Transactions of the VŠB Technical University of Ostrava, Mechanical Series No. 2, 2010, vol. LVI article No. 1805

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1 Transactions o the VŠB Technical University o Ostrava, Mechanical Series No. 2, 2010, vol. LVI article No Jose ZAPLETAL *, Filip VANČURA **, Karel NĚMEC *** LOW CYCLE RESPONSE AND FATIGUE LIFE OF ALUMINIUM ALLOY 7020 IN THE WHOLE AREA OF LIFETIME NÍZKOCYKLOVÁ ODEZVA A ÚNAVOVÁ ŽIVOTNOST SLITINY HLINÍKU 7020 V CELÉ OBLASTI ŽIVOTNOSTI Abstract In this article, cyclic hardening-sotening behaviours o high strength aluminium alloy 7020 in T6 condition are studied. By comparison o cyclic deormation curve and unidirectional curve was ound, that there is no signiicant change in cyclic response on tested material and that material can be accepted as cyclic stable. Experiments in high cycle area no discontinuity between low-cycle and high-cycle area was detected. Presented properties were not inluenced by the dierent loading requency (5 a 145 Hz) on dierent testing machines or low and high atigue area. Abstrakt Tato práce se zabývá studiem cyklického zpevnění-změkčení vysokopevné slitiny hliníku 7020 ve stavu T6. Z porovnáním cyklické deormační křivky s jednosměrnou křivkou bylo zjištěno, že u studovaného materiálu nedochází k výrazným změnám cyklické odezvy a můžeme jej považovat za cyklicky stabilní. Mezi nízkocyklovou a vysokocyklovou oblastí nebyla nalezena nespojitost. Uvedené chování neovlivnila ani rozdílná rekvence zatěžování (5 a 145 Hz) při provádění únavových zkoušek na různých zařízeních. 1 INTRODUCTION Alloys o AL-Zn-Mg type in the condition ater hardening belongs to the structural aluminium alloys with higher strength. Thanks to its good weldability, they are intended or stressed structural components and parts mainly in transport industry. The good weldability is stated by the low content o Cu and good corrosion resistivity in the T6 condition. The typical representative o the group aluminium alloy is the alloy EN AW-7020 (AlZn4.5Mg1) [1-4]. The aim o the work determination o cycle response o the alloy that is subject o study in low cycle area based on analyses o cycling hardening sotening curve. The secondary objective is determination o atigue lie curve on smooth testing specimens o 7020 alloy at symmetric loading tensile pressure at room temperature in the direction o orming o rod-shaped semi-product (L direction). The inal objective is to determination o Wöhler curve or the whole area o lietime (0.5 to 10 8 cycles) and alignment experimental measured data useul regression unction and determination o atigue limit σ C. 2 EXPERIMENTAL MATERIAL Tested samples or static and atigue tests were made rom 7020 aluminium alloy and supplied as rod-shaped with diameter D=20 mm rom the company Alcan Děčín Extrusion s.r.o. The material * Ing. Jose Zapletal, Brno UT, Faculty o Mechanical Engineering, Institute o Material Science and Engineering, Technická 2, Brno, Czech Republic, tel: (+420) , pepa.sanguis@centrum.cz ** Ing. Filip Vančura, Brno UT, Faculty o Mechanical Engineering, Institute o Material Science and Engineering, Technická 2, Brno, Czech Republic, tel: (+420) , yvancu01@stud.me.vutbr.cz *** Ing. Karel Němec, PhD, Brno UT, Faculty o Mechanical Engineering, Institute o Material Science and Engineering, Technická 2, Brno, Czech Republic, tel: (+420) , nemec.k@me.vutbr.cz 233

2 was heat treated to the T6 condition (artiicial ageing). Chemical composition determined through optical emission spectrometer is stated in table 1. Cylindrical testing samples were used or determination o basic mechanical properties at static loading in tensile mode. Cylindrical testing samples with screw heads (d 0 = 7 mm a L 0 = 12.5 mm) were used or determination o the low cycle parameters and or high cycle area experiments. Tests were done at room temperature. Tab. 1 Chemical composition o studied aluminium alloy Element Zn Mg Cu Si Mn Cr Fe Ti Al [wt.% ] Base 3 EXPERIMENTAL PROCEDURE Samples or static tensile tests were tested at computer controlled tension testing machine TiraTest The initial diameter o the cylindrical part was d 0 = 10 mm. The extension was measured at extensometer at initial measured length L 0 = 50 mm. The conditions o testes ulilled ČSN EN ISO The results o tensile tests and hardness measurement are stated in Tab 2. The samples or determination o low cycle parameters were loaded in computer managed electro hydraulic testing system INSTRON 8801 in the loading orce mode at sinusoidal wave required value with asymmetry P = 1 (R = 1, i.e. symmetric cycle). The constant requency o loading cycle = 5 Hz was maintained during testing. Deormation was measured with sensitive axial extensometer with measured length 12,5 mm. An extreme values o deormation and hysteresis loop were recorded in digital orm during the loading. The amplitude o the total deormation ε at was determined by the analyses o hysteresis look as the hal value peak to peak rom the total deormation. Number o cycles to ailure N was determined based on 3% change in eicient module E e against the values determined in hal lietime on the stated loading level. Experiments in high cycle area were made on high requency pulsator Amsler HFP 1478 with asymmetry o loading cycle P = 1. The loading requency as a unction o sample stiness was =145 ± 4 Hz. 4 RESULTS AND DISCUSIONS The microstructure o aluminium alloy 7020 on the orging direction (a) and upraight on (b) it is in Fig. 1a,b. The structure consists rom elongated polyedrical grains rom solid solution α and higher quantity o inclusions (Mn, Si, Fe base) which are directed through orming. Fig. 1 Microstructure o studied aluminium alloy The medium values o basic stress and deormation characteristics and hardness measurement determined rom 3 testing samples are stated in Tab

3 Tab. 2 Basic mechanical characteristic E [GPa] R m [MPa] R P0.2 [MPa] A [%] Z [%] HBW 5/ The comparison o cyclic and unidirectional deormation curve with denotation o the yield strength. direction is outlined in relation with amplitude o stress total deormation in Fig. 2. Points creating cyclic deormation curve are interlarded by the modiied Ramberg-Osgood unction (1) [5]. Its parameters determined by the regression analyses (at condition o method o least squares), i.e. modulus o elasticity E, parameter o stress σ 0 and exponent n are stated in Tab. 3. n σ σ E ε ( σ ) = + (1) σ Stress [MPa] CDK Tensile curve 0 0 0,002 0,004 0,006 0,008 0,01 0,012 0,014 0,016 0,018 0,02 Strain [-] Fig. 2 Comparison o cyclic and unidirectional deormation curve Curves o cyclic hardening sotening obtained rom hysteresis loop analyses are illustrated in Fig. 3. There is slight decrease o total deormation amplitude and ast decrease ε ap at the beginning o loading on the higher levels o loading cycle. The deormation response is stable on the lower levels o the loading cycle. The tested material can be considered as stable in total, the value o Manson s ratio R m /R p0,2 = 1.28 [6]. Total strain amplitude [-] 0,01 Stress 450 MPa Stress 400 MPa Stress 350 MPa Stress 300 MPa Stress 250 MPa Stress 200 MPa Stress 150 MPa 0,001 1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 1,E+06 Cycle Fig. 3 Curves o cyclic hardening - sotening Wöhler-Basquin curve was obtained as experimental data dependence o stress amplitude σ a on hal-cycle to ailure. The parameters atigue strength coeicient σ and atigue strength exponent b were determined based on that curve. [6]. 235

4 b σ a = σ (2N ) (2) The prevailing impact o elastic deormation part above the plastic deormation part was ound based on hysteresis loop analyses also in the area o low number o cycles to ailure. This eect is typical or aluminium alloy in over-ageing condition [7]. The amplitude o plastic deormation determined as hal o hysteresis loop width in N /2, was about value also or highest loading level 450MPa. For relation between number o hal-cycles to ailure and both part o deormation applies the equation (3) [7]. Parameters o power relation ε at 2N (Fig. 4b) are stated in Tab. 3. σ b c ε at = (2N ) + ε (2N ) (3) E ,0E-02 Stress amplitude [MPa] Total strain amplitude [ -] 100 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 N 1,0E-03 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 1,0E+06 N Fig. 4 (a) Wöhler-Basquin (a) and derived Manson-Coin (b) curve Tab. 3 Summary o acquired atigue parameters Wöhler-Basquin Manson-Coin Cyclic Curve σ b ε c E σ 0 n MPa MPa MPa Palmgren regression unction was used or alignment experimental determined data about relation stress amplitude and number o cycles to ailure on range 0,5 to 10 8 cycles. This unction accurately designates values o regression parameters, it has ainity between parameters and geometric waveorm or aluminium alloy. This unction has ormat [8, 9] b σ ( N ) = a( N + B) + σ The prevailing impact o elastic deormation part above the plastic deormation part was ound based on hysteresis loop analyses also in the area o low number o cycles to ailure. This eect is typical or aluminium alloy in over-ageing condition [7]. The amplitude o plastic deormation determined as hal o hysteresis loop width in N /2, was about value also or highest loading level 450 MPa. For relation between number o hal-cycles to ailure and both part o deormation applies the equation (3) [7]. Parameters o power relation ε at 2N (Fig. 4b) are stated in Tab. 3. (2) 236

5 LCF, 5 Hz HCF, 145 Hz UTS Palmgren 450 Stress amplitude [MPa] ,E-01 1,E+00 1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 1,E+06 1,E+07 1,E+08 1,E+09 N Fig. 5 Full area o atigue lietime and regression unction or alloy 7020 Although both types o tests were perormed on dierent testing machines and in dierent way o loading orce realization (hydraulic vs. resonance machines) there is no discontinuity between low-cycle and high-cycle area o atigue curve - as presented in Fig. 5. Change in requency o loading cycle rom 5 to 145 ± 4 Hz does not inluence the result described. Fig. 5 also presents very good alignment experimental measured data with Palmgren unction. Values o this regression parameters, i.e. parameters a, B, σ and exponent b are together with other important results (sum o squared deviations S and atigue limit σ C or 10 8 cycles) presented in Tab. 4. Tab. 4 Parameters o Palmgren regression unction Parameter a [MPa] b σ [MPa] B S [MPa 2 ] σ C [MPa] 1600,536-0, ,26 203, ,54 99,7 5 CONCLUSIONS Tested alloy, which is commercially available, has very good strength and plastic properties. It means that there is optimal precipitation hardening (condition T6). The microstructure consists rom elongated polyedric grains and high quantity o inclusions which are directed through orming. Based on development o cyclic hardening sotening curves can be concluded that there is decrease o total deormation amplitude on the higher level o loading cycle and that the deormation response is stable on lower levels. Based on comparison o cyclic deormation curve and unidirectional curve was ound, that there is no signiicant change in cyclic response on tested material and that material can be accepted as cyclic stable. Through the regression unctions material parameters o the Wöhler-Basquin curve σ = MPa a b = were determined. 237

6 The ull area o atigue lietime can be smooth by the Palmgren regression curve describing atigue properties mainly in the high-cycle area o tested aluminium alloy. No discontinuity between low-cycle and high-cycle area was ound at tested material, although the atigue tests were perormed on dierent machines. Presented properties were not inluenced by the dierent loading requency (5 a 145 Hz). Fatigue lie or smooth testing specimens determined rom calculation based on regression unction or 10 7 cycles is σ C = MPa and or 10 8 cycles is σ C = 99.7 MPa. ACKNOWLEDGEMENTS This work was supported by Ministry o Education, Youth and Sports (project 1M ) and Eco-Centre or Applied Research o Non-Ferrous Metals by Brno University o Technology. REFERENCES [1] SEDLÁČEK, V. Únava hliníkových a titanových slitin. Praha : SNTL, [2] MICHNA, Š. Encyklopedie hliníku. Prešov : Adin, [3] WANG, Q.Y., KAWAGOISHI, N., A CHEN, Q. Fatigue and racture behaviour o structural Al-alloys up to very long lie regimes. International Journal o Fatigue, 2006, roč. 28, č. 11, s [4] HRUBÝ, J., PETRUŽELKA, J., MLÝNKOVÁ, M. Development o aluminium alloys microstructure during hot orming. Sborník vědeckých prací VŠB - Technické univerzity Ostrava, Ostrava. č. 1, část 1, rok 2003, roč. XLIX, řada strojní, čl.1366, s , ISBN , ISSN [5] KLESNIL M., LUKÁŠ P.: Únava kovových materiálů při mechanickém namáhání, Academia, Praha, 1975 [6] MANSON, S. S.: Behaviour o Materials Under conditions o Thermal Stress. In Heat Transer Symposium. University Michigan Engineering Research Institute, 1953, s [7] POLÁK, J. Cyklická plasticita a nízkocyklová únavová odolnost kovových materiálů. Praha : Academia, [8] KOHOUT, J., VĚCHET, S. A new unction or description o atigue curves and its multiple merits. International Journal o Fatigue, 2001, roč. 23, č. 2, s [9] VĚCHET, S., KOHOUT, J., BOKŮVKA, O. Únavové vlastnosti tvárné litiny. Žilina : Žilinská univerzita v Žilině,