Available online at Fatigue S. Spanrad a *, J. Tong a

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Aville online t www.sciencedirect.com Procedi Procedi Engineering Engineering 2 (2010) 00 (2010) 1751 1759 000 000 Procedi Engineering www.elsevier.

Tong Deprtment of Mechnicl nd Design Engineering, University of Portsmouth, Anglese Building, Anglese Rod, Portsmouth PO1 3DJ, UK Received 26 Ferury 2010; revised 9 Mrch 2010; ccepted 15 Mrch 2010

1 Aville online t Procedi Procedi Engineering Engineering 2 (2010) 00 (2010) Procedi Engineering Ftigue 2010 Chrcteriztion of foreign oject dmge (FOD) nd erly ftigue crck growth in lser shock peened Ti-6AL-4V erofoil specimens S. Spnrd *, J. Tong Deprtment of Mechnicl nd Design Engineering, University of Portsmouth, Anglese Building, Anglese Rod, Portsmouth PO1 3DJ, UK Received 26 Ferury 2010; revised 9 Mrch 2010; ccepted 15 Mrch 2010 Astrct Foreign oject dmge (FOD) hs een identified s one of the primry life limiting fctors for fn nd compressor ldes, with the leding edge of erofoils prticulrly susceptile to such dmge. In this study, typicl erofoil specimen of Ti-6Al-4V lloy ws used. The leding edge of the specimen is representtive of generic erofoil geometry. The specimens were treted y lser shock peening (LSP) to generte protective residul stresses in the leding edge region prior to impct. FOD ws simulted y firing cuicl projectile t 250m/s hed-on t the leding edge using lortory gs gun. The specimens were then sujected to ftigue testing under high cycle (HCF), low cycle (LCF) nd comined LCF nd HCF loding conditions. Scnning electron microscopy (SEM) ws used to chrcterise the dmge fetures due to FOD. Crck initition nd erly crck growth due to FOD nd susequent ftigue loding were exmined in detil. An nlysis with cksctter electron (BSE) detector ws lso crried out to investigte the microstructul deformtion due to FOD, LSP, s well s the erly ftigue crck growth mechnism in complex residul stress field. c 2010 Pulished y Elsevier Ltd. Open ccess under CC BY-NC-ND license. Keywords: Bllisitc impct; Dmge chrcteristion; Titnium; Foreign oject dmge, SEM 1. Introduction Foreign oject dmge (FOD) to rotting components in modern ircrft engines is of gret concern in gs turine engines, where smll hrd prticles such s stones or metllic deris cn impct turine ldes during tkeoff nd lnding t high speeds up to 500m/s [1]. The result of the impct is loclised dmge in the form of notch or dent, often close to the leding edge, together with possile mteril loss, extensive plstic deformtion t oth mcro nd micro levels. Mny ftigue filures in jet engines hve een ttriuted to comintion of high frequency virtion loding nd prior dmge in-service events such s FOD, with significnt impliction on the reduction of component service life [1,2]. The evlution of FOD resistnce of mterils nd structurl components hs een crried out in typicl lde mteril, Ti-6Al-4V, in lortory test conditions. Nichols et l [3] were proly the first to study the effects of * Corresponding uthor. Tel.: fx: E-mil ddress: Sven.Spnrd@port.c.uk c 2010 Pulished y Elsevier Ltd. doi: /j.proeng Open ccess under CC BY-NC-ND license.

1752 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) 1751 1759 2 S. Spnrd et l./ Procedi Engineering 00 (2010) 000 000 FOD on titnium leding edge.

2 1752 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) FOD on titnium leding edge. They found tht the morphology of the dmge consisted of ters nd loss of mss in ddition to dents, nd dents with ters were more dmging in reducing ftigue life in the low-cycle ftigue (LCF) regime. More recent work t Berkeley [4,5] ws on the influence of FOD on high cycle ftigue (HCF) threshold. They found mrked reduction in ftigue life post FOD, compred with tht of undmged specimens. Impcts using hrd cues were crried out on pltes [7], generic erofoil specimens [8] nd more relistic erofoil shpe similr to rel compressor ldes [9,10]. The cuicl projectile ws considered more representtive of worst cse dmge seen in the engine. Whilst work t Oxford [9,10] ws more concerned with the reduction in ftigue strength due to FOD, work t Portsmouth [7,8] ws focused on erly ftigue crck growth due to FOD under more relistic loding conditions, including LCF, HCF nd comined LCF nd HCF. The im of this study is to chrcterize cuicl hed-on FOD impcts using SEM nd evlute the role of the dmge fetures in crck initition nd erly crck growth in lser shock peened Ti-6Al-4V simulted erofoil ldes. A comprison is lso mde etween well studied sphericl FOD dmge impcts [11,12] nd the present cuicl FOD impcts. 2. Experimentl procedures 2.1. Mterils nd specimen design The mteril used is titnium lloy Ti-6Al-4V with typicl microstructure of imodl distriution of primry phse nd lmellr colonies of +. The specimens were mchined from lnks for the production of fn ldes. The mteril ws first forged in the phse nd continued into the + phses, then rolled nd creep flttened in the + phse, s illustrted in Fig. 1. The mterils properties t room temperture re: Young s modulus, 103GP, yield stress, 860MP nd tensile strength, 980MP [7]. FOD dmge on ldes in ero-engines is known to occur mostly on the leding edge of n erofoil section, thus symmetricl, ellipticl-shped cross section ws used to simulte the generic form of erofoil components, see Fig. 2-. A four-point end geometry ws chosen to perform the ftigue tests. Fig. 1. The imodl microstructure of Ti-6AL-4V (s received).

3 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) Fig. 2. ) Geometry of four-point end erofoil specimen (dimensions in mm). ) schemtic illustrtion of hed-on impct in side view of the leding edge of the specimen LSP process A description of the physicl nd mechnicl mechnisms of the LSP process cn e found in [13]. The specimens were lser shock peened over the leding edge y Metl Improvement Compny (USA), using power density of 10 GW/cm 2, pulse durtion of 27 ns, spot size of 3 3 mm 2, 50% overlp nd 200% coverge. The totl LSPed re consists of 27 strikes nd mesure pproximtely 65 mm x 6 mm wide. To ensure dequte depth of the ensuing residul stress lyer, two lyers of identicl spot loction nd conditions were performed. A sketch of typicl lser spot pttern cn e found in [13] Simultion of foreign oject dmge Foreign oject dmge ws simulted y firing 3 mm hrdened steel cue onto the leding edge of the specimen t velocity of ~ 200m/s. A light gs gun (Oxford University, UK) ws used to fire the projectiles. The gun consisted of 2-litre gs cylinder connected vi pneumtic vlve to 2.5 m long sleeved rrel, s descried in [9]. The trget ws held in cross-vice which cn e rotted nd trnslted with micrometer precision. The rrel ws keyed symmetriclly to ensure no rottion of the projectile. Velocity ws mesured using photodiodes. The cue impcted edge first onto the leding edge of ech specimen in the centre of the guge length nd its trjectory ws t 0 0 to the impcted fce, s shown in Fig. 2. Figure 3 shows typicl FOD crter in shpe of V notch, representtive of the worst cse dmge oserved in service conditions. 2.4 Ftigue testing methods Ftigue testing of the simulted nd FODed irfoil specimens were crried out under four-point end, constnt mplitude (CA) loding in servo-hydrulic twin ctutor 100kN testing mchine. The support spn used in ll tests ws 107 mm nd the loding spn ws 57 mm, with ll rollers free to rotte nd move prt slightly s the specimen is loded. The conjoint ction of LCF nd HCF cycles, referred to s comined LCF+HCF loding, representing simplified flight spectrum s descried in [7-8] ws used. The test frequency for ll CA tests ws 80 Hz. When predefined step test lock of cycles ws reched, the test ws stopped, the stress rnge ws incresed y the designted percentge (either 3-5%), nd the test ws resumed for nother lock or filure, whichever occurred first. Crck initition nd growth ws monitored using direct current potentil drop technique. Visul mesurement of crck length ws lso performed on oth sides of the FOD notch, using the cetone replic method.

4 1754 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) Notch chrcteriztion Prior ftigue testing, the FOD notches were photogrphed in different orienttions for profiling nd mesurement of the notch depth y n opticl microscope. To chrcterize FOD notches scnning electron microgrph (SEM) study ws crried out using JEOL JSM SEM. A set of 5 specimens were chrcterized. The notches were exmined using vriety of orienttions, nd different stges in the ftigue filure progress to chrcterize dmge fetures, crck initition nd erly crck growth. These steps were ) prior ftigue testing, ) fter ftigue crck initition nd erly growth, nd c) fter ftigue frcture. The microgrphs of the two ltter steps provide insight into the loction nd nture of the crck initition nd erly crck growth, s well s the interction with the FOD dmge fetures. 3. Results nd discussion 3.1. FOD chrcteriztion The FOD fetures were chrcterized using the SEM nd the results re shown in Fig. 3, including the notch depth, loss of mterils (LOM), mteril sher nd mteril folding over the LE, het formtion resulting in fused res nd microcrcks Notch Depth The notch depths were mesured y positioning the SEM stge to chieve profile view of ech notch depth in x-direction. The verge notch depth of 9 FOD shot specimens with n verge speed of 197 m/s ws 1.43mm. The preferred crck initition sites were reported to e the deeper notch[2, 5]. This seems to e confirmed y the fretting deris due to ftigue ner the root of the FOD notch, shown in Figs Loss of mteril (LOM) nd mteril sher LOM nd mteril sher from the surfce of the notch re evident in ll FODed specimens (Figs 3, 4, 5, 6). Fig. 4 shows n overview of the impct site, including ll oserved impct fetures. LOM nd mteril sher re evident on the notch surfce in the form of sher dimples nd sher lips. Typicl ters (LOM) re clerly visile in Fig. 3. LOM is mostly found on the edges of the notch, s shown in Fig. 4. LOM my e ssocited with crck initition nd erly growth (Fig. 5e), lthough LOM lone does not pper to hve direct effect on the ftigue strength [6] Mteril folding nd het formtion Mteril folding over the leding edge lso occurred on ll FODed specimens nd cn e seen in Figs. 3,, 4, 5, 6. Mteril folding is result of mteril pile-up, due to the cuicl impct. When the cue strikes the LE, mteril is shuffled hed of the impcting cuicl edge. The shuffled mteril is prtly shered off t the eginning of the impct notch (LOM), or piled up preferly t the notch periphery (Figs. 3,, 4, 5, 6) s well s in the root of the notch (Figs. 3, 5). The folded nd piled-up mteril t the periphery of the notch cretes shrp notch, s shown in Figs. 3,, 5. The surfce of the folded mteril next to the root of the notch t the periphery shows different smoothness. The folded mteril is much smoother which indictes het formtion resulting in fused lyers of mteril, see Fig. 3. Another evidence of het formtion during the impct cn e seen in Fig. 3e, where the mteril ws folded in the root of the notch nd droplet structures cn e seen djcent to the res of incipient melting round the 'clevge' surfces [14]. Thirdly, microcrcks nd micro notches cn e seen in the re of re-fused mterils (Fig. 3, d) t the root of the notch. The origintion of these microcrcks will e explined s followed.

S. Spnrd, J. Tong / Procedi Engineering 2 (2010) 1751 1759 1755 S. Spnrd et l.

rim, loss of mterils (LOM), mteril sher nd folding.

Microcrcks nd intense sher nd t the se of the notch; e) Droplet structures s evidence of het formtion. 3.1.4.

3c, 5, c, e, ee). A typicl microcrck length (Fig.5ee) is out 40 m.

5 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) Micronotches Fold 3d 3c 3e c Sher lips d Sher nds, Micro crcks Sher direction Micronotch e Droplet structures Fig. 3: Scnning electron microgrphs of the impct site for hed-on impct, with typicl chrcteristics including mteril pile-up t crter rim, loss of mterils (LOM), mteril sher nd folding. ) Overview; ) Mteril folding nd pile-up ssocited with fusing during high speed impct; c) Mteril sher ssocited with LOM; d) Microcrcks nd intense sher nd t the se of the notch; e) Droplet structures s evidence of het formtion Sher nds nd micro crcks Mteril pile-up nd folding t the crter rim results in shrp notches (Figs. 3,, 5, 6) nd microcrcks (Figs. 3c, 5, c, e, ee). A typicl microcrck length (Fig.5ee) is out 40 m. Multiple microcrcks were lso found on the root surfce of the notch (Fig. 5), s well s sher nds [5]. These sher nds were induced due to high sher stress during impct. As shown erlier, the impct produces high temperture on the surfce loclly, which my result in strin-mismtch, leding to microcrcks. Micronoches, s identified in [5], on the surfce of the FOD notch my e result of plstic flow nd LOM, which cn e seen in Fig. 3,, 4, 5.

1756 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) 1751 1759 6 S. Spnrd et l./ Procedi Engineering 00 (2010) 000 000 Multiple crcks Fig.

testing. 3.2. Chrcteriztion of erly crck growth For the chrcteriztion of erly crck growth, the specimens were ftigue tested nd exmined using the SEM efore filure (Figs. 4, 6, c, e).

The pictures otined pre-ftigue testing were compred with those done post ftigue testing of the sme res (Fig. 6). 3.2.1.

6 1756 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) Multiple crcks Fig. 4: FOD notch surfce fter ftigue testing; ) Loose mterils due to fretting ftigue; ) Loose prticles ner the notch root t higher mgnifiction; c) Multiple crck inititions from the FOD indent post ftigue testing Chrcteriztion of erly crck growth For the chrcteriztion of erly crck growth, the specimens were ftigue tested nd exmined using the SEM efore filure (Figs. 4, 6, c, e). Susequently, they were ftigued until filure nd the two hlves of the specimen were exmined under the SEM (Figs 6, d, f). The pictures otined pre-ftigue testing were compred with those done post ftigue testing of the sme res (Fig. 6) Multiple crck growth nd fretting ftigue After ftigue crck length of out 7 mm, the FOD notch ws exmined using the SEM. An overview of notch is shown in Fig. 4. Bright prticles cn e seen primrily in the notch root, s well s on the surfce of the notch. Fig. 4 shows these prticles t higher mgnifiction nd Fig. 4c t 45degree ngle with respect to the notch plne. These prticles pper to e loose, possily due to fretting ftigue nd/or incresed level of crck closure s result of the residul stresses due to LSP nd FOD impct. The loose prticles vry in size, etween ~16 nd 68 m in length. Figs. 4c, show these prticles t the rim of the notch, where the crck pths re mrked out y these prticles, indicting multiple ftigue crcks, especilly ner the rim of the FOD notch Frcture surfce nlysis nd crck initition Figs. 5 nd c show the notch rims of oth ends of the notch root fter the FOD impct. Figs. 5 nd d correspond to the sme re of the FOD notch fter the ftigue frcture. The comprison shows tht the shrp notches t the notch rims my hve served s the crck initition sites.

1757 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) 1751 1759 S. Spnrd et l.

e /e/ f Mteril pile-up Microcrck Fig. 5: Frcture surfces pre- (, c, e) nd post- (, d, f) ftigue testing.

nd susequent crck growth ws detected in (); ) The shrp notch due to FOD served s crck crck initition site

due to mteril pileups nd mteril folds (Post testing); e) Microcrck formed t the crter rim fter FOD impct,

plnes. The stritions from the ottom side of the frcture surfce re continued onto the flke surfce.

To crete crck colescence more crck fronts re needed; one from the top nd one from the right rim side,

7 1757 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) Frcture surfce Fold Notch Notch c d Mteril pile-up Frcture surfce e /e/ f Mteril pile-up Microcrck Fig. 5: Frcture surfces pre- (, c, e) nd post- (, d, f) ftigue testing. ) Mteril pile-up nd folding over the leding edge creted shrp notch Ftigue from which microcrcks were formed nd susequent crck growth ws detected in (); ) The shrp notch due to FOD served s crck crck initition site (post testing); c) Mteril folds nd pile-ups occurred ner the rim of FOD notch; d) Two crck initition sites due to mteril pileups nd mteril folds (Post testing); e) Microcrck formed t the crter rim fter FOD impct, inset shows the crck t higher mgnifiction; f) Micro ftigue crck-growth t mteril pile-ups from the microcrck in (e) (post testing). On the frcture surfce (Fig 5) 'flke-like' mteril is evident which my e result of crck growths t different plnes. The stritions from the ottom side of the frcture surfce re continued onto the flke surfce. This indictes tht one crck initition ws developed from the left rim side. To crete crck colescence more crck fronts re needed; one from the top nd one from the right rim side, originted from the middle of the FOD notch with respect to the rims. This flke on the frcture surfce my well e result of multiple crck growths originted on different plnes. A reconstruction of the possile successive crck growth ptterns from the crck initition sites is shown in Fig. 5.

1758 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) 1751 1759 8 S. Spnrd et l./ Procedi Engineering 00 (2010) 000 000 crck inititions Flke Fig.

8 1758 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) crck inititions Flke Fig. 6: ) Ftigue stritions nd loose flkes s evidence of multiple ftigue crck growth on different plnes; ) FOD crter (left) nd frcture surfce (right). Arrows indicte the crck initition sites. Crck fronts re lso indicted Dmge fetures: Comprison of cuicl nd sphericl impcts For sphericl projectiles, FOD dmge, s chrcterized y size nd shpe of the indents nd loction of microcrcking, ws found to depend on the impct velocity. At velocities ove 250m/s, pronounced pile-up of mteril ws evident t the crter rim; dditionlly circumferentilly orientted sher nds were lso seen, emnting on the surfce of the impct crter; whilst such effects were not pprent for 200m/s. For impct velocities of 300 m/s, plstic flow of mteril t the crter rim resulted in multiple micro notches nd microcrcking. Therefore velocities elow ~250m/s do not necessrily induce ll the dmge processes ssocited with relistic simultion for FOD. The FOD dmge creted y cuicl projectile t n impct velocity of 200 m/s seems to e more detrimentl compred to sphericl FOD impcts t the sme speed. Dmge chrcteristics such s sher nds nd microcrcking were found only t higher impct velocities (>250m/s) for sphericl impcts, whilst using cuicl projectile these fetures re oserved t low velocity (200m/s). This seems to suggest tht the reltionship etween impct velocity nd dmge chrcteristics oserved in [5] for sphericl projectiles my e sujective of the projectile shpes. For V-notch creted y cuicl projectiles the stress concentrtions in the se of the notch nd t the rims re greter compred to those under sphericl impcts. The shrp notches creted due to circumferentilly piled-up mteril were fvourly orientted with respect to the pplied stress xis on susequent ftigue cycling. The microcrcks nd sher nds t root of the notch re predomintely orientted in the fvourle crck plne, prompting crck initition long the se of the notch. The multiple sher nds in the se of the notch might e responsile for plstic deformtion t microstructurl level hence residul stresses, which could then e responsile for multiple crck initition nd growth in the erly stge. 4. Conclusions Experimentl studies on FOD of generic erofoil specimen nd ftigue crck growth post FOD seem to suggest: 1. Dmge from high speed cue impct on leding edge of erofoil specimens my e chrcterised s mixture of notch indent, loss of mterils, mteril sher, mteril folding, het formtion, sher nds nd microcrcks. 2. Dmge fetures preferly occurred t certin res of the FOD notch, s shown in Tle Crck inititions seem to occur ) t the FOD-induced microcrcks nd micronotches ner the crter rims, where the stress concentrtion ws highest s result of mteril pile up nd LOM; nd ) in the notch root where surfce irregulrities such s sher-nds nd microcrcks were formed. 4. Multiple crck growth nd crck growth on different plnes my e result of non-uniform residul stress distriution creted y LSP nd FOD nd the irregulrities t the notch root.

9 S. Spnrd, J. Tong / Procedi Engineering 2 (2010) S. Spnrd et l./ Procedi Engineering 00 (2010) The FOD dmge creted y cuicl projectile seems to e more detrimentl compred to sphericl FOD impcts t given speed. 6. The reltionship etween impct velocity nd dmge chrcteristics oserved in [5] for sphericl projectiles my e sujective of the projectile shpes. Tle1. Typicl mteril dmge fetures oserved for hed-on FOD impct nd the loctions of their ppernce. Impct feture of hed-on impcts LOM Folding over the LE Mteril pile-up Sher-nds Are of Predominntly t the notch rims Notch rims Notch rims nd notch root Notch root, creted y the cue edge Acknowledgments The uthors wish to cknowledge the EPSRC nd the MOD of the UK for finncil support; Rolls -Royce eroengine division for the specimens nd the LSP tretment; The FOD simultion ws crried out t the University of Oxford. Helpful discussion with Dr John Schofield of Rolls-Royce is grtefully cknowledged. References [1] Nichols T. Criticl issues in high cycle ftigue. Int J ftigue, 1999;21(Supp. 1):S [2] Ruschu JJ, Nichols T, Thompson SR. Influence of foreign oject dmge (FOD) on the ftigue life of simulted Ti-6Al-4V irfoils. Int J Ftigue 2001;25: [3] Nichols T, Brer JP, Bertke RS. Impct dmge on titnium leding edges from smll hrd ojects. Exp Mech, 1980;20: [4] Peters JO nd Ritchie R.O Influence of foreign-oject dmge on crck initition nd erly crck growth during high-cycle ftigue of Ti- 6Al-4V. Engineering Frcture Mechnics, 2000; 67: [5] Peters JO, Ritchie RO. Foreign-oject dmge nd high-cycle ftigue of Ti 6Al 4V. Mterils Science nd Engineering 2001; A : [6] Mrtinez CM, Eylon D, Nichols T, Thompson SR, Ruschu JJ, Birkeck J, Porter WJ. Effects of llistic impct dmge on ftigue crck initition in Ti 6Al 4V simulted engine ldes. Mterils Science nd Engineering 2002; A325: [7] Ding J, Hll RF, Byrne J, Tong J. Ftigue crck growth from foreign oject dmge under comined low nd high cycle loding. Prt 1: Experimentl studies. Interntionl Journl of Ftigue 2007; 29: [8] Hll R, Byrne J, Zho T, Tong J. Influence of foreign oject dmge on ftigue crck growth of gs turine erofoils under complex loding conditions. Ftigue & Frcture of Engineering Mterils & Structures 2008; 31: [9] Nowell D, Duo P, Stewrt IF. Prediction of ftigue performnce in gs turine ldes fter foreign oject dmge. Interntionl Journl of Ftigue 2003;25: [10] Duo P, Liu J, Dini D, Golshn M, Korsunsky AM. Evlution nd nlysis of residul stresses due to foreign oject dmge. Mechnics of Mterils 2007; 39: [11] Bche MR, Evns WJ, Voice W. The resistnce to impct dmge nd susequent ftigue response of two titnium lloys. Mterils Science nd Engineering A333: [12] Bche MR, Brdshw C, Voice W. Chrcteristion of foreign oject dmge nd ftigue strength in titnium sed erofoil lloys. Mterils Science nd Engineering A354: [13] Ruschu JJ, John R, Thompson SR, Nichols T,. Ftigue crck nucletion nd growth rte ehvior of lser shock peened titnium. Int J Ftigue 1999; 21: [14] Engel, Klingele: Rsterelektronenmikroskopische Untersuchungen von Metllschäden. Köln, Gerling Institut für Schdenforschung und Schdenverhütung, 1974, P. 141.

Supplementary Figure 1 Supplementary Figure 2

Supplementary Figure 1 Supplementary Figure 2 Supplementry Figure 1 Comprtive illustrtion of the steps required to decorte n oxide support AO with ctlyst prticles M through chemicl infiltrtion or in situ redox exsolution. () chemicl infiltrtion usully