3-14

Mechanical Discussion on 950MPa Class Steel Plate Welded Joint for Penstock

Report 2: Effect of Welding Conditions

on Brittle Fracture Behavior and Fracture Assessment

T. Kawabata1, H. Sakaibori1, K. Onishi1 and M. Mochizuki2 1 Sumitomo Metal Industries, Ltd.

2 Osaka University

Abstract

Recently, high strength steel plate such as 950MPa class strength is focused as material of hydropower plant due to its effect on cost reduction and it has been already used to penstocks in Kanna-gawa and Omaru-gawa hydropower plants in Japan. However, it comes to issue that welding restrictions for preventing welding cracks and keeping strength are much severer than ordinary strength steel, for example, preheat condition or inter-pass-temperature range. If under-matching (lower stress in weld metal than base plate) is applied, it is possible that high strength steel plate becomes easier to use because restrictions in welding process may decrease by application of softer welding consumables. By applying softer welding, it is assumed that mechanical performances of the structure changes due to existence of heterogeneous region. However, quantitative effect of application of under-matching for high strength steel on deformation behavior and fracture assessment is not certain. In this report, in order to evaluate the effect of under-matching design on brittle fracture behavior, standard CTOD tests and wide plate tests with through-thickness notch by the welded joint using actual combination of 950MPa steel plate for penstock and envisioned welding consumables (950-570MPa class), were performed. As a result, welded joint with softer welding consumables showed higher resistance to brittle fracture. Furthermore, taking into consideration of allowable stress, sufficient load capacity was observed even in case of application of 570MPa class welding consumables.

Introduction

In recent years, hydropower plants have become increasingly large scales [1]. By this reason, higher strength steels have

been used (Figure 1). Since the advent of the 21st century, 950MPa class steel plates (HT950) to which the TMCP process is applied have been used in Japan [2]-[6].

0

1000

2000

3000

4000

5000

6000

1960 1970 1980 1990 2000 2010

HxD

(m x

m)

HT780HT680

490N/mm2

390N/mm2

HT950

HT590

Year of completion FIGURE 1: TREND OF EXPANSION IN SCALE OF PENSTOCK

However, the welding procedure for HT950 requires high level of skills of welding that is strictly controlled welding parameter (i.e. preheating, post heating and inter-pass temperature) for balancing avoidance of cold and hot cracking and maintenance of high strength. So the fabricator who can handle this steel plate and welding is limited. For further growth of application of HT950, technological development, which can reduce complication of welding, is desired. In order to solve this problem, a lot of trials of development of high strength welding material without preheating have been performed [7]. However, commercially successful result has not been generated yet.

On the other hand, mechanical investigation on allowance of application of softer welding material has been also performed [8]-[11]. From these results, the existence of soft area does not necessarily bring down the degradation of

Conference on High Strength Steels for Hydropower Plants - Takasaki

14 - 1

strength of structure. However, HT950 have not picked up yet.

In this series of investigation, HT950 steel plate with various strength of welding metal manufactured by using the realistic welding materials are tested in various aspect of fitness of penstock field. Finally critical assessment of applicable soft weld metal will be viewed.

In report1, change of welding deformation and strength on various strength of weld metal is investigated. In report2, effect of softer weld metal on brittle fracture toughness is investigated.

Experiment

Welded joint tested 26mm thick HT950 steel plate developed for penstock is used. The plate is manufactured by TMCP process. Microstructure of base plate is shown in figure 2. Flattened prior austenite grain boundary formed in rolling during unrecrystallized region is characteristic. Inner part of prior austenite grain boundary is quite fine and it consists mostly of tempered martensite. Tensile test results and Charpy impact test results are shown in Tables 1 and 2 respectively. Sufficient tensile property as 950MPa class steel and superior impact property taking consideration in design temperature 0 deg. C. are obtained. 4 welded joints are prepared by multi-pass welding as shown in Table 3 where welding condition is decided to control weld metal strength in envisioned tensile strength range. Mark1 is “matching” weldment and the same as current combination in actual hydropower plant. Mark2~3 is “undermatching” weldment with 780MPa and 570MPa class welding material respectively. Mark4 is the weldment in which 570MPa class welding material and narrow edge preparation (Figure 3) is used. Preheating and controlling inter-pass temperature is easier as the strength of welding material is lower as shown in Table 3. Table 4 is chemical compositions of base plate and weld metal of each weldment. Hardenability of mark 4 is higher than that of mark 3. This is due to dilution of C,Ni,Cr,Mo from base plate attributed to narrow groove. Macrostructure of each weldment is shown in Figure 4.

1/4t 1/2t FIGURE 2: MICROSTRUCTURE OF BASE PLATE

TABLE 1: TENSILE TEST RESULTS OF BASE PLATE

Specimen: JIS No.4 (φ14, G.L.=50mm) Direction 0.2%YS

(MPa)TS

(MPa) YR (%)

EL (%)

RA (%)

965 984 98.1 24.1 69.0L 970 988 98.2 24.3 65.71031 1032 99.9 20.9 62.3T 1031 1032 99.9 22.7 66.6

Specification[12] ≧885 950-1130 - ≧12 - * Sampling position; 1/2t

TABLE 2: CHARPY IMPACT TEST RESULTS OF BASE PLATE

Absorbed energy (J)

Brittle area (%) Dir. Temp.

(deg.C) Ind. Av. Ind. Av.

vTrs (deg.C)

280 0 270 0 L -55 291

280 0

0 -117

249 0 238 0 C -55 234

240 0

0 -108

Specification[12] -55 - ≧47 - - ≦-55 * Sampling position; 1/4t

TABLE 3: WELDING PARAMETER

Mark Edge preparation

Shielded gas

Welding consumable

preheat(deg.C)

Inter-pass temp.[aim](deg.C)

Current(A)

Voltage (V)

Speed (mm/min.)

Heat input(kJ/mm)

1 950MPa class 125 100~125[125] 2 780MPa class 100 100~220[175] 3

#1

4 #2

Ar80% -CO2 570MPa class N/A ~250[200]

270 31.0 310 1.62

TABLE 4: CHEMICAL COMPOSITIONS OF MATERIAL USED (RESULT OF CHECK ANALYSIS) (MASS%)

C Si Mn P S Ni others Ceqw Pcm Base plate 0.10 0.21 0.96 0.005 <0.001 1.49 Cu, Cr, Mo, V, Nb, B 0.55 0.26

1 0.08 0.29 1.20 0.004 0.002 2.52 Cu, Cr, Mo, V 0.65 0.28 2 0.07 0.39 1.10 0.010 0.001 2.41 Cu, Cr, Mo, V 0.51 0.24 3 0.06 0.35 1.40 0.011 0.005 0.27 Cu, Cr, Mo, V 0.43 0.19

Weld metal

4 0.07 0.34 1.30 0.010 0.003 0.52 Cu, Cr, Mo, V 0.47 0.21


14 - 2

26

55°

60°

14

3

9

26

16

13

7

3R 3

7

3

#1 #2

FIGURE 3: EDGE PREPARATIONS

Mark:1

Mark:2

Mark:3

Mark:4

FIGURE 4: MACROSTRUCTURE OF WELDED JOINT

Strength distribution of weldments Generally, welded joint includes large amount of heterogeneity in strength. In this study, hold of strength distribution is quite important for mechanical discussion of the effect of intentional change of strength of weld metal. Table 5 shows tensile test results of weldmetal. As lower the welding material used, the more 0.2%YS and TS decreases. The difference between strength of mark 4 and that of mark 3 has roots in the change of hardenability due to the difference of width of edge preparation and dilution. Matching ratio is calculated to be 1.02, 0.87, 0.68 and 0.73. Hardness of heat-affected zone (HAZ) also changes according to welding thermal history. For evaluation of hardness change during various thermal cycles, simulated thermal cycle test is performed. Thermal cycle is decided from the point of view of heat transfer [11]. Figure 5 shows hardness test result in case of 50mm thick-2.0kJ/mm-GMAW. Cooling rate of this time investigation is thought to be near this condition. Under wide range of peak temperature, hardness shows over 350Hv except 800-degreeC peak temperature conditions reflecting high hardenability of HT950. Figure 6(A) and (B) show hardness test results at grid point on actual weldment for detailed information. Average value of hardness of HAZ can be approximated to be 350 regardless of strength of weld metal.

TABLE 5: TENSILE TEST RESULTS OF WELD METAL

Mark 0.2%YS(MPa)

TS (MPa)

RA (%)

Sr [TSWM/TSBM]

926 999 64.3 1 929 1009 61.2 1.02

772 840 70.0 2 809 879 70.1 0.87

623 677 75.3 3 587 660 72.5 0.68

664 730 72.8 4 620 704 69.9 0.73

*Position: 1/4t in backing side **specimen:4mmΦ,G.L.=10mm

T1℃

1200℃

800℃

500℃

300℃ 200℃

t1sec

TT1-1200sec

T1200-800sec

T800-500sec

T500-300sec

T300-200sec

320

359350 351

294

357 357

303

356

327

270

290

310

330

350

370

1450 1200 1000 800

Hv1

kgf

1200 1000 800 1000 800 800- - - - 2nd peak temperatue

1st peak temperatue Unit:deg.C

FIGURE 5: HARDNESS CHANEGE OF HT950 AFTER VARIOUS SIMULATED WELDING THERMAL CYCLE

Mark1

0 10 20 30Distance from straight side fusion line [mm]

Hv9

8N

50Hv

Each line shows 300Hv

FIGURE 6(A): HARDNESS DISTRIBUTION AT GRID POINT (MARK1)


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Mark3

0 10 20 30Distance from straight side fusion line [mm]

Hv9

8N

50Hv

Each line shows 300Hv

FIGURE 6(B): HARDNESS DISTRIBUTION AT GRID POINT (MARK3)

Charpy impact properties of weldments Charpy impact tests notched in the center of weld metal, straight side fusion line and straight side fusion line plus 1mm are performed. As shown in Figure 7, taking into consideration of design temperature 0deg.C, all results are quite good. Viewing more detailed difference among conditions, vE0 of mark1 is relatively low. This can be concluded that high strength weld metal includes a lot of lattice defect therefore the resistance against ductile crack propagation is low. Also, viewing the results of fusion line, transition temperature tends to show better value as the strength of weld metal decreases. The mechanism of this tendency is the same as CTOD (Crack Tip Opening Displacement) test results and discussed below.

0

100

200

-120

-100

-80

-60

-40

vE0a

v.

[J]

vTrs

[deg

.C] (

plot

)

Weld metal

1 2 3 4 1 2 3 4 1 2 3 4

Fusion line HAZ1mm

vE0

vTrs

Mark→

FIGURE 7: CHARPY IMPACT TEST RESULTS OF WELDED JOINT

Standard CTOD tests of weldments A three point bending CTOD test compliant with BS7448 is the most common method for evaluation of the property against brittle crack initiation. Configuration of specimen is shown as Figure 8 (Bx2B size specimen with original thickness). Test temperature is zero deg. C that is design temperature of hydropower plants. Positions of notch are the

center of weld metal and straight side fusion line. As shown in Figure 9, fracture type of weld metal is 6 in all weldments, therefore, obtained critical CTOD values show some property related with ductile crack propagation property and does not show brittle crack initiation property. As well as Charpy test results in weld metal, weld metal of HT950 shows relatively low value. On the other hand, fracture type of fusion line is varied from 2 in HT950 to 6 in HT570. As the strength of weld metal decreasing, fracture type tends to change from brittle to ductile. This shows that combination of softer weld metal with HT950 base metal can enhance brittle crack initiation property and critical CTOD value. As shown in Figure 10, Sr, that is matching ratio (TSWM/TSBM) and Critical CTOD show relatively good relationship.

Figure 8: Configuration of CTOD test specimen used

Weld metal

1 2 3 4

Fusion line

Mark→

0

0.1

0.2

0.3

0.4

Crit

ical

CTO

D a

t 0de

g.C

[mm

]

1 2 3 4

Bx2B Local compression BS7448

6 6

6 6

6

6

6

6

2 2 2

4

4 4 6

6

Fracturetype

Required value[5]

FIGURE 9: CTOD TEST RESULTS OF WELDED JOINT


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0.6 0.8 1 1.2

0.05

0.1

0.5

Sr (TSWM /TSBM)

Crit

ical

CTO

D [m

m] <Fracture

type> 2 4 6

Base plate：HT950 BS7448 B×2B

HT570 Weld

HT570 Weld

Narrow groove HT780

Weld HT950 Weld

FIGURE 10: CORRELATION OF SR AND CRITICAL CTOD IN

THREE POINT BENDING TEST

Wide plate test with through thickness notch In order to evaluate fracture toughness in actual structure instance, Wide plate tests with through thickness notch are performed. Configuration of specimen is shown in Figure 11. Length of notch, 2C is set to be double of thickness, 52mm. According to the previous study [5], defect, which should be assumed in penstock fabricated with high strength steel, is semielliptical crack with length of a half of thickness and depth of a quarter of thickness. In case of this time weldments, size of assumed crack is 13mm in length and 6.5mm in depth. Through thickness with length of double of thickness which is adopted in this time experiment is severer than semielliptical crack despite the tip of through thickness defect is 0.1mm radius from the view of Weibull stress (definition of Weibull stress is described below) as shown in Figure 12. Measurement items of the test are force, clip gage displacement and overall strain as shown in Figure13. Test temperature is always kept to be 0 deg. C by spray of liquid nitrogen. Correlation of overall strain and net stress are shown in Figure 14. All specimens does not show brittle manner in all stage of loading. Numerical results are shown in Table 6. Considering that design stress of HT950 is 442.5MPa, sufficiently high net stresses are marked because all specimens fractured after all section yielding. Also CTOD values at maximum force are quite good. However, CTOD value, maximum force and Uniform elongation tend to be lower as strength of weld metal decreasing. However, judgment of necessity of uniform elongation has to be discussed as future task.

460

400 300

180

26

30゜

R60 100

With excess weld metal

・Notch location:FL(straight side)・Notch length:52mm ・Radius of notch tip:0.1mm

FIGURE 11: CONFIGURATION OF WIDE PLATE TEST SPECIMEN

0 100 200 300 400 500 6000

1000

2000

3000

4000

Gross stress [MPa]

Wei

bull

stre

ss(m

=20)

[M

Pa]

2000

2000

0.1R

26

Through thickness

notch 2C=52

2000

20002C=13

26

a=6.5

Surface crack

FIGURE 12: COMPARISON OF WEIBULL STRESS BETWEEN

THROUGH THICKNESS NOTCH SPECIMEN AND SEMIELLIPTICAL CRACK SPECIMEN (HOMOGENEOUS MODEL)

FIGURE 13: MEASUREMENT SYSTEM OF WIDE PLATE TEST


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0 1 2 3 4 5

0

500

1000

Overall strain, ε ∞ [%]

Net

stre

ss, σ

net

[MPa

]

mark1 mark2 mark3 mark4

Allowable stress of HT950

Test temp.:0 deg.C

FIGURE 14: CO-RELATION OF NET STRESS AND OVERALL

STRAIN IN WIDE PLATE TEST RESULT

TABLE 6: WIDE PLATE TEST RESULTS

Mark t [mm]

Angular distortion [mm/mm]

Pmax [kN]

σnet [MPa]

σgross [MPa]

δcr,av.[mm]

ε∞ at maximum

force [%]

1 26.27 2.37/460 6654 1021 844 2.42 2.13 2 26.28 2.72/460 6419 985 814 1.07 1.16 3 26.28 1.68/460 6105 937 774 1.14 1.16 4 26.28 2.57/460 6220 954 789 1.03 1.10

FEM analysis

FE-model Directing CTOD tests in which fracture type and critical CTOD value depends on matching ratio, FEM analyses are performed (Figure 15). FE-model consists of three layer, weld metal, HAZ and base plate, in which each part is homogeneous ignoring detailed hardness distribution created by various thermal histories for simplification. Size of each model is decided by the actual macrostructure. For simplification, symmetric shape to centerline of thickness is formed. Width of HAZ is 3mm without exception. Position of the center of specimen (position of initial crack) is set to be fusion line at straight side (Figure 16). Calculation model is set to be 1/2 model considering symmetric property to direction of thickness. Also process zone is set to be HAZ area at straight side for calculate Weibull stress by using result of FEM calculation. Correlation of each layer is determined by conversion by swift equation (Eq.1) using actual tensile test as shown in figure 17. Allocation of constitutive equation of each model is arranged in Table 7.

)1(1n

py ⎟⎟

⎠

⎞⎜⎜⎝

⎛+=αε

σσ

FIGURE 15: GENERAL OF FE-MODEL OF STANDARD THREE

POINT BENDING CTOD TEST

26

24 33

8

HAZ WM HAZ BM BM

Position of crack tip

Process zone

(calculation area of Weibull stress)

26

11 33

5

HAZ WM HAZ BMBM

Position of crack tip

Process zone

(calculation area of Weibull stress)

(a) mark 1~3 (b) mark 4 FIGURE 16: SIMPLIFIED THREE LAYER MODEL AND

CONFIGURATION OF GROOVE

0 0.5 1 1.5 20

500

1000

1500

2000

True strain

True

stre

ss [M

Pa]

HT570(WM)HT780(WM)

HT950(WM)

HT950(BM)

HT950(HAZ);Hv350

FIGURE 17: CONSTITUTIVE EQUATION USED

TABLE 7: ALLOCATION OF CONSTITUTVE EQUATION

Mark BM HAZ WM 1 HT950(BM) HT950(HAZ) HT950(WM)2 HT950(BM) HT950(HAZ) HT780(WM)3 HT950(BM) HT950(HAZ) HT570(WM)4 HT950(BM) HT950(HAZ) HT570(WM)

Discussion of FEM results Figure 18 shows relationship between clip gage displacement and force on FEM analysis. As softer the WM is, force shows lower value as is the case with experiment. For evaluation of


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the risk of brittle fracture, Weibull stress is calculated in accordance with the following equation [12] where i is the number of steps, j is the number of element and m is Weibull parameter (in this study, m is set to be 31[13]-[14]). Weibull stress is the concept that fracture risk is cumulative probability of very small region of fracture. Critical Weibull stress by this concept is considered to have transferability to different configuration and temperature. In this study, HAZ at straight side is put in as process zone by CTOD test results. Figure 19 shows relationship between CTOD calculated in accordance with BS7448 and Weibull stress. Comparing with Weibull stress at 0.10mm of CTOD, mark1, 2, 4, 3 are calculated in descending order. In other words, it can be said that probability of brittle fracture of under-matching weldment decreases if brittle fracture of HAZ is investigated. This is based on the phenomenon of deformation at crack tip. When one side of crack field is much softer than the other side, deformation at crack tip is concentrated to softer zone. Figure 20 shows contour line of distribution of equivalent plastic strain at crack tip at 0.10mm of CTOD. As is obvious from the figure, plastic strain is confined almost exclusively to softer weld metal zone in case of under-matching weldment.

)2()()(1)(

1

,,10

mzoneprocess

jji

mjiiw v

V ⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⋅= ∑ σσ

0 0.5 1 1.5 20

50

100

150

Clip gage displacement [mm]

Forc

e, P

[kN

]

mark 1mark 2mark 3mark 4

mark 1 mark 2

mark 3mark 4

FIGURE 18: P-VG RELATIONSHIP OF FEM ANALYSIS

0.001 0.005 0.01 0.05 0.10

1000

2000

3000

4000

CTOD compliant with BS7448

Wei

bull

stre

ss (m

=31)

[MPa

]

mark 1mark 2mark 3mark 4

mark 1 mark 2

mark 3 mark 4

FIGURE 19: COMPARISON OF WEIBULL STRESS BETWEEN

MATCHING RATIO AND GROOVE CONFIGURATION

Mark 1 Mark 3

HAZ WM HAZ WM

0.20.4

FIGURE 20: COMPARISON OF CONTOUR LINE OF EQUIVALENT

PLASTIC STRAIN AT 0.2MM OF CTOD

Conclusion

In this series of investigation, for reducing complication of handling HT950 class high strength welding material, application of softer weld material (down to 570MPa class) was investigated. In this 2nd report, especially brittle crack initiation property was investigated. Finally following conclusion has been obtained. ・ HAZ microstructure of HT950 is made harder than base

plate by transformation through wide range of welding thermal histories. So, actual weldment can be simplified to three-layer model, that is, base plate, HAZ and WM.

・ Hardness of WM comes under the influence of configuration of groove. Even in case of combination of HT570 class welding material, by application of narrow groove strength of weld metal can be enhanced through dilution of alloy element from base plate.

・ According to standard three point bending CTOD test results, under-matching weldment is of great advantage against brittle crack initiation. The minimum value of CTOD of application of HT570 class welding material is greater than 0.14mm whereas that of matching weldment is 0.09mm. The reason comes from distribution of deformation at crack tip, which is confined, to softer weld metal. This reason is backed by calculation of Weibull stress from FEM results.

・ Wide plate tests with through thickness notch, which is severer than the allowable defect of actual penstock structure, are performed for evaluation of brittle fracture behaviour of actual structure. As a result of them, every test always shows ductile manner and fractured after full section yielding. Even in case of HT570 weld metal, maximum stress of the weldment shows much higher than design stress of penstock

・ From these investigations, it is considered that the


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under-matching weldment, which brings in a major reduction of complication of welding process, warrants serious consideration for use in penstock. Also, in case of HT780, these results can be applied and useful. Application of Under-matching concept has a large amount of potential for further expanding of high tensile strength steel.

Acknowledgment

My heartfelt appreciation goes to Mr. Nozomu Watanabe, Nuclear Plant Service co., ltd. whose comments and suggestions were of inestimable value for our study. Special thanks also go to Mr. Takayuki Ueda and Mr. Kouji Hosoda, Sumitomo Metal Technology Inc. who exercised best effort for welding and large scale testing.

References

[1] Japan Electric Power Civil Engineering Association “One hundred years’ history of hydropower plants”, 1992.

[2] T.Kawabata, A.Nagayoshi, K.Arimochi, S.Okaguchi, Y.Matsukawa, and N.Watanabe ” Development of TMCP type HT100 steel plate with superior arrestability” [in Japanese], Pre-Prints of the National Meeting of JWS, Vol.67, (2000), 144-145.

[3] K.Onishi, H.Sakaibori, T.Kawabata and S.Okaguchi “Metallurgical basis of 890MPa class high yield strength steel(HT950) plates for penstock”, Proceedings of High Strength Steels for Hydropower Plants,(2005), 11.1-12.

[4] Y.Okamura, S.Yano, H.Inoue, K.Tanabe, K.Kawai and N.Watanabe " Development of heavy gauge 100 kg/mm2 class high tensile strength steel with high toughness and weldability by direct quench process : Study for 100 kg/mm2 class high tensile strength steel II” [in Japanese], Tetsu- to- Hagane, (1986), 72(5), .S614.

[5] T.Matsuura, Y.Kure, Y.Nishigami, I.Watanabe, M.Suzuki and S.Sakai “Weldability and weldment toughness of newly developed 100kgf/mm2 grade high tensile strength steel” [in Japanese], Quar.J.JWS, Vol. 3, No. 1, (1985) 124-131.

[6] Y.Nishiwaki, T.Maejima and K.Kubota “Study for the application of high tensile steel(HT-100) to penstock” , Journal of Construction Management and Engineering(VI), No.672, (2001),37-56.

[7] K.Hiraoka “Welding Technology for High Strength Steels” [in Japanese], Bulletin of the Iron and Steel Institute of Japan, (2008), 13(7), 477-482.

[8] K.Satoh, M.Toyoda, K.Ukita, A.Nakamura and T.Matsuura "Applicability of undermatching electrode to circumferential welded joint of HT80 penstock(1st report)” [in Japanese] , J.JWS, Vol.47, No.5,(1978) 283-288.

[9] K.Satoh, M.Toyoda, K.Ukita, R.Shimoda, A.Nakamura and T.Matsuura "Applicability of undermatching electrode to circumferential welded joint of HT80 penstock(2nd report)” [in Japanese] , J.JWS, Vol.47, No.10,(1978) 697-704.

[10] M.Toyoda, F.Minami, C.Ruggieri, C.Thaulow and M.Hauge “Fracture property of HAZ-notched weld joint with mechanical mis-matching – Part1”, Mis-Matching of Welds, ESIS 17,Mechanical Engineering Publications, London, (1994), 399-415.

[11] C.Thaulow, O.Ranestad, M.Hauge, Z.Zhang, M.Toyoda and F.Minami “FE calculations of stess fields from cracks located at the fusion of weldments” , Engineering Fracture Mechanics, Vol.57, No.6, (1997), 637-651.

[12] JESC H0001 (2000) “Technical Indication of 950MPa class high strength steel for Penstock”

[13] N.Yurioka and K.Kojima “A Predictive Formula of Weld Metal Tensile Strength” [in Japanese], Quar.J.JWS, Vol. 22, No. 1, (2004) 53-60.

[14] ISO27306 “Metallic materials-Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components”, (2009).

[15] T.Kawabata, M.Ohata and F.Minami “Critical Condition of Ductile Crack Extension and Its Critical Condition of Subsequent Brittle Fracture for High Strength Steel” [in Japanese], The Japan Society of Naval Architects and Ocean Engineers, (2007), 235-243.

[16] Y. Takashima, M. Ohata and F. Minami ”Weibull stress approach to correlation between Charpy impact energy and CTOD fracture toughness”, Proc. 9th European Mechanics of Materials Conference, EMMC9, Local Approach to Fracture, More--Sur-Loing, (2006), 81-86.


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