Noncontact Ultrasonic Thickness Measurements

Of Thin Aluminum She~t' *

C. Saleh + and D. A. Hutchins ++

+ Research Centre for Nuclear Techniques, BATAN++ Department of Engineering, University of Warwick

Coventry CV4 7 AL, England

Abstract

Noncontact Ultrasonic Thickness Measurements Of Thin Aluminum Sheets. A method has

been descnl>ed for the noncontact ultrasonic thickness measurements of thin aluminum sheets,using a Q-switched ruby laser generation concentric with an annular electro-magnetic acoustictransducer (EMAT) detection to imitate pulse~ho systems. The detected waveforms were analy­zed with the cuto()ff frequencies of the higher order Lamb modes. The results show in a goodagreement with micrometer measurements within 2% discrepancy.

Introduction

Industry in general has a continuing need for or interest in new and improvednondestructive evaluation (NDE) techniques to assess the integrity of structures forsafety,. economic and commercial significance. One of the commonly techniquesused in industry is thickness measurement. It is utilized on refinery and chemicalprocessing equipment, ship hulls, aircraft sections, gas pipes, pressure vessels, etc.,to evaluate the loss of thickness from external corrosion. In nuclear plants, thicknessmeasurement is usually made on fuel element cladding, reactor pressure vessels, coolantpump shafts and turbine rotors. Such measurements often be made with rapid scann­ing and without shutting down the process inside the equipment.

There are a number of methods available for NDE, but ultrasonic method isthe most widely used in major industries due to its simplicity [1]. In conventionalultrasonic methods, couplants (acoustic couplings) are usually required to provideeffective transfer of ultrasonic energy between transducers and parts being inspected[2]. This is a major limitation on extending inspections to such awkward problemsas rapid scanning testing, characterizing materials at elevated temperatures or inradioactive environment. To overcome these problems, several techniques for thenoncontact ultrasonic generation and detection in materials have been developed.These include the use of electromagnetic [3-7], electrostatic, and magnetostrictivetransducers for both generation and detection or the use of pulsed lasers for genera­tion [8-17] and interferometers for detection [18]. Combinations of such systemsare aJso possible in order to get optimum advantages [19] .

In this work, a noncontact pulse-echo system based on a pulsed laser generationand an annular electro-magnetic acoustic transducer (EM AT) detection will be appliedfor thickness measurements of thin (less than 1 mm) aluminium sheets. The aim is

• Research has been carried out at the Department' of Physics, Queen's University, Kingston,Ontario K7L 3N6, Canada.

32

to provide a noncontact inspection system suitable for industrial use. The thicknessdetermination method involves sending Lamb waves along the sample surface, and

the detected waveforms are analyzed with the cut-off frequencies of the higher orderLamb modes [20-22J. The results will then be compared with those of micrometermeasurements. The experiment can possibly be carried out using this system becausethe source and receiver are not in the same position. Furthermore, the pulsed lasersignal contains a wide bandwidth that would cause a number of Lamb wave modesto be generated.

Theory of Lamb waves

Lamb waves are ultrasonic waves propagated in a plate with two free surfaces

whose thickness is comparable to the wavelength. Lamb waves can be described as

superpositions of longitudinal and transverse waves for which particle displacementsOccur both in the directions ~f wave propagation and perpendicular to the plate Surface.Because of the resulting interference, Lamb waves have two basic modes, i.e., symme­

tric (s) modes and antisymmetric (a) modes (Fig. I). They are dispersive with theirphase velocity being a function of frequency. This makes analyzing time waveformsdifficult. Fig. 2 shows dispersion curves of the first six modes for aluminum. Note thatfor a given plate thickness and frequency, there are a fmite number of modes at which

Lamb waves can propagate. The two lowest order modes, i.e., ao and So can exist atall frequencies while all other modes have cut-off frequencies, below which they donot exist. The cut-off frequencies are given by the following formula [20J .

symmetric modes :

=

=

antisymmetric modes :

=

=

for n = I, 3, 5, ...

for n = 2, 4, 6, ...

for n = 2, 4, 6, ...

for n = I, 3, 5, ...

(1)

(2)

(3)

(4)

Here, .fc is the cut-off frequency, n is the mode number, h is the plate thickness,

vL and vT are the longitudinal and transverse w~ve velocities respectively. If vL andvT in the material are known and fc is obtained experimentally, then the materialthickness can be determined.

33

Material and Methods

A schemdI~ AfA4rAM M fk! !~~!PBtmUMB ift th~ @]l~~'im~nti~M.OWft ift lilJThe generating laser ultrasonic source was a Q-switchedlUby laser with a 30 ns pulse

duration and a maximum energy of 1.5 Joules. The laser beam was apertured andfocused with a lens through the centre of the EMAT, so that the laser source and

EMAT detector were concentric. Oil was coated to the aluminum surface to generatean impulsive vertical force [II]. The construction of EMAT detector used for this

purpose was an annular spiral pancake coil attached to a hollow cylindrical CoSmrare earth permanent magnet, with the magnetic field lines perpendicular to its face

(Fig. 4). Inner and outer diameters of the coil were 10.9 mm and 19.1 mm respec­tively, which were the same as those of the permanent magnet.

Three aluminum sheets, with thicknesses (measured by micrometer) of 0.80 mm,

0.55 mm and 0.50 mm respectively, were used in this experiment. Ultrasonic wave­

forms were taken from many different posie.Jns of each sample and recorded digitallyusing a Data Precision Data 6000 oscilloscipe. The oscilloscope was synchronoustriggered from the ruby laser itself using a photodiode. The recorded waveformswhere then transferred to an IBM PS/2 microcomputer for analysis and storage.

Results

Typical recorded time waveform and its spectrum of each sample are shown in

Figs. 5 and 6. Spectrum of the recorded time waveform can simply be obtained ina digital system by performing a Fast Fourier Transform (FFT) on the waveform.

Note from the spectra that there are distinct peaks appear above I MHz which corres­

pond to the higher order Lamb modes. A close inspection with aluminum thickness

of 0.80 rom indicates that the peaks at about 2.0,3,8 and 4.1 MHz correspond to the

a10 S2 and s, modes respectively. It is also evident from Fig. 6 ~hat the thinner the

sample the higher the cut-off frequencies of Lamb modes. Next, by taking vL= 6350 mls and vT = 3100 m/s in aluminum [I], and fc as determined from the experi-ments, the thicknesses of aluminum samples are summarized below.

micrometerLamb wave methoddiscrepancy

Sample (mm)(mm)(%)

1.

0.800±0.016 0.784±0.020 2

2.

0.550±0.019 0.540±0.011 2

3.

0.500±0.014 0.496±0.006 I

34

Discussion and Conclusion

Thickness measurements of thin aluminum sheets using the method of higher

order Lamb modes show a good agreement with micrometer measurements within2 % discrepancy. Micrometer measurements tend to be larger than the experimental

measurements because the surface roughness was not taken into consideration. To

evaluate properly the accuracy of this method, micrometer measurements would

have to be made on samples in which surface roughness was negligible comparedto the thickness, i.e., by polishing the surfaces of the samples. However, this methodmight be useful for thickness measurements of very thin materials where the echo

time of flight is comparable with the pulse length and access to only one of the sur­faces is necessary. The experiments using this method can easily be carried out without

coupling! problernsand this method lends itself to automatic inspection.

Reference

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35

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York (1973)

36

•

Direction of

wave propogotion••

-.-....--- • -.(Q)

Fig. 1 Lamb waves (a) symmetric mode (b) Antisymmetric mode.

(b)

37

,s

o ] 6 1 & 9 10

Fig. 2. Dispersion suryes of the first six modes oN ..amb waves for aluminum

transverse wave number

d

phase velocitytransverse velocity

211

Vt

half of the thickness

38

LASER

Photodiode

EM AT

SAMPLE

Fig. 3. Schematic diagram of the appratus

Data6000digital osdllo~cop~

Trigger

input

Amplifi ~r

Computer

0\M

CoSm magnet~

Fig. 4 Schematic diagram of the annular EMAT

40

a.8

a.'a.4GI

a.2""d :s~

a.1I...•

-;. -9.2E« -a.4

-a"r

II' , ' 1a

,

,..-11·1.11 a.4

a.81.21.62.aTiMe

<.1.0•.••.-5see>

1.9a.5

GI

8.a""d:s~...•- -II 5~ .E« -1.9

., u • ,

bI,,.,I

-1.;'a11.4

9.81.21.62.eTi ••••e

(.1.9""-5see>

11.6 9.4GI

9.2

~ :s~9.9

.... -~E -11.2«-9.4 CI

.,,.I-9·6.11

9.49.81.21.62.11

Ti ••••e (.1.9""-5see>

Fig. 5

Typical time waveform obtained from an aluminum sheet with thickness

of (a) 0.80 mm, (b) 0.55 mm, and (c) 0.50 mm 41