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Fatigue crack propagation in spheroidal-graphite cast irons with different microstructures
Fatigue crack propagation in spheroidalgraphite cast irons with different microstructures Keiro Tokaji, Takeshi Ogawa and Kazuhisa Shamoto* Department of Mechanical Engineering, Gifu University, 1-1 Yanagido, Gifu, Japan *Kawasaki Heavy Industries Ltd, 2-1-18 Nakamachidoori, Chuoh-ku, Kobe, Japan (Received 18 June 1993; revised 4 October 1993) Fatigue crack propagation (FCP) and crack closure have been investigated in four spheroidal-graphite cast irons (SGIs) with different microstructures: ferrite, pearlite, bull's eye and aus-ferrite (bainite). The FCP rates plotted against the stress intensity factor range showed a microstructure dependence due to differing contributions of crack closure. Therefore, the effect of microstructure disappeared as crack closure was taken into account, indicating that the intrinsic FCP resistance was the same for all the microstructures studied. Published FCP data on SGIs with a wide variety of microstructures are reviewed, and an overall discussion is developed on the effect of microstructure on FCP behaviour. (Keywords: crack propagation; crack closure; crack propagation resistance; threshold; spheroidal graphite cast iron; microstructure)
Table 1 Chemical composition (wt. %)
Spheroidal-graphite cast irons (SGIs) have recently been receiving increased attention as structural materials in various industries, because of their improved strength and ductility, which are comparable to those of steels. In particular, considerable interest has been shown in the property improvements obtained in ductile iron by austempering heat treatments. In order to establish the material strength and reliability in applications of SGIs to critical machine components, further understanding of their fatigue properties, such as fatigue strength and fatigue crack propagation (FCP), is needed. The FCP of SGIs has been found to be largely influenced by the microstructure, the size and volume fraction of the graphite, and the chemical composition t-11. In most cases, however, the effect of these variables tends to disappear as the FCP data are plotted in terms of the effective stress intensity factor range AKeff (after allowing for crack closure), indicating that they can affect crack closure behaviour, i.e. crack-driving force, but not the intrinsic FCP resistance. Of the above variables, microstructure is the dominating parameter in controlling the mechanical properties, and so the effect of microstructure on FCP behaviour is an important concern in selecting and processing SGIs. The present paper deals with this issue. In this study, FCP and crack closure are investigated in four SGIs with different microstructures: ferrite, pearlite, bull's eye and aus-ferrite. In addition, the published FCP data on SGIs with a wide variety of microstructures are reviewed. Based on the results 0142-1123/94/050344-07 © 1994 Butterworth-Heinemann Ltd
344 Fatigue, 1994, Vol 16, July
Material Ferrite Pearlite Bull's eye Aus-ferrite
C
Si
Mn
P
S
Mg
3.74 3.61 3.61 3.72
2.80 2.85 2.85 2.15
0.25 0.025 0.025 0.37
0.069 0.071 0.071 0.014
0.017 0.015 0.015 0.007
0.050 0.048 0.048 0.037
obtained, an overall discussion is developed on the effect of microstructure on FCP behaviour. MATERIALS AND PROCEDURES The materials used were four SGIs with different microstructures: ferrite, pearlite, bull's eye and ausferrite (bainite). The chemical compositions and material parameters are listed in Tables 1 and 2, respectively. Ferritizing was performed by heat treatment, which involved heating to 900 °C, holding at this temperature for 2 h, then slow cooling at 50 °C Table 2 Material parameters Average spheroidalArea fraction of graphite size Nodularity spheroidal graphite Material Ferrite Pearlite Bull's eye Aus-ferrite
(~m)
(%)
(%)
54 35 48 26
93 95 95 94
21 16 22 13
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al. Table 3
Mechanical properties
Material
0.2% proof stress, tro.2 (MPa)
Tensile strength, ~rB (MPa)
Elongation, ~b (%)
Reduction of area, ~ (%)
Elastic modulus, E (MPa)
Ferrite Pear~te Bull's eye Aus-ferrite
274 453 362 656
400 645 474 957
10 4 5 11
7 4 2
142000 146000 165000
h -1 to 200 °C and air cooling to room temperature. The iron with bull's eye microstructure was provided in the as-cast condition and was heated to 900 °C, held at this temperature for 2 h and then cooled in air to obtain the pearlitic microstructure. The ausferritic microstructure was produced by heating to 890 °C, holding at this temperature for 30 min, then quenching in a salt bath at 360 °C and holding for 2 h, followed by air cooling. The mechanical properties for all the microstructures obtained are given in Table 3. Compact tension specimens, 50.8 mm wide and 12.5 mm thick, were machined and then stress-relieved at 540 °C for 80 min in vacuum, except for the ausferrite. Tests were conducted at ambient temperature in laboratory air on 14.7 kN and 49 kN closed-loop servocontrolled electrohydraulic testing machines operating at a frequency of 10 Hz. Tests for increasing and decreasing stress intensity factor, AK, according to ASTM standards 12, were employed in the FCP experiments at a stress ratio R of 0.05. Tests at a constant
-4
-
o Ferrite o
O~ O
,x Pearlite [] Bull'seye O Aus-ferdte R=0.05
ZX~o
0.5
O
0
I
5
I
10
I
I
I
20
Maximum stress intensity factor Figure 2
Zl0
-5
Ferrite ix Pearlite [] Bull's eye <> Aus-ferrite R=0.05 0
;,,..,
±tJ 1 0
zl0
I
30 40 Kmax MPa
Crack closure behaviour
-3
-4
-5
-6
[
o
O
' Q
10
-
1.0
10
±t,.) 1 0
~10
-
-3
10
=10
-
-8
'
1
,
, ,,/~,1
,
10
Stress intensity factor range
,
~10 o
Ferrite Pearlite Bull's eye <> Aus-ferrite R=0.05 A []
~ -7 ~10
o
, ,,,,,
100 A K MPaV-m
Figure 1 Relationship between crack propagation rate and stress intensity factor at R = 0.05
10 I
10
Effective stress intensity factor range
100 AKeff MPaV-m
Figure 3 Relationship between crack propagation rate and effective stress intensity factor
Fatigue, 1994, Vol 16, July
345
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al.
10
-3
"10 "6 ;m E
,¢, I" '
t
-4
-5
w f,
i.°
Kmax=20MPa d~,/ AK-dec. I~d'f o Ferrite ~ . zx Pearlite e.~ [] Bull's eye <> A u s - f e r r i J < > f . .
Figure 4 Cleavage facets observed for the high AK regime in the bull's eye microstructure
maximum stress intensity factor ( g m a x = 20 MPa m ~) were also performed in order to obtain the FCP behaviour at high stress ratios. Prior to the experiments, a fatigue precrack was introduced more than 2 mm from the notch root under a constant AK of 18 MPa m i. Crack length and crack closure were automatically monitored using a compliance method in which the displacement at the crack mouth was measured with a clip-gauge extensometer. Crack closure was also monitored by a back-face strain gauge method (BFS). The Kop/ Kmax values (Kop = crack opening stress intensity) by these two methods were in good agreement to within -+ 5%.
=10 o
10
-8
I
~ ,7'1 ~<> I I I
11:0.05, AKeff
....... I
|
I I1[
10 Effective stress intensity factor range
The FCP rate d a / d N is presented in Figure 1 as a function of AK for the four microstructures. The FCP
346
SEM images of fracture surface (AKeff = 5 MPa m*): (a) ferrite; (b) pearfite; (c) bull's eye; (d) aus-ferrite
Fatigue, 1994, Vol 16, July
I
I
I
I
I I 11
100 AKeff MPad-m"
Figure 5 Relationship between crack propagation rate and stress intensity factor under constant Kmax
EXPERIMENTAL RESULTS
Figure 6
Ferrite Pearlite Bull's eye Aus-ferrite
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al. b e h a v i o u r clearly shows a m i c r o s t r u c t u r e d e p e n d e n c e . I n the low A K r e g i m e , t h e f e r r i t e has t h e highest F C P resistance; t h e p e a r l i t e a n d t h e aus-ferrite a r e the worst, a n d t h e b u l l ' s e y e exhibits an i n t e r m e d i a t e resistance b e t w e e n t h e m . I n t h e high A K r e g i m e , t h e bull's e y e indicates h i g h e r F C P r a t e s t h a n t h e o t h e r m i c r o s t r u c t u r e s . It can also b e s e e n in Figure 1 t h a t the n e a r - t h r e s h o l d F C P r a t e s i n c r e a s e a n d t h e t h r e s h o l d stress intensity factor r a n g e AKth d e c r e a s e s with i n c r e a s e in s t r e n g t h ( s e e Table 3). T h e crack c l o s u r e b e h a v i o u r is s h o w n in Figure 2 in t e r m s o f t h e r e l a t i o n s h i p b e t w e e n Kop/Kmax a n d Kmax. T h e K o p / K m a x v a l u e s a r e the s a m e in all the m i c r o s t r u c t u r e s at Km~x > 20 M P a m ~ ( A K > 19 M P a m i) a n d a r e a l m o s t c o n s t a n t i n d e p e n d e n t of Kmax. A t Km~x < 20 M P a m ~, h o w e v e r , t h e Kop / Km~x values i n c r e a s e m a r k e d l y with d e c r e a s i n g Km~x a n d indicate m i c r o s t r u c t u r e d e p e n d e n c e , in that crack closure is r e d u c e d as s t r e n g t h increases.
Table 4
Published data for fatigue crack propagation in SGIs
Number or Symbola 1-1 2-1 2-2 2-3 2-4 3-1 3-2 3-3 3-4 4-1 4-2 4-3 5-1 5-2 5-3 5-4 5-5 5-6 5-7 5-8 5-9 5-10 5-11 5-12 5-13 5-14 6--1 6--2 6-6 8-1 9-1 9-2 10-1 11-1 11-2 11-3 11-4 11-5 11-6 O A []
Figure 3 r e p r e s e n t s t h e F C P b e h a v i o u r p l o t t e d in t e r m s of AK~ff. T h e effect o f m i c r o s t r u c t u r e o b s e r v e d in Figure 1 c o m p l e t e l y d i s a p p e a r s , i n d i c a t i n g t h a t t h e c h a n g e in m i c r o s t r u c t u r e can a l t e r only c r a c k closure level, i.e. c r a c k - d r i v i n g force, a n d thus t h e intrinsic F C P resistance is t h e s a m e for all t h e m i c r o s t r u c t u r e s . T h e bull's e y e still exhibits h i g h e r F C P rates in the high A K r e g i m e ; this can b e a t t r i b u t e d to a static fracture m e c h a n i s m , b e c a u s e s o m e c l e a v a g e facets are seen on the f r a c t u r e surfaces, as shown in Figure 4, T h e r e l a t i o n s h i p s b e t w e e n da/dN a n d A K o b t a i n e d u n d e r c o n s t a n t Kmax a r e p r e s e n t e d in Figure 5. I n c o n s t a n t Kn~ax tests, stress r a t i o increases with d e c r e a s ing AK, a n d thus A K = AK~ff at A K < 10 M P a m i in all the m i c r o s t r u c t u r e s (i.e. c r a c k c l o s u r e - - f r e e b e h a v i o u r ) . T h e da/dN versus A K (AKeff) r e l a t i o n s h i p s are the s a m e i n d e p e n d e n t of m i c r o s t r u c t u r e a n d also coincide with t h o s e o b t a i n e d at R = 0.05. S E M images of t h e fracture surface in t h e n e a r -
Load ratio 0.1 0.05 0.05 0.05 0.05 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0,8 0,8 0.8 0.1 0.1 0.1 0.1 0.8 0.8 0.8 0.8 0.1 0.1 0.5 0.1 0.1 0.1 0.05 0.1 0.1 0.1 0.5 0.5 0.5 0.05 0.05 0.05 0.05
Matrix microstructure b
AKth
AK=n.~h
1
F
2
10
F+ B F + B F + B F + B F + B F + B F + B F+ B F + P B F + B F + P P F F + P P F F + P F+ M F + S F + B F + P F + M F + S F + B F + P F+ P F + P F F + B B F
11
B
9.0 10.0 12.7 13.2 14.7 18.8 16.3 15.5 12.3 10.2 4.9 14.8 11,8 8.5 6.5 4.0 3.2 4.1 10.0 10.3 12.8 14.5 4.4 4.2 3.9 4.0 12.1 10.3 5.6 13.0 9.0 5.0 12.4 8.1 3.0 11.5 5.4 3.0 8.8 11.7 7.8 10.3 8.3
5.3 ----5.9 4.6 4.5 3.5 3.7 3.5 3.3 2.7 3.5 2.5 4.0 3.2 4.1 3.3 3.8 2.6 3.6 4.4 4.2 3.9 4.0 2.8 3.9 3.2 -6.0 3.1 3.2 4.4 3.0 6.7 4.4 3.0 6.7 3.7 3.6 3.9 3.9
Reference
3
4
5
6 8 9
Present results
M F B M F F P F+ P B
(800 °C) (815 °C) (830 °C) (840 °C)
FCD50 FCD50 FCD50 FCD50 FCD50 FCD50 FCD60 FCD60 FCD60 FCD60 FCD60 FCD60 FCD60 FCD60 FCD50 FCD60 FCD50
aNumbers correspond to Figures 7 and 8
bF, ferrite; B, bainite; P, pearlite; M, raartensite; S, sorbite
Fatigue, 1994, Vol 16, July 347
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al.
10
-t...) 10
threshold regime at R = 0.05 are shown in Figure 6. Some intergranular facets of ferrite grains are observed in the ferrite and the bull's eye, but they do not significantly affect the FCP behaviour, as can be seen in Figure 3. In contrast, the pearlite and the ausferrite reveal a predominantly transgranular mode.
-3
-4
p..,,
REVIEW OF PUBLISHED DATA
FCP characteristics
-5
z10
-6
=10 o
~
-7
e]0
10
-8
1
10
Stress intensity factor range
100 AK MPaV-m
Figure 7 Relationship between crack propagation rate and stress intensity factor in SGIs with a wide variety of microstructures (Table 4). Open symbols indicate the present results
10
_t..) 10
-3
-4
p.,,
zlO
-5
Thresholds
o
-=_10
-6
INI
10 10
-7
-8
I
10
Effective stress intensity factor range
100 AKeff MPad'~
Figure 8 Relationship between crack propagation rate and effective stress intensity factor in SGIs with a wide variety of microstructures (Table 4). Open symbols indicate the present results
348
There have been many published results on the FCP behaviour of SGIs. For an overall understanding of the effect of microstructure, it is useful to review the literature here. Literature published in Japan was selected for this purpose and is listed in Table 41-11. The da/dN-AK relationships in SGIs with a wide variety of microstructures are shown in Figure 7: the numbers in the figure correspond to the data in Table 4. As can be seen from the figure, the martensite (11-2) and the bainites (4-2, 9-2) clearly show higher FCP rates over the whole AK regime, but the da/ dN-AK relationships for the other microstructures tend to fall in a relatively narrow scatter band, which also includes the present data (open symbols). A careful examination of Figure 7 indicates that the dual-phase microstructures such as ferrite-pearlite, ferrite-bainite and ferrite-martensite have an excellent FCP resistance compared with the single-phase microstructures, and that in the single-phase microstructures the FCP rates tend to increase with increasing strength. The latter trend is consistent with the present results described in the previous section. Figure 8 shows the FCP data plotted in terms of AKeff: AK = AK~fe at R = 0.50 and 0.80, because no crack closure may occur. Although the martensites still exhibit higher FCP rates, the scatter of the FCP curves is much narrower compared with Figure 7. This suggests that the intrinsic FCP resistance is almost insensitive to microstructure. In the near-threshold regime, some dual-phase microstructures tend to show excellent FCP resistance. This may be attributed to the reduction in crack-driving force resulting from marked crack deflections, i.e. fracture surface roughness.
Fatigue, 1994, Vol 16, July
The relationship between the threshold values Agth and AKen,th and microstructure is presented in Figure 9. The dual-phase microstructures show significantly higher Agth values: greater than 9 MPa mt. In the single-phase microstructures, however, most of the ferrites indicate higher Agth values, comparable to the dual-phase microstructures, but the bainites and martensites exhibit considerably lower values: less than 5 MPa mL The threshold values are also shown in Figure 10 as a function of tensile strength, era. There is a good correlation between AKth and era; the AKth values decrease with increasing era. This trend is quite similar to that in Figure 9, because the order of the microstructures in Figure 8 (ferrite --> pearlite --> bainite ---> martensite) corresponds to the increase in strength. As can be seen from Figures 9 and 10, the Ageff,th values are 3.5-4.0 MPa m t independent of microstructure or era. This implies that the above A g t h
Fatigue
crack propagation
l?l
F-B F-P F-M
::
F-E&bite(S)
Open symbol: AKtb Solid symbol: AKeff,tb
i-
8
0
0
0
0
I
I
I
I
I
Figure 9 Threshold values as a function of microstructure
25L
I
0 A
Ferrite
Pearlite Bainite Martensite q Dual-phZlX cl Opensymbol: AKth 0
Solid symbol
8
oo* l*
63
1500
Figure 10 Threshold values as a function of tensile strengh
dependence on microstructure to crack closure.
is primarily attributed
DISCUSSION Based on the present results and the review of published data, the macroscopic FCP mechanisms and resistance resulting from the change in microstructure of SGIs can be rationalized as shown in Figure II.
Single phase matrix Martensile
Bainite(B)
PearlIts
Microstructural
FerriIe(F)
i :
Dual-phase F-P
F-B
matrix
F-SISorbile)
unit size :+
StrenQlh .
: Fraclura
small4
low ‘*lpe)
surfacs roughness
; :
Crack closure
small* Apparent
low .
crack growth rasislance (da/dN-AK)
Similar magnitude
Depend an the combination the microstructure
ACKNOWLEDGEMENT The authors wish to thank Mr Y. Ohuchi for assistance with the experiments.
of
.
large
warge ; l high
I
FCP behaviour and crack closure were studied in four spheroidal graphite cast irons (SGIs) with different microstructures, and the published FCP data for SGIs with a wide variety of microstructures were reviewed. Based on these results, the effect of microstructure on the FCP behaviour was discussed. In the single-phase microstructures, the finer the microstructure is (and hence the higher the strength is) the lower the apparent FCP resistance (expressed in terms of AK) become. However, the intrinsic FCP resistance (after allowing for crack closure) is the same in all the microstructures. The dual-phase microstructures show excellent FCP resistance because of their enhanced crack closure, but the intrinsic FCP resistance is also insensitive to microstructure and is the same as that of the single-phase microstructures. Therefore, it is concluded that the intrinsic FCP resistance in SGIs is independent of the change in a wide variety of microstructures.
F-M
small4 high
In the single-phase microstructures, in general, the microstructural unit such as grain size would become finer with the order indicated in the figure. As is well known, the finer the microstructure is, the higher the strength becomes. In FCP behaviour, however, coarser microstructures would result in more remarkable deflections in their crack path, which lead to a rougher fracture surface, inducing enhanced crack closure. As a consequence, such microstructures indicate an excellent FCP resistance when the data are plotted in terms of AK. However, the intrinsic FCP resistance is insensitive to microstructure because the change in microstructure can alter only crack closure level, or crack-driving force. In the dual-phase microstructures, however, the crack path would be much more tortuous because the two phases have different metallurgical and mechanical properties, which induces enhanced crack closure and thus leads to higher FCP resistance than in the single-phase microstructures. As crack closure is also responsible for the excellent FCP resistance in this case, the intrinsic FCP resistance becomes independent of microstructure and is the same as that of the single-phase microstructures. As a conclusion, therefore, the intrinsic FCP resistance in SGIs does not depend on the change in a wide variety of microstructures.
AK&f@
1000 (JB MPa fi
500
Tensile strength
K. Tokaji et al.
0
l
0
cast irons:
CONCLUSIONS
8
otl’,,,,,‘,‘l’,‘,,,‘,“‘,,“,,‘l
:
in spheroidal-graphite
(orhigh) +
of essential crack growth resistance (da/dN- AKw)
Figure 11 Mechanism of fatigue crack propagation in SGIs
REFERENCES Kato, Y., Hirose, M. and Suzuki, S. Truns. Jpn Sm. Mech. Eng. 1985, 51, 1161 (in Japanese) Suzuki, H., Ueki, T. and Kobayashi, T. Tram. Jpn Sot. Mech. Eng. 1991, 57, 1029 (in Japanese) Sugiyama, Y., Asami, K. and Matsuoka, S. J. Sot. Marer. Sci. Jpn 1991, 40, 675 (in Japanese) Sugiyama, Y., Asami, K. and Kuroiwa, H. 1. Sot. Mater. Sci. Jpn 1991, 40, 65 (in Japanese) Sugiyama, Y., Asami, K. and Kuroiwa, H. Trans. Jpn Sot. Med. Eng. 1990, 56, 482 (in Japanese) Ito, S., Sugiyama, Y., Asami, K. and Yamada, S. J. Sot. Muter. Sci. Jpn 1987, 36, 369 (in Japanese)
Fatigue, 1994, Vol 16, July
349
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al. 7 8 9 10
350
Sugiyama,Y., Asami, K., Ito, S. and Yamada, S. J. Soc. Mater. Sci. Jpn 1988, 37, 776 (in Japanese) Takeuchi, S. and Yoshimura, H. Imono 1989, 61, 246 (in Japanese) Yamamoto, H., Yamada, S. and Kobayashi, T. lmono 1990, 62, 883 (in Japanese) Ogawa,T. and Kobayashi, H. Fatigue Fract. Eng. Mater. Struct. 1989, 10, 273
Fatigue, 1994, Vol 16, July
11 12
Hirose, Y. and Yajima, Z. In Preprints of 199th Meeting of Fatigue Committee Soc. Mater. Sci. Jpn, 1989, pp.1-6 (in Japanese) 'Annual Book of ASTM Standards', E647-88, American Society for Testing and Materials, 1988, p 636
Table 1 Chemical composition (wt. %)
Spheroidal-graphite cast irons (SGIs) have recently been receiving increased attention as structural materials in various industries, because of their improved strength and ductility, which are comparable to those of steels. In particular, considerable interest has been shown in the property improvements obtained in ductile iron by austempering heat treatments. In order to establish the material strength and reliability in applications of SGIs to critical machine components, further understanding of their fatigue properties, such as fatigue strength and fatigue crack propagation (FCP), is needed. The FCP of SGIs has been found to be largely influenced by the microstructure, the size and volume fraction of the graphite, and the chemical composition t-11. In most cases, however, the effect of these variables tends to disappear as the FCP data are plotted in terms of the effective stress intensity factor range AKeff (after allowing for crack closure), indicating that they can affect crack closure behaviour, i.e. crack-driving force, but not the intrinsic FCP resistance. Of the above variables, microstructure is the dominating parameter in controlling the mechanical properties, and so the effect of microstructure on FCP behaviour is an important concern in selecting and processing SGIs. The present paper deals with this issue. In this study, FCP and crack closure are investigated in four SGIs with different microstructures: ferrite, pearlite, bull's eye and aus-ferrite. In addition, the published FCP data on SGIs with a wide variety of microstructures are reviewed. Based on the results 0142-1123/94/050344-07 © 1994 Butterworth-Heinemann Ltd
344 Fatigue, 1994, Vol 16, July
Material Ferrite Pearlite Bull's eye Aus-ferrite
C
Si
Mn
P
S
Mg
3.74 3.61 3.61 3.72
2.80 2.85 2.85 2.15
0.25 0.025 0.025 0.37
0.069 0.071 0.071 0.014
0.017 0.015 0.015 0.007
0.050 0.048 0.048 0.037
obtained, an overall discussion is developed on the effect of microstructure on FCP behaviour. MATERIALS AND PROCEDURES The materials used were four SGIs with different microstructures: ferrite, pearlite, bull's eye and ausferrite (bainite). The chemical compositions and material parameters are listed in Tables 1 and 2, respectively. Ferritizing was performed by heat treatment, which involved heating to 900 °C, holding at this temperature for 2 h, then slow cooling at 50 °C Table 2 Material parameters Average spheroidalArea fraction of graphite size Nodularity spheroidal graphite Material Ferrite Pearlite Bull's eye Aus-ferrite
(~m)
(%)
(%)
54 35 48 26
93 95 95 94
21 16 22 13
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al. Table 3
Mechanical properties
Material
0.2% proof stress, tro.2 (MPa)
Tensile strength, ~rB (MPa)
Elongation, ~b (%)
Reduction of area, ~ (%)
Elastic modulus, E (MPa)
Ferrite Pear~te Bull's eye Aus-ferrite
274 453 362 656
400 645 474 957
10 4 5 11
7 4 2
142000 146000 165000
h -1 to 200 °C and air cooling to room temperature. The iron with bull's eye microstructure was provided in the as-cast condition and was heated to 900 °C, held at this temperature for 2 h and then cooled in air to obtain the pearlitic microstructure. The ausferritic microstructure was produced by heating to 890 °C, holding at this temperature for 30 min, then quenching in a salt bath at 360 °C and holding for 2 h, followed by air cooling. The mechanical properties for all the microstructures obtained are given in Table 3. Compact tension specimens, 50.8 mm wide and 12.5 mm thick, were machined and then stress-relieved at 540 °C for 80 min in vacuum, except for the ausferrite. Tests were conducted at ambient temperature in laboratory air on 14.7 kN and 49 kN closed-loop servocontrolled electrohydraulic testing machines operating at a frequency of 10 Hz. Tests for increasing and decreasing stress intensity factor, AK, according to ASTM standards 12, were employed in the FCP experiments at a stress ratio R of 0.05. Tests at a constant
-4
-
o Ferrite o
O~ O
,x Pearlite [] Bull'seye O Aus-ferdte R=0.05
ZX~o
0.5
O
0
I
5
I
10
I
I
I
20
Maximum stress intensity factor Figure 2
Zl0
-5
Ferrite ix Pearlite [] Bull's eye <> Aus-ferrite R=0.05 0
;,,..,
±tJ 1 0
zl0
I
30 40 Kmax MPa
Crack closure behaviour
-3
-4
-5
-6
[
o
O
' Q
10
-
1.0
10
±t,.) 1 0
~10
-
-3
10
=10
-
-8
'
1
,
, ,,/~,1
,
10
Stress intensity factor range
,
~10 o
Ferrite Pearlite Bull's eye <> Aus-ferrite R=0.05 A []
~ -7 ~10
o
, ,,,,,
100 A K MPaV-m
Figure 1 Relationship between crack propagation rate and stress intensity factor at R = 0.05
10 I
10
Effective stress intensity factor range
100 AKeff MPaV-m
Figure 3 Relationship between crack propagation rate and effective stress intensity factor
Fatigue, 1994, Vol 16, July
345
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al.
10
-3
"10 "6 ;m E
,¢, I" '
t
-4
-5
w f,
i.°
Kmax=20MPa d~,/ AK-dec. I~d'f o Ferrite ~ . zx Pearlite e.~ [] Bull's eye <> A u s - f e r r i J < > f . .
Figure 4 Cleavage facets observed for the high AK regime in the bull's eye microstructure
maximum stress intensity factor ( g m a x = 20 MPa m ~) were also performed in order to obtain the FCP behaviour at high stress ratios. Prior to the experiments, a fatigue precrack was introduced more than 2 mm from the notch root under a constant AK of 18 MPa m i. Crack length and crack closure were automatically monitored using a compliance method in which the displacement at the crack mouth was measured with a clip-gauge extensometer. Crack closure was also monitored by a back-face strain gauge method (BFS). The Kop/ Kmax values (Kop = crack opening stress intensity) by these two methods were in good agreement to within -+ 5%.
=10 o
10
-8
I
~ ,7'1 ~<> I I I
11:0.05, AKeff
....... I
|
I I1[
10 Effective stress intensity factor range
The FCP rate d a / d N is presented in Figure 1 as a function of AK for the four microstructures. The FCP
346
SEM images of fracture surface (AKeff = 5 MPa m*): (a) ferrite; (b) pearfite; (c) bull's eye; (d) aus-ferrite
Fatigue, 1994, Vol 16, July
I
I
I
I
I I 11
100 AKeff MPad-m"
Figure 5 Relationship between crack propagation rate and stress intensity factor under constant Kmax
EXPERIMENTAL RESULTS
Figure 6
Ferrite Pearlite Bull's eye Aus-ferrite
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al. b e h a v i o u r clearly shows a m i c r o s t r u c t u r e d e p e n d e n c e . I n the low A K r e g i m e , t h e f e r r i t e has t h e highest F C P resistance; t h e p e a r l i t e a n d t h e aus-ferrite a r e the worst, a n d t h e b u l l ' s e y e exhibits an i n t e r m e d i a t e resistance b e t w e e n t h e m . I n t h e high A K r e g i m e , t h e bull's e y e indicates h i g h e r F C P r a t e s t h a n t h e o t h e r m i c r o s t r u c t u r e s . It can also b e s e e n in Figure 1 t h a t the n e a r - t h r e s h o l d F C P r a t e s i n c r e a s e a n d t h e t h r e s h o l d stress intensity factor r a n g e AKth d e c r e a s e s with i n c r e a s e in s t r e n g t h ( s e e Table 3). T h e crack c l o s u r e b e h a v i o u r is s h o w n in Figure 2 in t e r m s o f t h e r e l a t i o n s h i p b e t w e e n Kop/Kmax a n d Kmax. T h e K o p / K m a x v a l u e s a r e the s a m e in all the m i c r o s t r u c t u r e s at Km~x > 20 M P a m ~ ( A K > 19 M P a m i) a n d a r e a l m o s t c o n s t a n t i n d e p e n d e n t of Kmax. A t Km~x < 20 M P a m ~, h o w e v e r , t h e Kop / Km~x values i n c r e a s e m a r k e d l y with d e c r e a s i n g Km~x a n d indicate m i c r o s t r u c t u r e d e p e n d e n c e , in that crack closure is r e d u c e d as s t r e n g t h increases.
Table 4
Published data for fatigue crack propagation in SGIs
Number or Symbola 1-1 2-1 2-2 2-3 2-4 3-1 3-2 3-3 3-4 4-1 4-2 4-3 5-1 5-2 5-3 5-4 5-5 5-6 5-7 5-8 5-9 5-10 5-11 5-12 5-13 5-14 6--1 6--2 6-6 8-1 9-1 9-2 10-1 11-1 11-2 11-3 11-4 11-5 11-6 O A []
Figure 3 r e p r e s e n t s t h e F C P b e h a v i o u r p l o t t e d in t e r m s of AK~ff. T h e effect o f m i c r o s t r u c t u r e o b s e r v e d in Figure 1 c o m p l e t e l y d i s a p p e a r s , i n d i c a t i n g t h a t t h e c h a n g e in m i c r o s t r u c t u r e can a l t e r only c r a c k closure level, i.e. c r a c k - d r i v i n g force, a n d thus t h e intrinsic F C P resistance is t h e s a m e for all t h e m i c r o s t r u c t u r e s . T h e bull's e y e still exhibits h i g h e r F C P rates in the high A K r e g i m e ; this can b e a t t r i b u t e d to a static fracture m e c h a n i s m , b e c a u s e s o m e c l e a v a g e facets are seen on the f r a c t u r e surfaces, as shown in Figure 4, T h e r e l a t i o n s h i p s b e t w e e n da/dN a n d A K o b t a i n e d u n d e r c o n s t a n t Kmax a r e p r e s e n t e d in Figure 5. I n c o n s t a n t Kn~ax tests, stress r a t i o increases with d e c r e a s ing AK, a n d thus A K = AK~ff at A K < 10 M P a m i in all the m i c r o s t r u c t u r e s (i.e. c r a c k c l o s u r e - - f r e e b e h a v i o u r ) . T h e da/dN versus A K (AKeff) r e l a t i o n s h i p s are the s a m e i n d e p e n d e n t of m i c r o s t r u c t u r e a n d also coincide with t h o s e o b t a i n e d at R = 0.05. S E M images of t h e fracture surface in t h e n e a r -
Load ratio 0.1 0.05 0.05 0.05 0.05 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0,8 0,8 0.8 0.1 0.1 0.1 0.1 0.8 0.8 0.8 0.8 0.1 0.1 0.5 0.1 0.1 0.1 0.05 0.1 0.1 0.1 0.5 0.5 0.5 0.05 0.05 0.05 0.05
Matrix microstructure b
AKth
AK=n.~h
1
F
2
10
F+ B F + B F + B F + B F + B F + B F + B F+ B F + P B F + B F + P P F F + P P F F + P F+ M F + S F + B F + P F + M F + S F + B F + P F+ P F + P F F + B B F
11
B
9.0 10.0 12.7 13.2 14.7 18.8 16.3 15.5 12.3 10.2 4.9 14.8 11,8 8.5 6.5 4.0 3.2 4.1 10.0 10.3 12.8 14.5 4.4 4.2 3.9 4.0 12.1 10.3 5.6 13.0 9.0 5.0 12.4 8.1 3.0 11.5 5.4 3.0 8.8 11.7 7.8 10.3 8.3
5.3 ----5.9 4.6 4.5 3.5 3.7 3.5 3.3 2.7 3.5 2.5 4.0 3.2 4.1 3.3 3.8 2.6 3.6 4.4 4.2 3.9 4.0 2.8 3.9 3.2 -6.0 3.1 3.2 4.4 3.0 6.7 4.4 3.0 6.7 3.7 3.6 3.9 3.9
Reference
3
4
5
6 8 9
Present results
M F B M F F P F+ P B
(800 °C) (815 °C) (830 °C) (840 °C)
FCD50 FCD50 FCD50 FCD50 FCD50 FCD50 FCD60 FCD60 FCD60 FCD60 FCD60 FCD60 FCD60 FCD60 FCD50 FCD60 FCD50
aNumbers correspond to Figures 7 and 8
bF, ferrite; B, bainite; P, pearlite; M, raartensite; S, sorbite
Fatigue, 1994, Vol 16, July 347
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al.
10
-t...) 10
threshold regime at R = 0.05 are shown in Figure 6. Some intergranular facets of ferrite grains are observed in the ferrite and the bull's eye, but they do not significantly affect the FCP behaviour, as can be seen in Figure 3. In contrast, the pearlite and the ausferrite reveal a predominantly transgranular mode.
-3
-4
p..,,
REVIEW OF PUBLISHED DATA
FCP characteristics
-5
z10
-6
=10 o
~
-7
e]0
10
-8
1
10
Stress intensity factor range
100 AK MPaV-m
Figure 7 Relationship between crack propagation rate and stress intensity factor in SGIs with a wide variety of microstructures (Table 4). Open symbols indicate the present results
10
_t..) 10
-3
-4
p.,,
zlO
-5
Thresholds
o
-=_10
-6
INI
10 10
-7
-8
I
10
Effective stress intensity factor range
100 AKeff MPad'~
Figure 8 Relationship between crack propagation rate and effective stress intensity factor in SGIs with a wide variety of microstructures (Table 4). Open symbols indicate the present results
348
There have been many published results on the FCP behaviour of SGIs. For an overall understanding of the effect of microstructure, it is useful to review the literature here. Literature published in Japan was selected for this purpose and is listed in Table 41-11. The da/dN-AK relationships in SGIs with a wide variety of microstructures are shown in Figure 7: the numbers in the figure correspond to the data in Table 4. As can be seen from the figure, the martensite (11-2) and the bainites (4-2, 9-2) clearly show higher FCP rates over the whole AK regime, but the da/ dN-AK relationships for the other microstructures tend to fall in a relatively narrow scatter band, which also includes the present data (open symbols). A careful examination of Figure 7 indicates that the dual-phase microstructures such as ferrite-pearlite, ferrite-bainite and ferrite-martensite have an excellent FCP resistance compared with the single-phase microstructures, and that in the single-phase microstructures the FCP rates tend to increase with increasing strength. The latter trend is consistent with the present results described in the previous section. Figure 8 shows the FCP data plotted in terms of AKeff: AK = AK~fe at R = 0.50 and 0.80, because no crack closure may occur. Although the martensites still exhibit higher FCP rates, the scatter of the FCP curves is much narrower compared with Figure 7. This suggests that the intrinsic FCP resistance is almost insensitive to microstructure. In the near-threshold regime, some dual-phase microstructures tend to show excellent FCP resistance. This may be attributed to the reduction in crack-driving force resulting from marked crack deflections, i.e. fracture surface roughness.
Fatigue, 1994, Vol 16, July
The relationship between the threshold values Agth and AKen,th and microstructure is presented in Figure 9. The dual-phase microstructures show significantly higher Agth values: greater than 9 MPa mt. In the single-phase microstructures, however, most of the ferrites indicate higher Agth values, comparable to the dual-phase microstructures, but the bainites and martensites exhibit considerably lower values: less than 5 MPa mL The threshold values are also shown in Figure 10 as a function of tensile strength, era. There is a good correlation between AKth and era; the AKth values decrease with increasing era. This trend is quite similar to that in Figure 9, because the order of the microstructures in Figure 8 (ferrite --> pearlite --> bainite ---> martensite) corresponds to the increase in strength. As can be seen from Figures 9 and 10, the Ageff,th values are 3.5-4.0 MPa m t independent of microstructure or era. This implies that the above A g t h
Fatigue
crack propagation
l?l
F-B F-P F-M
::
F-E&bite(S)
Open symbol: AKtb Solid symbol: AKeff,tb
i-
8
0
0
0
0
I
I
I
I
I
Figure 9 Threshold values as a function of microstructure
25L
I
0 A
Ferrite
Pearlite Bainite Martensite q Dual-phZlX cl Opensymbol: AKth 0
Solid symbol
8
oo* l*
63
1500
Figure 10 Threshold values as a function of tensile strengh
dependence on microstructure to crack closure.
is primarily attributed
DISCUSSION Based on the present results and the review of published data, the macroscopic FCP mechanisms and resistance resulting from the change in microstructure of SGIs can be rationalized as shown in Figure II.
Single phase matrix Martensile
Bainite(B)
PearlIts
Microstructural
FerriIe(F)
i :
Dual-phase F-P
F-B
matrix
F-SISorbile)
unit size :+
StrenQlh .
: Fraclura
small4
low ‘*lpe)
surfacs roughness
; :
Crack closure
small* Apparent
low .
crack growth rasislance (da/dN-AK)
Similar magnitude
Depend an the combination the microstructure
ACKNOWLEDGEMENT The authors wish to thank Mr Y. Ohuchi for assistance with the experiments.
of
.
large
warge ; l high
I
FCP behaviour and crack closure were studied in four spheroidal graphite cast irons (SGIs) with different microstructures, and the published FCP data for SGIs with a wide variety of microstructures were reviewed. Based on these results, the effect of microstructure on the FCP behaviour was discussed. In the single-phase microstructures, the finer the microstructure is (and hence the higher the strength is) the lower the apparent FCP resistance (expressed in terms of AK) become. However, the intrinsic FCP resistance (after allowing for crack closure) is the same in all the microstructures. The dual-phase microstructures show excellent FCP resistance because of their enhanced crack closure, but the intrinsic FCP resistance is also insensitive to microstructure and is the same as that of the single-phase microstructures. Therefore, it is concluded that the intrinsic FCP resistance in SGIs is independent of the change in a wide variety of microstructures.
F-M
small4 high
In the single-phase microstructures, in general, the microstructural unit such as grain size would become finer with the order indicated in the figure. As is well known, the finer the microstructure is, the higher the strength becomes. In FCP behaviour, however, coarser microstructures would result in more remarkable deflections in their crack path, which lead to a rougher fracture surface, inducing enhanced crack closure. As a consequence, such microstructures indicate an excellent FCP resistance when the data are plotted in terms of AK. However, the intrinsic FCP resistance is insensitive to microstructure because the change in microstructure can alter only crack closure level, or crack-driving force. In the dual-phase microstructures, however, the crack path would be much more tortuous because the two phases have different metallurgical and mechanical properties, which induces enhanced crack closure and thus leads to higher FCP resistance than in the single-phase microstructures. As crack closure is also responsible for the excellent FCP resistance in this case, the intrinsic FCP resistance becomes independent of microstructure and is the same as that of the single-phase microstructures. As a conclusion, therefore, the intrinsic FCP resistance in SGIs does not depend on the change in a wide variety of microstructures.
AK&f@
1000 (JB MPa fi
500
Tensile strength
K. Tokaji et al.
0
l
0
cast irons:
CONCLUSIONS
8
otl’,,,,,‘,‘l’,‘,,,‘,“‘,,“,,‘l
:
in spheroidal-graphite
(orhigh) +
of essential crack growth resistance (da/dN- AKw)
Figure 11 Mechanism of fatigue crack propagation in SGIs
REFERENCES Kato, Y., Hirose, M. and Suzuki, S. Truns. Jpn Sm. Mech. Eng. 1985, 51, 1161 (in Japanese) Suzuki, H., Ueki, T. and Kobayashi, T. Tram. Jpn Sot. Mech. Eng. 1991, 57, 1029 (in Japanese) Sugiyama, Y., Asami, K. and Matsuoka, S. J. Sot. Marer. Sci. Jpn 1991, 40, 675 (in Japanese) Sugiyama, Y., Asami, K. and Kuroiwa, H. 1. Sot. Mater. Sci. Jpn 1991, 40, 65 (in Japanese) Sugiyama, Y., Asami, K. and Kuroiwa, H. Trans. Jpn Sot. Med. Eng. 1990, 56, 482 (in Japanese) Ito, S., Sugiyama, Y., Asami, K. and Yamada, S. J. Sot. Muter. Sci. Jpn 1987, 36, 369 (in Japanese)
Fatigue, 1994, Vol 16, July
349
Fatigue crack propagation in spheroidal-graphite cast irons: K. Tokaji et al. 7 8 9 10
350
Sugiyama,Y., Asami, K., Ito, S. and Yamada, S. J. Soc. Mater. Sci. Jpn 1988, 37, 776 (in Japanese) Takeuchi, S. and Yoshimura, H. Imono 1989, 61, 246 (in Japanese) Yamamoto, H., Yamada, S. and Kobayashi, T. lmono 1990, 62, 883 (in Japanese) Ogawa,T. and Kobayashi, H. Fatigue Fract. Eng. Mater. Struct. 1989, 10, 273
Fatigue, 1994, Vol 16, July
11 12
Hirose, Y. and Yajima, Z. In Preprints of 199th Meeting of Fatigue Committee Soc. Mater. Sci. Jpn, 1989, pp.1-6 (in Japanese) 'Annual Book of ASTM Standards', E647-88, American Society for Testing and Materials, 1988, p 636