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Non contacting ultrasonics: a review of new approaches lead in CETIM H. WALASZEK Centre Technique des Industries Mécaniques (CETIM), 52, Avenue Felix Louat -BP 80067- 60304 Senlis, France D. CLORENNEC, D. ROYER Laboratoire Ondes et Acoustique, ESPCI-Université Paris 7, 10 Rue Vauquelin, 75231 Paris Cedex 05, France Contact

ABSTRACT

In industry, ultrasonic techniques enables to detect discontinuities inside of inspected parts, by analogue way than in medical field.
As in medical echography, a coupling medium in used to transmitt the energy from vibrating piezoeletric tranducer to the part. This medium, which oftenly consists in water or gel, reduces strongly the number of industrial cases of application of ultrasonic testing. So, hot or porous media cannot be controled by ultrasonics. Another type of limitations are testing on contaminated surfaces, or on parts subject to high speed motion, by example during rolling process.
The technical center for mechanical industries (CETIM, France) works presently in the field of non contacting ultrasonics. The techniques developed consist mainly in laser techniques, electromagnetic techniques (EMAT) and air coupled tranducers.
The present paper presents these different techniques and discusses the potentialities of its applications.

Introduction

Ultrasonic technique is mainly used in industrial field on echographic mode, like in medical field. Generaly, high frequency mechanical pulses are produced by piezoelectric devices, comonly called tranducers.
The resulting vibration which is called ultrasound is transmitted to the component under test by a coupling medium such as water, grease, oil or gel. The ultrasonic energy is reflected ever by backwall, or by eventual voids and discontinuities present in the material. The reflected energy returned to the transducer is converted in electric signal. Such signal is displayed on a screen, which allows monitoring of ultrasonic echoes reflected by the component under test.
The interest of the technique is its very good portability and the ability to test the material in its depth. More, the method needs access to only on side of the component, which is not possible with radiography. Nevertheless, the necessity to use coupling medium reduces the spectrum of applications in some generic applications which we are listed in following lines.
By example, porous material avoid to use couplant and some composite material (hoveycomb structures) cannot be tested by immersion. In this last case, the water may empty the discontinuities, that consequently do not produce echoes any more. Another limitation consists in difficulties in testing of hot temperature material, which produces vaporization of coupling medium, occuring transducer damage. Testing moving parts such as in rolling or milling process makes U.T testing very difficult because of instability of coupling medium induced by the motion. More, in some cases, components to be tested are manufactured at such high rate that does not allow traditional coupling application. Last limitation is necessity of performing coupling removal off the part after testing, which can be too much time consuming for some manufacturing processes.
Thus, there is a need for methods using alternative transmission of ultrasonic energy, overcoming the limitation of traditional coupling. CETIM, which the aim is to provide applied reseach for mechanical industries, leads several studies in this direction. The studied methods enabling non contact ultrasonic testing are discussed in following paragraphs :
• laser optical ultrasonic methods allow to generate and detect ultrasonic waves. If, these methods are not significantly influenced by the distance between optical system and the inspected part, they need stable position and are almost laboratory method
• electromagnetic methods (EMAT) are more site applicable but only when it is possible to maintain a small distance between the test object and the EMAT probe
• air coupling tranducer can work with greater distance, test object - probe, and give good performance on polymer composites.

Laser coupled ultrasonics

In the field of nondestructive evaluation, the need to detect surface breaking defects has motivated extensive work on the interaction of surface acoustic waves (SAW) with natural and artificial flaws [1]. Laser generation and detection of SAW (Rayleigh waves for instance) is potentially useful to investigate material surfaces. The considerable advantage of using lasers rather than conventional piezoelectric transducers is that this technique does not require any mechanical contact with the inspected surface [2]. At a sufficiently low absorbed power density, the acoustic waves are generated by the thermoelastic expansion. In this regime, a pulsed laser offers a wideband reproducible source. An optical interferometer is used as a large bandwith (0.01-20 MHz) non-contact point detector of the surface displacement associated with the Rayleigh wave. The sensitivity of the probe used enables us to detect displacements as small as 1 Å [3].
In this paper, we show how a slot on a cylinder influences the propagation of the Rayleigh waves.

Experimental results
The pulsed Nd:YAG laser is focused on the component. The acoustic energy is launched along a direction normal to this source. The angle between the Q-switched laser and the optical heterodyne probe is 90^o. The sample is a 25 mm diameter steel cylinder. The received signals are sampled and averaged in order to improve the signal-to-noise ratio. Then, they are stored in a computer and digitaly filtered in the 0.5-6 MHz range (fig.1).

Fig 1: Experimental set-up.

The longitudinal and shear wave velocities of the material are respectively 5880m/s and 3200m/s,. The theoretical Rayleigh wave velocity on a plane surface is found to be 2965m/s.

Fig. 2-a shows the mechanical displacement detected by the optical probe on the 25 mm diameter steel cylinder. The different echoes correspond to surface waves propagating around the cylinder in two opposite direction. As it is well know for Rayleigh waves, the amplitude decreases as where r is the propagation distance. The dispersive effect due to the surface curvature and the p/2 phase jump at the pole can be also observed [4].

Fig 2: Thermoelastic waveforms on a steel cylinder of diameter 25mm a) Time response, b) Spectrum

Fig. 2-b shows the Fourier transform of the signal. The resonance spectrum envelope is similar to the first echo spectrum. The maxima positions in the frequency spectrum correspond to different orders n of Rayleigh waves resonance on the cylinder. If n and n+1 are two successive resonance modes, the condition of constructive interference requires [5]:


(1)

where k_n is the wave number of the n_th modal resonance and R the cylinder radius. Assuming that f_n is the nth resonance frequency, we deduce from (1) the following formula :


(2)

which gives approximately the group velocity only related to the frequency difference Df.
At small n, the radius is of the order of the wavelength, the vibration penetrate the whole medium; at large n, the waves are confined close to the surface.

Fig. 3-a shows the mechanical displacement in the presence of a 0.25 mm deep, 0.2 mm wide artificial slot. The interaction between the Rayleigh pulse and the slot generates many waves (reflected and transmitted Rayleigh pulses, bulk waves generated by conversion on the tip of the slot).

Fig 3: Thermoelastic waveforms on a steel cylinder of diameter 25mm in the presence of a slot (depth: 0.25mm, with: 0.2mm). a) Time response, b) Spectrum

Fig. 3-b shows the fast Fourier transform of the previous signal. In the presence of a slot, the high frequency resonances are attenuated. The "cut-off" frequency (f_c) is proportional to the depth of the slot (h). The low-frequency waves propagation was not affected by the slot.
Other experiments with constant width slots (0.2mm) and variable depth (0.5mm,1mm) provide the following relation between the "cut-off" frequency and the depth of the slot :

f_c * h = cte In conclusion, this paper shows the possibility to inspect a section of the cylinder with a single laser shot and to determine the depth of the slot by measuring with the "cut-off" frequency.

Electromagnetic coupling (Electromagnetoacoustic called EMAT)

The principle is based on interaction between eddy current produced by a coil with permanent magnetic field produced by a magnet (fig. 4).


Fig 4: producing of ultrasound by EMAT

A high power electric pulse is sent in the coil. Both coil and magnet are placed near the surface of the material to inspect, which is to be conducting of electricity.
Pulsed eddy current and magnetic field produce pulsed Lorentz-force. This force results in propagation of ultrasound, from the region were eddy current were created, ie surface of the material. By reverse EMAT mechanism, echoes reflected by the material are detected by this tranducer and displayed or monitored like conventional ultrasonic signals. Trials performed in CETIM showed that EMAT generation of ultrasonic wave is specially interesting when coupling errors are prohibitted, like for stress measurement application, or when testing surfaces are hot. Generally, such system produces bulk shear waves but special arrangement allow to generate compressional waves too. In fig. 5 is presented echoes obtained in railway wheel,.


Fig 5: echoes on railway wheel

EMAT technique can also be used to produce surface wave, by using an array of electricity conducting elements placed at the surface of the area to be inspected.
Alternative high frequency current is sent in the array and eddy current which are produced at the material surface, combined with steady magnetic field B, produce Rayleigh wave. This wave propagates at the surface (fig. 6).
Such EMAT systeme was designed in CETIM and tested on rail, giving good signal to noise ratio. Sought application is stress measurement and surface defect detection.

Fig 6: production and detection of surface waves on a rail ( F=0.5 MHz) Left: principle ; right : echoe obtained

Air coupled ultrasonics

Coupling ultrasonics by air is generally not possible, because of the strong attenuation in the air. Nevertheless, air coupling remains possible when testing can be performed at low ultrasonic freqency. So, an example of application is testing of delamination on hoveycomb composites at 100 KHz. In this case, we used two focused transducers (fig. 7), with high power excitation. These transducers generate and detect ultrasonic wave in the material. If bulk waves propagate through the skin and the honeycom structure, plate waves propagate only in the skin of the structure (fig.7). Such arrangement allows acceptable resolution (some millimeters...), relatively to the low frequency used. Air coupling transducers are very interesting when total immersion in a tank or local immersion with water squirter are prohibitted.

Fig 7: generating ultrasonic waves with focused air transducers

Conclusion

Some of the methods presented in the paper such as laser or air coupling are already used in Aitcraft Industry, but not already spread out in mechanical industry .
As explained in the paper, CETIM is testing and adapting these methods and, quickly will make available for industrial application, the techniques which interest and applicability is proven for the mechanical industry. So, application of such development will enable significative extension of investigation field of ultrasonic NDT.

References

1. J.A. Cooper, RA Crosbie, R.J. Dewhurst, A.D.W. McKie and S.B. Palmer, IEEE
   Trans. Ultrason. Ferroelect. Control, 33 (1986) p.462.
2. C.B. Scruby and L.E. Drain, Laser Ultrasonics: Techniques and applications, Adam Hilger, Bristol , 1990.
3. D. Royer and E. Dieulesaint, IEEE, Ultrason. Symp. Proc. 1986 pp.527-30.
4. Y. Shui, D. Royer, E. Dieulesaint and Z. Sun, Ultrasonics symposium, 2-5 October, 1998, Chicago.
5. X. Jia, D. Royer and G. Quentin, Journal de Physique IV, Volume 2, Avril 1992.pp.789-792.
6. I.A. Viktorov , Rayleigh and Lamb waves, Plenum, New York ,1967.
7. D. Royer, C. Chenu, Experimental and theoretical waveforms of Rayleigh waves generated by thermoelastic laser line source, To be Published in Ultrasonics (2000).

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