Evaluation for Crack Depth Sizing Capabilities of Improved ...

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Evaluation for Crack Depth Sizing Capabilities of Improved UTTechniques by Numerical Simulation of Wave Propagation

Takashi Furukawa and Ichirou Kornura

Japan Power Engineering and Inspection Corporation *DE02105757X*

NDE Center

301h MPA-Seminar in conjunction with the 9th German-Japanese Seminar

Stuttgart, October 6 and 7 2004

Abstract

A flaw depth is one of the most important factors for the structural integrity

assessment. Several ultrasonic testing techniques have been applied to flaw depth Sizing. In

this paper, the capability of the flaw depth sizing technique was evaluated using a numerical

simulation system. The kind of the simulation was a large-scale finite element method

(F.E.M.). The explicit method and square elements made it possible to calculate a large-scale

analysis more than several ten million elements by personal computer. The input data of the

simulation system is a dimension of a test piece, elastic constant and density of the materials,

flaw size, flaw position and the condition of an ultrasonic testing (for example refraction angle,

frequency and probe position). The simulation results show the ultrasonic wave propagation

in the test piece and an A-scope display of UT. The capabilities of the following two sizing

techniques were evaluated using the simulation system; one was a mode-converted wave

method (about 30 degrees shear wave and about 70 degrees longitudinal wave) and another

was a tip diffraction echo technique using a longitudinal angle beam. The simulation results

suggest that the "Improved UT' is effective for crack depth sizing.

Keywords: Ultrasonic testing, Simulation, F.E.M., Flaw depth sizing

Introduction

Many cracks have been found in primary loop recirculation (PLR) piping made of

type 316L austenitic stainless steel (SS) on several Japanese BVR nuclear power plant. The

cracks in PLR piping were detected by an ultrasonic testing (UT) using a conventi onal angle

beam technique of 45 degrees shear wave. A tip echo technique using sear wave was applied

to measure a depth of each crack. A metallurgical investigation revealed that the cracks found

in the PLR piping were stress corrosion cracking (SCO. Actual depth of each crack was also

investigated. Most of data measured by UT was almost the same compared with te actual

flaw depth; however some data was undersized 111.

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The metallurgical investigation also revealed that the cracks appeared in the surface

of the heat affected zone (HAZ) and has grown up toward the weld metal. Most crack growth

stopped around the fusion lines. Some crack tip, however, propagated into weld metal 2.

Longitudinal wave was selected for the tip echo technique to improve SCC depth sizing

accuracy in type 316L SS. A phased array technique and other useful technique were also

applied for SCC depth sizing. The flaw depth sizing capability of these techniques that were

generically called "Improved UT'techniques were verified by a Round Robin Test using PLR

piping with SCC 1.

A numerical simulation system was applied for theoretical evaluation of SCC depth

sizing capabilities of the Improved UT techniques in this paper. The simulation results of

ultrasonic wave motion in weld metal and A-scope display suggested that the Improve UT

were effective for SCC depth sizing of 316L SS.

Principle of numerical simulation system

The simulation system applied in this paper is a large-scale two-dimensional FEM by

explicit method. A large-scale analysis more than several ten million elements is required for

the simulation of UT. Limitation of the squared element shape and specialized for te

ultrasonic propagation make large scale and low resource possible. A model of isotropic and

homogeneous materials is defined by elastic constant and density. A grain axis direction is

added to the model at anisotropic materials. The grain axis direction of each element was

allocated on the model of anisotropic and heterogeneous materials. A weld metal structure

model tat was devised by J. A. Ogilvy 31 is selected in this simulation. Elastic constant and

density used in this paper are shown in Table .

Table I Elastic constant and density

Austenitic stainless steel base metal C11=C22=266 GPa,

C12= 114 GPa, C33= 76GPa

Density = 79 x 103 kg/M3

Austenitic stainless steel weld metal Cii=263 GPa, C22=216GPa,

C12= 145 GPa, C33= 129GPa

Density = 79 x 103 kg/M3

Principle of flaw depth sizing, the tip echo technique

Figure shows an example of the simulation results of the tip echo technique. A

thickness is 20mm. The refraction angle, frequency and transmitted wave mode are 45

degrees, MHz and share wave, respectively. Figure I (a) shows a calculated A-scope display

with corner reflected echo and wave front. Diffracted wave at the tip of flaw and calculated tip

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echo signal are shown in Figure 1 (b). The amplitude of the tip echo is very weak compared

'th that of the corner echo. Flaw depth (d) is a function of the difference beam path

distance between the corner echo (Wl) and the tip echo (W2) and the refraction angle (0).

d (WI - W2)cosO

won

Defect Model Defect ModelCorner Echo

Tip EchoW1 W2

_7

0 5 1 0 1 20 25 30

0 5 I 1 5 20 25 30 B.. Path Di stance

Beam Path Distance (m)

(a) Corner echo (b) Tip echo

Figure Principle of the flaw depth sizing technique (Tip echo technique)

Improved UT technique

Typical techniques of the Improved UT are the tip echo technique with focused

longitudinal angle beam and a mode converted wave method. The simulation results of the tip

echo technique using focused longitudinal angle beam is shown in Figure 2 A detail of the

simulation model and its condition are the same that of in Figure 7 and Table 2 Longitudinal

wave and shear wave are displayed in white and black color gradations, respectively. Figure 2

(a) shows that the longitudinal wave propagates into the weld metal model clearly. The tip

diffraction wave is generated (Figure 2 (a)) and the tip echo is detected (Figure 2 b)).

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L

rack Model

S

rack Model

(a) Wave front of crack tip diffraction

Tip Echo

E

0 5 1 15 20 25 30

Beam Path Distance (m)(b) Simulated tip echo

Figure 2 Simulation results of the tip echo technique using longitudinalangle beam (The crack tip is located in weld metal model)

The principle of the mode converted wave method is shown in Figure 3 This method

is applied to grasp whether the flaw depth is shallow or deep. Specifications of ultrasonic

probe are 2MHz or 4MHz and 70 degrees longitudinal angle beam probe. Waves generated

from the probe are both longitudinal (about 70 degrees) and shear wave (about 30 degrees),

schematically shown in the left of Figure 3 (a). When the share wave hits a back wall at about

30 degrees, a longitudinal wave generates from the inner surface, which is called "secondary

creeping wave". The secondary creeping wave is extremely sensitive to very hallow flaw. If a

flaw becomes deep, the flaw reflects the mode converted longitudinal wave directly as shown

in Figure 3 b). This echo is called "mode converted wave echo". An estimate of the flaw depth

can be obtained by observing the absence or presence of the secondary creeping wave and

mode converted wave echo.

51. -

2nd Creeping waveCreeping wave echo(Longitudinal ave--

2nd Creeping wa(Longitudinal wav bout 30 degrees I

Share Wave 1,

Defect

(a) Schematic diagram of beam path of secondary creeping wave and its echo

Mode converted waveecho

degreesShar ve

Defect

(b) Schematic diagram of beam paths of the mode converted wave and its echo

Figure 3 Principle of secondary creeping wave and mode-converted wave method

Evaluation of Improved UT by simulation analysis

Mode converted wave method

The model of the dimension and grain axis direction of weld metal is shown in Figure

4. Wave motion and A-scope signal was calculated using seven kind of Raw depth (2mm, 4mm,

8min, 10mm, 14min, 16mm, 18mm) model and two kind of the flaw location (located in HAZ

and deep flaw tip located in the weld metal) model. The refraction angle was 70 degrees

longitudinal and about 30 degrees shear wave. The nominal frequency was 4MHz A

dimension of each element was 0026 mm x 0026 mm, and a total number of the elements

were about 3 million. Figure shows examples of the simulation results. The echo patterns of

shallow (2mm flaw depth) and deep flaw (8nim flaw depth) are shown in Figure 5. The

secondary creeping wave was present whether the flaw is shallow or deep. On the other hand,

the mode converted wave was absent in the shallow flaw. When the flaw was deep, the mode

converted wave echo was present.

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Probe:

Weld Metal Grain axis direction 70 degrees Vwithin a weld 4 MHz

EE

C)CI4

Base Metal

Figure 4 Schematic diagram of simulation model for secondary creeping wave andmode-converted wave method

Figure 6 shows the relationshi of the flaw depth and the echo amplitude of the mode

converted wave. The result in Figure 6 N is similar to in Figure 6 (a), so that the simulation

results suggest that the mode converted wave is not affected by weld metal structure and

detected at 4mm 20% thickness) over flaw depth.

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Defect o el (Height:10% thickness) Defect Model (Height:10% thickness)

r ........ I..... .... I , ': 1; III I- 11 ... � �- , , r ........ I......... I .... '. I .... .. 1.

(D De A Creeping wave

412n" Creeping waveecho

(D

Ca

E

0 20 40 60 80 100 120 0 20 40 60 80 100 T20

Beam Path Distance (mm) Beam Path Distance (m)

(a) Simulation results of a small flaw (flaw depth =2 mm, 1 0% t)

L

Defect 0 el (Height:40% thickness) Defect Mo el (Height 40% thickness)

'�e ii�'�I ''� '1',' ;n'l' 'I . ...... ...'�e I reeping wave cl)

C e�.,t 1,�;,ht I.' 'MO C.ne r e� W.,.'_0

41 2nd Creeping wave Mode-converted

echo Wave echo

CD

co

0

0 20 40 60 80 100 120 0 20 40 60 80 too 120

Beam Path Distance (mm) Beam Path Distance W(b) Simulation results of a large flaw (flaw depth =8 mm, 40% t)

Figure 5 Simulation results of the secondary creeping wave and mode-converted wave method

applied to both small and large flaw

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Defect Model (Height:10% thickness) Defect l\To+del (Heiight:10% thickness)igh '';O ...........

t .'';n 'ew ng wave'De �'e' I W

Z32nd Creeping wave

CLecho

N

co coE E0 0

0 20 40 60 80 ]Do 120 0 20 40 60 80 100 120

Beam Path Distance (mm) Beam Path Distance (m)(a) Simulation results of a small flaw (flaw depth =2 mm, 10% t)

1- 7 L

Defect Model (Height:40% thickness) D fect el (Height:40% thickness) S

W Defect He i ght 40% t. Defect Height 40% t, Abele Converted Wave

4_1 d 412n Creeping wave Mode-convertedecho C' wave echo

JE

CU coFE E

0

0 20 40 60 80 100 120 0 20 40 60 80 100 120

Beam Path Distance (nu) Beam Path Distance )(b) Simulation results of a large flaw (flaw depth =8 mm, 40% t)

Figure 5 Simulation results of the secondary creeping wave and mode-converted wave methodapplied to both small and large flaw

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Freq. 4MHz, defect in HAZ

200%

180%160% ---------------------

+j 140% ------------CL

---------------F 120%

-a 100% -- ------- 4------- --

80% - ------- -----------E 60% -----------

0 40% ----------- --------------------------------20%

0%

0 5 0 1 5 20

Flaw Depth Ed] (mm)

(a) Flaw location is in HAZ

Freq. 4MHz, defect tip in weld

200%

CD 180%160%

140%E 120%<

-0 100%a)L-4 80% ---------------------

E 60% --- - --------------- ---------------------0 40% ---------------------- ---------- ----------z I

20% ---------------------0%

0 5 0 1 5 20

Flaw Depth [d] (mm)

(b) Deep flaw tip location is in weld metal model

Figure 6 Simulation result of the relationship between mode-converted echoamplitude and flaw depth

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Tip echo technique

The simulation model of the tip echo is shown in Figure TA thickness of the model

was 20mm and the flaw depth was 8mm. The flow tip was located in weld metal. The nominal

frequency was 5MHz A dimension of each element was 0017 mm x 0017 mm, and a total

number of the elements were about million. Figure shows the tip echo simulation of a 45

degrees shear wave. Wave front and A-scope signals of Y=31.Omm, Y=32.Omm and Y=33.Omm

are shown in Figure (a), (b) and (c), respectively. The tip echo at Y=31.0mm and Y33.Omm

is not detected because the tip diffraction wave was affected by anisotropic and heterogeneous

weld metal structure.

Grain axis directionwithin a we Id Probe:

Weld Metal Y 45 degrees5 MHz

EE

C)

Base Metal

Figure 7 Schematic diagram of simulation model for tip echo technique

The simulation results of the longitudinal wave are shown in Figure 9 These are 45

degrees longitudinal wave at Y=32.5mm (Figure 9 (a)), Y=33.5mm (Figure 9 N) and

Y=34.5mm (Figure 9 0). The tip echo was detected clearly at each probe position. These

results confirm that the tip echo technique using the longitudinal wave is effective if the crack

tip is located in the weld metal of SS for depth sizing.

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3.5 micro sec. 3.5 micro sec. 3.5 micro sec.

5 micro sec. 5 micro sec. 5 micro sec.

6.5 micro sec. 6.5 micro sec. 6.5 micro sec.

200 - 200 - Tip Echo 2 -Tip Echo

150- Tip Echo 150 150 -

100- 100 100 -

50 50 50

0 A.-A 00 5 to 15 20 0 5 10 15 20 0 5 10 15 20

Beam Path Distance (mm) Beam Path Distance (mm) Beam Path Distance (mm)

(a) Probe position: Y=31.0mm (b) Probe position: Y=32.Omm (c) Probe position: Y=33.Omm

Figure 8 Simulation results of the tip echo technique used a shear wave 45 degrees

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1.5 micro sec. 1.5 micro sec. 1.5 micro sec.

2.5 micro sec. 2.5 micro sec. 2.5 micro sec.

3.5 micro sec. 3.5 micro sec. 3.5 micro sec.

4.5 micro sec. 4.5 micro sec. 4.5 micro sec.)OXIO )OXIO,

Tip Echo TipEcho Tip Echo150 150 150

100 100

50 50 50

0 0 00 5 10 1 5 20 0 5 10 15 20 0 5 1 0 15 20

Beam Path Distance W Beam Path Distance (mm) Beam Path Distance (mm)

(a) Probe position: Y=32.5mm (b) Probe position: Y=33.5mm (c) Probe position: Y=34.5mm

Figure 9 Simulation results of the tip echo technique used a longitudinal wave 45 degrees

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Conclusion

An influence of the anisotropic and heterogeneous weld metal structure was

investigated in this simulation. The numerical simulation results suggest that the "Improved

UT" is effective for the depth sizing capability of SCC in 316L SS.

References

[11 H. Tokuma, T. Fukuda and T. Furukawa, "SCC Experiences and NDE Technologies on

Recirculation Pippins in BWRs", 5th International Workshop on the Integrity of Nuclear

Components ASINCO (Asian Society for Integrity of Nuclear Components), April 2004

[21 K. Kumagai, et al., "EVALUATION OF IGSCC GROWTH BEHAVIOR OF 316NG PLR

PIPING IN BWW', Proc. ASME/JSME PVP2004, 2004

[31 J. A. Ogilvy, "Computerized ultrasonic ray tracing in austenitic steel", NDT

INTERNATIONAL, Vol. 8, No.2, pp.67-77 1985