non-contact evaluation of thickness reduction of plates ...iv conferencia panamericana de end buenos...

IV Conferencia Panamericana de END

Buenos Aires – Octubre 2007

Non-contact evaluation of thickness reduction of plates and pipes using

EMAT-generated guided wave

Ik-Keun Park, Yong-Kwon Kim and Jin-Hyuk Lee

Seoul National University of Technology

Seoul, 139-743, Korea

+82-970-6332

+82-977-4507

[email protected]

Yong-Sang Cho

Korea Electric Power Research Institute

Daejeon, 305-380, Korea

Jun-Hyun Lee

Pusan National University

Pusan, 609-735, Korea

Abstract

The evaluation of thickness reduction in structures such as plates and pipes by guided

waves is presented. Ultrasonic guided wave techniques have been widely studied and

successfully applied to various non-destructive applications with the advantage of long

range inspection. Non-contact methods for ultrasonic wave generation and detection

have been of a great concern and highly demanded in the guided wave techniques due to

their capability of wave generation and reception on surface of high temperature or on

rough surface. A non-contact EMAT (Electro-Magnetic Acoustic Transducer) technique

for guided waves is illustrated with some applications on monitoring of corrosion

simulated thickness reduction in structures. The EMAT technique is successfully

applied for the non-contact generation and detection of guided waves in the structures.

Promising features based on the dispersive behavior of selected wave modes are found

to quantify thickness reduction. The experimental results show that the mode cutoff

measurements provide a qualitative scheme to measure thinning defect while the change

in group velocity can be used as a quantitative parameter.

1. Introduction

It has been reported that in 1968, Wallance developed a non-contact technique using

EMAT (Electro Magnetic Acoustic Transducer) to generate and receive ultrasonic

waves in a metallic material with the lift-off of several mm from the surface of the test

material. In 1970 and 1980s, the technical advance and various applications of EMAT

IV Conferencia Panamericana de END Buenos Aires – Octubre 2007 2

have been carried out by other researchers such as R.B. Thomson (3) and B.W. Maxfiled

(5, 6).

EMATs are able to generate and receive ultrasonic waves without contacting the surface

of test materials by the interaction of magnetic field and Eddy current which are induced

from magnets and coils into the test material. Thus, EMATs do not require any pre-

process or removal of coating or insulation on the test surface. But, it has been reported

that the effective lift-off should be less than 2 mm due to the rapid exponential decay of

the electromagnetic force from the surface.

Also another advantage with EMATs is that various wave modes can be generated

easily by change in the arrangement of magnets and coils. For example, shear horizontal

wave (SH wave) is not easy to generate with conventional PZT transducers due to the

coupling problem. But, this problem can be overcome easily by using EMATs since

EMATs do not require any coupling media between the transducer and the test material.

Ultrasonic guided waves have been noted as an effective method to inspect joints or

weld in plates and pipelines. It has been reported that the guided wave techniques are

successfully applied to inspection of the airplane (12)

. In the inspection of the corrosion

in a aluminum plate using guided waves, it is found that many features of the Lamb

waves such as the cutoff frequency, group velocity, and transmission and reflection

factors are sensitive to the thickness reduction of a plate (13).

In this paper, the use of EMAT is proposed as a new efficient non-contact NDE tool to

monitor the thickness variation of corrosion-simulated plates and pipes.

2. Understanding on the Ultrasonic Guided Wave Technique

As well known from earlier works, elastic waves propagating along thickness, d=2h of a

plate satisfy the Rayleigh-Lamb frequency equation.

1

22

24

)tan(

)tan(±

−−=

kq

pqk

qh

ph................................................................................................. (1)

Where the signs “+” and “-” on the right hand side are for symmetric and anti-

symmetric modes, respectively. p and q in eqn.(1) represent the corresponding wave

vector components of the partial longitudinal and shear modes in thickness direction as

illustrated in the equation below.

2

2

2 kC

pL

−

=

ω............................................................................................................. (2)

2

2

2 kC

qT

−

=

ω............................................................................................................. (3)

Where ω is the angular frequency and k is the wave number (11). Dispersion curves for a plate are numerically obtained from the eqn. (1). Fig. 1 shows

the phase and group velocity dispersion curves of a steel plate. Mode cutoff regions are


also shown. Cp, Cg, f and d denote the phase velocity, group velocity, frequency and

thickness, respectively.

Fig. 1 Dispersion curves for a steel plate (CL=5.8mm/sec, CT=3.2mm/sec), S=

symmetric modes, A=antisymmetric modes.

Thickness reduction of plates can be detected based on the measurement of various

dispersion characteristics of guided wave modes. Especially, the phase velocity of a

narrow band signal, which is a function of the wavelength of EMAT, is a key

experimental parameter to select a desired mode at a given frequency. Alternatively,

other modes can also be generated by frequency tuning. The propagation characteristics

of guided wave modes need to be carefully observed to quantitatively evaluate various

thickness reductions. In this paper, among those features (13)

, the mode cutoff and group

velocity changes are used to assess the thickness reduction.

3. Principles and Features of Lamb-EMAT

The EMAT generation and reception of ultrasonic waves involve the three following

mechanisms: the Lorentz's effect, the magnetic effect and the magnetostrictive effect.

The Lorentz's effect exists in a conductive material while the magnetostrictive effect

appears in a ferro-magnetic material.

When the magnetic field, B, and Eddy current, Je, induced by the coil near the surface

are applied to the conductive material, Lorenz force is generated in the normal direction

to both magnetic filed and Eddy current. This force moves the material particle in the

direction that the force is applied. By applying the alternative current, the force

direction is also changed alternatively and consequently, the stress waves are generated

and propagated. (Fig. 2)

N

S

N

S

N

S

Jc

Je F

BF = Je × B

Fig. 2 Wave generation mechanism of the EMAT

0 1 2 3 4 5 60

1000

2000

3000

4000

5000

6000

7000

8000

9000

AAAA0000

SSSS0000

AAAA1111

SSSS1111

SSSS2222

fd[MHzfd[MHzfd[MHzfd[MHz·mm]mm]mm]mm]

AAAA2222

0 1 2 3 4 5 60

1,000

2,000

3,000

4,000

5,000

6,000

fd[MHzfd[MHzfd[MHzfd[MHz·mmmmmmmm

AAAA0000

SSSS0000

AAAA1111

SSSS1111

SSSS2222

AAAA2222

mode cutoff

region

Cg[m

/sC

g[m

/sC

g[m

/sC

g[m

/s

Cp[m

/s]

Cp[m

/s]

Cp[m

/s]

Cp[m

/s]


In order to generate Lamb waves with EMATs, the coil and permanent magnets are

arranged as shown in Fig. 2. A permanent magnet is located above the meander coil

which is designed to have alternative direction of the current flow in each turn. The

center frequency of the EMAT is determined by the spacing of the coil. Usually, the

spacing of coil is 1/2 of the wavelength. This arrangement of the coil and magnets

results in Lamb wave in the conductive materials in which, the particle movement is

parallel to the surface.

4. Thinning Defect Monitoring

Fig. 3 shows the experimental setup for detection of thinning defect in aluminum, steel

plate and steel pipes using the Lamb wave. Fig. 4 shows a pair of EMATs and coils

used to generate and receive the Lamb waves.

Fig. 3 Schematic of Lamb-EMAT system for detection of thickness reduction in

specimens

Fig. 4 A pair of EMATs and coils

In order to consider the case of thickness reduction, the EMATs were placed on the

opposite side of the simulated corrosion defects. One of the EMATs was used as a

transmitter and the other was used as a receiver. The distance between the EMATs is

250 mm. The test specimens are shown in Fig. 5. In case of an aluminum plate whose

thickness is 2 mm and whose size is 1200 x 400 mm as shown in Fig. 5a, three artificial

defects to simulate thickness reduction due to corrosion were machined. The area of

each defect is 50 x 50 mm and the depths of the defects are 10, 20 and 30% of the plate

thickness. In case of steel plates whose thickness is 1 mm and whose size is 800 x 300

mm as shown in Fig. 5b, eight artificial defects simulating different thickness reductions

in the range between 3% and 20% of the plate thickness were machined. In case of

carbon steel pipes whose thickness is 4.5 mm and length is 2000 mm as shown in Fig.

5c, various defects were machined. The circumferential width of all defects is 30 mm.

The lengths of the defects are 30, 40, 50, 60 and 70 mm, and the thicknesses are 3, 5, 7,

10, 15, 20, 25 and 30% less the pipe thickness.


50mm*50mm△d

△d/d=10%

△d/d=20%

△d/d=30%

50mm*50mm△d

△d/d=10%

△d/d=20%

△d/d=30%

(a) (b) (c)

Fig. 5 Dimensions and locations of simulated hidden corrosion defects in three

specimens.

The mode cutoff is one of the characteristics of the Lamb wave: a wave mode

disappears or its amplitude decays with the thickness reduction. In order to enhance the

detectibility of thickness reduction with mode cutoff, it is ideal to choose the Lamb

wave modes near mode cutoff regions (Fig. 1). The experiment results of mode cut-off

due to increasing thickness reduction are shown in Fig. 6. In case of an aluminum plate,

S1 was chosen as a mode sensitive to thickness reduction in terms of the mode cut-off.

Fig. 6a shows the waveforms of S1 mode at the center frequency of 1.4 MHz in the 2

mm-thick aluminum plates with no defect, 10, 20, and 30% thickness reduction,

respectively. In case of steel plates and pipes, A1 was chosen as a mode sensitive to

thickness reduction in terms of the mode cut-off. Fig. 6b and 6c show the waveforms of

A1 mode. The center frequencies are 1.8 MHz and 0.49 MHz in the 1 mm-thick steel

plates and 4.5 mm-thick pipes, respectively.

As shown in Fig. 6, it is found that the S1 and A1 modes disappear when the thickness

is sufficiently reduced. Therefore, S1 and A1 modes turn out to be useful modes to

detect the presence of thickness reduction in the plate and pipe considered here.

However, the mode cutoff cannot provide any quantitative evaluation of thickness

reduction (8, 9, 10 and 13)

.

Since the phase and group velocities of guided waves change as functions of frequency

and specimen thickness, the change in group velocity can be used for the evaluation of

thickness reductions described below.

In order to obtain a correlation between the change in group velocity and thickness

reduction, it is better to select the mode whose group velocity is discernibly changed

due to thickness (Fig. 1). Therefore, S0 and A1 modes were selected for evaluation of

thickness reduction.

As shown in Fig. 7, it is found that S0 and A1 modes change the group velocity due to

increasing thickness reductions. In case of an aluminum plate, S0 mode was chosen as a

mode sensitive to thickness reduction. Fig. 7a shows the waveforms of S0 mode at the

center frequency of 1.21 MHz in the 2 mm-thick aluminum plate with no defect, 10, 20,

and 30 % thickness reductions, respectively. In case of steel plates and pipes, A1 mode

was chosen as a mode sensitive to thickness reduction in terms of the change in the

group velocity. Fig. 7b and 7c show the waveforms of A1 mode. The center frequencies

are 2.09 MHz and 0.62MHz in the 1 mm-thick steel plates and 4.5 mm-thick pipes,

respectively (8, 9, 10 and 13)

.


-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

Defect-free

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

dd /∆ =10%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

dd /∆ =20%

x 10-3

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dd /∆ =30%

-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

Defect-free

-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

Defect-free

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

dd /∆ =10%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

dd /∆ =10%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

dd /∆ =20%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

dd /∆ =20%dd /∆ =20%

x 10-3

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dd /∆ =30%

x 10-3

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dd /∆ =30%dd /∆ =30%

Time [sec.]

Am

plit

ude [

V]

-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

Defect-free

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

dd /∆ =10%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

dd /∆ =20%

x 10-3

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dd /∆ =30%

-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

Defect-free

-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

-0.15-0.1

-0.05

00.050.1

0.150.2

0.250.3

Defect-free

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

dd /∆ =10%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

dd /∆ =10%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

dd /∆ =20%

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

-0.15-0.1

-0.050

0.050.1

0.150.2

0.250.3

0.35

dd /∆ =20%dd /∆ =20%

x 10-3

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dd /∆ =30%

x 10-3

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

-0.15-0.1

-0.050

0.050.10.150.20.250.30.35

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

dd /∆ =30%dd /∆ =30%

Time [sec.]

Am

plit

ude [

V]

(a) (b) (c)

Fig. 6 Change in the amplitude of mode due to increasing thickness reductions in

(a) aluminum plate (S1 mode), (b) steel plates (A1 mode) and (c) pipes (A1 mode)

x 10

0.25-0.2- 0.1-0.05

00.050.10.150.20.25

0.25-

- 0.1-0.05

00.050.10.150.20.25

0.25

-0.1

- 0.1-0.05

00.050.10.150.20.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 1.2 1.4 1.6 1.8

x 10

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0

x 10

0 0.2 0.4 0.6 0.8 1.0

x 10-6

Defect-free

dd /∆ =10%dd /∆ =10%

dd /∆ =20%dd /∆ =20%

dd /∆ =30%dd /∆ =30%

Time [sec.]

Am

plit

ude [

V]

2

x 10

0.25-0.2- 0.1-0.05

00.050.10.150.20.25

0.25-

- 0.1-0.05

00.050.10.150.20.25

0.25

-0.1

- 0.1-0.05

00.050.10.150.20.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.25-0.2

-0.15-0.1

-0.050

0.050.1

0.150.2

0.25

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 1.2 1.4 1.6 1.8

x 10

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0

x 10

0 0.2 0.4 0.6 0.8 1.0

x 10-6

Defect-free

dd /∆ =10%dd /∆ =10%

dd /∆ =20%dd /∆ =20%

dd /∆ =30%dd /∆ =30%

Time [sec.]

Am

plit

ude [

V]

2

(a) (b) (c)

Fig. 7 Change in the group velocity due to increasing thickness reductions in (a)

aluminum plate (S0 mode), (b) steel plates (A1 mode) and (c) steel pipes (A1 mode)

(a) (b) (c)

Fig. 8 Results of the thinning defect monitoring: (a) aluminum plate, (b) steel

plates, and (c) steel pipes

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.51

1.5

2

2.5

3

3.5

4Group velocity

Gro

up v

elo

city [

Gro

up v

elo

city [

Gro

up v

elo

city [

Gro

up v

elo

city [

㎜㎜㎜㎜// //

ffff·d [d [d [d [㎒㎒㎒㎒·

Defect freeDefect freeDefect freeDefect free 10%10%10%10%

20%20%20%20%

30%30%30%30%

AAAA0000

SSSS

AAAA0000

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

㎷㎷㎷㎷]] ]]

Time [Time [Time [Time [μμμμsec]sec]sec]sec] 20 40 80 100 0 60 120 140 180 200 160

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

㎷㎷㎷㎷]] ]]


0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

Defect freeDefect freeDefect freeDefect free

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 0 8

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

3%3%3%3%

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 0 8

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

5%5%5%5%

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 0 8

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

7%7%7%7%

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 0 8

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

10%10%10%10%

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 0 8

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

12%12%12%12%

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 1

- 0 . 0 8

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

15%15%15%15%

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0 - 4

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

17%17%17%17%

0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2

x 1 0- 4

- 0 . 0 6

- 0 . 0 4

- 0 . 0 2

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

20%20%20%20%

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

㎷㎷㎷㎷]] ]]


Time [Time [Time [Time [μμμμsec]sec]sec]sec]

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

0 0 . 5 1 1 . 5

x 1 0- 4

- 1 5 0

- 1 0 0

- 5 0

0

5 0

1 0 0

1 5 0

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

Am

plit

ude [

㎷㎷㎷㎷]] ]]

Defect freeDefect freeDefect freeDefect free

3%3%3%3%

5%5%5%5%

7%7%7%7%

10%10%10%10%

12%12%12%12%

15%15%15%15%

17%17%17%17%

20%20%20%20%

20 40 80 100 0 60 120 140 180 200 160


The observed changes in group velocity caused by thickness reduction are plotted on

dispersion curves (Fig. 1) in Fig. 8. The theoretical and experimental results show

similar trends. According to the experimental results of Fig. 7 and Fig. 8, it is possible

to evaluate thickness reduction by the comparison between change of group velocity

and theoretical group velocity. However, the dispersion curves of the structure to be

tested should be determined prior to the field test because the Lamb wave velocity is

highly dependent on the geometry and material properties.

5. Conclusions

In this paper, the thickness reduction in thin plates and pipes using a non-contact guided

wave technique was evaluated. The EMATs were successfully utilized to generate and

receive Lamb waves in an aluminum plate, steel plates and steel pipes. The dispersive

behavior of S1, S0 and A1 modes are used for evaluation of thickness reduction. The

experimental results show that the mode cutoff of S1 and A1 modes provide a

qualitative measurement to detect the presence of thinning defect and it is also possible

to evaluate the thickness reduction quantitatively by measuring the change in the group

velocity of S0 and A1 modes.

References and footnotes

1. B Y Ahn, Y J Kim, Y G Kim and S S Lee, ‘Development of an EMAT System for Detecting Flaws in Pipeline’, Journal of KSNT, Vol 24, No 1, pp 15-21, 2004

2. M Hirao and H Ogi, ‘EMATs for Science and Industry Noncontacting Ultrasonic Measurements’, Kluwer Academic Publisher, 2003

3. J S Lim and R B Thompson, ‘Precise velocity measurement using EMATs’, KSNT/SC0009, pp 54-58, 2002

4. J H Lee, I K Park, Y K Kim, Y S Cho and D H Kim, ‘Health Monitoring of Gas Pipes using Noncontact Lamb-EMAT’, KSNT/SC0044, pp 252-256, 2007

5. B W Maxfield and C M Fortunko, ‘Design and Use of Electromagnetic Acoustic Wave Transducers(EMATs)’, Materials Evaluation, Vol 41, No 12, pp 1399-1408,

1983

6. B W Maxfield, A Kuramoto and K Hulbert, ‘Evaluating EMAT Designs for Selected Applications’, Materials Evaluation, Vol 45, No 10, pp 1166-1183, 1987

7. R Murayama, ‘A Survey of Electro- magnetic Acoustic Transducer’, J. JSNDI Vol 52, No 2, pp 63-67, 2002

8. I K Park, Y K Kim, Y Cho, Y S Ahn and Y S Cho, ‘A Study on the Behavior of Ultrasonic Guided Wave Mode in a Pipe Using Comb Transducer’, Journal of

KSNT Vol 24, No 2, pp 142-150, 2004

9. I K Park, T H Kim, H M Kim, Y K Kim, Y S Cho and W J Song, ‘Evaluation of Hidden Corrosion in a Thin Plate Using a Non-Contact Guided Wave Technique’,

Journal of Key Engineering Materials, Vol 321-323, pp 492-496, 2006

10. I K Park, Y K Kim, T H Kim, W J Song and Y S Cho, ‘Sizing Evaluation of Thinning Defects Using Noncontact EMAT Technique’, The 7th Far-East

conference on Nondestructive Testing, pp 203-209, 2006

11. J L Rose, ‘Ultrasonic Waves in Solid Media’, Cambridge University Press, 1999


12. J L Rose and L E Soley, ‘Ultrasonic Guided Waves for Anomaly Detection in Aircraft Components’, Materials Evaluation, Vol 58, No 9, pp 1080-1086, 2000

13. D Tuzzeo and F Lanza di Scalea, ‘Noncontact Air-Coupled Guided Wave Ultrasonics for Detection of Thinning Defects in Aluminum Plates’, Res. Nondestr.

Eval., Vol 13, No 2, pp 61-77, 2001

14. W Zhu, Rose, J N Barshinger and V S Agawala, ‘Ultrasonic Guided Wave NDT for Hidden Corrosion Detection’, Res. Nondestr. Eval., Vol 10, No 4, pp 205-225, 1998

non-contact evaluation of thickness reduction of plates ...iv conferencia panamericana de end buenos...

Documents