ISIJ International, Vol. 37 (1 997), No. 12, pp. 1170-1 179

Review

Subsurface CrackStrength Alloys

Generation in High-cycle Fatigue for High

Osamu

Nationai

UMEZAWAand Kotobu NAGAl

Research Institute for Metals. Sengen. Tsukuba, Ibaraki-ken, 305 Japan.

(Received on May6. 1997.• accepted in final form on May26. 1997)

In order to progress in the understanding of fatigue fracture process for high strength alloys, the previousstudies of the subsurface fatigue crack generation are ~eviewed. A change in crack initiation mechanismfrom specimen surface to specimen interior often introduces a plateau ("knee") followed by a rather sharpdrop in the shape of S-N curve. Various subsurface crack origins are related with microstructural crackingsand pre-existing defects. The subsurface initiation site is formed as a Stage I crack. Anewexplanation ofmicrocrack growth is proposed for the subsurface crack generation process. The subsurface crack size is

the most important parameter to determine howthe crack becomesa fatal crack. The size highly dependson the maximumcyclic stress range, which implies a AKth threshold controiling mechanism.Thedislocationstructures in high-cycle fatigue are fairly planar for both austenitic steel and titanium alloy.

KEYWORDS:fatigue crack initiation, Stage I (Modell) crack; high-cycle fatigue; defect; fish-eye; thresholdstress intensity factor range; Iow temperature; titanium alloy; high strength steel; austenitic steel.

l. Introduction

The high-cycie fatigue fracture process is generallyclassified into the four stages which are damage,crackinitiation (Stage I), crack propagation (Stage II) andrapid fracture (Stage 111). In the "damage" stage offatigue, Iocalized accumulation of strain is introduced bycyclic stress (strain). In the absence of defects, it is

believed that a fatlgue crack is initiated at the specimensurface. The extrusion-intrusion mechanisml) well ex-plains this behavior, and persistent slip bands (PSBs)play an important role in the mechanism.2) Even if acrack starts at a pre-existing defect in the specimeninterior, the PSBSare supposedto be active and producethe extrusion-intrusion at the free surface. Whydoes the

main crack select the internal defect instead of the ex-trusion-intrusion flaw at the specimensurface? This canbe accounted for by a difference in stress con centra-tion magnitude around them. Engineering metalllc ma-terials have somesingularities or heterogeneity In their

microstructure. Microscopically localized deformation is

always related to the microstructural heterogeneity such

as non-metallic inclusions, pores, second phase partic]es

andgrain boundaries, which are potential nuclel for local

stress concentration. Acomparatively larger pre-existing

defect in the specimeninterior mayintroduce higher stress

concentration than the surface flaws. Especially at lowercyclic stress, the difference is considered to becomemoredlstinct, since the extrusion-intruslon mechanismbe-

comesless active; the formation of PSBSneeds a plastic

strain amplitude over l0~5 2) and the size of the pre-existing defect Is unchangeable. Onthe contrary, when

C 1997 ISIJ 1170

the pre-existing defect is very small, the specimen mightbe immunefrom the defect-nucleation-crack at anycyclic stress level.3) Thus, it is considered that the site

or Stage I crack which is selected for fatal crack is al-

ways competitive and depends on its size, shape, Ioca-tion in specimen and stress concentration at the tip.

In these 30 years a numberof investlgators have foundthat subsurface (internal) fatigue crack initiation occurs.Subsurface crack initiatlon in high-cycle fatigue is usuallyassociated with the presence of inclusions or pores.Especially in high strength steels, effects of non-metallicinclusions on fatigue crack Initiation at both specimensurface and specimen interior, fish-eye, have beeninvestigated.4 ~ 11)Murakami12) reviewed previous majorpapers regarding on the fatigue fracture re]ated with smalldefects and/or non-metallic inclusions, and proposed aquantitative evaluation of the defects on fatigue strength.

Furthermore, the subsurface crack Initiation due tomicrostructural cracking was reported for carburizedsteels.6,7) Table 1Iists the major reports on the subsur-face crack inltlation in steels at or above room tem-perature. 7~ I I , 13 21)

Titanium alloys are relatively "clean" material; in this

sense subsurface crack initiation is not expected.

However, subsurface crack initiation whlch is notassociated with pre-existing defects has been oftenreported for various titanium alloys at and below roomtemperature under cyclic axial loading conditions.22 ~ 38)

Minagawa39) reviewed the feature and mechanismoffatigue fracture in titanium alloys. Table 2summarizesthe results of subsurface crack initiation In tltanium al-

10ys and its origin at room temperature in the ref-

ISIJ International, Vol. 37 (1 997), No. 12

Table I .Steels showing subsurface crack initiation (fish-eye) and its origins.

MaterialsFatigue tests Cycles range appeared

subsurface fractureSubsurface crack ori~in References

Srin steel

JIS SUP7, SUP9A, SUP12

Tool steel

JIS SKD61, SKDI l

Carburized steel

JIS SCr420JIS SNCM439

Carbonan low allo steel

JIS SB450, SCMV2(RT, 473, 573, 673 K)

Martensitic stainless steel

rotating bendingaxial, R=-1 1Oreversed torsion, R=-lrotating bendingaxial, R=-1 1Orotating bendingrotating bending

rotating bendingaxial, R=-l

axiaVultrasonic, R=O

106-

1010

/ rotating bendlng,axial, R=-1 1O

l06-

109

6lO -65xlO -

108l08

106-

109

l08-

1010

non-metallic inclusion / carbide

non*metallic inclusion /carbide

non-metallic inclusion / Stage I cracknon-metallic inclusion

non-metallic inclusion / intergranular facet

X20CrMoV121 non-metallic inclusion

[9],[10],[13]

[14],[15]

[9] ,[ IO],[ 16]

[17]

[7]

[8]

[ll],[18],[19]

[20],[2l]

[22]

Table 2. Titaniumorigin.

alloys showing subsurface crack initiation in high-cycle fatigue at room temperature and its

Maleria! Fatigue test Cyc!es range appearedsubsurface fracture

Subsurface c"ack origin References

Ti-6A1-4V allo

IMI 318

mil l-annealed barmill-annealed barbi-modalfme lamellar

coarse acicular

Widmanstatten

as cast

cast + HIPPIM

Other a~~J~~~~i~~l~Ti-4AI-4Mo-2Sn-O 5Si (IMI 550)Ti-5A1-2.5Fe (P/M)

Ti-8M0-8V-2Fe-3AI

axial, R=Oaxial. R=-1 /Oaxia~ultrasonic, R=Oaxial, R=-1 / 0.2axial. R=-1 / 0.2

axial, R=-1 /Oaxia], R=*1 /Oaxial, R=O. 1axial. R=O. 1axial, R=Ol

axial, R* Oaxial, R=O, l

axial, R=O, l

l07-

3xl07l06

-I08

l07-

1010

l06-

I07 / R=0.2

l06 -107 /R*0.2l06

-I08

l06-

I08

5xl 04-

2xl052xl05

-I06

4xl05-

I07

3xl06-

2x I043xl07

-I07

l05-

I07

cleavage facet (ce phase)cleavage (OO~)pundeterrnined

undeterminedundeterminedcleavage (OOI)p / colony facet

cleavage (OOI)~ / colony facet

porefacet (o; colony /GBoc)

facet (c( phase)

cleavage facet (o; phase)facet (c( phase/GBo;)

intergranular

[23]

[24]

[22]

[26]

[26]

[24],[25]

[24],[25]

[27]

[28]

[29]

[23 J[30]

[31]

erences.22~31) Furthermore the authors have studiedhigh-cycle fatigue properties of structural materials atcryogenic temperature, i.e. Iiquid nitrogen temperature(77 K) and liquld helium temperature (4 K). Someof thetest materials showed subsurface crack initiation atcryogenic temperatures.32~38,40-43) Table 3shows thesummarized results of fatigue crack initiation site andsubsurface crack origin at cryogenic temperatures. Nagaiet al.44) reviewed the deformation and fracture In

cryogenic fatigue. Cryogenic temperature fatigue is mostfavorable to characterize the subsurface crack generationand to discuss its mechanism,since the subsurface crackinitiation prevails moreat lower temperature supposedlydue to higher static strength and is detected shorter cycles

to failure below at 77K than at room temperature.In this paper, phenomenological aspects of the sub-

surface crack generation in high-cycle fatigue for highstrength alloys as listed in Tables 1-3 are presented mainlywith respect to its microstructural features and mech-anisms.

2. Wohler Fatigue Diagrams

2.1. Subsurface Crack Initiation Dueto MicrostructuralCracking

Thefirst slgnificant observation concerning subsurfacecrack initiation is that it is dominant in long-life rangeand at lower temperature, while the surface crack ini-

tiation occurs in high peak stress tests and in short-lifetests; therefore most of the materials clearly exhibit twokinds of fatigue crack generation. It is revealed that thecrack initiation site shifts from the surface to the interlor

at lower stress in an S-N curve. At that time the S-Ncurve has temporal fatigue limit, "knee". For example,the S-N curve of a Ti6A1-4V alloy shows a plateaurange ("knee") between 106 and 107 cycles as illustratedin Fig. 1.22) There is a rather sharp drop in the fatiguestrength between 107 and 108 cycles, thereafter the curveffattens out and reaches a fatigue limit at 1010 cycles.

Thedrop in fatigue strength is associated with a changein the mechanismof fatigue damageas illustrated in Fig.2(a). However, in the case of someaustenitic steels, it is

hardly seen a "knee" In thelr SN curves 40.41) It is

1171 C 1997 ISIJ

ISIJ International, Vol. 37 (1 997), No. 12

Table 3. Crack initiation site In low temperature fatigue tests (axial, R=0.01) and their subsurface crack origin.

Material Crack initiation site

(Nf=104-

107cycles )Subsurface crack origin References

293 K 77 K 4KTltanium allo sTi-Fe-OTi-5Al-2.5SnTi*6Al-4V mill -annealedTi-6Al-4V a+PannealedTi-6Al-4V pannealed

A t nrtlc steel solution treated

SS>I

S>I

SS>Is>IS>I

SUS316LNSUS316LN+200/0 cold-rolled

32Mn-7Cr-O.1N25Mn-5Cr-1Ni24Cr- 15Ni-4Mn-O

. 33N25Cr*13Ni-0,35N

Al min mallo

SSS

S/IS>Is>I

Ss>Is>IS>IS>IS>l

s>Is>I

A2219-T87

s>I

IIS

s>IS>I

S/1 S/1 S/I

transgranular (oe grain)

transgranular (fine oe phase)transgranular (oe phase)transgranular (oe phase)transgranular (o; phase/colony)

pore /Cr, Morich zoneCr, Morich zoneintergranular

Al203 inclusion

intergranular

AlN precipitate / intergranular

precipitate / intergranular

[32]

[33],[34],[35]

[36],[37]

[32],[38]

[32],[38]

[35],[40]

[40]

[41]

[42]

[41 J[42]

[43]

S: surface crack initiation; I: subsurface crack initiation;

S>1: the initiation site shifts from surface to interior at lower stress;

S/ l: both type crack initiation are detected

~~o

~;:

,~

XE~$

~

1200

lOOO

800

600

400

200

Ti-6Al-4V mill*annealed

: R=0, at roomtemperature

j Frequcncy : 60 Hz, 150 H7, 20 kHZ

- - - --;,,knee"

- ~~-

surface crack initiationsubsurface crack initiation

Specimen: mechanicaily polishcd, electropolisbcdEnvironment : air. vacuum

f,l0-5 •1'orrlj 4NNaCl

10

Fig. l.

3 l04 1o I llOs

l06

107

108

J09

1O IOFatigue cycles, Nf

The S-Ncurve for mill-annealed Ti6A1~1;V alloys at

room temperature (after Atrens el a!.22)).

considered that curve 2 in Fig. 2(a) relatively shifts toshorter life region and is close to overlap with curve l,

becausc the curves I and 2 are independent of eachother.45)

2.2. Effect of Pre-existing Defects

The surface hardening treatment such as carburizing,nitriding and induction hardening for steels leads to thedevelopment of residual compressive stresses in surfaceregions. In such treated materials, the fish-eye fracture(subsurface crack initiation) occurs at lower stress level

or in long-1ife range. Moreover somehard steels alsoexhibit the fish-eye fracture.9 I i'22) Then, the SNcurveof those materials shows the same trend with that ofTi-6A14V alloy mentioned earlier. The schematicdiagram of the S-N curve having fish-eye fracture is

represented in Fig. 2(b).10) The drop in fatigue strengthfor the steels is associated with achangein the mechanismof fatigue damageas well as for Ti-6AI~IV alloy.

Whenthere is a pre-existing defect with a larger size

in the specimen interior, the shift from curves 2to curve2' in Fig. 2(a) can be considered.45) That meansthe size

and shape of internal cracks or pre-existing defects havean important role on the S-N curve.

~~t)

Q)

c-

oho~ie)

~:*

(a)

curve I : surface

\

fatiguefracture mechanism

~~c:'

e

curve I : extrusion-intrusion

curve 2 : microcracking

ourve 2' : pre-existing defect

curte 2: subsurfaoe

*\ microcracking\ 5- 40 um\Ilkneelt \

curve 2' :* subsurface~pre-existing defect (inclusion, pore)~] 30- IOOunl

(b)

A: surfacec:

\subsurface defect

\ aa/HV=l\.71

B: surface inclusion knee" \\

Numberof cycles to failure

(logarithmic indication)

Fig. 2. Schernatic diagram showing the S-Ncurves dependedon the fatigue fracture mechanisms; (a) Ti-5Al2.5SnELI alloy at 4K45) and (b) fish-eye fracture in hardsteels. I o)

2.3. Effect of Fatigue Test Conditions

Even in the case of Ti-6AI~,V alloys, comparison ofS-Ndata amongthe literatures has somedifficulties. Inthe first place, most of the fatigue studies have beencarried out in rotating bending, i.e. maximumstress atspecimensurface,46,47) or only to 107 cycles with surfacecrack initiation.48.49) Secondly, in several cases noobservations or only a limited numberof analyses havebeen reported concerning fatigue initiation sites. Then,available data are alrnost limited for the literatures listed

in Table 2. In c(-p titanium alloys the subsurface crack

C 1997 ISIJ 1172

ISIJ International. Vol, 37 (1997), No. 12

Table 4. Effect of environment on subsurface fatigue crack initiation for titaniumalloys.

Material Environment References

Ti-6Al-4V (IMI 318)

Ti-6Al-4V (mill-annealed)

Ti-6Al-4V (bi-modal)

Ti-6Al-4V (fine lamella)

Ti-4Al-4M0-2Sn-0.5Si OMI550)

dry argon, air, H20saturated air

specimensurface fmish, air, NaClsolution

vacuum,air, RD/TDvacuum,air

dry argon, air, H20saturated air

[23]

[22]

[26]

[26]

[23]

Fig. 3. SEMmic]'ographs of fatigue rracture surfaces andsubsurface crack initiation sites of samples failed at 4K; (a), (c) intergr'anular cracking in 32Mn--7Cr-O I~l

steel41]; (b), (d) Al203 inclusion in 25Mn-5Cr-1Nis, teel,42] Photographs (c) and (d) are magninedimagesat initiation sites in photographs (a) and (b), re-

spectively,

initiation under high-cycle axial fatigue loading occursregardless of testing 'and metal]urgical conditions, i.e.

temperature, frequency, stress ratlo R=(Tmi~/a~.*, en-vironment, manufacturing process, impurities concen-tration and microstructure, as summarizedin Tables 3,

4.22.23.26,36) The effects of frequency, environment andspeclmen surface can be hardly detected on the S-Ncurve in the region of subsurface crack initiation.22.23.26)

However, the meanstress possibly plays a major role onthe subsurface crack initlation behavlor.26,50,51) UnderR=- l condition, the subsurface crack initlation andthe knee in an S-N curve are difficult to detect at orbelow 107 cycles.26) Then, the fatigue behavior over 107

cycles is still unclear, and it Is necessary to accLmnrlatethe long-iife test data up to 1010 cycles.

Onthe other hand, fish-eye fracture in steels has beenreported in both axial and rotating bending tests, butnot in reversed torsion test, as shown in Table l. Thetrend of S-N curves is almost the samebetween axial

and rotating bending, but the fatigue strength in therotating bending test is about 1.5 times as high as thatIn the axial one. Io) Here is almost no effect of stress ratioin axial lo'ading on the fish-eye fracture behavior.10)

Fig. 4. Intergranular facets observed at the origin of casefish-eye; (a) fish-eye and (b) origin (after Masudaeta/. 7)).

At cryogenic temperatures the subsurface crackinitiation typically occurred as shownin Table 3. Then,it can be detected in wider ranges for both cyc]ic stress

and numberof cycles than at room temperature.

3. Subsurface Fatigue Crack Initiation Site

3.1. Subsurface Crack Origin

The morphology of subsurface crack initiation site

seems to be associated wlth their microstructure. TheSEMphotogr'aphs in Fig. 3 represent the vicinity ofsubsurface fatigue crack initiation site in samples whichfailed at 4K for austenitic steels.41'42) Figure 3(c) is anexample of the crack initiation site intergranularlyformed, and Fig. 3(d) shows the site associated withnon-metallic inclusion. Each subsurface crack initiationsite has a mlcrostructural origin as listed in Tables 1-3.

Austenitic steels exhibit various types of subsurface crackorigins such as intergranular, non-metallic partlc]es andsegregation, under cryogenic fatigue tests as shown in

Table 3. Generally the subsurface crack initiation formsa facet or facets due to a microstructural cracking,although internal fatigue origins in some materialshave been invariably associated with the presence ofdefects such as non-metallic particles. Furthermore, the

morphology and size of subsurface crack initiation sites

are also varied by stress level. As the peak cyclic stressdecreases, the size of the initiation site Increases, and thesite consists ofrnore facets. In modernsteel making, the

numberof inclusions in steels and their size have beenminimized, and relatively "c]ean" steels can be obtained;therefore fish-eye fracture in the "clean" hard steel is notexpected. However, subsurface crack initiation notassociated with pre-existing defects has been reportedinstead of fish-eye fracture.6.7.lo) The initiation site

exhibits a facet, as shownin Fig. 4.

1173 C 1997 ISIJ

ISIJ International, Vol.

3.2. Microcrack Growth and Coalescence

Neai et a/.23) observed the subsurface crack Initiatlon

sltes of oe-P titanium alloys and concluded that its orlgln

was a result of simple cleavage. The facet at the origin

of fatigue fracture was featureless, but results by Patonet al.52) and authors37) suggest that the facet wascreated

by a process of microcrack growth involving a large

numberof cycles and not by the Instantaneous spreadof the origina] facet. For example. Fig. 5 shows that

detailed observation classifies the subsurface crackinitiation sites of Ti-6AI~~V alloy into "Facet" and"Border" parts. The Border envelops a Facet and has aductile fracture surface at inter-Facets shownin Fig. 5(b).

Furthermore, two regions Aand Bare found in Facets.

Region A is a circular part of about 4umin diameter.This circular part touches a Border, and shows fragi]e

cracking surface. Region Bis of the Facets except region

A. It gives an appearance like microcrack growth in aradial pattern from region A. However, its appearanceis obviously different from that of the fatigue crackpropagating plane (Stage II) in low AKregion. Regions

A and Bon a Facet shownIn Flg. 5(a) are not on the

same plane against the principal stress axis. Mostlyonly one region AIs found in a subsurface crack initiation

site.

Severa] sub-cracks (microcracks) in the vicinity of asubsurface crack initiation site were found in the

longitudinal cross section of the failed specimen undervarious stress levels. Thelr crack length was over a fewmicron-meter. It is considered that sub-cracks werecaused by cycllc deformation or by p]astic zone at the

main crack tip. All sub-cracks were nucleated in the cc

phase, and in manycases they touch c(-p interfaces as

37 (1 997), No. 12

AFacet

oe-P boundary

microcracking

and its growth

Fig. 5. The morphology of subsurface crack initiation sites

(a)(c), sub-cracks (d)-(f), and illustration showing the

mech'anism of microcrack growth (g) for Ti-6A1~~Vsamples which failed at cryogenic temperatures.37)

@1997 ISIJ 1174

shownin Fig. 5(d). Thevarious kinds of sub-cracks wereobserved In their morpho]ogy. A one-to-one corre-spondencebetween illustration (g) and sub-cracks in Fig.

5can be excellentiy explained for a formation processof Stage I crack; a microcrack is initiated in an e( grain;

the mlcrocrack grows in the c( graln, and continuouslyextends into the neighbor pplate and into the next c(

grains, until it finally forms the initiation site with acritical size.

3.3. Stage I Crack Formation from Pre-existing Defects

Effects of non-metal]ic inclusions on hlgh-cycle fatiguestrength and on subsurface fatigue crack initiation be-havior are discussed both for the spring steels9'10) andfor austenitic steels at cryogenic temperatures.42) In aspring steel, Stage I cracks were observed in multipledirections near inclusion.9) The fatigue cracks associat-

ed with inclusions formed through the debonding ofincluslons and the microcrack initiation in the ma-trix.4'5.42) Then, the crack initiation site associated wlthpre-existing defects in specimeninterior is produced fromnot only the particles but also Stage I cracks. Thewholesize of the initiation site involving Stage I cracks is larger

than that of the defect itself. This suggests that the natureof the internal crack generation process is almost the

samewith the subsurface crack generation resulting frommicrocracking in matrix.

3.4. Location ofCrack Initiation Site in Fracture Surface

The location of fish-eye fracture in steels is limited to

near surface for rotatlng bending test, but is scatteredinto deeper in specimen interior for axia] one. In the caseof rotating bending test, the stress gradient in specimenapparently exlsts, and the maxirnumstress is at speclmensurface. Hence, only axial loading is mentioned below.

The general explanation on the most likely place toInitiate the Stage Tcrack says that near the specimensurface is predominant site due to macroscopic defor-

mation localization.53.54) In addltlon that, the localizedtensile plastic deformation at the surface is expected in

tension-tension loading which leads to the developmentof residual compressive stresses in surface regions.55) Infact, the internal sites of Ti-6AI~}V alloys were mostlyplaced with about 120,am (4.5 mmin waist diameter)56)

or 300400~m(2 mlTl in waist diameter)26) jnterlor fromthe surface. In the P-quenched alloy, however, the site

(colony facet) was sometimes located at the center ofspecimen.24) In austenitic steels, on the other hand, thesite wasplaced anywherefrom the vlcinlty of specimensurface to the center in specimen, and there is no tenden-

cy for a favorable depth of the location.41'42) Yoko-yamaet a/.32) have investigated the distribution of sub-Surface crack initiation sites for various titanium alloys.

The p-anne'aled Ti-6Al4V alloy with acicular micro-structures exhibited a wider distribution range for in-

itiation sites, i,e. 50700,nn (3mmin gage diameter),

and its initiation sites consisted of large colony facet. Onthe contrary, the location of initlation sites for equiaxed

onew'as in a narrow range, i,e. 50l 50 /am. TiFe-O alloy

has larger elongated c( grains than Tl-6A1~}V one, andexhibited the locatlon of sites placed anywherefrom the

ISIJ International, Vol. 37 (1997), No. 12

vicinity of specimen surface to the center in specimen.These phenomenamight be related with the size ofinitiatlon slte or facet and the stress level, respectively.

4. Threshold Condition of Stage I Crack

4. I .Attempts to Evaluate the Crack Initiation Site Size

The value of stress intensity factor at a tip of Stage Icrack played an important role on startlng propagationas Stage 11 crack. To determine the size of internal crackinitiation site, Murakamil2) proposed that the area ofpre-existing defects themselves such as non-metallicinc]usions wasquantified and that "~//~T!':area"

wasdefined

as a crack length (2a) parameter. Figure 6 shows anexample of the re]ationship between normal stress and

~,~~size of pre-existing defect, area, for the fish-eye frac-

ture in spring steels.9)

In the case of microstructural cracklng (facet), sub-surface crack initiation site appears flat at low mag-

1000

\\1+T\~4~~~~rh ~:r_\AIK"{~:~.l

)~f+~},(\ \\/ AK+~=8~~

S~*f***

Heat A

8ao ~6aa

o k30T1-42aTo 5aoT~-4goT

AA~~Ef~~~~~AA~~Origin of fatigue crack

~*~ *,L~

*4oa ORotating bending-No mark surface

~ Reversed torsion I fnclusion near surface

/ interna[ inclusion2

nification and is inclined to the principal stress axis

as shownin Fig 7. Then, the subsurface crack initiationsite is defined as the whole of the inclined area whichdoes not placed on the fatigue crack propagating plane.

To quantify the size of the Stage I crack, authorss7)

adopted a crack iength parameter 'f " as the minor axisof an orthographic projection of the subsurface crackinitiation site on the main crack propagating plane. Theminor axis is almost parallel to the initia] crackpropagating dlrection and the main crack propagatingpiane is almost perpendicular to the principal stress axis.

In the case of postulating the equivalent crack projected

on the propagating plane, f~ corresponds to its length.

Three-dimensional indication

tprincipal stress

~~:

~

7LE

V)

10 10\1~7~~ (~m)

Relationship between normal stress and ~area (after

Abee! a/.9]).

l rrf r k initi ti n t / r x] tln fct

IProjected on propagated plane

~~ rrarea

Fig. 6. Fig. 7.

fs

) lsubsurface crack size

~~Definition of subsurface crack

Jarea, as a geornetric p'arameter.Slze 56) 211ld,

.''fq*'

~~~

~~

~~

V

l20

100

80

60

40

20

o

(a) O

Internal

A*0.5)o

e

CI:

A:

25Mn-5Cr, interior

25Mn-5Cr,surface

2SCr-13Ni, AlN25Cr-13Ni, intergranular

AKlmax= 2.5 MPa~TI i R~J) l

Cl~

~~;

~~~

L;~cs

~~,,

~s;~

(,~

o 500 1000 1500 2000

l20

lOO

80

60

40

20

o

(c)

R=0.01

::CM'::;CN:'

* =32M*-7C*

oooo

AKlmax= 3~MPa~T,i~lo

g~ioocJ

~

IEPo~ ~-

o

O 1500 2000 O 500 1000 1500 2000500 lOOO

Fig. 8.

Maximumstress range, Aomax(MPa)

Size of the crack initiation sites associated with pre-existing defects (a), and microstructural crackisteels (c). Solid lines showthe calculated threshold crack initiation site size.

gfor Ti6AI~IV alloys (b) 'and aus. tenitic

1175 @1997 ISIJ

ISIJ International, Vol. 37

Using ,f, as well as~//;!1;;,are~, the size of crack initiation

site (Stage I crack) can be related to the maximumstress

range for the subsurface crack initlation sites caused bycryogenic fatigue, as shownin Fig. 8.

4.2. Fatigue Limit of the Material Containing SmallDefect

Lankford et al.4) have reported that the fatigue life in

4340 steel was more closely related to the diameter ofinclusions than to whether the nucleating particle was10cated at the surface or subsurface. Whenpluralmicrocracks could be nucleated and grow, whichmicrocrack becomesfor a main crack is always com-petitive and depends on the size and the stress level.

Kitagawa et al.58) and Murakamil2) proposed that thefatigue limit of the material containing pre-existingdefects was determined from the relationship betweenthe defect size and threshold stress range as illustrated

In Fig. 9. These ideas are based on the assumption that

a small defect such as an inclusion is equivalent to amicrocrack (Stage I crack) and that the mlcrocrackgrowth is retalned under the fatigue limit. In the rangeof 2a (crack length) > d* (threshold defect slze), AKthvalue determines the fatigue limit threshold stress range.In 2a d., on the other hand, the threshold stress rangeis almost equal to the fatigue limit of the materlal withoutdefects.

Secondary, the assumptions as follows are usua]1y

madefor the simplification. The deformation modeatcrack tip was the Mode11 In Stage I and the ModeI in

Stage II. Whena Stage I crack becomesa critlcal size,

only principal stress controls its propagation. Then, anapproximate equatlon to give the maximumstress in-

tensity range, AKl~'**' at the crack tip is roughly rep-resented as follows;

AKl*" =AAcF~..~1~7y;;7c2a

.................(1)

2a=~//!1~~area: Murakamil2)

where A(T~.* is maxlmumcyclic stress range, and A is

the coefficlent depending on the shape and location ofthe site. According to the equatlon, the critlcal condltionthat a pre-existing defect provides a fatal crack in a givenstress range has been evaluated.9,12)

4.3. Dependenceof Initiation Site Size on Stress RangeAsmentioned in Sec. 3.3, it shouid be noted that Stage

(1997), No. 12

I crack size is not always equal to pre-existing defect size.

In Fig. 6, AKI value of sorne data is obviously lower thanAKth Value. It is believed that the Stage I crack slze is

larger than defect size itself. Generally the microcrackitself does not always provided a critical main crack size.

The microcrack needs to grow untii it finally forms theinitiation site of a critical size as well as the Stage Icrack generation resulting from a mlcrostructural crack-ing.37,41) It is also necessary to take into account thethree-dimensional shapeof the defect (Stage I crack) andof the radius of curvature at the tip. Hence, it maynotbe true that the defect size Is equal to the nucleus of maincrack or the threshold defect size. Especially in the rangeof 2a c!., it is considered that the size gap between thepostulated nucleus of maincrack and the defect is greater.

Figure 8 represents the size of Stage I crack andestlmated AKl*** value, using .~ as the crack length

parameter. In this estimation the temperature depen-dence of AKl*'** value was neglected, and AKl~'* valuefitted for a]1 data. The dependenceof the initiation site

size on the maximumstress range coLuld be accountedfor by an assumption that the microcrack growth is

controlled by the critical condition, AKl***=constant.

1~~~

a)eJ)S:

~CF$:~ ~~~q'~S~'eJ)~~

'- ~:'~~O~!~~~,D

~)~~{

H

lOOO

lOO

10 dc 100 1000Pre-existing crack size, 2a (um)

Fig. 9. Dependenceof threshold conditions for fatigue crackgrowth of a microcrack on stres's range.

@1997 ISIJ 1176

5. Fatigue Deformation Structures and MicrocrackGeneration

Therelevant deformation structures analyzed by TEMhave been examined for some austenitic steels andtitanium alloys.24,41.59~61) The dominant deformationmodeis the primary slip system in each material, andthe dlslocation motion is planar, especlally at low stresslevel at 4and 77 K. Cross slip of dislocations is stronglysuppressed, and dislocation movementsare restricted totheir s]ip planes. Deformation twinning, stacking faults

and transformations such as e-martensite are not majorfactors in cyclic deformation. As mentioned below, thedislocation structure is changed by the applied stresslevels and fatigue cycles.

5.1. Austenitic Steels

At higher stress level, slip bands are mainly observed.At the lower stress level, on the other hand, micro-siip

or dislocation pile-ups are blocked or sharpiy iocalized

at grain boundaries. Figure 10 shows an example ofdislocation structure of a sample failed at 4K for

25Crl3Ni steel. Co-pianar slip on (1 1l)- I0> systemoccurs predominantly. The secondary (conjugate) slip

is also operative, and the interaction between primaryslip bands and secondary ones is recognized. Then thedislocation movementsare restricted on their slip planes,

and the cross slip of dislocations is strongly suppressed.

The dislocation arrangement is developed mainly bypile-ups or co-planar arrays which are piled up andblocked at the grain boundary as shown in Fig. 10.

Although the slip bands do not form remarkably,micro-slip bandsare widely spaced. Micrographs in Figs.lO(b), lO(c) represent the strain concentration at a grainboundary In which two incoming systems are involved.

Evenat the lower stress level, dislocations weregeneratedin a grain with more than one s]ip systems, while their

ISIJ International, Vol. 37 (1997), No. 12

~~

~/,~' 1~

~{,~

~~iY~~~,,.. ~~~~~~h h'~~~~'

~~ ~ '~#; '~~~~~~, ~9~fl

'

~

Fig. 10. Dislocation sti'uctures of a cyclically defonTled 25Cr--1 3Ni-0.35h] s'leel after 2090770 cycles at 4K ((Tn =485 MPa).(c) are the magnified image of pile-ups in (a)

Photographs (b) and

density wasrather low. Theplastic deformation in thosesteels was highly inhomogeneous, and the co-planardislocation pile-ups produced the localized deformationand/or strain concentration at a grain boundary whichare considered to be a potentia[ source of a microcrackingin a specific incompatibility of grain boundary.

5.2. Titanium Alloys

TEMstudies have beendone for sometltanium alloys

to characterize the dislocation structure of fatiguefractured samples.24.59~61) At low stress amplitudeswhere subsurface crack initiation was appeared, slip

occurs primarily on prism planes and is blocked at c(-f3

interfaces or grain boundaries as shown in Fig. Il(a).

TEMstudies confirmed that the glide process is extremelyplanar. Dislocation pile-ups in the titanium alloys arenot unique to cyclic loadlng, but should be moreseverein cyclic loading.50) Steele et a!.50) considered that fatiguecrack initiation due to dislocation pile-ups should befavored at subsurface locations, since at surface sites

stress build-ups would be moreeasily relax.

Ruppenet a/.24) proposed that the piled-up disloca-

1177

tlons induced slip in the /3 phase under cyclic loadingand that the sllp in pphase could lead to the formationof a Cottrell cleavage knife (OOl)/; and thereby provided

a site for subsurface crack initi'ation. It is considered that/3 phase is softer than o( phase, because the fitted part onpphaseshowsa moreductlle mannerin subsurface crackinitiation site 'as shown in Fig. 5(b). The dislocationdensity in pphase is a]so higher than that of c( phaseand multiple slip systems are operative as shownin Fig.

1l(b). In two-phase materials, piastic deformation startsfirst in a "softer"' phase, and the two-phase interface

becomesa site for dislocation piling. However, those donot provide the reason for cleavage crack in fi phase,Mostly microcracklng or facet in subsurface crackinitiation site is fitted to a( phase region as shown in

Fig. 5.

At high stress amplitudes, on the other hand, py-ramidal slip becomesoperative and as a consequencea morecompatible type of plastic deformatlon betweenthe two phases can develop. Under these circumstancesthe tendency for subsurface crack initiation might bered uced

.

5o)

O1997 ISIJ

ISIJ International. Vol. 37 (1 997), No. 12

Fig. Il. Dislocation structures in cryogenic fatigue for (a)

Ti5Al-2.5Sn alloy (at 77 K, a.=505 MPa, 12280cycles) and for (b) pphase in a Ti-6Al4V alloy (at

4K, (T. =426MPa,1699330 cycles).

6. Summary

The subsurface or internal crack generation of Stage

I crack is very commonin high-cycle fatigue in lowtemperature range for high strength alloys. Althoughrelatively "clean" alloys minimized pre-existing defects

have been obtained, the subsurface crack initiation maynot be avoidable in long-life range over 107 cycles.

Therefore, investigations on fatigue behavior over 107

cycles and fatigue limit for high strength alloys arenecessary. Onthe other hand, there is a possibility thatthe location and distribution of the subsurface crackinitiation sites are related with the size of the site or facet

and stress level. Furthermore, heterogeneous micro-plasticity due to planar slip and restricted system is

considered to play an Important role on making thesubsurface crack. Then, it should be progressed in theunderstanding of damagestage in high-cycle fatiguefracture process.

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