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21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy 21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy

Evaluation of Pre-strain Effect on Abnormal Fracture Occurrence in Evaluation of Pre-strain Effect on Abnormal Fracture Occurrence in Drop-Weight Tear Test for Linepipe High Charpy XV Portuguese Conference on Fracture, PCF 2016,Steel 10-12with February 2016, Paço de Energy Arcos, Portugal Drop-Weight Tear Test for Linepipe Steel with High Charpy Energy a

a a

b

c

d

Toshihiko Amanoa, Taishi Fujishirobof , Yasuhiro c, Takehiro d * of an Thermo-mechanical modeling a highShinohara pressure turbineInoue blade Toshihiko Amano , Taishi Fujishiro , Yasuhiro Shinohara , Takehiro Inoue * Materials Reliability Research Lab., Research & Development, Nippon Steel & Sumitomo Metal Corporation(NSSMC), Hyogo 660-0891, Japan airplane gas turbine engine Materials Reliability Research Lab., Research & Development, Nippon Steel & Sumitomo Metal Corporation(NSSMC), Pipe &Tube Research Lab., Research & Development, NSSMC, Hyogo 660-0891, Japan Hyogo 660-0891, Japan b b

Pipe &TubecResearch Research & Development, NSSMC, Hyogo 660-0891, Japan Kimitsu RLab., & D Lab., NSSMC, Kimitsu Chiba 299-1141, Japan c b 299-1141, KimitsuLab., R&D Lab.,aNSSMC, Kimitsu Chiba Plate & Shape Research Research & Development, NSSMC, Futtsu Japan Chibac 293-8511, Japan d Plate & Shape Research Lab., Research & Development, NSSMC, Futtsu Chiba 293-8511, Japan a Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal Abstract b IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Abstract Portugal c Brittle fracture control is one of the most important subjects in natural gas transmission pipeline in order to maintain CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Brittleintegrity fractureover control is one of the most subjects natural isgas transmission pipeline in order to maintain structural several decades. The Dropimportant Weight Tear Test in (DWTT) widely used as the test method to evaluate the Portugal d

P. Brandão , V. Infante , A.M. Deus *

structural over several decades. The Dropsteels. Weight Tear Test widely frequently used as theoccurs test method evaluate resistance integrity against the brittle fracture for linepipe However, an (DWTT) abnormalisfracture duringtothe DWTTthe in resistance the brittle fracture linepipe steels. However, abnormal fracture frequently during the DWTT in recent highagainst toughness line pipe steels.for The abnormal fracture is alsoanknown as inverse fracture. The occurs abnormal fracture is defined recent high toughness pipe steels. Thehammer abnormal fracture is also known although as inversethe fracture. abnormal defined as Abstract the cleavage fractureline is observed at the side in DWTT specimen ductileThe fracture firstlyfracture initiatesisfrom the as the tip cleavage fracture is observed at the hammer in DWTT specimen the ductile from the notch side. Many studies for abnormal fracture side appearance/behavior havealthough been carried out infracture order tofirstly clarifyinitiates the mechanism notch side. Many studies for abnormal fracturethe appearance/behavior have been carried out in order to clarify the mechanism of During the tip abnormal fracture occurrence and to ensure prevention of long brittle fracture propagation for pipelines. their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, of theInabnormal fracture occurrencepre-straining and to ensure prevention of longside brittle fracture propagation for pipelines. this study, a compressive at the theSuch impact hammer in the DWTT specimen was evaluated under quasiespecially the high pressure turbine (HPT) blades. conditions cause these parts to undergo different types of time-dependent Inload thisconditions. study, a compressive pre-straining atwere the impact hammer side in totheprint DWTT specimen was evaluated quasistatic The specimen’s surfaces electrolytically-etched circle patterns with 5 mm inunder diameter in degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict static load conditions. The specimen’s surfaces were electrolytically-etched to print circle patterns with 5 mm in diameter in order to measure plastic strain. Charpy impact specimens were taken from the quasi-static loaded and unloaded DWTT specimen the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation order to measure plastic strain. Charpy specimens were taken The from the quasi-static loaded andthat unloaded than DWTT specimen to company, measure the of impact pre-straining on toughness. testflight results show of model the werepossible used toinfluence obtain thermal and mechanical data for three impact different cycles. In ordermore to create 2the%3D to measure the possible influence of pre-straining onCharpy toughness. The impact test results show that moreonthan 2 %property of the compressive pre-strain gave 7 to 10 % decrease of the upper-shelf energy. The effect of pre-straining needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and materialtensile properties were compressive pre-strain gavepresent 7 to 10experiments % decrease of the Charpy upper-shelf energy. The effect of pre-straining on tensile property was also evaluated. that the occurrence of abnormal fracture hammer can be3D obtained. The dataThese that was gathered was fed indicate into the FEM model and different simulations werenear run,the first with a side simplified was also evaluated. These present experiments indicate that occurrence of and abnormal fracture near the hammer side cangas be attributed to the compressive pre-straining. Furthermore, the the chevron-notched the3D pre-cracked DWTTs partial rectangular block shape, in order to better establish the model, and then with the real mesh obtained fromand the the blade scrap. The attributed to theconducted compressive pre-straining. Furthermore, the chevron-notched and the pre-cracked DWTTs and the partial gas burst test were in order to compare the brittle-to-ductile transition temperature. Based on these experiments, the effect overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a burst test configuration were conducted inthe order to compare thetransition brittle-to-ductile transitionthe temperature. Based on these experiments, the effect of model notch brittle-to-ductile temperature correlation can be useful inonthe goal of predicting turbine blade life, given and a set of FDR data. between DWTTs and pipe test were of notch configuration on relationship the brittle-to-ductile transition temperature the correlation between DWTTs and pipe test were discussed. In addition, the between the pre-straining and theand abnormal fracture appearance was considered. discussed. In addition, the relationship between the pre-straining and the abnormal fracture appearance was considered. © 2016 The Authors. Published by Elsevier B.V. © 2016 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific © 2016, (Procedia StructuralbyIntegrity) Hosting byCommittee Elsevier Ltd.of AllPCF rights2016. reserved. © 2016 PROSTR The Authors. Published Elsevier B.V. Committee Peer-review under responsibility the Scientific of ECF21. Peer-review under responsibility of theof Scientific Committee of PCF 2016. Peer-review under responsibility of the Scientific Committee of ECF21. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation. Keywords: DWTT; Linepipe steel; Brittle fracture; Abnormal fracture apperance ; Pre-strain; Partial gas burst test Keywords: DWTT; Linepipe steel; Brittle fracture; Abnormal fracture apperance ; Pre-strain; Partial gas burst test

* Corresponding author. Tel.: +81-6-7670-5875; fax: +81-6-6489-5794. * Corresponding Tel.: +81-6-7670-5875; fax: +81-6-6489-5794. E-mail address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.

2452-3216 © 2016 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby Scientific Committee of ECF21. Peer-review underauthor. responsibility the Scientific Committee of ECF21. * Corresponding Tel.: +351of218419991.

E-mail address: [email protected]

2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 © 2016, PROSTR (Procedia Structural Integrity) Hosting by Elsevier Ltd. All rights reserved. Peer review under responsibility of the Scientific Committee of PCF 2016. 10.1016/j.prostr.2016.06.055

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1. Introduction Brittle fracture control is one of the most important subjects in natural gas transmission pipeline in order to maintain structural integrity over several decades. The Drop Weight Tear Test (DWTT) is widely used as test method to evaluate the resistance against brittle fracture for linepipe steels. However, abnormal fracture which is also known as inverse fracture frequently occurs during DWTT in recent high toughness line pipe steels. The abnormal fracture is defined as the cleavage fracture is observed at the hammer side in DWTT specimen although the ductile fracture firstly initiates from the notch tip side. Many studies for the abnormal fracture appearance/behavior have been carried out in order to clarify the mechanism of abnormal fracture occurrence and to ensure the prevention of long brittle fracture propagation for pipelines. Progress for measuring technique and analysis technique during DWTT such as continuous shoot using high-speed camera contribute to the understanding abnormal fracture occurrence. The mechanism of abnormal fracture occurrence has been steadily become clear such studies. In this study, compressive pre-straining at the impact hammer side in DWTT specimen was evaluated under quasistatic load conditions. The specimen’s surfaces were electrolytically-etched to print circle patterns with 5 mm in diameter in order to measure plastic strain. Charpy impact specimens were taken from the quasi-static loaded and unloaded DWTT specimen to measure the possible influence of pre-straining on toughness. Furthermore, the DWTTs with several types of notch such as chevron notch, pre-cracked notch and the partial gas burst test were conducted in order to compare the brittle-to-ductile transition temperature. Based on these experiments, the effect of notch configuration on the brittle-to-ductile transition temperature and the correlation between DWTTs and pipe test were discussed. In addition, the relationship between the pre-straining and the abnormal fracture appearance was considered. Nomenclature AFA API CN DNV DWTT

Abnormal Fracture Appearance American Petroleum Institute Chevron Notch Det Norsk Veritas Drop Weight Tear Test

PN SA SATT SMYS SPC

Pressed Notch Shear Area Shear Area Transition Temperature Specified Minimum Yield Stress Static pre-cracked

2. Material and test procedures 2.1. Material Table 1 shows material used in this study. API 5L X65 Grade UO linepipe steel manufactured by thermomechanical control process (TMCP) is prepared. The dimensions of the pipe are 609.6 mm (24in.) in outer diameter (OD), 19.1 mm in wall thickness (WT) and 9000 mm in Longitudinal length. Table 1 also summarizes the material properties of the base metal with respect to the tensile property and the Charpy absorbed energy. The yield strength (YS) and the tensile strength (TS) in Table 1 were obtained from  8.9 mm round-bar tensile specimens in the transverse direction. The yield stress is defined at the 0.5 % total strain. Fig. 1 shows the Charpy test results. Charpy upper shelf absorbed energies of the pipe tested at + 0 oC was 382 J. Table 1. Material in this study Pipe ID

Grade

KP251

X65

Pipe size

Tensile properties

Charpy upper shelf energy

WT (mm)

OD (mm)

YS (MPa)

TS (MPa)

tEL (%)

Cv (J)

19.1

610

532

590

28.2

382

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3

400

Shear area fraction [%]

Absorbed energy [J]

450 350 300 250 200 150 100 50 0 -140

-120

-100

-80

-60

-40

Temperature [OC]

-20

0

20

40

100 80 60 40 20 0 -140

-120

-100

-80

-60

-40

Temperature [OC]

-20

0

20

40

Fig. 1. Charpy test results for base metal of the pipe; (a) absorbed energy and (b) shear area fraction

2.2. Drop-weight tear test (DWTT) In order to investigate the effect of notch configurations on DWTT results with respect to the brittle-to-ductile transition curves, the shear area and the abnormal fracture appearances, three kinds of DWTT specimens were prepared. Fig. 2 shows the specimen configurations and notch types. The pressed notch DWTT (PN-DWTT) specimen is a standard specimen in API 5L/DNV standard. The chevron notch DWTT (CN-DWTT) is also standardized in API. The static pre-cracked DWTT (SPC-DWTT) is prepared using PN-DWTT specimen by conducting static three-point bending test. The details for static pre-cracking method are shown in following section. All DWTT specimens have initially same notch depth. All specimens were flattened and taken from transverse direction of pipe. Fracture surfaces were observed to evaluate shear area fraction and abnormal fracture appearances. (a)

(b)

Notch Angle 45 o±2o

PN

notch ((b))

Notch Depth 5.1±0.51 76.2±3

CN

Radius 0.025 Notch Depth 5.1±0.51 90o

305

SPC 19.1

Unit:mm

Notch Depth 5.1±0.51 Static pre-crack

Fig. 2. (a) Dimension of DWTT specimens and (b) notch configurations; PN, CN and SPC

2.3. Static three-point bending test The static three-point bending tests were conducted at room temperature in order to prepare the SPC-DWTT and the pre-strained materials. Fig. 3 shows a method of statically pre-cracking and measuring degrees of pre-straining. Static pre-crack was induced in the standard DWTT specimens with pressed notch (PN-DWTT) which had dimensions of 76.2 x 305 x 19.1 mm in transverse direction (Fig. 2 (a)). As shown in Fig. 3 (b), the load was applied until a drop in maximum load of approximately 1.25 % (Wilkowski et al., 2012). The surface of the PN-DWTT specimens was electrolytically-etched to print circle patterns with 5 mm in diameter in order to measure the plastic strain (Fig. 3 (c)). Fig. 4 shows an example of the strain distribution after the static three-point bend test in terms of (a) the plastic strain in the traverse direction, (b) the plastic strain in the propagation direction and (c) the equivalent plastic strain. The equivalent plastic strain eq was evaluated from the strain components by Eq. (1);

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2  x 2   y 2   xy 2   z 2 3

 eq  z 

425

(1)

1 1 ( x  1)( y  1)

(2)

Where, x is the strain in the traverse direction, y is the strain in the propagation direction (see Fig. 3(c)) and z is the strain in the through-thickness direction. z was obtained considering the volume constancy condition in Eq. (2). The Charpy V-notch impact specimens were taken from quasi-static loaded and unloaded DWTT specimens to evaluate the effect of pre-straining on the absorbed energy. The round-bar type tensile specimens whose diameter was 8.9 mm were also taken from loaded and unloaded DWTT specimens. These pre-strained specimens were taken from several locations whose degrees of pre-straining were -4.5 to 4.6 % for the Charpy specimens and were -6 to 12.0 % for the tensile specimens. The Charpy impact tests were conducted at room temperature which is approximately 23 °C because there are not enough specimens to evaluate effect of pre-straining on transition curve. Tensile tests were also conducted at room temperature. (a)

(b)

(c)

y

1.25% Load drop

x

 5 mm

PN-DWTT

Load

x: Traverse direction Y: Propagation direction

Static pre-crack Displacement

Static pre-crack

Fig. 3. Method of preparing pre-strained materials; (a) Static three-point bending test, (b) Load vs. displacement curve and (c) circle patterns on specimen surface to measure the plastic strain (a)

(b) 80

Strain (%) 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 -5.0 -6.0

70 60 50 40 30 20 Y Z

10 X

Strain

0 -40 -30 -20 -10

0

10

20 30

40

(c) 80

Strain (%) 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 -5.0 -6.0

70 60 50 40 30 20 Y Z

10 X

Strain

0 -40 -30 -20 -10

0

10

20 30

40

Equivalent plastic Strain (%) 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 -5.0 -6.0

80 70 60 50 40 30 20 Y Z

10 X

0 -40 -30 -20 -10

0

10

20 30

40

Fig. 4. Strain distribution after static three-point bending test; (a) traverse direction (b) propagation direction and (c) equivalent plastic strain

2.4. Partial gas burst test A partial gas burst test was conducted at low temperature in order to investigate fracture behavior of pipe material and to evaluate a local strain during the crack propagation. Fig. 5 shows illustrations of the partial gas burst test. In this test, the pipe body at upper geometrics was cooled by using liquid nitrogen because a surface notch was machined at upper geometrics of the pipe as fracture origin. The configuration of surface notch is shown in Fig. 5. The stepped notch is adopted in this test. The length and depth of deep notch section are 500 mm and 10 mm, respectively. The depth of deep notch section was determined so that fracture occurs at the pressure correspond to 0.80 SMYS using the Charpy energy-based equation for axial part-through-wall crack in pipe (Kiefner et al., 1973).

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5

The cooling baths were set up on the test vessel. In order to control the pipe surface temperature uniformly, the solenoid valves were used. In this test, it was possible to obtain fracture behaviors under two different test temperatures in one burst test because temperatures in the cooling baths were separately controlled between the west side and the east side. The pipe wall temperatures were measured at 5 mm under the pipe surface in the drilled holes because the pipe surface was affected fluid injections. The test vessel contained 85 % cooled water (not frozen) and 15 % air gap. After keeping the target temperature, the temperatures were held over 20 minutes, and then the test pipe was pressurized until fracture occurs. The nitrogen gas was used as the pressurized medium. Numerous thermocouples, timing wires, pressure transducer, strain gauge and scribed grid were setup to measure the pipe wall temperature, fracture speed, burst pressure, local strain during crack propagation and plastic strain. The test was conducted at our original burst test facility in Japan. Initial notch

700 500

5 Notch depth 10 19.1 (WT)

Liquid nitrogen

West side Nitrogen gas

East side

Cooling bath Thermocouple

Strain gauge

Sleeve pipe

Seam weld

85 % water

4500 9000

Unit in mm

Fig. 5. Illustrations of partial gas burst test

3. Experimental results and Discussions 3.1. Comparison of brittle-to-ductile behavior between DWTTs and partial gas burst test Table 2 summarizes the main results of the partial gas burst test. Fig. 6 shows the setup of the test pipe before the test and the fracture appearances after the test. The burst pressure was 23.9 MPa which corresponds 84 % SMYS (381 MPa of hoop stress). The burst pressure was a little higher than target pressure. In this test, both brittle crack and ductile crack appeared at the ligament of the initial stepped notch depending on pipe wall temperatures. In the west side, whose temperatures were controlled at between -11 to -21 oC, the ductile fracture was initiated and propagated into the pipe body in fully ductile mode. On the other hand, in the east side, whose temperatures were controlled at between -17 to -38 oC, the single brittle fracture was initiated in the thicker ligament of the initial steeped notch and propagated into the pipe body. Then the single brittle fracture was quickly arrested and change to a shear ductile fracture. The maximum fracture speeds were 290 m/s in the west side and 390 m/s in the east side, respectively. The maximum compressive strain in the west side before reaching the propagating crack which was measured at 40mm away from crack front was approximately 1.3 %. This was lower than that observed in PNDWTT using the high speed camera (Fujishiro et al., 2012; Sakimoto et al., 2013). It seems that a large compressive strain was induced by bending deformation in PN-DWTT specimens. Table 2. Summary of partial gas burst test Grade

Pressure (MPa)

Hoop stress (MPa)

X65

23.9

381

side

Fracture appearance

Temperature where SA% measured (oC)

Shear lip area fraction (%)

West side

Fully Ductile

-11

100

East side

Brittle fracture quickly change to ductile fracture

-18

65-100

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Toshihiko Amano et al. / Procedia Structural Integrity 2 (2016) 422–429 "Toshihiko Amano et. al." / Structural Integrity Procedia 00 (2016) 000–000 (a)

(b)

427

(c) Fracture direction

West

West East

East (d) Fracture direction

Fig. 6 partial gas burst test result; (a) before the test, (b) after the test, (c) fracture path in west side and (d) fracture path in east side

Shear area fraction [%]

The DWTT results are shown in Fig. 7. The 85 % shear area transition temperature (SATT) using DNV SA% rating method which includes ductile fracture from notch due to abnormal fracture appearance (AFA) behavior in rating for the PN-DWTT, the CN-DWTT and the SPC-DWTT were -30 oC, -25 oC and -13 oC, respectively. The brittle-to-ductile transition curves in the CN-DWTT and the SPC-DWTT shifted toward a higher temperature side compare to that in the PN- DWTT due to reducing the crack initiation energy from the total energy. However, the abnormal fracture appeared regardless of types of notch in the transition region. The shear area fractions (SA%) in the pipe burst test are also plotted in Fig. 7 in order to compare with that in several notched DWTT specimens. The SA% in the pipe burst test is defined shear lip thickness fraction rated same evaluation area as the DWTT SA rating. As shown in Fig. 7, the SA% in the pipe burst test result located near the brittle-to-ductile transition curve in the SPC-DWTT specimen. Fig. 8 compares fracture appearances at -10 oC and -20 oC for the PN-DWTT specimen, the CN-DWTT specimen, the SPC-DWTT specimen and the tested pipe. As shown in Fig. 8, the clear shear lips can be observed at -20 oC for the tested pipe and it is similar to the normal fracture appearances, which means the brittle crack initiates at the root of notch, in the SPC-DWTT specimens. On the other hand, the abnormal fracture appearances appeared both at -10 o C and -20 oC for the PN-DWTT specimens. It seems that the pre-straining occurred at the hammer impact side due to higher resistance against crack initiation at root of the notch. 100 80 60

PN-DWTT CN-DWTT SPC-DWTT AFA SA85% Pipe burst test

40 20 0

-80

-60

-40

-20

0

20

40

Temperature [OC] Fig. 7. Correlation of shear area between DTTs and pipe burst test (a)

PN-DWTT

SPC-DWTT Normal fracture

Abnormal fracture Appearance (AFA) Shear area : 100 % Tested Temp. : -10 o C

Shear area : 96 % Tested Temp. : -10 o C

SPC

Pipe burst test

Shear area : 100 % Temp. : -11 o C

Fig. 8. Comparison of fracture appearances; (a) test temperature of -10 oC and (b) test temperature of -20 oC

Toshihiko Amano et al. / Procedia Structural Integrity 2 (2016) 422–429 Author name / Structural Integrity Procedia 00 (2016) 000–000

428 (b)

PN-DWTT

CN-DWTT

SPC-DWTT

Shear area : 100 % AFA

Shear area : 96 % Normal fracture

Shear area : 92 % Tested Temp. : -20 o C

Shear area : 100 % Tested Temp. : -20 o C

Pipe burst test

Shear area : 65 to 70 %

Shear area : 66 %

Shear area : 59 % Tested Temp. : -20 o C

7

SPC

Shear area : 70 to 100 % Temp. : -18 o C

Fig. 8. Comparison of fracture appearances; (a) test temperature of -10 oC and (b) test temperature of -20 oC (Con’t)

3.2. Effect of pre-strain on abnormal fracture occurrence in DWTT Fig. 9 shows the results of the Charpy impact tests and the tensile test for the non-strain and the pre-straining materials. The all tests were conducted at the room temperature. As shown in Fig. 9 (a), for the both compressive and tensile pre-strainings, more than 2 % pre-straining gave 7 to 10 % decreases of the Charpy upper-shelf energy. On the other hand, with regard to tensile properties, as the degree of pre-straining were increased, the tensile strengths were increased, see Fig9 (b). However, the yield strength of the materials subjected to within 5 % compressive pre-straining slightly decreased although the yield strength of the tensile pre-strained materials increased. It seems that decreasing of the yield strength in compressive pre-straining may contribute to a large deformation and local embrittlement prior to crack initiation. Further investigation such as brittle-to-ductile transition curve of Charpy test obtained from pre-strained materials is needed to relate abnormal fracture occurrence to embrittlement behavior caused by pre-straining. 700 650

YS, TS [MPa]

400 350 300 250

600 550 500

YS TS

450

200 -6.0

-4.0

-2.0

0.0

2.0

4.0

400 -10.0

6.0

-5.0

0.0

5.0

10.0

15.0

Pre-strain in traverse direction [%]

Pre-strain in traverse direction [%]

Fig. 9. (a) Charpy absorbed energy and (b) Tensile properties of pre-strained specimens tested at room temperature (a)

(b)

Compression Tension

80

Distance from notch side [mm]

Absorbed energy [J]

(b)

450

Distance from notch side [mm]

(a)

With AFA Without AFA

70 60 50 40 30 20 10 0

-10

-5

0

5

10

15

20

25

Plastic strain in traverse direction[%]

30

35

Compression

80

Plastic strain measured at  partial gas burst test

Tension

With AFA Without AFA

70 60

(c)

AFA

50 40 30 Nornal fracture

20 10 0

-10

-5

0

5

10

15

20

25

30

Plastic strain in traverse direction[%]

35

Fig. 10. Plastic strain near the crack front for the SPC-DWTT; (a) after the static three-point bending test (b) after the DWTT

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Fig. 10 comparers the plastic strain in the traverse direction near the crack front for the SPC-DWTT tested at -10 C in between the specimen with AFA and without AFA. Approximately 4.0 % of the compressive pre-straining occurred after the static bending test in the specimen appeared AFA in DWTT while 2.0 % of the compressive prestraining occurred in the specimen without AFA in DWTT. Fig. 10 (b) shows the plastic strain after the SPC-DWTT. Fig. 10 (b) also shows the plastic strain measured in the partial gas burst test using the scribed grid. The plastic strains in the SPC-DWTT specimen without AFA were lower than that in pipe. On the other hand, for the SPCDWTT specimen with AFA, the plastic strains at both the hammer impact side and the root of notch side which appeared the brittle fracture were larger than that measured at the fully ductile fracture region. Therefore, the abnormal fracture occurs easily because the range of strain in hammer impact side was relatively high.

o

4. Conclusions In this study, the DWTTs with the pressed notch (PN), the chevron notch (CN), the pre-cracked notch (SPC) and the partial gas burst test were conducted in order to compare the brittle-to-ductile transition temperature. The effect of pre-straining near the impact hammer side in the SPC-DWTT specimen was also evaluated at static three-point bending test. Charpy impact specimens were taken from quasi-static loaded and unloaded DWTT specimen to measure the possible influence of pre-straining on toughness. Based on these experiments, the relationship between the pre-straining and the abnormal fracture appearance was considered. The main conclusions obtained in this study are as follows;  The brittle-to-ductile transition curves in the CN-DWTT and the SPC-DWTT shifted toward a higher temperature side compare to that in the PN- DWTT. However, the abnormal fracture appeared regardless of types of notch in the transition region.  The fracture appearances in the pipe burst test were similar to that in the SPC-DWTT specimen which is easy to initiate the brittle fracture due to reducing crack initiation energy while the abnormal fracture occurred at near hammer impact side.  More than 2 % of compressive and tensile pre-straining gave 7 to 10 % decreases of the Charpy upper-shelf energy. Further investigation such as brittle-to-ductile transition curve of Charpy test obtained from prestrained materials is needed to relate abnormal fracture occurrence to embrittlement behavior caused by prestraining.  The abnormal fracture appearance located at above the 85 % SATT occurred due to the pre-strain effect and the deflection due to bending prior to crack initiation. Acknowledgements The authors would like to thank Nippon Steel & Sumitomo Metal Corporation for support in preparing this reports and permission to publish the results. The authors also wish to acknowledge Nippon Steel & Sumikin Technology Co. in the execution of the DWTT and the partial gas burst test. References American Petroleum Institute (API), 1996. API Recommended Practice for Conducting Drop-Weight Tear Tests on Line Pipe. API RP 5L3. Fujishiro, T., Hara T. and Aihara, S., 2012. Effect of plastic deformation on occurrence of abnormal fracture during DWTT, 9th International Pipeline Conference (IPC2012), Calgary, Canada, Paper #IPC2012-90165. Hasenhütl, A., Erdelen-Peppler, M., Kalwa, C., 2014. Inverse fracture – What is it all about?, 10th International Pipeline Conference (IPC2014), Alberta, Canada, Paper #IPC2014-33476. Hwang, B., Lee, S., Kim, Y.M., Kimi, N.J., Yoo J.Y. and Woo, C.S., 2004. Analysis of abnormal fracture occurring during drop-weight tear test of high-toughness line-pipe steel. Materials Science and Engineering, A368, 18-27. Kiefner, J. F., Maxey, W. A., Eiber, R. J., and Duffy, A. R., 1973. Failure Stress Levels of Flaws in Pressurized Cylinders. ASTM STP 536, 461481. Sakimoto, T., Igi, S., Oi, K., Aihara, S., 2013. The pre-straining effect on inverse fracture in DWTT for high toughness linepipe, 6th International Pipeline Technology Conference, Ostend, Belgium, Paper# S20-02. Wilkowski, G., Shim, D-J., Hioe, Y., Kalyanam, S., and Brust, F., 2012. How new vintage line-pipe steel fracture properties differ from old vintage line-pipe steels, 9th International Pipeline Conference (IPC2012), Calgary, Canada, Paper #IPC2012-90165.