Fractographic analysis of fatigue cracking in spheroidal graphite cast irons

Fractographi i: graphite

anafysis offatigue

cracking

cast irons

1. introduction The study of frac@_u-e sur”i@ces m a mx~~scopic scale as a means uf assessing I&e fractnre resistance and quatity 0-s metalTic maicrTaIs dals5 back to the middle ages [I]. More xxsntiy, &mi:t fifty years ago: siuJirs were ma& e2j of fracCuxz prCNXs~es on ik ItliCKsscopiC sZdf2. Since th4X. however, significant improvc~2ents in the investigative techniques and insmments have been evidenced with the develapmcnt of both the transmission elecrrm micmscopes 43334) and tfre scanning electron microscopes fSEMZ. In many instances, the fact that the prixess of fatigue crack e.xtensicn can occur during 11significant part of an engineering compment’s lifetime, elevates the theoretical and pm&A importance of such studies. Indeed, modern-day advances in the field of Linear Elastic Fracture Mechanics (LEFM) have yielded reliable methods of apply0167-8442/92/SO5.00

0 1992 - Eisevier Science Publishers

B.V. Ali nghts reserved

in spher&ikd

rtiportcd in sc!tne of thcsc references. it was shown in [4] that a high degrre of’gratihitl: nodutc and matrix &Xol@on was &dent. in the fatigue process a.nd that the graphite nsdules locally retarded the fatigue crack giowth rates. They also identified the following CCHS

tqvc

&I.

hccn

fatigue crack, growth mechanisms; intergranular fracture, cleavage,. ductile striations and microvoid coalescence. The growing fatigue crack front was clearly attracted towards the graphite nodules [S]. When this happens, a complicated schedule of local crack blunting and re-initiation of crsck extension occurs. The effects of R-ratio on the threshold fatigue characterjstics have been observed [6] to have three .different fatigue failure intergranular failure, transgranular modes, quasi-cleavage and ductile striated growth, occurred with their proportions changing with AK level. Intergranular failure usually occurred around graphite nodules and tended to reach maximum proportions at intermediate AK levels while the. area fraction of graphite spheroids on the fatigue fracture, surfaces was much higher than that recorded metallographically. Endurance fatigue life study on both flake and spheroidal graphite cast irons [7] observed that a high. proportion of graphite nodules were evidenced on the fatigue f&t&e surfaces, and complete decohesion of the graphite nodule-matrix interfaces tias commonly observed. Also, graphiiic irons exhibited a fatigue failure mechanism that was progres&e brittle cleavage of the metallic matcix, viz., in nodular irons, com.plete decohesion of the graphite nodule-matrix interface occurred while in flake irons the fatigue crack propagated through, the .glaphite flake initiating brittle cleavage failure. Indeed, simiIar obsetiatibns ,have been tepoced in a tensile loading study on flake cast irons [S]; Recently; findings [9] showed that the main ‘fatigue mechanism was ductile striated growth which contained some areas of quasi-cleavage and intergranular failure which o%urred mainly around the graphite nodule& These &ted: fractographic- studies have shown sonie ,inconsistencies ,3tid. by no m&ins represent + detailed’ fractographic picture of the fatigue crack ehe,$sion ,brocess. As a result, the present paper ‘represents ati -‘atfetipt to. report a detailed ,fr?ctographiCi. $tidy bf the .ftitigu& :fract
f:~es of two sphcruidal graphite cast irons at various R-ratios in an air environment.

2, Specimen preparaGin The bulk chemiial composition %wt of the two spheroidal graphite cast irons utilised in the present paper, designated SGl and 332 can be found in Table 1. kter conventional metallographic preparation, the mean linear intercept of the Polygonal Ferrite Grains, dcu were measured by lineal analysis. The area fraction of al! the microconstituent phases were assessed by a point counting technique which, involved at least 1.000 points in a single operation. The distribution and average size of the graphite nodules in both cast irons were determined by quantitative meiallographic techniques and the various metallographic details are listed in Table 2. Fatigue crack growth tests were conducted at three separate R = Kmin/Kmax values and full details of the testing procedures and experimental results are listed in [6]_ Selected test specimen were broken open at liquid nitrogen temperatures after fatigue testing and the ,fatigue fracture surfaces were carefully cleaned in alcohol in an ultrasonic bath for IO mins before being subjected to a detailed scanning electron microscope, SEM examination. This included a detailed ‘area fraction assessment of the different types of failure modes and graphite nodules prevalent on the fatigue surfaces at differing crack lengths, ‘i.e., different stress intens& range (AK) levels. Each fractographic assessment *as conducted ‘over an area of about OS square millimetres and was achieved using a grid super imposed on the SEM screen and pointcounting. Sbme of these selected fatigue surfaces were etched lightly in a solution of 2% nital in an attempt to estabiish any correspondence between particular failure mode/s and microstructural coristituent/s.

Table 1 Chemical composition of graphite car;t irons in %wt

SGl SG2

C

Si

3.i6 3.42

2.43 2.24

.Mn 0.40 0.41

S

P

Mg

0.006 _O.“$

0.035 0.031

OB52, 0.031

5s

tw

9E

LP

SL

ZE

w

2

LI’O

S65’5

09.5 sow

EZ’O SIX0

Z?X 1%

Fig. 3. InWrgranular fatigue crack extension (left to right) and R = 0.91 local secondary cracking in SG1; JK = 3.3 MPaG. and da /dN = IO-’ mm/c.

shown in Fig. 3. From this figure and that of Fig. 4, it can be Seen that secondary intergranular separation from ihe ptimary fatigue crack is com-

Fig.

5.

Examples

spheroid-ferrite matrix decohesion with (a) decohesion at top of nodule; (h) decohesion occurred along bottom of nodule.

Fig. 4. Incidence of intergranular (SC11 crack extension (left to right) along ferrite grain boundaries at threshold AK level forAK=1.9MPa&,

R=0.91,dt~/dN=lO-~

mm/c.

moniy observed, and intergranular crack extension is evident even at threshold fatigue crack growth rates, typically IO-’ mm/c (see Fig. 4). It has been generally observed that when a growing fatigue crack encountered a graphite spheroid decohesion of about a half of the spheroid-ferrite matrix interface occurred after which the crack further extended by growth along certain ferrite grain boundaries. No incidences of a crack extending through the graphite spheroid were recorded and instances of decohesion of the spheroid-ferrite interface are illustrated in Fig. 5. Also evident in these figures are incidences of secondary cracks which emanated from the graphite spheroid-ferrite matrix interface. At intermediate stress intensity range levels, i.e., about 4.5 MPa\/;;; for the high R-ratio data, incidences of small amounts of intergranular damage have been observed ahead of she main crack tip, see Fig. 6. Figure 6(a) shows a view of the intergranu-

Fig. 6. lntergranular damage tSGl$ ahead of the clrimap crack tip at inremediite mack gTlMh rates and R = 11.91with growth left to right. (a) crack rip extending from a graphite nodule: (b) intergranular damage along ferrite grain bndaries (top right 1ahead of main crack tip (hnttwn kft 1.

lar nature at the main crack front white Fig. tXb1 shows local&d intergranular cracking (top right) about one ferrite grain size in advance of the main crack tip (bottom leftl. At near final failure AK, much secondary cracking. sometimes erratic in nature, was evident in many instances. Such secondary cracking, which issued from both spheroid-ferrite matrix boundaries, bottom left Fig. 7(a). and from the primaq crack edges. Fig. 7(b). The fatigue crack profile details for microstructure SG2 (the predominantly pearlitic microstructure) are illustrated in Figs. 8 through 11. Crack extension by extendir?g from one spheroid to an adjacent one in the predominantly pearlitic microstructure SG2 is shown in Fig. 8. In these cases, crack growth occurred by the crack etiend-

Fig. 8. Crack extension (left IO right, tc?_jumpin; frwn one nodule to an adjacent nodule IS.33 ahere crack path is along fwite grain boundaries or ferrite-war:ite phase boundaries

dccohcsion in the locality elf a sphctoid. Again. *c~ntJ:u-y cracking is cvidt2nt in Fig. H. I’ho dCt;tilCd ~;IIUI’C cd’ the intcraetion of’ the growing fittiguc cr;rck and the microstructuritl phitscs arc given in I:ip. 1). l;ig:urc Y(a) shows the growing crack cxtcndinp along ti fcrrite grain boundary. However, when the crack extends into the pcarlitc phase, the crack direction was deviated locally by about 45”. In Fig. 9(h), which illustrates crach growth through the pearlite microconstituent, it is evident that no deviation of the crack front was observed during the passage through one pearlite colony to another. However, when the growing crack approaches a graphite spheroid, it suddenly deviated towards the spheroid. Figure Y(c) shows evidence of secondary cracking within the pearlite phase and it can bc seen that crack front deviation occurred when the crack crossed a pearlite colony boundary. Figure 9(d)

Fig. 9. Crack growth (left to right) characteristics

6G2)

shows the growing fatigue crack passing through with thr *:dpc of a pcarlitc colony. apparently little local crack &&tWt. ;tird back into the ferrite phase. At intermediate to high fatigue crack growth rates. i.e., greater than 10”‘J mm/c, it was common to observe more than one crack between closely adjacent graphite spheroids and such an incidence is illustrated in Fig. 10. Again, note that both cracks extend bctwecn the spheroids in an intergranular fashion. Upon encountering the second spheroid, both crack cause dccohesion around the spheroid perimeter and eventually joined to form one common crack path. Another feature which was common in the intermediate fatigue crack growth regime was cvidcnce of damage in the form of small cracks which was somewhat removed from the main fatigue crack profile, see Fig. 11(a). A detailed view of this damage is shown in Fig. 11(b) where it can be seen that, in the pearl&e phase, damage

with R = 0.91. Ia) Note crack extension along

ferrite grain boundaries

and

through a pearlite colony. Note deviation in crack direction as pearlite

colonies. Note little deviation

nodule matrix interface. (cl Deviations

in crack direction

it enters pearlite phase. tb) Crack extension through different at colony boundaries. Also note large deviation as crack is attracted to

in crack direction as it encounters pearlite colony boundaries. discrete pearlite colony,

td) Crack extension through a

Fig. 10. Crack extension (left to right) between adjacent graphite nodules LSG2) with R = 0.91 and crack path mainly along ferrite grain boundaries.

tended to trccut at pcarlitc L=UIWI~ hrundarics. Such damage ranged in kc from Irr grn to al~ut ff#I @rn maximum and could tvc Ir~,rtr’d a~ much as 0.5 mm away from the main crack path.

Fig. 1I. Secondary damage removed from main fatigue crack (left to right) in SGZ. (a) Note damage in the form of cracks about 0.2 to 0.3 mm from main crack (top left). fh) Detailed view of secondsFy cracks within the pearlite phase. Note damage appears IObe at pearl&e colony boundaries.

A general view of the fatigue frdcturc \urfac’c of the microstructure SGl at a near threshold AK value of 2.7 MPav’G (R = (1.91) is given in Fir. 12(a). This figure shons that the dominant failure mode was ductile striated crack growth together with isolated facets of intergranuiar failure which tend to occur around or adjacent to graphite spheroids. In this particular location. the fracture surface contained 1 lk intergranular failure facets. A detailed view of ;hc nature of the

ir~ferirrariu!x failure facets that tcndsd to sufrouni spheroidal graphite particles is given in Fig. 12(b). This figure shows five discrete grains (top right) which have $ailctd intcrgranularly, the grains being about 20 to SO pm in size. At intermediate 4K lcvcls, i.e., about 4 MPav~, Fig. 13 illustrates isolated intcrgranular facets within the dominant ductile fatigue failure mode. The nature of the ductile faiiure was clearly defined by heavy slip type lines, which is typical of a planar slip process. A detailed view of the slip lines, see Fig. 13(b): shows the highly directional and prafiled appearance where the average line spacing was about 2 pm. isolated transgranular In a few instances, cleavage facets were evident at intermediate 4K levels and an example of this is given in Fig. 14.

Fig. 13. Fatigue fracture surface SG1 with R= 0.91 and AK = 4 MPafi where crack grows upward. (a) View of isolated Intergranular failure facets surrounded by ductile striated crack growth showing heavy slip lines. (b) Detailed view of ductile striated growth. Slip or line spacing about 2 X lo-’ mm average.

In this particular instance, a flat transgranufar cleavage facet about 60 ~.lrn in size. was Initiated from the spheroid-matrix interface. X detailed montage Cif crack kXtcJlSitXl ZlCliXS 23 ~I-~@32 nodule is shown in Fig. 15. This figure illustrates the prepocderance of intergranular failure around the nod&e which is surrounded almost rompIelie@ by hi@& direction& slipped ductile srriated failure. Details of the fatigue crack failure surfaces for microstructure SGI at few R-ratio of 0.15 are given in Figs. 16 through 22. At a dK level of about 13 MPa$&Y, extensive intergranular failure was observed in Fig. 16. In rhjs case, 36% of the fatigue surface had this failure mode with the remainder being ductile striated fatigue failure. A detailed view of an intergranular location is shown

14x sp4rcrc&ll ani_4t.fK! rnarrk

1;tnrrl41:cr I’t~rr~~reilriil wv;th quilt i’I!Rl’lrll~lf? CJvPtllllC ~v’tld!,; ti?s:t I’Liq.C crl’ d L was the :~ppcal”~mx of’ t’lat, c!i:;:rvayc-like rs_giorb witlrin which striations or li~rus of’ pIasticit)i occurred perpendicular to zhe river pattern lines which extended in the general direction of macrocrack growth. Figure I9 illustrates a montage of this type of failure mode occurring around a graphite nodule. This figure illustrates the muhi-directional nature of the varisus lined or striated locations and the different spacings that existed between these lines. Indeed, Fig. 20 illustrates a schematic of the various direction and average line spacing that were assessed from Fig. 19. From this figure, it can be seen that the average line spacing varied from 0.7 pm to 5.3 pm with growth directions varying over almost 180”. It may also be observed the line spacing was greatest just before the groivins fatigue crack front encountered the graphite nodule while after passage through, or around the nodule, the line spacing was about five times lower. Indeed, if such Iine spacings can be related to fatigue crack extension per load cycle, then this observation implies that when a growing fatigue crack ~~co*~~+P~~ nn?~*l~ r.wntvth UI^LClil 9 u LI” ..sYII, *ho L1._ ~rarlr _a-_._ -_.,. Fig. 16. Fatigue fracture surface SGl with R = 0.15 and AK = 13.5 MPaqz where crack grows upward. Cal General view of extensive intergranular failure amount to 36% of fracture surface. (b) Detaiied view of intergranular failure.

in Fig. 16(b) in which the grain size varied between 20 to SO pm. A general observation was that the low R-ratio tests exhibited much more intergranular failure than the high R-ratio tests in microstructure SGl. A general view at intermediate crack growth rates, see Fig. 17, shows a surprising amount of graphite nodules prevalent on the fatigue fracture surface. Indeed, at this particular location, the area percentage of spheroids was assessed at 23% which was about two and a half times that assessed metallographically Es]. A detailed montage of crack extension around a specific graphite spheroid at near threshold AK levels, see Fig. 18, illustrated: ?? The preponderance of intergranular facets associated with the spheroid, ?? The highly directional nature of the ductile fatigue locations, and

rates can be locaily retarded. The detailed nature of these highly directional, flat, cleavage-like fatigue regions is shown in Fig. 22. From this figure, the marked reguIar nature of these lines are evident and are even imprinted on the ductile ridges or river patterns of this fracture process> see Fig. 21(b). In this instance, the zverage line spacing was assessed at about 2 IJm. At near fast failure bK levels, a significant amount of true transgranular cieavage facets were observed and one incidence, where a flat cleavage facet was initiated from the top of a buried graphite nodule, is shown in Fig. 22. Fractographic details of the predominanrly pearlitic microstructure, SG2, are illustrated in Figs. 23 through 27. In this pzticular microstructure, it was evident that The incidence of isolated flat, transgranular cleavage facets, see Fig. 23, was somewhat increased, and The extent of intergranular failure was significantly reduced, see Fig. 24, compared to that of the ferritic microstructure SGL

Fig. 17. Fatigue fracture surface of SGl with R = 0.15 and significant extent of spheroids on fracture amounting to 23% of fracture area (white line: 200 pm).

The highly directional nature of the ductile striated fatigue locations are shown in Fig. 25. This highly directional nature could be the result of the pearlite Iamellae which 3actured along cementite locations during fatigue crack extension. Indeed from Fig. 25, it appears that this is the case with the boundary being a pear& coloary

process to that observed in the predominantly ferritic microstructure, viz., flat, cleavage-like facets which contained very distinct regula: striztions or crack arrest lines, see Fig. 27. These line patterns are highly regular and are imprinted upon the ductile tear ridges, or river patrern lines, prevaleni ~359 27rh fiat facet. At this particuiar location, ;he average line spacing ;ria~ 2.6 pm. One fatigue fracture surface ol’73R = 0.15 test coaducted on predominately pearfitic microszurture SG2 was selected and etched iE ;i 24 nizal solution in an effwt to gain further insight intc the interaction be%een fatigue crack exte~~siw and specific microstructural features, Delaiii~ ,of

Fig. 18. Fatigue fracture surface SGi at near threshold AK level and R = 0.15.

the et&cd

fatigue

Figs. 25 through intci’fir;uInlar titr

I;.trnslirrc,

32. Figure

f:ICCt %WCii1fed abuut

are givenin 28 shows a smooth

fracture YLlrfaec

3

pm

Wit11 3 tf;iCk thick,

nnd

a

CCI?I(sIIdiscrete

pearhtc co16my. This particular- region of intergranular failure is probably associatcd with a ferritic-peattitic interphase boundary. The remainder of the fatigue fracture surface is ductile fatigue associated with the pearlite phase. Flat isolated transgranular cleavage facets, ‘I=.,100 ,mn in size, were observed to occur in the fet-rite phase, see Fig. 29(a). However, the initiation site for such facets was coarse cementite carbides

Fig. 19. Montage

of crack extension

upward at approximateiy

associated with the edge of peariitc colonies, see Fig. 24(b). Figure 30 illustrates the edge of such a rramgranular cleavage face{ prevalent within the ferrite phase. The cleavage !:acct terminated when it eccountercd the pearlite phase, see top RHS of Fig. 30(a). However, at another paint, see Fig. 30(b), the cleavage failure continues through the pearhte and across a peariite colony boundary. Crack extension across the ferrite phase is shown in Fig. 31 when it is evident that both intergranular failure and a flat cleavage-like failure mode which contains striations or lines of crack arrest-type behaviour coexist adjacent to

45’ around MPav&.

a graphite

nodule in SGl with R = 0.15and AK = 11.4

Fig. 21. Farigue fracture surfaz 931 at intermediate II; levels and R = 0.15 where crack grows from I& to sight. hi General view of slip lines on a cleavage-like fatigue frac0xe surface. (bj Detaited view of slip fines average spacing AXY_I~ 1 .um.

Fig. 23. Details of large transgranular cleavage facets on the Fatigue fracture surface SG2 with R = 0.91 and AK= 3.6 MPqG where crack grubs from right Lo left.

Fig. 26. Transgranu\ar clca~age and ductile striated SG2 b&cture modes v&h R = n.91 and AK = f MPar&. fal General VLZW= ib) Detaifed view nC ductile striated crack gro’kfb. .%xr-’ age spacing about 1 pm.

Fig. 24. Incidence of small extent uf irttergranuiar SG2 failure, typically d fehf percent wtth R = 0.91 and dK = 3 MPa& where crack groM-5upward.

Fig. 25. Details of dominant ductile striated fatigue crack growth (upward) with R = 0.91, AK = 4 MPa& and highly directional nature of the striations X2.

one arkother. fn this case, the average line spacing was assessed at about 2.3 ~.rm. Note also that the top of Fig. 31 shows ductile fatigue crack growth associated with the pearlite phase. FinaHy, a monta.ge of rhe etched fatigue i%acture surface, see Fig_ 33 shows an intergranular facet Itrrp half1 associated wirh a coarse pe3rfite cofony. Within this facet, the crack has efther ~MxU~ arresled or jumped up to a new atomic fracture plane. Outwith this intergranular facet, ductile fatigue crack extension is observed in association with different colonies of the pearlite phase.

The fractographically assessed extent of intergranular failure facets for the different R-ratio test for the predominantly ferritic S.G. cast iron, SGI, is given in Fig. 33. From this figure, it is

evident that beiow a dK level of about 8 SIPa&, the extent of intergranufar failure remains fairly constant at about 12% Above the 4K level, a significant increase suddenly occurred up to a maximum value of - 43% at a dK vafue of - 22 MPa$&. Above this SC value, the extent of intergranuiar failure or decohesion dramatically decrease until at a MC approaching 16 MPay% it was only about LIE%. A similar plot for the predominantly pearlik SG, cast iron micrflstructure, SG2, is shown in Fig, 34. It is immediately obvious from this figure that the extem of intergranular failure mode is significantly less tlza~ that found in mlcrosrrueture SGl inasmuch thzl ?? At MC fevels less than about 9 MPat~%: the average intergranufar failure was or& a kw percent, and * A maximum of about 12-% fnCergran~;br failure accurred at a 1K kvel .f-tE- I3 MP&G. $MKRI& &th Figs. 33 and 34 show ~some scatter, the trends in each case are rnscsnsitive :o &s R-ratia K&Ie. r-llCWeWr, When this data iS &7rtcd

Fig. 29. Details of flat transgranular cleavage facet SG2 with R = 0.15. (a) General view of cleavage facet. (b) Detailed view of initiation size.

ture SGl depending upon the R-ratio. At a low R-ratio, the maxima was observed to occur at a K,,, level of - 13 MPa&n while in the case of the R = 0.5 and R = 0.91 data, the maxima oclevels of 24 and 37 MPa& curred at K,,, respectively. In the case of microstructures SG2, see Fig. 36, although no maximas are evident, the data points are again separated in terms of Rratio. From the fractography of the fatigue surfaces, a flat, cleavage-like failure mode, within which highly defined striations or plasticity line marking were observed running perpendic-uiar to the r%er pattern lines which were roughly parallel to the macrocrack growth direction, was evident in both microstructures over the total dK testing range. In the case of the ferritic SG cast iron microstructure, SGl, the average line marking spacing with these Bat fracture regions was assessed over the whoie AK testing range for the R = 0.15 test and

Fig. 30. Details

of the edge of a flat tramgranular crack SG2 with R = 0.15.

cleavage

this data is presented in Fig. 37. From this figure, it is evident that over the 4K range, 8 to 16 MPaG, no real trends were observed and the lime spacing values reside between about 0.5 ,urn

Fig. 31. Intergranular failure associated with the ferrite phase while ductile striated growth in the pearlite phase SG2.

From the few

viz.,

Fig. 32. Montage of general nature of.fatigue extension SG2.

general

foregoingfraceogsaphicai details, a obsemations can readily be made3

0

I

0 E

,

I

4

2

a

6

/

!O

12

IS

b

18

~K~MFa-$iT)

Fig. 33. Fractographically

assessed

% intergranular

failure facets SGl at variotis R-rati

0.1, it can be seen that for steefs, the maximum amount of intergranuk3r failure occurred over a AK range of about 16 to 20 MPa&. in the case of the present data for the spheroidal graphite

I

2

6

of d K let-t:.

micro~trwtwe~, the maximas occurred at somewhat lower AK levels, viz., 12 to 13 MPa$x_ Suggested in 1111is that transgranular planar slip, **hi& t.eads to slip localisation, is essential in

1

4

values TS a function

I

0

10

It?

L

r

4

14

16

18

AK(MrlP+il

Fig. 34. Fractographically

assessed

% intergranular

failure facets SG2 at various R-ratio values as a fmctian

of AK level.

A

-.‘

promotingintwgranuiar

f%Iure when the qclic plastic zone size, R,,,,, approached the structural parameter size (grain size), d. Indeed, when this cquali+ is ackievc& extcnslvc slip band activity caused increased slip reversal and shp band fatigue damage which are effective processes for the dislacation transport of an active species (hydrogen) from the air environment, and dislocation piie-up leading to stress concentrations. Thus, it can be stated that the observed relationship between the maximum in the extent of intergranular failure and Ran, = da exists in situations where deformation Iocahsation along active shp pIanes occurs ahead of the crack tip. This variation of the extent of intergranular failure with AK level can also be appropriately characterised in terms of the average crack velocity, da/dt, or L’, which is easily obtained by multiplying the crack growth rate, da/dn, by the loading frequency, V, i.e.,

Fig. 37. Relationship between average line spacing within the cleavage facets SGI and the stress intensity range value for I! = 0.15.

da

da

X=%X

/

5ok

!_egend

Pm=

A R=091 DR=0.50

SGI

&E&II

i OR=O~LS 40

IA R=0.9!

SG2 ? ?R=0,50 * R=O.L5

L_

&

L

1

2

4

6

I

8

IO

12

Id

AKIMi’@) Fig. 38.

Percent intergranular

SGl and SG2 failure as a function of d K level.

16

18

20;

b

13’

“[‘py

g-;.&;

%jq:;l(;g-p”l (.&[;I

fryIn;

ii.]<: presfJgjl

iifudy,

fCSf$., !lh&flCf with r>rfrLx data rcpurberl rhc Iiic~I’;rtrrri:, lITI,: :~llrwi:n i, Fi$_;. 41 1,I I - 135. l;r.tlfty thiti i’iprre, it Wfl he: Scc:U Ill;tt thC Kll~lXilll~~S

I;” .-- ff. 15

in

lisl* tlj~,ist~uls exhibit ::r~)cl ~antmor~;~lit~ innWltli:h ihat fhG m.nxint11m CS~CiYlof intcrgram.ilitr faihm ~KXW~TXIat a cm& wlocibj value appraaching 10 -3 nrmjsec. As wclI as exhibitiilg the infIuencc of crack velocity on the incidence of intergranukir failure in fatigue tests conducted in an air awirunment, tm such intcrgranuiar separation were observed 1121 in a vacuum environment. This joint dependency on environment and crack velocity strongly indicate that intergranular failure occurred by an environmental assisted cracking, EAC, process which is controlled by the diffusion characteristics of an active species in the region of the crack tip enclave. In this particular case, the active species could probably be hydrogen which is generated by the reaction of atomically fresh fatigue surfaces with water molecules in the air environment. Once generated, hydrogen can diffuse into some critical region near or at ihe crack tip and cause a reduction in the cohesive strength of grain boundaries; such events have been postulated [14-161. This type of process would be controlled by that supply rate of hydrogen to the crack tip vicinity, i.e., at high crack velocity values, there would be insufficient time for hydrogen to diffuse to the critical region, hence, intergranular failure would be reduced while at low crack velocities, the rate of hydrogen production would be too low to trigger off grain boundary decohesion. From such an explanatF-3q the observed existence of intergranular failure maximas at certain crack velocities is readily appreciated. From Figs. 39 and 40, it is evident that the maximum extent of intergranular failure in the present study occurred at lower AK levels than those observed for steels, and occurred at much higher crack velocity values at which no intergranular failure in steels would be possible. The former observation can be explained by the fact that graphite spheroids act as preferential sites of intergranular failure associated with bulls-eye ferrite formation. This, together with the passible reduction in grain boundary cohesion strength that results from contamination by possibly carbon or phosphorous atoms that diffused from the spheroid, could result in easy intergran-

Another general fractographic observation was the preponderance of graphite spheroids on the fatigue fractures surfaces inasmuch that in some instances, over 20% of spheroids were observed while the random metallographic assessment was less than 10% Other workers [5,6,14] ha;le also reported an enhanced preponderance of graphite nodules or flakes on fatigue fracture surfaces. Indeed, finite element analysis [5] predicted that particles having a lower modulus than the surrounding matrix, i.e., soft particles like graphite nodules, attract and accelerate fatigue cracks extending into their vicinity by increasing the stress intensity at the crack tip. Suggested also is that, in two dimensions, the area of particle catchment (the number of particles on a fatigue fracture surface) is increased by four times. (This assuming that all cracks within a distance of 4R [where R = particle radius] and at angles up to 60” arc affected). From the present study, it would appear that the area of particle catchment was increased by a factor of about two rather than four. A schematic of the interaction behveen an extending fatigue crack encountering a graphite spheroid is i!!ustrated in Fig. 42. This figure details the progressive processes involved, viz., crack approaching nodule, crack deflection, decohesion at nodule-matrix interface, re-initiation and continued crack extension through the matrix. Fracture modes, other than intergranular faifure, have been commonly observed, viz., ductile fatigue crack growth and transgranular cleavage. The former fatigue failure mode was shown to be represented by zones of a more diversified topography which included regions of more ductile slip band separation, or coarse planar slip, and the more commonly reported finer ductile striated fatigue process. Instances of coarse planar type slip are exhibited in Figs. 13, 15 and 18. Such a faceted appearance is the result [14] of a glide plane decohesion mechanism where reversed slip on a limited number of slip planes ahead of the extending crack front weakens the cohesive strength of atomic bonds in the crack tip enclave

1151. When the siip planes are svcaken~_! to a sufficient degree. loca! separations occur at low stresses which cause the coarse crysta!fographic (faceted) appearance KS]. Instances of ductile striated or finer sfipped regions are ~much more common7 see Figs. 25 and 26. A di&ocation h.as been proposed 1171 to describe the transiriorr oi’ planar slip to wavy slip which is also the point at which faceted growth is repiaced by ducti’le striated growth, i.e., stage fl &Ggue crack growth occurs at the expense of stage I growth. In general, faceted growth was much iess common than ductile striated growth and tended to occur at lower AK levels although evidence of their coexistence at specific regions were observed. The latter faihrre mode, viz., transgranular cleavage type fracture, was also evident on the fatigue fracture surfaces in the form of isolated, flat, fan-shaped, facets: and as extensive regions

&!cl;lc ?wialit.ms. what is rile possibility that they may i‘)LI twiillc s2riahions? Wri~lQcfatigue striations WUVCTit-St rcportcL1 i;a [ I!Z.;?,,?] for tligfl slt’cngth

;,:3untinunt a61~y.s. d;GAIy, CYiKk

Legend ;5

Fotlgue

Onto

n

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IO

I5

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Fig. 42. Comparison of the macro fatigue crack growth rate data with the fractographically assessed average crack arrest or line spacing as a function of AK.

this trend was consistent ever the total A-K range of testing. The average line spacing data and the average crack growth rate data for microstructure SGl at 67 z 0.15 is shown in Fig. 42 as a function of stress intensity range, AK. For this figure, it can be seen that at AK levels approaching 8 MPa&, the average line spacing is some three orders of magnitude greater than the average recorded macro-crack growth data. Even ai dK value approaching lo-12 MPavG, the line spacing value were still an order of magnitude greater while at the maximum dn” values of 16 MPaG, the line spacing exhibited good commonality with that fatigue crack growth rate data being about 3 to 5 X 10W3 mm. From the definition given for ductile striations, it is evident that at low AK levek, these markings were not striations. Also, at such low AK ievels, if the markings were striations crack velocity values [Crack Velocity = Crack Growth Rate Times Frequency (105 HZ)] of between 9 mm/min to 45 mm/min should have been observed: clearly no such crack velocity data were recorded experimentally. Having demcnstrated that these line markings were not

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crack arrest markings can have a much greater spacing than ductile type striations kimiiar to the ptcsent observations). In the case of miid stec[ [24]? however, brittle striations were Lznly observed in a liquid mercury environment while ductile striations were observed in air and salt watt: environments. Hence, in the present study, the observed crack arrest markings were not brittle striations as they can ordy be induced by powerful material-environment interactions. The fractographic details of BCC metals C2.51 involved cleavage failure that can occur by differing amounts of plastic deformation, inasmuch that with increasing plasticity, a more complex cleavage relief is observed. Indeed, with increasing plastic deformation, we can get the following succession, viz., smooth, mirror-like cleavage; cleavage with steps; cleavage with periodic relaxation and cleavage with tear ridges. Atso exhibited in this particular paper were instances of striated or crack arrest markiczs within cleavage facets which were strikingly similar to those observed in the present study. Vasilev suggested that this striated type relief was the resuIt of comparatively slow growth of an extending cIeavage crack that underwent periodic relaxation. This type of cleavage by periodic relaxation type growth was termed the FrideI-Orlov mechanism [26-281. Basically, this mechanism involves the periodic stoppage of a growing cleavage crack due to relaxation, i.e., plastic relaxation retards the motion of the cIeavag= crack, viz., when the crack veiocity becomes less than some criticai va’lue, the crack suddenly stops growing and the crack tip becomes bhmted (arrest markings are formed). Crack growth will re-occur when the stress is increased above some level to reinitiate cleavage failure and such processes of crack arrest ensued by crack reinitiation and SC on produce striations on cleavage fracture surfaces. A schematic illustration of the Fridel-Orlov mechanism is shown in Fig. 43. From the foregoing evidence, it is suggested that the line or crack arrest markings prevalent on cleavage facets observed in both spheroidal graphite microstructures in the present study are the result of perisurfs

aR=O

LlrrCSt

(a!

( b) Fig. 43. Schematics ticity anti resding

CRACK

CRACK

SURFACE

PROFILE

of Fridel-Orb mechanism &wing p!ascomours by crack tip ~oppagcs. ia) crack surface: Cb) crack prdiie.

odic relaxation effects, i.e., the Pride&Orlo!; mechanism. Interestingly enough, over a decade ago, the presence of fine paralle? markings superimposed on ductiie-like facets were found [28] on Ihe fatigue fracture surfaces of a high strength aiuminum alloy. These markings occurred at veq low 4K levels and had an average Iine spa&g of about 5 x lo-’ mm which was insensitive to 4iK level. Similar parallel fracture markings were observed 1301 in a fatigue study of a Xi-i\l’b eutectic composition. In this case, the average spa&g of the parallel fracture markings waz about ‘7x fO_’ mm over the 4K range 15 to about 40 MPaJf% while the ductile striation spacing agreed well with the recorded average macro fatigue crack growth data over the dK range 30 ~a about 60 iMPa&. These fine par,allei fracture markings represented shp offsets C29,31] that were produced in the wake of the extending crack front and were not related to any crack growth increment. The fine parahef markings are different from the line or crack arrest markings obsertled in the present investigation inasmuch that they were behveen 10’ to 10” times finer in spacing, and imprinted on a ductile fracture surface whiIe

A somewhat lengthy fractographic study, both qualitative and quantitative, has been conducted on the fatigue fracture surfaces of two spheroidal graphite cast iron microstructures, ferritic and pea&tic ia nature, subjected to different toad ratio (-ii-ratio) fatigue q&es. at has been shown that A Ggnific;mi zmowxt of inlergranuiar failure or &cohesion was prei;alent in both ferritic and Fearlitic spheroidal graphite microstructures at !ow ho intelmediate AK levels. This static f&L me mode attained a maximum in both microstrwtures ati a specjfic AK level ifi& tended x0 coincide with the reversed plastic zox sire approaching ferrite grain size dimcnsiorss. The maximum extent of intergranular &lhiri’ ir. holh spheroidal graphite micsostru~tcures occurred at lower .IK icveis and at muc:rl Easter crack velociq values than ;hose reported for steels.

esterrt OF intersganu~ar failure w2s muck greater in extent in the ferritic microstructure than ir_ the predcminantf~! pearItic microstrucThe

tWX. A mwh higher preponderance a! graphite spheroids or nod&s were prevalent on the fatigue fracture surfaces cif both microstmctures than would hzve been expected from metalngraphic indications, This resuIted from the fact that soft particles (graphite nodules1 which have a much lower elastic modulils than the surrounding matrix, attract and acceIerate fatigue cracks extending in their vicinity by raising the applied stress intensity at the crack tip.

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1,i‘q.W crar:k cxlct.t.4it\tl, nssociaied with pkirrur

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Ilrrt.!trasir:t~~sf ruclurc~~ + h ~rcncr’rll~~atrrr’&ol?scxvcct ~Wc:I’ 1tw tutd Ah’ tcslitrg, WL~S the occurrence of flat, range of ctc;.tvagc-rypc regions within which regular par:tllcI crack arrest lirlcs ~NCTC very prominent. The average spacing of the crack arrest lines was 2-3 pm and this was insensitive to the 9 i< IeveI, i.e., they bore no relationship to the crack increment per fatigue cycle. !t is suggested that such crack arrest lines or marking observed in both microstructures are the result of cleavage failure that extended between periodic relaxation events which is cohectively known as the Fridel-Orlov mechanism. EcvcXh in

Acknowledgements

The author would like to thank David J. Bulloch, Scaw Metals, Ltd., South Africa, for performing the Fatigue Crack Growth Tests and Dave McManus and Stan Terras, both of the Microscopy Unit, University of the Witwatersrand, South Africa, for sample preparation and advice on quantitative fractographic technrques,

[If V. Biringuccid,De La

Pirotechnia, IS40, translated by C.S. Smith and M.T. Gnudi, The Pirotechnia of Vannoccio Biringuccio, Am. Inst. Min. Meiuii. Pet. Exg. !!?62!. [Zj C.A. Zapffe and G.A. Moore, Fatigue in rock formations, Am. hst. Min. Mefall. Pet. Eng. 154 (1943) 335-342. of nodular iron? J. I31 W.L. Bradleyy, Toughness properties Metals (1985) 74-76. and V.V. Kalaida, 141 A.J. Krasowsky, 1.V. Kramerenko Fracture toughness of nodular graphite cast irons under static, impact and cyclic loading, FangLie Frucr. Eng., Mater. Strud. 10 (1987) 223-237. and W.J. Plumbridge, FaIS1 A.J. Padkin, M.F. Brereton tigue crack growth ir! @o phase alloys, Mater. Sci. Techrd. 3 (1987) 217-223. [61 D.J. Bulloch and J.H. Builoch, Influence of R-ratio and microstructure on threshold fatigue crack growth characteristics of spheroidal graphite cast irons, Jjtf. J. Pres. I/es. Piping 36 (1989) 289-314. [7] P.A. Blackmorr and K. Morton, Structure-property rela-

The slow fatigue crack growth and threshold behavior of a medium carbon steel, Eq. &ct. .&&&I.7 I19751 69-77. [III KS. Ravichandran, E.S. Dwarakadasa and R. Kishore, Some considerations on the occurrence of intergranular fracture during fatigue crack growth in steels, Mat. Sci. Org. 83 (1986) Lll-L16. WI P.E. Irvine and A. KurzfeId, Intergranular failure at near threshold fatigue crack growth rates, Metal Sci. (1979) 495-502. f131 J.H. Bulloch and D.J. Bulloch, Influence of carbon content and tempering temperature on fatigue threshold characteristics of steels, submitted to ht. J. Pressrrre Vess. Piping. and D. Meyn, Electron Fractogr@p, [I41 C.D. Beacham ASTM STP 436 (1968) 59-71. '115!M. Geil and G.R. Leverant, Trans. Am. hr. M&z. Med. Pet. Eirg. 232 (1968) l&69-;X74. of hydrogen ml C.E. Price and L.B. Traylor, Fractography and mercury embrittlement in nickel 200, CorrorionNACE 43 (19871229-237. J. _4~pl. t171 D.O. Swenson. Cleavage failure in materials, P&ics 4Q (1969) 3467-4572. [181 H.C. Burghard and N.S. Stoloff, Cleavage phenomena and topographic features, Electron Fractography, ASiX UP 436 (19681 32-43. [19] E.K. Friddie and F.E. Waikil The effec; ofgiaifi SIzc 0I; the occurrence of cleavage fatigue failure in 16 stainless steel, J. Mats. Sci. Letters 11 (1976) 386-389. f201 C.J. Beevers, R.J. Cooke, J.F. Knott and R.O. Ritshie. Some considerations of the influence of sub-critical cleavage growth during fatigue crack propagation in steeb., ilfer. Sci. 9 <19751 119-126. 1211 J.H. Bulloch and R+ Kennedy, Influence of stress ratio and volume fraction martensite on the threshold fatigue crack growth of granular bainites, Res. Mechanica 15 (1985) 289-314. [22] P.J,E. Forsyth and D.A. Ryder, Fatigue striations in aluminum alloys, Itlera&rgia 63 (1961) 117-126. I231 P.J.E. Forsyth, CA. Stubbington and D. Clark, Fatigue striations in various materiaIs, J. Inst. Metals 90 (19611962) 238-248. I241 R.J.H. Wanhill, Formation of brittle striations, Corrosion NACE 31 I19751 66-74. [25] A.D. Vasiiev, Scanning electron fractography of body centred cubic (BCC) metals, Scanning Electron Microscopy III (1986) 917-948. [26] J.J. Golman, C. Knudsen and W.R. Wlesh, Cleavage cracks and dislocations in LiF crystals, J. Appl. Phys. 29 Cl9SS) 601-608. [27] G. Fridel, “Strain hardening and crack propagation”, in Atomic Mechanisms of Fracture (Metallugizdat: Moscow, 1963) pp. 504-513.