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Materials Science and Engineering A 483–484 (2008) 239–241

Abnormal fracture appearance in drop-weighttear test specimens of pipeline steel

Zheng Yang a,∗, Chang-Boo Kim a, Yaorong Feng b, Chongdu Cho a

a Department of Mechanical Engineering, Inha University, Incheon 402-751, Republic of Koreab Tubular Goods Research Center, CNPC, Xi’an 710065, PR China

Received 6 June 2006; received in revised form 11 September 2006; accepted 11 September 2006

bstract

The abnormal fracture appearance (AFA), which is frequently observed on the drop-weight tear test (DWTT) fracture surface of high performanceipeline steels, makes the DWTT technique difficult to utilize. Different DWTT specimens are tested at different temperatures. The cleavage regionsf different AFA result from different phenomena during specimen fracture. All of the specimens exhibiting AFA should not be discarded as invalid

pecimens. Some new approaches are proposed to prevent or reduce the occurrence of AFA. The evaluation of DWTT specimens exhibiting AFAs re-examined considering the coupled effects of deformation, material behavior and fracture on the fracture appearance of high performanceipeline steel. 2007 Elsevier B.V. All rights reserved.

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eywords: Pipeline steel; DWTT; Abnormal fracture appearance

. Introduction

The drop-weight tear test (DWTT) has been widely used as atandard method of evaluating the resistance to fracture propaga-ion in pipelines [1,2]. But abnormal fracture appearance (AFA)s frequently observed on the DWTT fracture surface of higherformance pipeline steels [3,4]. These steels tend to undergouctile fracture at the notch root, followed by a brittle behaviors the fracture propagates across the specimen. AFA makes theWTT technique difficult to employ and some approaches haveeen made to prevent such AFA. However, no satisfactory solu-ion has been proposed yet. In this study, the intrinsic causes ofFA are analyzed by considering the coupled effects of deforma-

ion, material behavior and fracture on the fracture appearance ofigh performance pipeline steel, in order to determine whetherertain specimens exhibiting AFA qualify for DWTT.

. Different types of AFA

The test materials used in the present investigation are API70 pipeline steels directly cut from different pipes. The DWTT

∗ Corresponding author. Tel.: +82 32 8607383; fax: +82 32 8681716.E-mail address: zyang@mail.xjtu.edu.cn (Z. Yang).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.09.182

pecimen has a size of 76.2 × 305 × B mm (B is the pipe thick-ess) in accordance with the API RP 5L3 [5] specification. Theength of the specimen is taken in the circumferential directionf the pipe. The specimens are tested at different temperaturesy using the DWTT testing machine with maximum energyapacity of 30,000 J and hammer velocity of 6.9 m/s.

The DWTT specimens have a long fracture path and willroduce different fracture appearances. Ductile crack initiations a prerequisite for the appearance of the abnormal fracturehenomenon, however, only beyond a specific toughness level.WTT specimens with different thicknesses may all produceFA (Fig. 1). A shear crack starts from the notch, extends over

ome distance, and then transforms into a cleavage fracture. AFAan be observed on fracture surfaces of DWTT specimen withifferent notches (Fig. 2). The AFA cannot be fully suppressedven with a chevron notch or an electrical discharge wire-cutotch, whose corresponding fracture-initiation load and energyan be substantially reduced.

The AFAs of DWTT specimen can be classified under threeypes according to the causes of cleavage fracture appearance onracture surface. In the first type, the so-called “inverse fracture”

f a cleavage fracture mode occurs in the hammer-impactedone of the fracture surface where the width of fracture sur-ace increases abruptly (Fig. 3a). In the second type of AFA, aarge cleavage fracture zone begins from the center to the end

240 Z. Yang et al. / Materials Science and Engineering A 483–484 (2008) 239–241

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Fig. 1. The abnormal fracture appearance of different thickness specimens.

f fracture surface (Fig. 3b). The area of the cleavage fractureone is large and its residual thickness is clearly larger than thepecimen original thickness. In the third type of AFA, a largeregion of cleavage fracture appears only in the center of the frac-ure surface (Fig. 3c). The cleavage fracture zone is close to the

otch and the residual thickness of the cleavage fracture zone ismaller than the original thickness of the specimen at the fractureurface.

ig. 2. The abnormal fracture appearance specimens of (a) pressed notch, (b)hevron notch and (c) electrical discharge wire-cut notch.

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ig. 3. The abnormal fracture appearance specimens of (a) the first type, (b) theecond type and (c) the third type.

. Methods to prevent or reduce the occurrence of AFA

The AFA problem can be partially solved by inserting intohe DWTT specimen a sharp notch, such as a static precrack,

fatigue precrack, an electron-beam weld, a chevron notch,tc. [4], instead of the conventional standard pressed notch. Aharp notch can indeed reduce the fracture-initiation energy andaximal load during fracture as the region influenced by the

ammer-impact can be reduced. Whereas this method can onlyrevent or reduce the occurrence of the second type of AFA, theleavage fracture of the first AFA type occurs in the hammer-mpacted region of the fracture surface. The first AFA type,hich tends to become more serious as the material toughnessf specimen increases, originates from the enhanced transverseompressive strain which is produced by the coupled effect ofammer compression, bend of specimen and the frictional forcef test machine supports, at the hammer-impact region of frac-ure surface before fracture. The rolling supports of the test

achine can reduce the frictional force and the transverse com-ressive strain of specimens during fracture. It can prevent oreduce the occurrence of the first AFA type. The third type ofFA is caused by the change of stress state and dynamic effecturing crack propagation and is impossible to avoid by modify-ng the notch shape and/or the test method. In fact, this fracture

ype corresponds to the intrinsic fracture behavior of materialsnd it is not justified to regard specimens exhibiting the thirdype AFA as invalid and to discard them in view of the API RPL3 [5] specification.

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Z. Yang et al. / Materials Science

. Recommended methods to evaluate the DWTTpecimen with AFA

The cleavage regions of the first and second type of AFAre caused by excessive compression deformation and locallyncreasing thickness. They cannot occur during the fracture of aursting pipeline. In the fracture process, the three-dimensionalffects of the crack tip stresses decrease the pipe wall thickness.he transverse strain, local compression of hammer and speci-en bending make the specimen residual thickness larger than

he original thickness during fracture. The fracture surface ofhe second type of AFA cannot be evaluated with the API stan-ard because the cleavage region has been compressed and itsoughness has degraded. Modifying the notch to a sharp notch toeduce the fracture-initiation energy can hamper the second typef AFA. In the case of the first type of AFA, the cleavage fractureegion is small on the hammer-impact side and frequently liesutside the effective region of evaluation on the fracture surface.his type of specimen can safely be treated as a valid specimenisregarding the abnormal fracture surface.

The third type of AFA manifests the fracture characteristic ofhe material itself. The speed of fracture and the stress state ofrack tip vary during crack propagation and this can produce the

hange of ductile-brittle behavior. This should be regarded as aalid specimen but the cleavage fracture in the effective regionf evaluation should not be included in the shear area percentating.

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. Conclusions

The AFAs of DWTT specimen can be classified under threeypes according to the causes of cleavage fracture appearancen fracture surface. The rolling supports of test machine onlyan prevent or reduce the occurrence of the first AFA type.he second type of AFA should be prevented by employing aharp notch. The third type of AFA is caused by the changes oftress state and dynamic effects during crack propagation, andccounts for the intrinsic fracture behavior of the material undernvestigation. This third mode is impossible to avoid by modi-ying the notch shape and the test method. Samples showing therst and third type of AFA can be considered as valid in order

o perform the quantitative analysis, regardless of the abnormalracture and cleavage fracture surface.

eferences

1] N. Nozaki, K. Bessyo, Y. Sumitomo, I. Takeuchi, A. Yamashita, SumitomoSearch 26 (1981) 76–90.

2] B.N. Leis, R.J. Eiber, Proceedings of the 1998 Meeting on Onshore PipelineTechnology, Istanbul, Turkey, International Business Conferences/London

Eng. A368 (2004) 18–27.4] K. Seifert, Mater. Testing 26 (1984) 277–280.5] API Recommended Practice 5L3, API, 1996.