Lisboa, system is +/50kN. Anymachine load ratio can be Flat and specimen types that are normally used in fatigue testing can be used. The new testing offers newselected. possibilities for round investigations of material properties in the very high cycle fatigue Portugal c used. Theregime. new testing machine offersEngineering, new possibilities for investigations of material inRovisco the very cycle several fatigue CeFEMA, Department of Mechanical Instituto Superior Técnico, Lisboa, Av. Pais,high 1, 1049-001 Lisboa, (VHCF) Compared to other systems used in the field of VHCFUniversidade testing thedeproperties RUMUL GIGAFORTE provides (VHCF) regime. Compared to other issystems in the consumption fieldPortugal of VHCF the RUMUL provides several advantages. The size of the machine smaller used and energy lesstesting compared to a servoGIGAFORTE hydraulic system. The actually advantages. The size of the machine is smaller and energy consumption less compared to a servo hydraulic system. The actually tested material volume is larger than the material volume that is tested on ultrasonic systems. The testing frequency of 1000Hz tested material is larger than the material that is tested ultrasonic systems. The years testing of machine 1000Hz allows normallyvolume continuous testing, without stoppingvolume for cooling down theon specimen. In the past three thefrequency new testing Abstract allows normally used continuous testing,atwithout stopping of forthe cooling down the specimen. the pastinthree years the new testing was intensively for example the laboratory Fraunhofer institute IWS In Dresden Germany. Effects of the machine 1000Hz was intensively at the laboratory of the Fraunhofer institute IWS in Germany. of theup1000Hz testing frequencyused on for the example fatigue behaviour of the material were observed. This talkDresden shows some exampleEffects of heating of the During their operation, modern aircraft ofengine components are subjected to increasingly demanding operating conditions, testing frequency on the fatigue behaviour the material were observed. This talk shows some example of heating up of the specimen related to the 1000Hz testing frequency and highlights some of the found frequency related effects on fatigue strength. especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent specimen related to the 1000Hz testing frequency and highlights some of the found frequency related effects on fatigue strength. degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict © the 2018 The Authors. Published by Elsevier B.V.data records (FDR) for a specific aircraft, provided by a commercial aviation behaviour of HPT blades. Flight © 2018creep The Authors. Published by Elsevier B.V. © company, 2018 The under Authors. Published by Elsevier B.V. Peer-review responsibility of the ECF22 organizers. wereresponsibility used to obtain and mechanical data for three different flight cycles. In order to create the 3D model Peer-review under of thethermal ECF22 organizers. Peer-review responsibility ECF22 needed forunder the FEM analysis,ofa the HPT bladeorganizers. scrap was scanned, and its chemical composition and material properties were Keywords: New Fatigue Testing Machine; Frequency effects simulations were run, first with a simplified 3D obtained. TheResonant data that was gathered was fedGiga intocycle the (VHCF); FEM model and different Keywords: Newblock Resonant Fatigue Testing Giga cycle Frequency effects rectangular shape, in order to Machine; better establish the (VHCF); model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a can be useful in the goal of predicting turbine blade life, given a set of FDR data. 1. model Introduction

1. Introduction ©‘‘There 2016 The Published Elsevier B.V. materials” [1]. Studies on damage mechanism on higher number of is Authors. no infinite fatiguebylife in metallic Peer-review under responsibility of the Scientific PCFStudies 2016. ‘‘There is no infinite fatigue life in metallic materials” [1]. on damage mechanism onby higher number of load cycles, in the range of up to 1010 cyclesCommittee and more ofcould well proof the finding published Claude Bathias load cycles, in the range of up to 1010 cycles and more could well proof the finding published by Claude Bathias and others. Thanks to the development of faster testing technics and the shortening of testing time a large number of Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. and others. Thanks to the development of faster testing technics and the shortening of testing time a large number of

* Corresponding author. Tel.: +4152 672 43 22; * E-mail Corresponding Tel.: +4152 672 43 22; address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the ECF22 organizers. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.112

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basic research activities took place in the recent decades. Ultrasonic fatigue testing systems work on the resonant frequency at about 20kHz, and require relatively small specimens with a specific geometry. Ultrasonic fatigue studies showed that a fatigue limit in the traditional sense does not exist in the Gigacycle regime. Cracks may occur subsurface or on the surface, and may start for example from inclusions in the material [1]. Subsequent with higher testing frequency, an old question of fatigue testing is high-lighted and cannot be neglected: “What is the effect of the testing frequency on fa-tigue life?” Testing on very high testing frequency may lead to different damage mechanism than under real loading condition for example of an engine component. Since inclusions and imperfection play an important role in VHCF the manufacturing process has a significant effect on fatigue life in the Gigacycle regime. Particularly for relatively inhomogeneous materials the testing of material volumes that represents the scatter of the manufacturing process is a concern. In 2014 RUMUL could present a new resonant fatigue testing machine, with a testing frequency of 1000Hz. The dynamic load of maximum 50kN peak-peak is produced with an electromagnetic system, similar to established resonant fatigue testing systems which typically run on testing frequencies from about 40 up to 250Hz. The static portion of the load is provided by two mechanical spindles, the maximum load of the system is +/- 50kN. Any load ratio can be selected. Flat and round specimen types that are normally used in fatigue testing can be used. The new testing machine offers new possibilities for investigations of material properties in the very high cycle fatigue (VHCF) regime. Compared to other systems used in the field of VHCF testing the RUMUL GIGAFORTE provides several advantages. The size of the machine is smaller and energy consumption less compared to a servo hydraulic system. The actually tested material volume is larger than the material volume that is tested on ultrasonic systems. The testing frequency of 1000Hz allows normally continuous testing, without intermittently stopping the test for let the specimen cool down. In the past four years the new testing machine was intensively used for example at the laboratory of the Fraunhofer-Institut für Werkstoff- und Strahltechnik IWS in Dresden in Germany. It is used for testing material samples and small components as well. Some effects of the 1000Hz testing frequency on the fatigue behaviour of the material were observed [2]. Recently the IWS laboratory developed a small salt spray chamber and mounted it on the GIGAFORTE to preform fatigue testing under corrosive atmosphere.. 2. Effects of the loading frequency on fatigue life What is the effect of the frequency of an alternating load on fatigue life and fatigue testing? This question is probably present since beginning of fatigue testing. And it is clear there are frequency effects. For lower frequencies the effects can be neglected very often, however the sometimes unknown magnitude of some effects led to a quite conservative approach of limiting the testing frequency in some areas of fatigue testing. The frequency effects can be divided in three areas: Temperature and environment as extrinsic factors and strain rate as intrinsic factor [2, 3]. The effects may superimpose, and affecting fatigue life in the same or opposite direction. 2.1. Temperature A higher material temperature lowers usually the fatigue life as the ultimate strength of a material is related to the temperature. Some materials show a temperature de-pending crystallographic transformation that affects the material properties and fatigue life. Maintaining the specified temperature range is therefore a basic requirement for fatigue testing. A material specific basic damping is always present when deforming a solid material. Microscopic plastic deformation during cyclic loading leads to additional damping and it is almost completely transferred to heat. The damping energy and correspond-ing heat that is produced per load cycle and volume is constant for an even axially loaded specimen. The produced heat per time is proportional to the frequency. The resulting material temperature depends as well on the present heat loss, for example the heat flow to the fixture and to the ambient atmosphere. Convectional cooling can be used to control the temperature during testing. Some material do not show the above described linear relation between temperature and testing frequency, with higher frequency the temperature does not increase as expected [4]. This finding may point to hardening (resp. softening) mechanism that belongs to the category “strain rate” in this context.

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2.2. Environment There are time related mechanisms such as oxidation, corrosion or creep that may a play role for the formation of a new surface during crack initiation and propagation. Depending on the relevance of such mechanisms a significant frequency effect can be expected. For example, it is reported that some investigations show a significant frequency dependency of Aluminium alloys on fatigue life. The Aluminium alloy AW-5083 shows almost now frequency effect at 20kHz on fatigue life in inert atmosphere but in air [3]. 2.3. Strain rate The strain rate is proportional to the testing frequency. During loading in resonant condition the strain rate is not constant. It follows a sinus function. It is thought that the strain rate of irreversible deformation could affect fatigue life significantly. An influence of the testing frequency at 20kHz on fatigue life could be found on quenched and tempered steel 50CrMo4 depending on the strength of the material. It was concluded that the found correlation of fatigue life and testing frequency is related to the strain rate and is typical for cubic body centred metals. The frequency effect is mainly seen on the left side, of finite life of the S–N curve. [3]. For metastable austenitic steel (1.4301, AISI 304) a frequency effect related to the transformation of crystallographic structures was found during testing at 1000Hz with the GIGAFORTE. The analyses showed that higher amounts of strain-induced Martensite and lower plastic strain amplitudes are observed when the cyclic experiments are carried out at lower frequency, promoting higher fatigue strengths [2, 5]. 3. Temperature records RUMUL could look into heating up behaviour of material samples in the last year. The specimens have been provided by interested laboratories. For Temperature re-cording a type K thermocouple was attached on the specimen. Compressed air was used to mitigate heating up if required. Load ratio was selected -1 for all tests. Material

Specimen

Nodular Iron

round, cyl. and hour glass, w. thread round, hour glass, w. thread

9% Cr-steel

Gauge diam. 7 mm 8 mm

Ti alloy

round, hour glass, w. thread

5 mm

Ferritic steel HV30 ~ 220

round, cyl. w/o thread

8 mm

1)

2)

Testing condition

Freq.

load increasing 0.2*106cycles / step compr. air cooling load increasing, 2*106cycles / step compr. air cooling load increasing 0.5*106cycles / step no cooling load increasing, 106cycles / step no cooling, (Fig.2)

1111 Hz 1024 Hz

Load amplitude 8.5 kN (220 MPa)

Temp.

18.6 kN (373 MPa)

26°C

996 Hz

5.52 kN (280 MPa)

1023 Hz

17 kN (337 MPa)

38°C

54°C1) 35°C 62°C2)

cooling temporarily off temperature is not stabilizing, probably softening effect

4. Summary and Outlook The RUMUL GIGAFORTE is an efficient tool for testing very high number of load cycles in a reasonable time. Common specimen types and sizes can be used. Depending on material and load the specimen may heat up. The heating is usually low or moderate and can be mitigated with compressed air cooling, continuous testing is possible. In Fig. 1 the 1000Hz Fatigue Testing Machine RUMUL GIGAFORTE is shown with small sound enclosure, whereas in Fig. 2 RUMUL GIGAFORTE is shown with round specimen without thread, thermocouple attached with tape.

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The possibility to test on 1000Hz testing frequency may help to further evaluate frequency effects and further enhance the confidence on fatigue data in the high and very high cycle regime.

a) b) Fig. 1 a) The 1000 Hz Fatigue Testing Machine RUMUL GIGAFORTE; b) detailed view. References 1. International Journal of Fatigue 93 (2016) 215. In Memoriam Claude Bathias 1938–2015, Haël Mughrabi, Professor Emeritus of Materials Science and Engineering, University of Erlangen-Nürnberg, 91058 Erlangen, Germany 2. Einfluss der Prüffrequenz auf die Rissinitiierung und das Ermüdungsrisswachstum im HCF/VHCF-Bereich am Beispiel des Stahls 1.4301, Tagung Werkstoffprüfung 2016, M. Zimmermann et al. Institut für Werkstoffwissenschaft, Technische Universität Dresden 3. Frequency effect and influence of testing technique on the fatigue behaviour of quenched and tempered steel and aluminium alloy. International Journal of Fatigue 93 (2016) 224–231 N. Schneider et al. State Material Testing Institute and Institute for Materials Technology, Technische Universität Darmstadt, Grafenstr. 2, 64283 Darmstadt, Germany 4. Neue Ansätze für eine Schädigungsenergiehypothese auf der Basis thermischer Messungen, 6. Sitzung des DVM Arbeitskreises Betriebsfestigkeit in Darmstadt, Klaus Stärk. 5. Influence of loading frequency and role of surface micro-defects on fatigue behaviour of metastable austenitic stainless steel AISI 304. D.F. Pessoa et. al. Fraunhofer-Institut für Werkstoff -und Strahltechnik IWS, 01277 Dresden, Germany and Institut für Werkstoff-wissenschaft, Technische Universität Dresden, 01069 Dresden, Germany