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Effect of transversal loading on the fatigue life of cold-drawn duplex stainless steel
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Procedia Structural Structural IntegrityIntegrity Procedia1300(2018) (2016)584–589 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Effect of transversal loading on the fatigue life of Effect of transversal loading on the fatigue life of cold-drawn steel XV Portuguese Conference on Fracture, duplex PCF 2016,stainless 10-12 February 2016, Paço de Arcos, Portugal cold-drawn duplex stainless steel Thermo-mechanical modeling of adehigh pressure a a Mihaela Iordachescu *, Maricely Abreu , Andrésturbine Valienteaa blade of an a a Mihaela Iordachescu *, Maricely de Abreuengine , Andrés Valiente airplane turbine Materials Science Dpt., E.T.S.I. Caminos, Universidadgas Politécnica de Madrid, Prof. Aranguren St., 28040, Madrid, España a
Materials Science Dpt., E.T.S.I. Caminos, Universidad Politécnica de Madrid, Prof. Aranguren St., 28040, Madrid, España
a
P. Brandãoa, V. Infanteb, A.M. Deusc*
AbstractaDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Abstract Portugal b Department of Mechanical Engineering, Instituto Técnico,heavily Universidade de Lisboa, Av. Rovisco Pais, steel 1, 1049-001 The IDMEC, paper gives new insights on failure behavior of Superior high-strength, cold-drawn duplex stainless wires Lisboa, when Portugal heavily cold-drawn duplex stainless steel wires when The paper gives new insights on failure behavior of high-strength, simultaneously subjected to static transverse and longitudinal loadings, with the latter ones being of a fully tensile or cyclic c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, simultaneously subjected to static transverse and longitudinal loadings, withinto the strands latter ones of a fully tensile cyclic nature. The wires might experience such combined actions when incorporated of prebeing or post-tensioned cableor systems, Portugal nature.used Thein wires might experience such combined actions when incorporated intoare strands precontact or post-tensioned cablethe systems, today a wide spectrum of construction applications. The transversal loads due toofthe forces between wires, todaymainly used in a wide of construction The transversal loads are the dueexperiments to the contact forces between wires, and occur by spectrum longitudinal tensioning ofapplications. strands. In order to reproduce them, were made with athe specially and mainly occurassuring by longitudinal tensioning of strands. In order to reproduce them, the experiments made with a or specially designed device the control of the locally applied transversal compression during the wire were loading in simple cyclic Abstract designedThe device assuring the control of thebi-axial locally loading applied did transversal compression the wire loading in cyclic tension. results concerning the static not show significantduring differences concerning thesimple failureorload of tension. The results static bi-axial loading did not show significant differences concerning the failure load of duplex stainless steelconcerning wiresmodern whenthe compared with that of currently used prestressing eutectoid wires: on this operating basis, an empirical During their operation, aircraft engine components are subjected to increasingly demanding conditions, duplex stainless steel wires when compared with that of currently used prestressing eutectoid wires: on this basis, an empirical especially the high pressure turbine (HPT) Such conditions cause these to undergo different types of time-dependent fracture criterion predicting the critical load blades. combinations was formulated. The parts simultaneous action of transverse compressive fracture criterion predicting the critical load combinations was formulated. The simultaneous action of transverse compressive degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict loading and fatigue tensile loading of 200 MPa stress range produces a nominally infinite lifetime of lean duplex wires for the creep of HPT blades. Flight records for aa nominally specific aircraft, provided bylean a commercial aviation loading and behaviour fatigue tensile loading 200 MPadata stress range(FDR) produces infinite lifetime of duplexbewires for combinations of the compressive andofmaximum tensile loads experimentally determined. These combinations could roughly company, used tobyobtain thermal and mechanical data for fatigue three different flightthan cycles. In to create the 3D model combinations of the compressive and maximum loads experimentally determined. These could be capacity roughly described as were those given compressive loads ortensile maximum tensile loads higher 50%combinations of order the tensile bearing forthose the FEM a HPTloads bladeorscrap was scanned, and its chemical material properties were described as given analysis, by compressive maximum tensile fatigue loads higher composition than 50% of and the tensile bearing capacity of needed lean duplex wire. Thewire. data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D of obtained. lean duplex block shape, in order to better B.V. establish the model, and then with the real 3D mesh obtained from the blade scrap. The © rectangular 2018 The Authors. Published by Elsevier © 2018 The Authors. Published by Elsevier B.V. overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a © 2018 The under Authors. Published by B.V. Peer-review responsibility of Elsevier the ECF22 organizers. Peer-review under responsibility of the ECF22 organizers. model can be useful in the goal of predicting turbine blade Peer-review under responsibility of the ECF22 organizers. life, given a set of FDR data. Keywords: cold-drawn duplex stainless steel wires; tensile-compression static loading; static compression-tensile fatigue; fatigue life; failure © 2016 The Authors. Published bysteel Elsevier Keywords: cold-drawn duplex stainless wires;B.V. tensile-compression static loading; static compression-tensile fatigue; fatigue life; failure micro and macro-mechanisms; Peer-review under responsibility of the Scientific Committee of PCF 2016. micro and macro-mechanisms; Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.
* Corresponding author: orcid.org/000-0003-0545-4581; Tel.: +34 910-673-309; * E-mail Corresponding orcid.org/000-0003-0545-4581; Tel.: +34 910-673-309; 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.2018.12.096
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Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 584–589 M. Iordachescu, M. de Abreu, A. Valiente/ Structural Integrity Procedia 00 (2018) 000–000
585
1. Introduction Today, pre or post-tensioning tendon-cable systems using multiple strands are incorporated into a vast array of structures such as bridges, buildings, cryogenic liquefied natural gas (LNG) tanks, dams, nuclear power stations, retaining walls, tunnels, and wind towers (Dywidag, 2017 and BBR VT, 2009). The strands are made from 7 individual high-strength, cold-drawn eutectoid wires, 6 helically wound outer the central straight one (EN ISO 15630-3). Essentially, they carry tensile loads, but transverse loads from different sources may appear due to construction tolerances and misalignments, wind loads or temperature changes. These are generally accompanied by bending stresses in the anchorage systems or in their couplers used to form arc-shapes along the prestressing length. In both situations, the bending stresses might be of the same order of magnitude as the axial tensioning stresses and the results in reduced fatigue strength of strands (Cullimore, 1972, McTyer and Evans, 2017, Dywidag, 2017, BBR VT, 2009). Regarding this issue, additional data is required not only for structural design purposes but also for development and certification of new generations of prestressing wires. This paper gives new insights on the failure behaviour of two types of high-strength, heavily cold-drawn duplex stainless steel wires when simultaneously subjected to static transverse and longitudinal loadings, with the latter ones being of a fully tensile or cyclic nature. These are potential candidates for pre or post-stressing purposes, assuring similar strength levels, higher damage tolerance and significantly higher resistance to stress corrosion than the eutectoid steels wires (Valiente and Iordachescu, 2012, Iordachescu et al, 2015). The experiments were performed with a specially designed device to control a locally applied transversal compression during the wire loading in simple or cyclic tension. Thus, the results concerning the static bi-axial loading of the duplex stainless steel wires could be compared with those of currently used prestressing eutectoid wires: on this empirical basis, a unique fracture criterion is proposed. Scanning electron microscopy (SEM) was used to determine the corresponding failure macromechanisms of the tested wires as well as the differences in the fatigue damage when static transverse loading was combined with a longitudinal cyclic one. Nomenclature DSS ES LDS P P0 Pm PFmax Q T-QL F-QL
high-alloyed duplex stainless steel wire prestressing eutectoid steel wire low-alloyed duplex stainless steel (lean duplex) wire tensile load maximum load in simple tension tensile bearing capacity under transverse loading maximum applied tensile fatigue load transverse, constant compression load tensile test under transverse loading fatigue test under transverse loading
2. Wire materials and testing methods The studied high-strength wires, of 4 mm diameter, were manufactured by cold drawing from two distinct classes of duplex stainless steels: the first a low-alloyed (LDS) and the second a high-alloyed (DSS) one. In addition, a current prestressing eutectoid steel wire (ES), of the same diameter, was used in the experiments for comparative purposes. Table 1 summarizes their mechanical properties. These were experimentally obtained by tensile testing wire samples of 350 mm length, at room temperature. The tests were performed with a 200 kN servo-hydraulic machine by using a constant crosshead speed of 1 mm/min, with the elongations being measured on a 12.5 mm gauge length with a conventional clip-on extensometer. The data were further used as a reference for evaluating the tensile bearing capacity of wires under static bi-axial loading. The first testing method employed in this study, namely the tensile test under transverse loading (here referred as T-QL), was aimed at assessing the sensitivity to transverse loading induced by the contact of a tensioned wire with a rigid boundary. In this view, a compression load was perpendicularly applied on the wire surface and held constant
Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 584–589 M. Iordachescu, M. de Abreu, A. Valiente / Structural Integrity Procedia 00 (2018) 000–000
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during its tensile tensioning up to failure. One of the effects that the test sought to reproduce was the local compression that may occur in service between the wires of a loaded prestressing strand. Fig. 1 shows the test arrangement used in the experiments. Commercial wedge grips for prestressing wires were used to fix the specimens, of 350 mm length, in the vertical loading frame of the 200 kN servo-hydraulic machine which provided the tensile loading. The transverse compression was applied and held constant by the actuator and the support plate of a small loading frame, which remained attached to the wire through the compression load, without sliding or axial force transmission. This was assured by a pulley counterweight system by maintaining the horizontality of the frame and balancing its weight (Fig 1a). The actuator of the small loading frame is a hydraulically driven piston connected to an air-oil pressure converter that assures controlled thrust through the air-pressure regulator mounted to the outlet of a compressed air cylinder. Thus, on the piston side, the compression load was applied perpendicularly to the tested wire axis by using a small wire-sample of same type (Fig 1b); the other side of the tested wire was longitudinally supported on a V-shape die, of 30 mm length. The die shape and the counterweight system of the horizontal frame prevented the lateral displacement of tested wire, once fixed in wedges (Fig. 1c). Details capturing the main testing sequences, namely the wire fixing, bi-axial loading and failure are given in Fig. 1c, Fig. 1d-1 and Fig. 1d-2. These were acquired with an optical acquisition system VIC-2D, and allowed obtaining the elongation data corresponding to the longitudinal tensioning of the wire. Table 1. Mechanical properties of studied wires Elastic modulus, [GPa]
Yield strength, Rp0.2 [MPa]
Tensile strength, Rm [MPa]
Maximum uniform elongation, Agt [%]
Reduction of area, RA [%]
LDS (1.4482)
180
1350
1820
2.3
51
DSS (1.4462)
160
1429
1660
2.2
70
ES (1.4482)
205
1640
1740
3.2
50
Fig. 1. Tensile testing of wire specimens under transverse loading: a) general view of the test arrangement; b) loading mode; c) frame and actuator for transverse loading; d-1) transversely compressed zone before tensioning; d-2) wire´s failure
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The second testing method used in the present research, namely the fatigue test under transverse loading (here referred as F-QL), was designed to reproduce the fatigue behaviour of the wires when simultaneously subjected to cyclic tensile loading and static compressive transverse loading. The same test arrangement given in Fig. 1 was employed, with the only difference being given by the cyclic nature of the tensile load by applying the stress rage of 200 MPa, recommended by the International Federation for Structural Concrete, FIB (2005) for fatigue testing of prestressing wires. The tests were performed for different combinations of the maximum tensile fatigue load and the transverse load PFmax – Q. According to (FIB, 2005), with the tests not exhibiting in fatigue failure before 2∙106 load cycles were considered as resulting in infinite fatigue life; although most of tests were continued up to 5∙10 6 load cycles. Lastly, macro-microscopic analyses of the fracture surfaces were performed in order to identify the fracture features and the corresponding damage mechanisms of wires when subjected to T-QL and F-QL tests.
Fig. 2. Tensile load vs. percentage elongation dependency on constant transverse compression loads Q, of: a) LDS; b) DSS; c) ES wire specimens
Fig. 3. a) Macroscopic fracture features of the LDS wire specimens when subjected to axial tensioning and transverse loading; b) Experimental tensile bearing capacity of the wires vs. transverse load
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3. Experimental results 3.1. Effect of the transverse compression load on the tensile resistance of the wires The effect of the transverse compression load Q on the tensile behavior of LDS, DSS and ES wires is presented in Fig. 2. The plot of each T-QL test was obtained by coupling the recorded tensile load with the percentage elongation measured by post-processing the captured image sequence up to maximum load with the video image correlation software VIC-2D. The measured elongation corresponds to a virtual 12.5 mm gage length centered at the contact with the wire used for applying the transverse load, Q. The plots show that the tensile bearing capacity of the wires decreases as Q increases. The elongation under maximum load also exhibits a trend to increase with Q. The fracture features corresponding to relevant values of the load Q are given in Fig. 3a. Accordingly, the fracture initiates at one end of the compression induced contact area and roughly propagates along a plane forming about 40º with the wire axis (Fig. 4a). Pure tensile necking coexists with this failure mechanism for low values of Q, which fully disappears with the increase of Q. Fig. 3b shows that the bearing capacity Pm of the wires depends on the applied transverse load Q, and a unique linear dependence, of negative slope of 0.565 may be defined when Pm and Q are expressed as fractions of corresponding maximum tensile load P 0 that each wire class can sustain in simple tension. The T-QL fracture of the analyzed wires is activated by a common failure mechanism of shear plastic collapse. Fig. 4b and Fig. 4c illustrate this, the first image showing at macroscale the shear fracture of one of the broken LDS wires in the T-QL test, and the latter, a higher magnification detail indicating the ductile morphology of fracture initio. 3.2. Effect of transverse compression load on the fatigue life of the LDS wire The effect of transverse compression load Q was also noted in the F-QL tests made on several LDS wire specimens. Several fatigue tests, made under the stress range of 200 MPa, resulted in failure for load cycles less than 2∙106. Fig. 4d shows the macrofractograph of one of the broken specimens, illustrating that Q determines the fracture initiation site at one end of the compression induced contact area. This end becomes a strong spot stress concentrator for the applied cyclic tensile loading, from which a peculiar fatigue-cracking plane develops. This plane and the wire axis form almost the same angle of about 40º previously found in the fracture paths of the T-QL tests. Such an inclination plane contrasts with the transverse fatigue cracks that are usual in tensile wires. The beach marks of this atypical fatigue crack growth are shown at higher magnification scale in Fig. 4e. Fig. 4f gathers the fatigue test data of LDS wires in the non-dimensional axial – transverse load diagram PFmax/P0 – Q/P0, in which PFmax is the maximum tensile fatigue load, Q is the transverse compression load and P0 is the maximum load sustained in simple tension. For comparison purposes, the diagram also contains the previously determined static failure locus. The horizontal line AB corresponds to the yielding load of the LDS wires in simple tension, as the limit load admitted by FIB (2005) for fatigue testing of prestressing wires. Then, the curve BC defines the 200 MPa endurance limit locus of LDS wires when simultaneously subjected to tensile fatigue loading and static transverse compression. The diagram indicates that the fatigue life depends on the combination of PFmaxQ values. The fatigue life of the wires remains nominally infinite for transverse loads Q lower than 40 % of P0. However, the fatigue failure of the wires is triggered for values of Q higher than 50% P0 when combined with tensile fatigue loads whose maximum value PFmax also surpasses 50% P0 for 200 MPa stress range. The data in Fig.4f suggest that values of Q and PFmax respectively higher and lower than 50% P0 would not result in fatigue failure for less 2∙106 load cycles. An explanation consistent with this would be that the strong compressive forces prevent the conversion of the superficial damage produced by the spot stress concentrator into a fatigue cracking initiator. 4. Conclusions The experimental results concerning the static bi-axial loading of duplex stainless steel wires did not showed significant differences regarding failure load when compared with that of prestressing eutectoid wires of the same diameter. On this basis, an empirical fracture criterion predicting the critical load combinations has been formulated.
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Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 584–589 M. Iordachescu, M. de Abreu, A. Valiente/ Structural Integrity Procedia 00 (2018) 000–000
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The simultaneous action of transverse compressive loading and fatigue tensile loading of 200 MPa stress range produces nominally infinite lifetime of the wires for combinations of the compressive and maximum tensile loads experimentally determined. These combinations can be roughly described as those given by compressive loads or maximum tensile fatigue loads higher than 50% of the tensile bearing capacity of the wire.
Fig. 4. a) Sketch of tensile/fatigue failure of wires under transverse load; b) macroscopic image of the LDS wire fracture in the T-QL test; c) ductile shear fracture morphology of broken wires subjected to T-QL; d) macroscopic image of the LDS wire fracture in the F-QL test; e) beach marks of shear fatigue fracture propagation on the wires broken under F-QL; f) Fatigue test results of LDS wires in the non-dimensional axial – transverse load diagram (Q - applied transverse compression load, PFmax - maximum applied tensile fatigue load, P0 - maximum load in tension)
Acknowledgements The authors gratefully acknowledge the financial support obtained from the Spanish Ministry of Science and Innovation through the project BIA 2014-53314–R and the collaboration with INOXFIL S.A. who kindly provided the high-strength, lean and duplex steel wires. References BBR HiAm CONA Strand stay cable system, BBR VT International, 03.2009, www.bbrnetwork.com Cullimore M.S.G., The Fatigue strength of high tensile wire cable subjected to stress fluctuations of small amplitude, IABSE publications 1972, http://doi.org/10.5169/seals-24940 DYWIDAG multistrand stay cable systems, DYWIDAG-SYSTEMS International, 04 178-1/07.17 -web sc, dywidag-systems.com/emea DYWIDAG bonded post-tensioning using strands, DYWIDAG-SYSTEMS International, 04 160-1/06.17-web sc, dywidag-systems.com/emea EN ISO 15630-3, Steel for the reinforcement and prestressing of concrete - Test methods - Part 3: Prestressing steel, ISO, 2010 Iordachescu M., De Abreu M., Valiente A., Effect of cold-drawn induced anisotropy on the failure of high strength eutectoid and duplex steel wires. Eng Fail Anal 2015; 56:412-421. McTyer K., Evans D.W., Mechanical direct shear tests of cables – combined stress relationships, Proceedings of the 17th Coal Operators' Conference, Mining Engineering, N. Aziz, B. Kininmonth (eds.), University of Wollongong, 8-10 Feb 2017, 171-182. Valiente A., Iordachescu M., Damage tolerance of cold drawn ferritic-austenitic stainless steels wires for prestressed concrete. Constr Build Mater 2012; 36:874-880. FIB bulletin 30, Acceptance of stay cable systems using prestressing steels, FIB 2005, ISSN 1562-3610
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Procedia Structural Structural IntegrityIntegrity Procedia1300(2018) (2016)584–589 000–000
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ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Effect of transversal loading on the fatigue life of Effect of transversal loading on the fatigue life of cold-drawn steel XV Portuguese Conference on Fracture, duplex PCF 2016,stainless 10-12 February 2016, Paço de Arcos, Portugal cold-drawn duplex stainless steel Thermo-mechanical modeling of adehigh pressure a a Mihaela Iordachescu *, Maricely Abreu , Andrésturbine Valienteaa blade of an a a Mihaela Iordachescu *, Maricely de Abreuengine , Andrés Valiente airplane turbine Materials Science Dpt., E.T.S.I. Caminos, Universidadgas Politécnica de Madrid, Prof. Aranguren St., 28040, Madrid, España a
Materials Science Dpt., E.T.S.I. Caminos, Universidad Politécnica de Madrid, Prof. Aranguren St., 28040, Madrid, España
a
P. Brandãoa, V. Infanteb, A.M. Deusc*
AbstractaDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Abstract Portugal b Department of Mechanical Engineering, Instituto Técnico,heavily Universidade de Lisboa, Av. Rovisco Pais, steel 1, 1049-001 The IDMEC, paper gives new insights on failure behavior of Superior high-strength, cold-drawn duplex stainless wires Lisboa, when Portugal heavily cold-drawn duplex stainless steel wires when The paper gives new insights on failure behavior of high-strength, simultaneously subjected to static transverse and longitudinal loadings, with the latter ones being of a fully tensile or cyclic c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, simultaneously subjected to static transverse and longitudinal loadings, withinto the strands latter ones of a fully tensile cyclic nature. The wires might experience such combined actions when incorporated of prebeing or post-tensioned cableor systems, Portugal nature.used Thein wires might experience such combined actions when incorporated intoare strands precontact or post-tensioned cablethe systems, today a wide spectrum of construction applications. The transversal loads due toofthe forces between wires, todaymainly used in a wide of construction The transversal loads are the dueexperiments to the contact forces between wires, and occur by spectrum longitudinal tensioning ofapplications. strands. In order to reproduce them, were made with athe specially and mainly occurassuring by longitudinal tensioning of strands. In order to reproduce them, the experiments made with a or specially designed device the control of the locally applied transversal compression during the wire were loading in simple cyclic Abstract designedThe device assuring the control of thebi-axial locally loading applied did transversal compression the wire loading in cyclic tension. results concerning the static not show significantduring differences concerning thesimple failureorload of tension. The results static bi-axial loading did not show significant differences concerning the failure load of duplex stainless steelconcerning wiresmodern whenthe compared with that of currently used prestressing eutectoid wires: on this operating basis, an empirical During their operation, aircraft engine components are subjected to increasingly demanding conditions, duplex stainless steel wires when compared with that of currently used prestressing eutectoid wires: on this basis, an empirical especially the high pressure turbine (HPT) Such conditions cause these to undergo different types of time-dependent fracture criterion predicting the critical load blades. combinations was formulated. The parts simultaneous action of transverse compressive fracture criterion predicting the critical load combinations was formulated. The simultaneous action of transverse compressive degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict loading and fatigue tensile loading of 200 MPa stress range produces a nominally infinite lifetime of lean duplex wires for the creep of HPT blades. Flight records for aa nominally specific aircraft, provided bylean a commercial aviation loading and behaviour fatigue tensile loading 200 MPadata stress range(FDR) produces infinite lifetime of duplexbewires for combinations of the compressive andofmaximum tensile loads experimentally determined. These combinations could roughly company, used tobyobtain thermal and mechanical data for fatigue three different flightthan cycles. In to create the 3D model combinations of the compressive and maximum loads experimentally determined. These could be capacity roughly described as were those given compressive loads ortensile maximum tensile loads higher 50%combinations of order the tensile bearing forthose the FEM a HPTloads bladeorscrap was scanned, and its chemical material properties were described as given analysis, by compressive maximum tensile fatigue loads higher composition than 50% of and the tensile bearing capacity of needed lean duplex wire. Thewire. data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D of obtained. lean duplex block shape, in order to better B.V. establish the model, and then with the real 3D mesh obtained from the blade scrap. The © rectangular 2018 The Authors. Published by Elsevier © 2018 The Authors. Published by Elsevier B.V. overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a © 2018 The under Authors. Published by B.V. Peer-review responsibility of Elsevier the ECF22 organizers. Peer-review under responsibility of the ECF22 organizers. model can be useful in the goal of predicting turbine blade Peer-review under responsibility of the ECF22 organizers. life, given a set of FDR data. Keywords: cold-drawn duplex stainless steel wires; tensile-compression static loading; static compression-tensile fatigue; fatigue life; failure © 2016 The Authors. Published bysteel Elsevier Keywords: cold-drawn duplex stainless wires;B.V. tensile-compression static loading; static compression-tensile fatigue; fatigue life; failure micro and macro-mechanisms; Peer-review under responsibility of the Scientific Committee of PCF 2016. micro and macro-mechanisms; Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.
* Corresponding author: orcid.org/000-0003-0545-4581; Tel.: +34 910-673-309; * E-mail Corresponding orcid.org/000-0003-0545-4581; Tel.: +34 910-673-309; 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.2018.12.096
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Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 584–589 M. Iordachescu, M. de Abreu, A. Valiente/ Structural Integrity Procedia 00 (2018) 000–000
585
1. Introduction Today, pre or post-tensioning tendon-cable systems using multiple strands are incorporated into a vast array of structures such as bridges, buildings, cryogenic liquefied natural gas (LNG) tanks, dams, nuclear power stations, retaining walls, tunnels, and wind towers (Dywidag, 2017 and BBR VT, 2009). The strands are made from 7 individual high-strength, cold-drawn eutectoid wires, 6 helically wound outer the central straight one (EN ISO 15630-3). Essentially, they carry tensile loads, but transverse loads from different sources may appear due to construction tolerances and misalignments, wind loads or temperature changes. These are generally accompanied by bending stresses in the anchorage systems or in their couplers used to form arc-shapes along the prestressing length. In both situations, the bending stresses might be of the same order of magnitude as the axial tensioning stresses and the results in reduced fatigue strength of strands (Cullimore, 1972, McTyer and Evans, 2017, Dywidag, 2017, BBR VT, 2009). Regarding this issue, additional data is required not only for structural design purposes but also for development and certification of new generations of prestressing wires. This paper gives new insights on the failure behaviour of two types of high-strength, heavily cold-drawn duplex stainless steel wires when simultaneously subjected to static transverse and longitudinal loadings, with the latter ones being of a fully tensile or cyclic nature. These are potential candidates for pre or post-stressing purposes, assuring similar strength levels, higher damage tolerance and significantly higher resistance to stress corrosion than the eutectoid steels wires (Valiente and Iordachescu, 2012, Iordachescu et al, 2015). The experiments were performed with a specially designed device to control a locally applied transversal compression during the wire loading in simple or cyclic tension. Thus, the results concerning the static bi-axial loading of the duplex stainless steel wires could be compared with those of currently used prestressing eutectoid wires: on this empirical basis, a unique fracture criterion is proposed. Scanning electron microscopy (SEM) was used to determine the corresponding failure macromechanisms of the tested wires as well as the differences in the fatigue damage when static transverse loading was combined with a longitudinal cyclic one. Nomenclature DSS ES LDS P P0 Pm PFmax Q T-QL F-QL
high-alloyed duplex stainless steel wire prestressing eutectoid steel wire low-alloyed duplex stainless steel (lean duplex) wire tensile load maximum load in simple tension tensile bearing capacity under transverse loading maximum applied tensile fatigue load transverse, constant compression load tensile test under transverse loading fatigue test under transverse loading
2. Wire materials and testing methods The studied high-strength wires, of 4 mm diameter, were manufactured by cold drawing from two distinct classes of duplex stainless steels: the first a low-alloyed (LDS) and the second a high-alloyed (DSS) one. In addition, a current prestressing eutectoid steel wire (ES), of the same diameter, was used in the experiments for comparative purposes. Table 1 summarizes their mechanical properties. These were experimentally obtained by tensile testing wire samples of 350 mm length, at room temperature. The tests were performed with a 200 kN servo-hydraulic machine by using a constant crosshead speed of 1 mm/min, with the elongations being measured on a 12.5 mm gauge length with a conventional clip-on extensometer. The data were further used as a reference for evaluating the tensile bearing capacity of wires under static bi-axial loading. The first testing method employed in this study, namely the tensile test under transverse loading (here referred as T-QL), was aimed at assessing the sensitivity to transverse loading induced by the contact of a tensioned wire with a rigid boundary. In this view, a compression load was perpendicularly applied on the wire surface and held constant
Mihaela Iordachescu et al. / Procedia Structural Integrity 13 (2018) 584–589 M. Iordachescu, M. de Abreu, A. Valiente / Structural Integrity Procedia 00 (2018) 000–000
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during its tensile tensioning up to failure. One of the effects that the test sought to reproduce was the local compression that may occur in service between the wires of a loaded prestressing strand. Fig. 1 shows the test arrangement used in the experiments. Commercial wedge grips for prestressing wires were used to fix the specimens, of 350 mm length, in the vertical loading frame of the 200 kN servo-hydraulic machine which provided the tensile loading. The transverse compression was applied and held constant by the actuator and the support plate of a small loading frame, which remained attached to the wire through the compression load, without sliding or axial force transmission. This was assured by a pulley counterweight system by maintaining the horizontality of the frame and balancing its weight (Fig 1a). The actuator of the small loading frame is a hydraulically driven piston connected to an air-oil pressure converter that assures controlled thrust through the air-pressure regulator mounted to the outlet of a compressed air cylinder. Thus, on the piston side, the compression load was applied perpendicularly to the tested wire axis by using a small wire-sample of same type (Fig 1b); the other side of the tested wire was longitudinally supported on a V-shape die, of 30 mm length. The die shape and the counterweight system of the horizontal frame prevented the lateral displacement of tested wire, once fixed in wedges (Fig. 1c). Details capturing the main testing sequences, namely the wire fixing, bi-axial loading and failure are given in Fig. 1c, Fig. 1d-1 and Fig. 1d-2. These were acquired with an optical acquisition system VIC-2D, and allowed obtaining the elongation data corresponding to the longitudinal tensioning of the wire. Table 1. Mechanical properties of studied wires Elastic modulus, [GPa]
Yield strength, Rp0.2 [MPa]
Tensile strength, Rm [MPa]
Maximum uniform elongation, Agt [%]
Reduction of area, RA [%]
LDS (1.4482)
180
1350
1820
2.3
51
DSS (1.4462)
160
1429
1660
2.2
70
ES (1.4482)
205
1640
1740
3.2
50
Fig. 1. Tensile testing of wire specimens under transverse loading: a) general view of the test arrangement; b) loading mode; c) frame and actuator for transverse loading; d-1) transversely compressed zone before tensioning; d-2) wire´s failure
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The second testing method used in the present research, namely the fatigue test under transverse loading (here referred as F-QL), was designed to reproduce the fatigue behaviour of the wires when simultaneously subjected to cyclic tensile loading and static compressive transverse loading. The same test arrangement given in Fig. 1 was employed, with the only difference being given by the cyclic nature of the tensile load by applying the stress rage of 200 MPa, recommended by the International Federation for Structural Concrete, FIB (2005) for fatigue testing of prestressing wires. The tests were performed for different combinations of the maximum tensile fatigue load and the transverse load PFmax – Q. According to (FIB, 2005), with the tests not exhibiting in fatigue failure before 2∙106 load cycles were considered as resulting in infinite fatigue life; although most of tests were continued up to 5∙10 6 load cycles. Lastly, macro-microscopic analyses of the fracture surfaces were performed in order to identify the fracture features and the corresponding damage mechanisms of wires when subjected to T-QL and F-QL tests.
Fig. 2. Tensile load vs. percentage elongation dependency on constant transverse compression loads Q, of: a) LDS; b) DSS; c) ES wire specimens
Fig. 3. a) Macroscopic fracture features of the LDS wire specimens when subjected to axial tensioning and transverse loading; b) Experimental tensile bearing capacity of the wires vs. transverse load
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3. Experimental results 3.1. Effect of the transverse compression load on the tensile resistance of the wires The effect of the transverse compression load Q on the tensile behavior of LDS, DSS and ES wires is presented in Fig. 2. The plot of each T-QL test was obtained by coupling the recorded tensile load with the percentage elongation measured by post-processing the captured image sequence up to maximum load with the video image correlation software VIC-2D. The measured elongation corresponds to a virtual 12.5 mm gage length centered at the contact with the wire used for applying the transverse load, Q. The plots show that the tensile bearing capacity of the wires decreases as Q increases. The elongation under maximum load also exhibits a trend to increase with Q. The fracture features corresponding to relevant values of the load Q are given in Fig. 3a. Accordingly, the fracture initiates at one end of the compression induced contact area and roughly propagates along a plane forming about 40º with the wire axis (Fig. 4a). Pure tensile necking coexists with this failure mechanism for low values of Q, which fully disappears with the increase of Q. Fig. 3b shows that the bearing capacity Pm of the wires depends on the applied transverse load Q, and a unique linear dependence, of negative slope of 0.565 may be defined when Pm and Q are expressed as fractions of corresponding maximum tensile load P 0 that each wire class can sustain in simple tension. The T-QL fracture of the analyzed wires is activated by a common failure mechanism of shear plastic collapse. Fig. 4b and Fig. 4c illustrate this, the first image showing at macroscale the shear fracture of one of the broken LDS wires in the T-QL test, and the latter, a higher magnification detail indicating the ductile morphology of fracture initio. 3.2. Effect of transverse compression load on the fatigue life of the LDS wire The effect of transverse compression load Q was also noted in the F-QL tests made on several LDS wire specimens. Several fatigue tests, made under the stress range of 200 MPa, resulted in failure for load cycles less than 2∙106. Fig. 4d shows the macrofractograph of one of the broken specimens, illustrating that Q determines the fracture initiation site at one end of the compression induced contact area. This end becomes a strong spot stress concentrator for the applied cyclic tensile loading, from which a peculiar fatigue-cracking plane develops. This plane and the wire axis form almost the same angle of about 40º previously found in the fracture paths of the T-QL tests. Such an inclination plane contrasts with the transverse fatigue cracks that are usual in tensile wires. The beach marks of this atypical fatigue crack growth are shown at higher magnification scale in Fig. 4e. Fig. 4f gathers the fatigue test data of LDS wires in the non-dimensional axial – transverse load diagram PFmax/P0 – Q/P0, in which PFmax is the maximum tensile fatigue load, Q is the transverse compression load and P0 is the maximum load sustained in simple tension. For comparison purposes, the diagram also contains the previously determined static failure locus. The horizontal line AB corresponds to the yielding load of the LDS wires in simple tension, as the limit load admitted by FIB (2005) for fatigue testing of prestressing wires. Then, the curve BC defines the 200 MPa endurance limit locus of LDS wires when simultaneously subjected to tensile fatigue loading and static transverse compression. The diagram indicates that the fatigue life depends on the combination of PFmaxQ values. The fatigue life of the wires remains nominally infinite for transverse loads Q lower than 40 % of P0. However, the fatigue failure of the wires is triggered for values of Q higher than 50% P0 when combined with tensile fatigue loads whose maximum value PFmax also surpasses 50% P0 for 200 MPa stress range. The data in Fig.4f suggest that values of Q and PFmax respectively higher and lower than 50% P0 would not result in fatigue failure for less 2∙106 load cycles. An explanation consistent with this would be that the strong compressive forces prevent the conversion of the superficial damage produced by the spot stress concentrator into a fatigue cracking initiator. 4. Conclusions The experimental results concerning the static bi-axial loading of duplex stainless steel wires did not showed significant differences regarding failure load when compared with that of prestressing eutectoid wires of the same diameter. On this basis, an empirical fracture criterion predicting the critical load combinations has been formulated.
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The simultaneous action of transverse compressive loading and fatigue tensile loading of 200 MPa stress range produces nominally infinite lifetime of the wires for combinations of the compressive and maximum tensile loads experimentally determined. These combinations can be roughly described as those given by compressive loads or maximum tensile fatigue loads higher than 50% of the tensile bearing capacity of the wire.
Fig. 4. a) Sketch of tensile/fatigue failure of wires under transverse load; b) macroscopic image of the LDS wire fracture in the T-QL test; c) ductile shear fracture morphology of broken wires subjected to T-QL; d) macroscopic image of the LDS wire fracture in the F-QL test; e) beach marks of shear fatigue fracture propagation on the wires broken under F-QL; f) Fatigue test results of LDS wires in the non-dimensional axial – transverse load diagram (Q - applied transverse compression load, PFmax - maximum applied tensile fatigue load, P0 - maximum load in tension)
Acknowledgements The authors gratefully acknowledge the financial support obtained from the Spanish Ministry of Science and Innovation through the project BIA 2014-53314–R and the collaboration with INOXFIL S.A. who kindly provided the high-strength, lean and duplex steel wires. References BBR HiAm CONA Strand stay cable system, BBR VT International, 03.2009, www.bbrnetwork.com Cullimore M.S.G., The Fatigue strength of high tensile wire cable subjected to stress fluctuations of small amplitude, IABSE publications 1972, http://doi.org/10.5169/seals-24940 DYWIDAG multistrand stay cable systems, DYWIDAG-SYSTEMS International, 04 178-1/07.17 -web sc, dywidag-systems.com/emea DYWIDAG bonded post-tensioning using strands, DYWIDAG-SYSTEMS International, 04 160-1/06.17-web sc, dywidag-systems.com/emea EN ISO 15630-3, Steel for the reinforcement and prestressing of concrete - Test methods - Part 3: Prestressing steel, ISO, 2010 Iordachescu M., De Abreu M., Valiente A., Effect of cold-drawn induced anisotropy on the failure of high strength eutectoid and duplex steel wires. Eng Fail Anal 2015; 56:412-421. McTyer K., Evans D.W., Mechanical direct shear tests of cables – combined stress relationships, Proceedings of the 17th Coal Operators' Conference, Mining Engineering, N. Aziz, B. Kininmonth (eds.), University of Wollongong, 8-10 Feb 2017, 171-182. Valiente A., Iordachescu M., Damage tolerance of cold drawn ferritic-austenitic stainless steels wires for prestressed concrete. Constr Build Mater 2012; 36:874-880. FIB bulletin 30, Acceptance of stay cable systems using prestressing steels, FIB 2005, ISSN 1562-3610