Fatigue behaviour of pre-strained commercially pure titanium K. Kusukawa, K. Takao Yuge National College of Maritime Technology, Yuge-cho, Ehime

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1 Fatigue behaviur f pre-strained cmmercially pure titanium K. Kusukawa, K. Taka Yuge Natinal Cllege f Maritime Technlgy, Yuge-ch, Ehime Abstract Rtating bending fatigue tests have been carried ut n a ntched cmmercially pure titanium pre-strained by 0% and 10% in tensin. The result f fatigue tests shws that a pre-strained titanium has lng fatigue life cmpared t an annealed ne in the range f finite life. Cnversely, the fatigue limit f pre-strained decreases by 12%. The ntch sensitivity f an annealed ne is very lw, hwever, it increases due t pre-straining. N nn-prpagating macr crack is bserved, but sme micrscpic nn-prpagating cracks are bserved n a sharp ntch with a radius f 0.25mm. The crack initiatin behavir was investigated thrugh successive bservatins. On the surface f annealed ne, multiple micr cracks initiated alng slip bands, within a few grains in cntact at a ntch and jin tgether t grw beynd grain bundaries. On the ther hand, fr a pre-strained ne micr cracks initiated alng bundaries f defrmatin twins within a single grain, jin t be a main crack leading t a final fracture. 1 Intrductin Pure titanium has an excellent resistance t crrsin and a high strength t weight rati. It has been used frequently fr heat exchangers in pwer statins r reacting cntainers in varius chemical plants. Pure titanium has a hexagnal clse-packed lattice, hwever, it has gd plastic frming characteristics and it is easy t be subjected bending r cld frming. Fr example, members f a plate-type heat exchanger are wrked by cld press, and a cndenser tube is rlled. These members are defrmed plastically in advance. Therefre, it is imprtant t make clear the effect f pre-st raining n fatigue characteristics fr the practical use f a pure titanium. Hwever. there has been few reprts n this prblem *.

2 876 Lcalized Damage It has been cnfirmed that ntch sensitivity is extrardinarily lw and n nn-prpagating fatigue crack exists in an annealed pure titanium ^. This seems t be related t a crystal structure and it is caused by large cnstraint t micrstructural defrmatins between grains. In pure titanium, twin develps during pre-straining in additin t defrmatin due t slip. It is expected that twin frmatin affects fatigue behavir. The present research deals with the investigatin n the effect f pre-straining n fatigue crack initiatin and prpagatin n a ntched and an unntched pure titanium during rtating bending tests. 2 Materials and Experimental Prcedure The materials used fr this research were tw kinds f cmmercially pure titanium bar (JIS TB35, with diameter f 15mm). Chemical cmpsitins are shwn in Table 1. Difference in each cmpsitin is nly slightly, therefre we treated them as the same metal. Specimen cnfiguratin and details f ntches are shwn in Fig.l. Sme f specimens were annealed at 1073K fr Ihr in vacuum after turning. They were machined circumferencial ntches. The thers were pre-strained in tensin by 10%(accuracy f the strain was ±0.4%) after annealing in vacuum. Pure titanium is highly anistrpic in plastic defrmatin, the shape f crss sectin changes frm circle t ellipse after prestraining. In rder t mdify their shapes, they were turned the parallel prtin by abut 0.2mm and machined ntches. All specimens were chemically plished befre fatigue tests. The mechanical prperties f bth materials are shwn in Table 2. Fatigue tests were perfrmed using a catilever-type rtating bending fatigue machine (with capacity f 100 TV m) perated at the speed f abut 1780 rpm. The surfaces f specimens were replicated successively t bserve initial cracking behavir by using ptical- and electrn-micrscpes(sem). 3 Experimental results and discussin 3.1 Fatigue lives and ntch sensitivity Figure 2 shws the S-N curves fr an annealed metal. It was reprted by ne f the authrs ^ that mannealed pure titanium exhibits a cnsiderably sharp bent in an S-N curve and a relatively distinctive fatigue limit. Fr ntched specimens they als exhibit sharp bents at abut 10^ cycles fr all ntch radii. The specimen with ntch radius p = 0.25mm did nt fail up t TV = 2 x 10^ cycles under the stress amplitude f its fatigue limit. Figure 3 shws similar S-N curves fr a pre-strained specimen tgether with the results fr annealed nes withut data plts. Pre-strained specimens exhibit fatigue limits in the same way as annealed nes. In the range f finite life, the fatigue strengths f pre-strained specimens were higher than thse f annealed nes fr all ntch radii. Cnversely, the fatigue limits were lwer than thse f annealed nes by abut 12%.

3 era' t in c^ M< (D CA 0^ P 5 gj 30) a; (T> a- w ^ Stress amplitude ^ MPa r Ul O (Jl O U X C ZC C 3 0" (D t O^ 0 C1 $ ' 0 i >/, " O 0) -? f I ~Q "^ "O 0 : 9 O -* CJi. KD ; en 3 3 E 3 0 ', f ' -] A 1 >J U g. ^z II O * 03 7T D Q. '. r~\. =,.,. l, l, : 2. 4> i re 03 3n ± 01 r cr> 00 Annealed en CJ> " -1 (t 0 ^ -h O 03 Ultimate Elngatin tensi le strength a.(hpa) j *(%) *1 0> <-> O3 Q >-»».-J s? 0] (D t r chanical prperties f 0) 3 B) Q- CJ1 CND cn cr zc 3 Table 1 Chemical cmpsitins f materials in wt%. S a t t -J Dynamic indentatin hardness Hvd:50gf 1 % 00 ^1 <I

4 878 Lcalized Damage 2250 =200 D, 150 T3 ii CL E 50 c/).+- /=5mm P=1mm - P=0.25mm Annealed,p=5mm p=1mm,0=0.25mm ^ 10* 10^ 10 10^ Number f cycles t failure Brken Unbrken Fig.3 S-N curves fr a pre-strained specimen. 03 CL 400 Annealed 10* 1100 Prestrained E c /P mm~* Fig.4 Relatin between the elastic maximum stress at fatigue limit and the reciprcal f ntch radius. Figure 4 shws the relatin between the elastic maximum stress Kt&w ( VrnaxiKt stress cncentratin factr,<j^ :fatigue limit) at a ntch, and the reciprcal f a ntch radius, l/p, in ther wrds, the index meaning ntch insensitivity and the ne meaning ntch sharpness, respectively. An annealed pure titanium shws very lw ntch sensitivity cmpared t a prestrained ne. 3.2 Crack initiatin behavir Figure 5 shws an example f successive bservatins n the surface f the annealed specimen with p 5mm under <j^ = l70mpa, a slip band is bserved in the grain where a crack initiates later at N = 7.0 x 10*. The density f a slip band increases with lading cycles and several small cracks initiate alng them(at TV = 8.0 x 10* ). The main crack starts t grw after jining tgether with ther cracks that initiate in grains in cntact at almst the same number f cycles. The fundamental aspect f crack initiatin prcess in this material is almst identical

5 Lcalized Damage 879 t the ne as already reprted '\ hwever, the number f slip bands in ne grain fr the present metal is less than the frmer ne. This indicates that slip cncentrates in a lcalized regin in a grain. Figure 6 shws a similar bservatin fr a pre-strained specimen with p = 5mm under a^ l70mpa. There are many slip bands and twins n the surface. A fatigue crack initiates at a twin bundary. Three small cracks (marked ) initiate alng a twin bundary, then they jin t be a single crack leading t final fractrue. Partridge * reprted that fatigue cracks initiate at the twin fragment interface f pure magnesium. In case f an annealed titanium, n nn-prpagating micr crack appear and nly slip bands are bserved fr bth a ntched and an unntched specimen under stress amplitudes f respective fatigue limits. In case f a pre-strained ne, hwever, nn-prpagating micrcracks within grains are bserved in all ntches. 3.3 Small crack prpagatin behavir Figure 7 shws the relatin between crack grwth rate and crack length fr bth materials under a a = l7qmpa. Crack grwth tests were carried ut n a pre-strained specimen with a drill pit (diameter and depth are the same, 0.6mm ) and a smth specimen fr an annealed titanium. It is well knwn that bth micrstructure and stress gradient affect grwth f small cracks, especially, grwth rates drastically decrease in a pure titanium when a crack tip appraches a grain bundary ^. At the same crack length, the average grwth rate f a pre-strained specimen is lwer than that f an annealed ne. Crack initiatin lives f the same ntch p = 5mm, under <j«hompa are abut the same( TV = 8 x 10^) fr bth materials. Therefre, the increase in fatigue life fr a pre-strained specimen is thus due t the decrease in crack grwth rate. 3.4 Effect f pre-straining n ntch sensitivity Fr an annealed titanium, a fatigue crack starts t grw by jining multiple micrcracks which initiate alng the slip bands within a few grains in cntact. At a fatigue limit, n nn-prpagating micr crack appears and nly slip bands are bserved. Thus a fatigue limit means the limit f crack nucleatin. This is the same as in a ntched specimen. On the ther hand, fr a pre-strained ne, fatigue cracks initiate at pint-like small regins alng twin bundaries. At a fatigue limit, nn-prpagating micrcracks appear even in the case f a sharp ntch. Cnsidering abve behavir, schematic illustratins f the relatin between stress distributin and grain diameter at fatigue limits f ntched specimens fr an annealed- and pre-strained titaniums are shwn in Fig.8. Fr a prestrained specimen, surface layer at the ntch where an average stress is equal t the stress amplitude at the fatigue limit f smth specimen cr^, is thinner than the ne f an annealed specimen, because fatigue cracks initiate at pint like small regins. Therefre, cr^ax/^w decreases, in ther wrds, ntch sensitivity increases due t pre-straining. The result shwn in Fig.4 is reprduced in Fig.9 after nrmalized by

6 880 Lcalized Damage

7 Lcalized Damage a=170mpa Q) O Annealed 10-8 ^T Pre-strained Ntch Grain (a)pre-strained Crack Length 1 mm Ntch Grain (b)annealed Fig.7 Relatin between crack prpagatin rate and crack length. T?- rig.8 r Schematic i. -n illustratin *.- f r stress, distributins near ntch rt at fatigue limit. D ^1 D, Annealed </> 0) T3 Maximum sensitivity ^ ^ I z /P Fig.9 Cmparisn f ntch sensitivity with Al-ally. cru,, in rder t cmpare ntch sensitivites f bth titaniums with the ne f Al-ally *> which is a typical material giving rise t pint-like cracking behavir. The curve representing a ntch sensitivity f a pre-strained specimen appraches the ne f Al-ally. This leads that the ntch sensitivity increases by pre-straining, due t the change in crack initiatin prcess frm definite

8 882 Lcalized Damage regin type t pint-like type. Fr practical use f pure titanium defrmed heavily due t plastic frming, annealing may be recmmended in rder t imprve fatigue ntch characteristics. 4 Cnclusin Rtating bending tests were carried ut in rder t investigate the effect f pre-straining n fatigue ntch characteristics f a cmmercially pure titanium. The fllwing cnclusins were btained. 1. The fatigue strength in the range f finite life increases, but the fatigue limit decreases. 2. Fr an annealed titanium, fatigue cracks initiate alng the slip bands with in a few grains in cntact and jin tgether. Fr a pre-strained ne, cracks initiate alng twin bundaries. 3. Crack grwth rates f pre-strained titanium is lwer than that f an annealed ne at the same stress amplitude and the same crack length. 4. The ntch sensitivity increases due t pre-straining, because fundamental aspect f crack initiatin prcess change frm definite regin type t pint-like type. References 1. Owens,J.P.,Watsn,P.& Plumtree,A. Mechanical Behaviur f Material, Prc. f ICM.,Kyt,1972, Taka,K.,Sgh,M. & Matsumt,K. Effect f grain size n ntch sensitivity in fatigue f cmmercially pure titanium, Trans. Jpn Sc. Mech. Eng. 1986,52, (in Japanese). 3. Taka,K. & Nisitani,H. Fatigue crack initiatin and ntch sensitivity f cmmercial purity titanium, Trans. Jpn Sc. Mech. Eng. 1984,50, (in Japanese). 4. Partridge,P.G. Cyclic twinning in fatigued clse-packed hexagnal metals, Philsphical Magagine,1965,12, Taka,K. & Kusukawa,K. Prpagatin behavir f small fatigue cracks initiated at the ntch in cmmercially pure titanium, Trans. Jpn Sc. Mech. Eng. 1994,60, (in Japanese). 6. Taka,K. & Nisitani,H. Relatin between the characteristics f crack initiatin f metal and their ntch sensitivity in fatigue, Jurnal f Sc. Mat. Sci. Jpn 1987,36, (in Japanese).