Ros Kiri Ing

Time-reversed Lamb waves

Lamb waves are extensively involved in plate structure inspection because of their guided nature. However, their dispersive nature often limits their use in flaw detection. In this paper we show that the use of a time-reversal mirror (TRM) allows to automatically compensate for the dispersive nature of Lamb waves. Experiments showing the spatial and temporal behavior of time-reversed Lamb waves, demonstrate the ability of TRMs to self-focus and to recompress dispersive pulses. This is demonstrated in a set of experiments in which a broadband ultrasonic laser source is used to simulate a point Lamb wave source and an optical interferometer is used to map the time reversed elastic field. We also show that TRM may work in pulse echo mode and allows to detect and to focus along large 2-D plates on any flaws located in the inspected area.

Time recompression of dispersive Lamb waves using a time reversal mirror-application to flaw detection in thin plates

In this paper, the time reversal processing is applied to compensate for the effect of dispersive Lamb waves. A time reversal mirror, made of a 32 element transducer array is coupled to a Duralumin plate with a Plexiglas wedge. In the first experiment, a Nd-YAG laser is used to create a point source of Lamb waves. The transmitted pulse is then recorded and time-reversed by the array and both time recompression and spatial refocusing are observed at the source point. In the second part of this paper, flaw detection in plate is performed with a pulse echo time reversal technique and we show the effectiveness of this technique to enhance the signal to noise ratio.

Transfer and Green functions based on modal analysis for Lamb waves generation

This work presents an easy way to deduce the tensorial transfer and Green functions for Lamb waves generated in isotropic elastic plates. These functions could be applied to obtain the response of each propagating mode in the ensemble of excited modes arising from any sort of pulsed excitation (wedge transducers, lasers, etc.). The transfer function is based on modal analysis development. Not only is it easy to manipulate but also allows the avoidance of laborious calculations for each kind of Lamb waves source. Theoretical predictions are compared with those of Viktorov [I. A. Viktorov, Rayleigh and Lamb Waves (Plenum, New York, 1967)] and with experimental measurements of Lamb waves generated by the wedge-transducer method.

Real-time focusing using an ultrasonic one channel time-reversal mirror coupled to a solid cavity

Focusing and beam steering is achieved by using a time-reversal process and a single transducer coupled to a solid cavity that is immersed in water. This low-cost technique makes it possible to focus acoustic energy anywhere on a 3D domain with a spatio-temporal resolution comparable to that of multiple transducers array. A short pulse is emitted from a transducer stuck at the surface of the solid cavity. The multiple-scattered field is measured in front of the solid cavity using a hydrophone needle at a reference point. This signal is then time reversed and remitted by the transducer. Around the reference point, one can observe a spatio-temporal recompression. The sidelobe level as well as the focal width no longer depend on the transducer aperture but on the dimensions of the solid cavity and the multiple paths covered by the acoustic waves in the solid. Moreover, it is shown how the experimental impulse responses on the front face of the cavity can be used to control the emitting ultrasonic field. This...

Tangible interactive interface using acoustic time reversal process

Time reversal in acoustic is a very efficient solution to focus sound back to its source in a wide range of material including reverberating media. It expresses the following physical properties: a wave still have the memory of its source location. The concept presented in this paper consists in detecting the acoustic waves in solid objects generated by a simple human touch. In a second step, the information related to the source location are extracted from a virtual time reversal experiment in the computer. Then, an action (turn on the light or a CD player) is associated to each location. Thus, the whole system transforms solid objects into interactive interfaces. Compared to the existing acoustic techniques, it presents the great advantage to be simple and easily applicable on inhomogeneous objects with any shape. The number of possible touch locations at the surface of objects is shown to be directly related to the mean wavelength of the detected acoustic wave.

Detection and imaging in complex media with the D.O.R.T. method

The D.O.R.T. method is an appropriate method to detect defects in complex and noisy solid samples. This method applies to any type of transducer array and linear waves. It is an efficient mean to separate the echo of a defect from the microstructure contribution or from another defect. The detection of defects in titanium alloy containing grain structure are shown. Results illustrating the detection at 5 MHz of flat bottom holes (FBH) with different diameters located at 140 mm depth in a titanium alloy sample are shown. Then we show that this method applies to imaging with Lamb waves. The detection of 0.6 and 0.8 mm holes in a 1 mm thick duralumin plate is presented.

Surface and sub-surface flaws detection using Rayleigh wave time reversal mirrors

The time reversal process is used to detect surface and sub-surface flaws and leads to a self focusing process of Rayleigh waves. The surface waves are generated with an array of linear transducers coupled to a specimen surface with a Plexiglas wedge. The advantages of such a technique are demonstrated with the detection of surface and sub-surface holes of subwavelength dimensions. In a pulse-echo measurement, the signal to noise ratio is exponentially increased when the self focusing repetitive process is used Measurements are calibrated with an optical interferometer and results are compared with a surface focusing model to point out the sensitivity of such a detection method.

Acoustic imaging device with one transducer.

This paper presents a low profile imaging device using only one piezoelectric transducer and a microphone. The transducer is glued to an aluminum plate of non-regular geometry that acts as an acoustic cavity. Beam steering is achieved, and the acoustic waves should be focused anywhere in front of the plate. Finally, using a single microphone receiver working in echographic mode, our imaging device is able to locate any object placed in front of it.

Focusing and beamsteering of laser generated ultrasound

The generation of acoustical waves in solids by pulsed laser irradiation has the advantage of being contactless and nondestructive in the case of the thermoelastic mode. Unfortunately, acoustical characteristics of such a source are limited. To modify these parameters, a phased array of thermooptical sources was made by moving the laser beam, and control of beam directivity was obtained. Achievement of such an array is accomplished with an acoustooptic deflector synchronized with a large pulse dye laser. The influence of the velocity and geometry of the moving source is studied, and comparison with a motionless source is made.<<ETX>>

Measurement of thickness or plate velocity using ambient vibrations

Assuming the Greens function is linear with respect to the boundary conditions, it is demonstrated that flexural waves detected by a point receiver and a circular array of point receivers centered on the previous receiver are proportional regardless location of the source and geometry of the plate. Therefore determination of plate velocity or thickness is done from the measurement of ambient vibrations without using any emitter. Experimental results obtained with a plate of non regular geometry excited with a single transducer or a remote loudspeaker are shown to verify the theoretical approach.