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Ultrasonic testing characteristicsof weld transverse crack

Xiaolei Sun, Chao Qi, Shunli Zhao, Zhong Sun

(Offshore Oil Engineering Co., Ltd., Qingdao 266000, China)

Abstract: This paper introduced the process of ultrasonic

testing for the finding and judging of transverse crack in

some FPSO project. The difference of echo wave shape and

amplitude between longitudinal and transverse ultrasonic

scanning was introduced, and the echo signal’s characteristic

of transverse crack was analyzed, which could be the

reference for the quality inspection of other steel structure

projects.

Key words: steel structure; ultrasonic testing; transverse crack

DOI: 10.7512/ j.issn.1001-2303.2017.13.27

0 IntroductionCrack is one of the most harmful weld defects in marine steel

structure. It can be divided into several kinds according to its extension

direction, such as longitudinal crack, transverse crack, radial crack etc.

Transverse crack usually appears in the weld boundary, which was

caused by the concentrated stress, improper welding procedure or

metallurgical factors etc., then extend to the weld and the heat affected

zones because of the incisive notch in the end of the crack and highly

concentrated stress.

At present, there are few researches on the inspection of finding

transverse crack. Ultrasonic testing has more advantages than other

traditional testing methods (such as X-ray detection, magnetic particle

testing and eddy current testing) in this field. In this paper, the process of

ultrasonic testing and defects detected was analyzed which was based

on the transverse crack finding by ultrasonic testing in the supported

steel structure of some FPSO.

1 Weld basic informationThe base material is DH36 with double bevel-groove, the thickness is

37.5mm.The weld method is FCAW with TWE-711Ni as filler metal. As

the welding environment temperature was -2℃, the weld preheat and

wind protection was adopted. Non-destructive testing was acted when

the welding was completed after 48 hours. Magnetic particle testing

was used to examine the weld surface, the result was acceptable.

The result of longitudinal ultrasonic scanning was acceptable, but a

few of echo waves were found by transverse ultrasonic scanning, the

transverse crack was judged tentatively.

Xiaolei SunEmail: sunxiaol@mail.cooec.com.cn

2 Ultrasonic Testing2.1 Preparation

2.1.1 Equipment

OLYMPUS USM35 was adopted for ultrasonic testing; the frequency

of probe is 2.5MHz.

2.1.2 Calibration

(1) The calibration of ultrasonic velocity is carried out by the first and

second back echo reflected fromФ100mm arc surface in IIW reference

block referring to AWS D1.1.

(2) The probe refraction angle is confirmed by the maximum

reflection from the Φ50mm hole in IIW reference block.

(3) The basic sensitivity is confirmed by the maximum reflection

from the Φ1.6mm hole in IIW reference block.

(4) The scanning sensitivity is basic sensitivity +6dB.

2.1.3 Standard

AWS D1.1 specifies that scanning pattern D shall be used when weld

is ground flush for transverse crack inspection as showing in Fig 1.

Acceptance criteria: any crack shall be unacceptable, regardless of

size or location.

2.2 Testing process

2.2.1 Magnetic particle testing (MT)

MT was adopted for the weld surface testing, no indicated signals

appeared during cross-magnetizing with oil-based magnetic suspension

after contrast aid applied.

2.2.2 Ultrasonic testing (UT)

(1) Analysis of reflection from crack

The ultrasonic reflection of crack in planar reflector is more sensitive

Xiaolei Sun is a project manager in COOEC and acquired master degree in 2009. Qualification certificates that he has involved several different fields, such as CWI for weld inspection, PCI for coating inspection, PMP for project management and etc. He is mainly engaged in the project management and quality inspection of marine steel structure, and he also has done some researches on special welding technique for non-metallic material, such as glass, ceramic, intermetallic compound, etc. He has written more than 20 papers which have been already published.

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Fig.4 transverse crack

Fig.2 longitudinal scanning

Fig.3 transverse scanning

Fig.1 UT scanning patterns

than hole in cylindrical reflector, which could be confirmed by the sound pressure

formula,

For long horizontal hole, P= (1)

For flat-bottom hole, P= (2)

P represents the sound pressure of wave source, S represents the square of wave

source, S ’ represents the square of flat-bottom hole, D represents the diameter of

long horizontal hole, λ is wave length, X represents the distance from flat-bottom hole

to wave source.

When the probe angle was 70°, the long horizontal hole was 1.58mm and the

hole depth was 12mm, the diameter of equivalent rounded crack obtained from

formula (1) and (2) is 1.34mm which will be the same height to basic sensitivity.

(2) No indicates were found during longitudinal scanning, the scanning range

should guarantee that the sound beam could cover the whole weld section;

(3) Defects were indicated during transverse scanning acted, the echo was spiculate

and the height was beyond the full screen display. Continuous defects were indicated

when the probe moved along the weld. Two indications appeared in screen at the same

time occasionally, which could be concluded that the cracks were arranged tightly. The

depth of defects was mostly in the range from7 to 15mm, as shown in Fig 3.The location

was marked on weld where the depth of defect was about 20mm, which could be

inferred that the crack extended to the contrast side of this weld. Not any or very small

indication was displayed when the probe moved in the opposite direction,

(4) The same results were obtained when the same operation (1)and (2) performed

on the contrast side of the weld .The depth of defect was also about 20mm where had

already be marked, which proved the extention of crack.

2.3 Verificaion

About 10mm of the weld was moved out by gouging and grinding where transverse

defects were found by UT. The transverse crack could be indicated clearly and distinctly by

MT as shown in Fig.4.

3 RepairIn order to make the proper repairing welding procedures, some test specimens were

cut down from the components which the transverse crack was existed in. Physicochemical

tests were performed for these specimens. Repairing welding procedures and new welding

procedures for the products were established on the basis of the related tests results and

analysis.

Both ultrasonic testing and magnetic particle testing were performed on the welds

which were repaired by the new welding procedures after the weld cooled down to

ambient temperature, 48hours and 72hours after welding. The testing results were

acceptable without any transverse crack and other weld defects.

4 Conclusions(1) The accuracy of ultrasonic testing for transverse crack was verified by using

magnetic particle testing after gouging.

(2) During ultrasonic transverse scanning, the characteristics of echo will be changed

obviously when changing the scanning direction, it could be concluded that the detected

crack has a fixed angle to the weld surface.

(3) Characteristics of transverse cracks detected by ultrasonic testing are obvious. Both

qualitative analysis and ration analysis for transverse crack could be carried out quickly

when the ultrasonic testing is performed strictly according to the related standard.