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Significance of fracture facet size in cleavage fracture process of welded joints
Materials" Science and Engineering, A176 (1994) 385-391
385
Significance of fracture facet size in cleavage fracture process of welded joints Tadashi Ishikawa and Toshiaki Haze Oita R&D Laboratory, Nippon Steel Corporation, 1 Nishinosu, Oita 870 (Japan) (Received May 3, 1993)
Abstract The cleavage fracture process was investigated in terms of both the fracture toughness and microstructure, especially focusing on the roles of the brittle second-phase panicles (as crack initiation sites) and matrix microstructures. Using the microstructure, combining various sizes of ferrite side-plate (FSP) which correspond to various fracture facet sizes, and non-metallic inclusions as easily identified crack initiators of unstable cleavage fracture, the following aspects were revealed. Cleavage fracture initiation can be dominated both by the size of crack initiators (such as non-metallic inclusions) and by the fracture facet size (which is determined by the coarser phase of the microstructure, such as FSPs), since the fracture facet size dominates the critical condition for the propagation of microcracks. The microstructure for a finer fracture facet size can prevent unstable cleavage fracture, even if relatively large crack initiators exist. Therefore, crack arrestability is the principal factor in the process of cleavage fracture to failure.
1. Introduction
The mechanism of cleavage fracture has been widely studied. The initiation of cleavage fracture on the microscopic scale can be described by Griffith's theory and micromechanics [1, 2]. Meanwhile, the continuum theory approach, such as the K , concept or crack tip opening displacement (CTOD) concept, has been widely used for the evaluation of fracture toughness and the safety design of structures. Furthermore, to clarify the effect of the microstructures on the fracture toughness, the relationships between fracture toughness and microscopic fracture stress have been investigated, since the microscopic fracture stress can be linked to microstructural parameters through Griffith's formula ]3, 4]. Ritchie et al. [3] first proposed the concept of characteristic distance (where the local stress level exceeds the microscopic fracture stress) to interpret the relationships between fracture toughness and microscopic fracture stress [3]. Subsequently, statistical models suggested that the characteristic distance could be regarded as a measure of the sampling volume of the crack initiation site [5-7]. However, the physical meaning of characteristic distance has not been clarified yet, since the continuum view in the statistical model cannot provide any further physical insight into the fracture process in steels and welded joints with complex microstructures. 0921-5093/94/$7.00 ,%~1710921-5093(93)02542-B
Cleavage fracture can be generated by microcracks originated from cracked inclusions, and can propagate macroscopically to unstable failure. Clarification of the mechanism in such a process of cleavage fracture can provide the metallurgical basis of fracture toughness used for the evaluation of a material. Figure 1 is a schematic diagram of the stress distribution ahead of a crack. An increase in K value causes the characteristic distance X0 to increase. Assuming that cleavage fracture occurs at an inclusion where the local stress exceeds the microscopic fracture stress, the larger characteristic distance causes a flatter stress gradient at the crack initiation point. Since the stress gradient affects the crack propagation behaviour in Griffith's crack theory, the characteristic distance can be translated relative to the stress distribution for crack propagation. It is expected that cleavage fracture should be generated at a larger cracked inclusion, even if the applied K value is smaller than the fracture toughness. However, the critical condition of macroscopic propagation can be dominated both by the microstructure areound the crack initiation site and by the stress gradient ahead of the crack initiation site. Therefore, this study focuses especially on the propagation process from microcracking to macroscopic failure, using microstructures with various sizes of ferrite side-plate (FSP), which correspond to various fracture facet sizes, and non-metallic inclusions as © 1994 - Elsevier Sequoia. All rights reserved
386
T. lshikawa, 7". Haze
/
Fracture facets in cleavage fracture
easily identified crack initiators of unstable cleavage fracture.
tion provided identifiable crack initiation sites surrounded by various fracture facet sizes. 2.2. Mechanical testing To simulate the actual fracture behaviour in the welded joints, large-scale tensile testing was performed. Because of the large distance between the loading points, the loading system can be regarded as being similar to that in actual structures subjected to a uniform tensile stress field. Dynamic measurements of the stress changes during the test confirmed that the applied stress can be maintained during the propagation of a crack through a specimen. Figure 2(a) shows a specimen for large-scale tensile tests. The discontinuities in the load vs. displacement curves indicate crack initiation during the test. The fracture toughness K c was calculated using the gross fracture stress a at crack initiation and the initial notch length 2 a, according to
2. Experimental details 2.1. Materials
The microstructures were designed to enable the roles of the brittle second-phase particles (as crack initiation sites) and the matrix to be clarified independently. Three nominally identical welded joints A, B and C were produced by one-side, submerged-arc welding, using three charges of steel with slightly different chemical compositions. The chemical compositions and the mechanical properties of the base plate are given in Table 1. The one-side, submerged-arc welding process with a heat input of about 15 kJ mm- 1 formed a grain-coarsened, heat-affected zone (GCHAZ) through the thickness of the weldment. Different chemical compositions provide various fracture facet sizes in the G C H A Z of the welded joint. Since inclusions possibly exist in centre-line segregation, the combination of the C G H A Z and centre-line segrega-
f ...................
t
K c = a(:ra)l/2{2 W/:ra tan(:ra/2 W )}1/2
where W is the width of a specimen. To analyse the fracture behaviour in the large-scale tensile tests using microscopic fracture mechanics, the microscopic fracture stress was evaluated by the method of Griffith and Owen [8], using blunt-notchedtype, slow-notched bend testing (as shown in Fig. 2(b)). Furthermore, to evaluate the fracture arrest toughness in the welded joints, compact crack arrest (CCA) testing was carried out, using the specimen shown in Fig. 2(c).
.........,
,.
* i
.............................
I
i
,
............ / 2.3. Microscopic observation
Laroe X 0
Small X 0
(1)
Scanning electron microscopy (SEM) observation with energy-dispersive X-ray analysis (EDXA) reveals the initiation site of cleavage fracture in the fractured specimen. In some cases, the fracture surfaces were slightly etched by Nital to observe the microstructure corresponding to the crack initiation site. In other cases, the fractured specimen was sectioned at the initiation site, confirming the location of the sectioning
Xo
Xo
Fig. 1. Schematic diagram of stress distribution ahead of a crack with stress intensity factor K.
TABLE 1. Chemical composition and mechanical properties of base plates Plate
A B C
Thickness (ram) 25 25 25
aYield stress. bTensile stress. CEIongation.
ypa (MPa)
Chemical composition (wt.oYo) C
Si
Mn
P
S
Ti
0.09 0.12 0.10
0.31 0.21 0.19
1.52 1.41 1.48
0.009 0.006 0.008
0.001 0.002 0.003
0.007 0.007 0.013
461 479 460
(MPa)
El c (%)
553 573 550
25 27 23
TS
b
T. lshikawa, T. tta=e
: ;-'~31.7 :40
~!= ":"
31.7~" 40
/
Fracture fi~cets in cleavage fracture
387
: -;
(b) Slow-notched band s~ecimen
Fig. 3. Example of macrostructure of welded joint. ~
O. Imn~
25 (I) Centre-notched lido plntm t i l t
°pecimmn
(¢) CCA t i l t
°plCillOn
Fig. 2. Mechanical test specimens: (a) centre-notched wide plate specimen: (b) slow-notched, bend test specimen: (c) CCA specimen.
by SEM observation, in order to clarify the microstructure at or around the initiation site. To evaluate the size of the cleavage fracture facets, SEM stereo observation was carried out, using a pair of SEM images tilted at 0 ° and at 3 ° through a stereoviewer. The SEM images through the stereoviewer enabled observation of the angles formed by adjacent facets, as well as identification of the tear ridge which surrounded the facet colonies. We define the effective facet here as the region which was surrounded by the tear ridges. The diameter of the effective facet unit was evaluated using the image analyser and the fractographs through the stereoviewer.
3. Results and discuss,on 3.1. Microstructure in the local brittle zone Figure 3 shows an example of welded joints. A G C H A Z is observed along the fusion line through the thickness in the joint. Figure 4 shows the microstructures of the GCHAZ. The microstructure of joint A consists of a mainly FSP microstructure. Massive ferrite and cementite regions are observed in joint B. A FSP microstructure is not observed in joint C. 3.2. Microscopic fracture stress and microstructure Cleavage fracture originated from an inclusion or second-phase particle, such as island martensite, which was easily identified from SEM observation. Figure 5 shows an example of fracture surfaces, where an inclusion was found at the origin of the cleavage crack initiation site in the fractured slow-notched, bend test specimen. Figure 6 shows the values of the microscopic fracture stress obtained by slow-notched, bend testing, which are plotted against the diameter of the initiation sites identified on the fracture surfaces by SEM.
Fig. 4. Microstructures in G C H A Z of welded joints: (a) joint A: {b)joint B: (c)joint C.
Although the facet sizes near the crack initiation site in these joints vary from 50 to 500 /~m, the estimated effective surface energy (yp) in these joints is approximately 14 J m -2 which is the same value as that
388
T. lshikawa, T. Haze
/
Fracture facets in cleavage fracture
(
Fig. 5. Example of crack initiation site observed in slow-notched, bend specimen in tractured condition.
1,500
b~
0¢ O////// //~ /.////
o.~
l, 000
/
Tp =14J/m~
N
•.,
9J/m2
500
/,z /////
• •
A
ot size 400-'-490(pm) i
B
150"-.250Cue)
~"
•
C
/,./
o
60"40
Cu,)
0 D, 5
1/
,/d-
(pro-o. 5)
Fig. 6. Relationship between microscopic fracture stress and size of the initiation site in fractured, slow-notched, bend specimens with various fracture facet sizes.
obtained by Curry and Knott for simple ferrite steel [9]. Therefore, it is confirmed that the microscopic fracture stress obtained by blunt-notched-type specimens is dominated mainly by the size of the crack initiation site. 3.3. Fracture toughness K c a n d microstructure Figure 7 shows an example of the fractured surface of a large-scale test specimen. T h e crack initiated from the cracked inclus!,on and propagated through the
Fig. 7. Example of (a) fracture initiation site observed on (b) coarse fracture facet in large-scale test specimen.
7. lshikawa, T. Haze
/
Fracturefiwets in cleavagefracture
specimen. Figure 8 shows the relationships between the Kc values at the first fracture in the large-scale tensile tests and the longitudinal diameters of the initiation sites (such as inclusions), which were clearly
~150 =-100
e\~
I Steel B
==:::
Steel A
0 3 5 10 20 Diameter of Initiation Site (p.m) Fig. 8. Relationships between fracture toughness Kc and size of crack initiation site in fractured large-scale tensile test specimen for joints A, B and C. Facet size range: A, A, 40(I-490/~m; e, B, 150-250 #m; m, C, 50-80 #m.
Fig. 9. Comparison between (a) coarse fracture facet and (b) fine fracture facet observed in large-scale tensile test specimen.
389
observed by SEM. This figure clearly suggests that the larger inclusion size caused the lower K c value. Although K,. is strongly affected by the size of the initiation site, the relationship between K c and the size of the crack initiation site depends on the individual joints. Figure 9 shows an example of the difference in fracture facet size. Therefore, the fracture toughness is also strongly affected by the fracture facet size, which was determined by the microstructure of the matrix around the crack initiation sites. 3. 4. Arrested microcrack initiated at inclusion No pop-ins were detected in the large-scale tensile test for joint C, although inclusions should have existed as crack initiation sites. However, some short arrested cracks of about 100 # m in diameter were observed on the fracture surface of the specimen, as shown in Fig. 10. This evidence suggests that microscopically initiated cleavage fracture cannot propagate in the matrix with finer fracture facets. 3.5. Relationship between crack arrest toughness and microstructure The pop-in phenomenon is caused by the difference between the fracture toughness in the local brittle zone and that in the surrounding region. The fracture arrest
'Fig. 10. SEM fractography images of short arrested crack observed in fine-grained, heat-affected zone of joint C. Cleavage crack was initiated from the inclusion but arrested: (a) arrested microcrack: (b) detail of (a).
390
T. lshikawa, T. Haze L. S, T. GO-THR POP-IN
Joint A
•
~
B
•
0
/
Fracture facets in cleavage fracture
Kcm bY CCA ~,
150
Z
,t
Range of Kca
50
-
,t
2
0
lO0
I
I
I
200
300
400
500
Facet Size (Urn)
Fig. 11. Possible range of fracture arrest toughness, predicted by pop-in behaviours in large-scale tensile test, together with K~,, values obtained by CCA test.
toughness is larger than the K c value at the initiation of the pop-in when a crack arrests, because the applied stress is maintained during the pop-in phenomenon. In contrast, the fracture arrest toughness is less than the K,, value at the fracture initiation when pop-in is not observed. Figure 11 shows the possible range of the fracture arrest toughness, as predicted by the pop-in phenomenon in the large-scale tensile tests, as a function of the equivalent diameter of the effective facet surrounded by the tear ridge. Also in Fig. 11 are plotted the Kc~ values of the fracture arrest toughness obtained by C C A testing. The Kc, values obtained by the CCA test correspond to the range of the fracture arrest toughness estimated from the popqn,:phenomenon. As seen in Fig. 11, the microstructure with finer effective grains has a larger fracture arrest toughness (short crack arrestability). Microscopic observation revealed that coarser fracture facets were observed in a lath-type microstructures as FSPs, while finer fracture
Fig. 12. Secondary microcracks observed near initiation site for (a) joint A and (b) joint B.
7~ lshikawa, f. Haze
Initiation
%,,
I
=-
Propagation
/
Fracture fiwets in cleavage ~acture
Fai lure
[
1
i
Micro-LGriffith Mochani,~s theory
II POP-IN
~:i:C:Macr0sc0p~ == ======================= =
K concept
1
Fig. 13. Schematic diagram of mechanisms in cleavage fracture process.
facets were observed in acicular ferrites, as shown in Fig. 12. It also reveals that joint B has arrested microcracks more frequently than joint A. To improve the short crack arrestability, it seems to be beneficial to decrease the portion of FSPs in the microstructure of the welded joint. Therefore, the crack arrestability is the principal factor in the process of cleavage fracture to failure, as shown schematically in Fig. 1 3.
4.
Conclusions
The cleavage fracture process was investigated in terms of both the fracture toughness and microstructure, especially focusing on the roles of the brittle second-phase particle (as crack initiation sites) and matrix microstructures. Using the microstructure combining various sizes of FSP which correspond to
391
various fracture facet sizes, and non-metallic inclusions as easily identified crack initiators of unstable cleavage fracture, the following aspects are revealed. ( 1) Cleavage fracture initiation can be dominated not only by the size of crack initiators, such as non-metallic inclusions, but also by the fracture facet size, which is determined by the coarser phase of the microstructure, such as FSPs. (2) The fracture facet size determines whether microcracks generated can be propagated as unstable fracture or can be arrested. The microstructure related to a finer fracture facet size can prevent unstable cleavage fracture, even if relatively large crack initiator exists. (3) Crack arrestability is the principal factor in the process of cleavage fracture to failure.
References 1 A. A. Griffith, Philos. Trans. R. Soc. London, Ser. A, 221 (1921)496. 2 D.A. Curry and J. F. Knott, Met. Sci., 12 (1978) 511. 3 R. O. Ritchie, J. F. Knott and J. R. Rice, J. Mech. l'hvs. Solids, 21 (1973)395. 4 J. P. Naylar and E R. Krahe, Metall. Trans. A, 5 (1974) 1699. 5 D.A. Curry and J. F. Knott, Met. Sci., 13 (1979) 341. 6 A. Fontaine, E. Maas and J. Tulou, IRSII) Rep. REI25& March 1986. 7 K. Wallin, T. Saario and K. Torronen. Met. Sci., 18 (1984) 13. 8 J. R. Griffith and D. R. J. Owen, J. Mech. Phys. Solids, 21 (1973)395. 9 D.A. Curry and J. E Knott, Met. Sci., 10 (1976) 1.
385
Significance of fracture facet size in cleavage fracture process of welded joints Tadashi Ishikawa and Toshiaki Haze Oita R&D Laboratory, Nippon Steel Corporation, 1 Nishinosu, Oita 870 (Japan) (Received May 3, 1993)
Abstract The cleavage fracture process was investigated in terms of both the fracture toughness and microstructure, especially focusing on the roles of the brittle second-phase panicles (as crack initiation sites) and matrix microstructures. Using the microstructure, combining various sizes of ferrite side-plate (FSP) which correspond to various fracture facet sizes, and non-metallic inclusions as easily identified crack initiators of unstable cleavage fracture, the following aspects were revealed. Cleavage fracture initiation can be dominated both by the size of crack initiators (such as non-metallic inclusions) and by the fracture facet size (which is determined by the coarser phase of the microstructure, such as FSPs), since the fracture facet size dominates the critical condition for the propagation of microcracks. The microstructure for a finer fracture facet size can prevent unstable cleavage fracture, even if relatively large crack initiators exist. Therefore, crack arrestability is the principal factor in the process of cleavage fracture to failure.
1. Introduction
The mechanism of cleavage fracture has been widely studied. The initiation of cleavage fracture on the microscopic scale can be described by Griffith's theory and micromechanics [1, 2]. Meanwhile, the continuum theory approach, such as the K , concept or crack tip opening displacement (CTOD) concept, has been widely used for the evaluation of fracture toughness and the safety design of structures. Furthermore, to clarify the effect of the microstructures on the fracture toughness, the relationships between fracture toughness and microscopic fracture stress have been investigated, since the microscopic fracture stress can be linked to microstructural parameters through Griffith's formula ]3, 4]. Ritchie et al. [3] first proposed the concept of characteristic distance (where the local stress level exceeds the microscopic fracture stress) to interpret the relationships between fracture toughness and microscopic fracture stress [3]. Subsequently, statistical models suggested that the characteristic distance could be regarded as a measure of the sampling volume of the crack initiation site [5-7]. However, the physical meaning of characteristic distance has not been clarified yet, since the continuum view in the statistical model cannot provide any further physical insight into the fracture process in steels and welded joints with complex microstructures. 0921-5093/94/$7.00 ,%~1710921-5093(93)02542-B
Cleavage fracture can be generated by microcracks originated from cracked inclusions, and can propagate macroscopically to unstable failure. Clarification of the mechanism in such a process of cleavage fracture can provide the metallurgical basis of fracture toughness used for the evaluation of a material. Figure 1 is a schematic diagram of the stress distribution ahead of a crack. An increase in K value causes the characteristic distance X0 to increase. Assuming that cleavage fracture occurs at an inclusion where the local stress exceeds the microscopic fracture stress, the larger characteristic distance causes a flatter stress gradient at the crack initiation point. Since the stress gradient affects the crack propagation behaviour in Griffith's crack theory, the characteristic distance can be translated relative to the stress distribution for crack propagation. It is expected that cleavage fracture should be generated at a larger cracked inclusion, even if the applied K value is smaller than the fracture toughness. However, the critical condition of macroscopic propagation can be dominated both by the microstructure areound the crack initiation site and by the stress gradient ahead of the crack initiation site. Therefore, this study focuses especially on the propagation process from microcracking to macroscopic failure, using microstructures with various sizes of ferrite side-plate (FSP), which correspond to various fracture facet sizes, and non-metallic inclusions as © 1994 - Elsevier Sequoia. All rights reserved
386
T. lshikawa, 7". Haze
/
Fracture facets in cleavage fracture
easily identified crack initiators of unstable cleavage fracture.
tion provided identifiable crack initiation sites surrounded by various fracture facet sizes. 2.2. Mechanical testing To simulate the actual fracture behaviour in the welded joints, large-scale tensile testing was performed. Because of the large distance between the loading points, the loading system can be regarded as being similar to that in actual structures subjected to a uniform tensile stress field. Dynamic measurements of the stress changes during the test confirmed that the applied stress can be maintained during the propagation of a crack through a specimen. Figure 2(a) shows a specimen for large-scale tensile tests. The discontinuities in the load vs. displacement curves indicate crack initiation during the test. The fracture toughness K c was calculated using the gross fracture stress a at crack initiation and the initial notch length 2 a, according to
2. Experimental details 2.1. Materials
The microstructures were designed to enable the roles of the brittle second-phase particles (as crack initiation sites) and the matrix to be clarified independently. Three nominally identical welded joints A, B and C were produced by one-side, submerged-arc welding, using three charges of steel with slightly different chemical compositions. The chemical compositions and the mechanical properties of the base plate are given in Table 1. The one-side, submerged-arc welding process with a heat input of about 15 kJ mm- 1 formed a grain-coarsened, heat-affected zone (GCHAZ) through the thickness of the weldment. Different chemical compositions provide various fracture facet sizes in the G C H A Z of the welded joint. Since inclusions possibly exist in centre-line segregation, the combination of the C G H A Z and centre-line segrega-
f ...................
t
K c = a(:ra)l/2{2 W/:ra tan(:ra/2 W )}1/2
where W is the width of a specimen. To analyse the fracture behaviour in the large-scale tensile tests using microscopic fracture mechanics, the microscopic fracture stress was evaluated by the method of Griffith and Owen [8], using blunt-notchedtype, slow-notched bend testing (as shown in Fig. 2(b)). Furthermore, to evaluate the fracture arrest toughness in the welded joints, compact crack arrest (CCA) testing was carried out, using the specimen shown in Fig. 2(c).
.........,
,.
* i
.............................
I
i
,
............ / 2.3. Microscopic observation
Laroe X 0
Small X 0
(1)
Scanning electron microscopy (SEM) observation with energy-dispersive X-ray analysis (EDXA) reveals the initiation site of cleavage fracture in the fractured specimen. In some cases, the fracture surfaces were slightly etched by Nital to observe the microstructure corresponding to the crack initiation site. In other cases, the fractured specimen was sectioned at the initiation site, confirming the location of the sectioning
Xo
Xo
Fig. 1. Schematic diagram of stress distribution ahead of a crack with stress intensity factor K.
TABLE 1. Chemical composition and mechanical properties of base plates Plate
A B C
Thickness (ram) 25 25 25
aYield stress. bTensile stress. CEIongation.
ypa (MPa)
Chemical composition (wt.oYo) C
Si
Mn
P
S
Ti
0.09 0.12 0.10
0.31 0.21 0.19
1.52 1.41 1.48
0.009 0.006 0.008
0.001 0.002 0.003
0.007 0.007 0.013
461 479 460
(MPa)
El c (%)
553 573 550
25 27 23
TS
b
T. lshikawa, T. tta=e
: ;-'~31.7 :40
~!= ":"
31.7~" 40
/
Fracture fi~cets in cleavage fracture
387
: -;
(b) Slow-notched band s~ecimen
Fig. 3. Example of macrostructure of welded joint. ~
O. Imn~
25 (I) Centre-notched lido plntm t i l t
°pecimmn
(¢) CCA t i l t
°plCillOn
Fig. 2. Mechanical test specimens: (a) centre-notched wide plate specimen: (b) slow-notched, bend test specimen: (c) CCA specimen.
by SEM observation, in order to clarify the microstructure at or around the initiation site. To evaluate the size of the cleavage fracture facets, SEM stereo observation was carried out, using a pair of SEM images tilted at 0 ° and at 3 ° through a stereoviewer. The SEM images through the stereoviewer enabled observation of the angles formed by adjacent facets, as well as identification of the tear ridge which surrounded the facet colonies. We define the effective facet here as the region which was surrounded by the tear ridges. The diameter of the effective facet unit was evaluated using the image analyser and the fractographs through the stereoviewer.
3. Results and discuss,on 3.1. Microstructure in the local brittle zone Figure 3 shows an example of welded joints. A G C H A Z is observed along the fusion line through the thickness in the joint. Figure 4 shows the microstructures of the GCHAZ. The microstructure of joint A consists of a mainly FSP microstructure. Massive ferrite and cementite regions are observed in joint B. A FSP microstructure is not observed in joint C. 3.2. Microscopic fracture stress and microstructure Cleavage fracture originated from an inclusion or second-phase particle, such as island martensite, which was easily identified from SEM observation. Figure 5 shows an example of fracture surfaces, where an inclusion was found at the origin of the cleavage crack initiation site in the fractured slow-notched, bend test specimen. Figure 6 shows the values of the microscopic fracture stress obtained by slow-notched, bend testing, which are plotted against the diameter of the initiation sites identified on the fracture surfaces by SEM.
Fig. 4. Microstructures in G C H A Z of welded joints: (a) joint A: {b)joint B: (c)joint C.
Although the facet sizes near the crack initiation site in these joints vary from 50 to 500 /~m, the estimated effective surface energy (yp) in these joints is approximately 14 J m -2 which is the same value as that
388
T. lshikawa, T. Haze
/
Fracture facets in cleavage fracture
(
Fig. 5. Example of crack initiation site observed in slow-notched, bend specimen in tractured condition.
1,500
b~
0¢ O////// //~ /.////
o.~
l, 000
/
Tp =14J/m~
N
•.,
9J/m2
500
/,z /////
• •
A
ot size 400-'-490(pm) i
B
150"-.250Cue)
~"
•
C
/,./
o
60"40
Cu,)
0 D, 5
1/
,/d-
(pro-o. 5)
Fig. 6. Relationship between microscopic fracture stress and size of the initiation site in fractured, slow-notched, bend specimens with various fracture facet sizes.
obtained by Curry and Knott for simple ferrite steel [9]. Therefore, it is confirmed that the microscopic fracture stress obtained by blunt-notched-type specimens is dominated mainly by the size of the crack initiation site. 3.3. Fracture toughness K c a n d microstructure Figure 7 shows an example of the fractured surface of a large-scale test specimen. T h e crack initiated from the cracked inclus!,on and propagated through the
Fig. 7. Example of (a) fracture initiation site observed on (b) coarse fracture facet in large-scale test specimen.
7. lshikawa, T. Haze
/
Fracturefiwets in cleavagefracture
specimen. Figure 8 shows the relationships between the Kc values at the first fracture in the large-scale tensile tests and the longitudinal diameters of the initiation sites (such as inclusions), which were clearly
~150 =-100
e\~
I Steel B
==:::
Steel A
0 3 5 10 20 Diameter of Initiation Site (p.m) Fig. 8. Relationships between fracture toughness Kc and size of crack initiation site in fractured large-scale tensile test specimen for joints A, B and C. Facet size range: A, A, 40(I-490/~m; e, B, 150-250 #m; m, C, 50-80 #m.
Fig. 9. Comparison between (a) coarse fracture facet and (b) fine fracture facet observed in large-scale tensile test specimen.
389
observed by SEM. This figure clearly suggests that the larger inclusion size caused the lower K c value. Although K,. is strongly affected by the size of the initiation site, the relationship between K c and the size of the crack initiation site depends on the individual joints. Figure 9 shows an example of the difference in fracture facet size. Therefore, the fracture toughness is also strongly affected by the fracture facet size, which was determined by the microstructure of the matrix around the crack initiation sites. 3. 4. Arrested microcrack initiated at inclusion No pop-ins were detected in the large-scale tensile test for joint C, although inclusions should have existed as crack initiation sites. However, some short arrested cracks of about 100 # m in diameter were observed on the fracture surface of the specimen, as shown in Fig. 10. This evidence suggests that microscopically initiated cleavage fracture cannot propagate in the matrix with finer fracture facets. 3.5. Relationship between crack arrest toughness and microstructure The pop-in phenomenon is caused by the difference between the fracture toughness in the local brittle zone and that in the surrounding region. The fracture arrest
'Fig. 10. SEM fractography images of short arrested crack observed in fine-grained, heat-affected zone of joint C. Cleavage crack was initiated from the inclusion but arrested: (a) arrested microcrack: (b) detail of (a).
390
T. lshikawa, T. Haze L. S, T. GO-THR POP-IN
Joint A
•
~
B
•
0
/
Fracture facets in cleavage fracture
Kcm bY CCA ~,
150
Z
,t
Range of Kca
50
-
,t
2
0
lO0
I
I
I
200
300
400
500
Facet Size (Urn)
Fig. 11. Possible range of fracture arrest toughness, predicted by pop-in behaviours in large-scale tensile test, together with K~,, values obtained by CCA test.
toughness is larger than the K c value at the initiation of the pop-in when a crack arrests, because the applied stress is maintained during the pop-in phenomenon. In contrast, the fracture arrest toughness is less than the K,, value at the fracture initiation when pop-in is not observed. Figure 11 shows the possible range of the fracture arrest toughness, as predicted by the pop-in phenomenon in the large-scale tensile tests, as a function of the equivalent diameter of the effective facet surrounded by the tear ridge. Also in Fig. 11 are plotted the Kc~ values of the fracture arrest toughness obtained by C C A testing. The Kc, values obtained by the CCA test correspond to the range of the fracture arrest toughness estimated from the popqn,:phenomenon. As seen in Fig. 11, the microstructure with finer effective grains has a larger fracture arrest toughness (short crack arrestability). Microscopic observation revealed that coarser fracture facets were observed in a lath-type microstructures as FSPs, while finer fracture
Fig. 12. Secondary microcracks observed near initiation site for (a) joint A and (b) joint B.
7~ lshikawa, f. Haze
Initiation
%,,
I
=-
Propagation
/
Fracture fiwets in cleavage ~acture
Fai lure
[
1
i
Micro-LGriffith Mochani,~s theory
II POP-IN
~:i:C:Macr0sc0p~ == ======================= =
K concept
1
Fig. 13. Schematic diagram of mechanisms in cleavage fracture process.
facets were observed in acicular ferrites, as shown in Fig. 12. It also reveals that joint B has arrested microcracks more frequently than joint A. To improve the short crack arrestability, it seems to be beneficial to decrease the portion of FSPs in the microstructure of the welded joint. Therefore, the crack arrestability is the principal factor in the process of cleavage fracture to failure, as shown schematically in Fig. 1 3.
4.
Conclusions
The cleavage fracture process was investigated in terms of both the fracture toughness and microstructure, especially focusing on the roles of the brittle second-phase particle (as crack initiation sites) and matrix microstructures. Using the microstructure combining various sizes of FSP which correspond to
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various fracture facet sizes, and non-metallic inclusions as easily identified crack initiators of unstable cleavage fracture, the following aspects are revealed. ( 1) Cleavage fracture initiation can be dominated not only by the size of crack initiators, such as non-metallic inclusions, but also by the fracture facet size, which is determined by the coarser phase of the microstructure, such as FSPs. (2) The fracture facet size determines whether microcracks generated can be propagated as unstable fracture or can be arrested. The microstructure related to a finer fracture facet size can prevent unstable cleavage fracture, even if relatively large crack initiator exists. (3) Crack arrestability is the principal factor in the process of cleavage fracture to failure.
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