Time domain model and experimental validation of non-contact surface wave scanner.

This paper presents a time-domain model for the prediction of acoustic field in an air-coupled, non-contact, ultrasonic surface wave scanner, which includes an air-coupled Emitter, the Propagation space, and an air-coupled Receiver (EPR). The computation is divided into three steps, with each step being modeled in the time domain by its spatio-temporal transfer function. The latter are then used in turn, to find the pulse response of the overall system. The model takes the finite size of the aperture receiver, the attenuation in both air and the tested solid sample, as well as the electric response of the emitter-receiver set he into account. The attenuation is characterized by a causal time-domain Green's function, allowing wideband attenuation of a lossy medium, obeying the power law αω=α0ωη,0⩽η⩽2, to be used. The model is implemented numerically using a Discrete Representation approach. It is then validated quantitatively by comparing the predicted acoustic field with experiment. The prediction error for three typical field features, the system's impulse response, the on-axis field distribution, and the directivity pattern, is globally smaller than 3%. In order to obtain this high level of accuracy in the model, the parameters characterizing the solid sample used during the experiment were measured experimentally, with a specifically developed experimental setup. Overall, the proposed model is approximately 100 times faster than 3D FEM with an equivalent spatio-temporal resolution. In parallel, a simplified model is proposed, which neglects the attenuation in air and assumes the emitter inclination angle to be perfectly adjusted. This approach makes it possible to further shorten the computational time by a factor of about ten, whilst maintaining good accuracy. Thanks to its computational efficiency, the proposed model can be used to formulate various recommendations concerning the scanner settings, in particular the inclination angles of the emitter and receiver, and their distance from the sample.

A time-domain model and experimental validation of the acoustic field radiated by air-coupled transducers.

This paper presents a time-domain model for the prediction of an acoustic field in an air-coupled, non-contact, ultrasonic setup, which includes an air-coupled Emitter, the Propagation space and an air-coupled Receiver (EPR). The model takes into account the finite size of the aperture receiver, attenuation in air, and the electric response of the emitter-receiver set he. The attenuation is characterized by a causal time-domain Green's function, allowing the wideband attenuation of a lossy medium obeying the power law α(ω)=α0ωη,1⩽η⩽2 to be included. The electrical response is recovered experimentally using a procedure which includes the deconvolution of air absorption effects. The model is implemented numerically using a discrete representation approach. In order to study the influence of receiver size and attenuation, five different computational approaches are proposed; each of these is evaluated quantitatively, by comparing the predicted acoustic field with the experimentally measured signal. The prediction error is studied in both the near and far fields, for three typical field features: the system's impulse response, the on-axis field distribution, and the directivity pattern, for the case of air-coupled transducers operating at two different central frequencies, namely 50kHz and 350kHz, with a 10mm diameter wideband receiver. It is shown that when the attenuation in air, the receiver size, and the accurately recovered electric response he, are correctly taken into account, the model allows the system's impulse response to be accurately predicted, with on-axis errors ranging between 0.2% in the far field and 1% in the near field. In the near-field area and within the far field -3dB beam spread width, the error is generally greater than on the axis, but globally remains smaller than 1%. Inclusion of the size of the receiver dimension in the model appears to be crucial to the accuracy of the near field predictions, and an approximate criterion is proposed for the evaluation of the influence of receiver. The procedure used to recover the electric response he is also presented in detail. The results obtained from this study are used to formulate various recommendations related to EPR modelling.

Non-destructive characterization of surfaces and thin coatings using a large-bandwidth interdigital transducer.

This paper deals with non-destructive testing of thin layer structures using Rayleigh-type waves over a broad frequency range (25-125 MHz). The dispersion phenomenon was used to characterize a layer-on-substrate-type sample comprising a thin layer of platinum 100 nm thick on a silicon substrate. The originality of this paper lies in the investigation of different ways of generating surface acoustic waves (SAWs) with large bandwidth interdigital transducers (IDTs) as well as the development of a measuring device to accurately estimate the SAW phase velocity. In particular, this study focuses on comparing the performance (in terms of SAW amplitude and bandwidth) of different excitations imposed on IDTs. The three types of excitations are burst, impulse, and chirp. The interest of chirp excitation compared to the other two types was clearly demonstrated in terms of the SAW bandwidth and amplitude of displacement. With these IDT transducers, measurements could be performed over a wide frequency band (20-125 MHz), and consequently, dispersion curves could be obtained over a wide frequency band with a range of velocity variations in the order of 100 m/s. Under these conditions, an extremely accurate estimate of the phase velocity as a function of the frequency could be obtained using a Slant Stack transformation. Finally, from these experimental dispersion curves and theoretical dispersion curves, an accurate estimate of the thickness of the layer could be obtained by inversion. This estimated thickness was then confirmed using profilometer measurements.

Effective and rapid technique for temporal response modeling of surface acoustic wave interdigital transducers.

Surface Acoustic Wave Interdigital Transducers (SAW-IDT) has a considerable application potential for characterization of properties of thin layers, coatings and functional surfaces. For optimization of these SAW-IDTs, it is necessary to study various SAW-IDT configurations by varying the number of electrodes, dimensions of the electrodes, their shapes and spacings. The finite element method (FEM) is generally used to model such transducers but results are obtained in several hours (or days). Thus it is necessary to implement effective and rapid technique for SAW-IDT modeling. In this study, we develop simulation tool based on Spatial Impulse Response model. Therefore, we reduce considerably computing time and results are obtained in a few seconds. In order to validate this method, theoretical and experimental results are compared with finite element method. The results obtained show a good concordance and confirm effectiveness of suggested method. In additional, this method requires less computer memory.

Generation of broadband surface acoustic waves using a dual temporal-spatial chirp method.

Wideband surface acoustic wave (SAW) generation with a spatial chirp-based interdigital transducer was optimized for non-destructive characterization and testing of coatings and thin layers. The use of impulse temporal excitation (Dirac-type negative pulse) leads to a wide band emitter excitation but with significantly limited SAW output amplitudes due to the piezoelectric crystal breakdown voltage. This limitation can be circumvented by applying a temporal chirp excitation corresponding in terms of frequency band and duration to the spatial chirp transducer configuration. This dual temporal-spatial chirp method was studied in the 20 to 125 MHz frequency range and allowed to obtain higher SAW displacement amplitudes with an excitation voltage lower than that of the impulse excitation.

Surface acoustic wave characterization of optical sol-gel thin layers.

Controlling the thin film deposition and mechanical properties of materials is a major challenge in several fields of application. We are more particularly interested in the characterization of optical thin layers produced using sol-gel processes to reduce laser-induced damage. The mechanical properties of these coatings must be known to control and maintain optimal performance under various solicitations during their lifetime. It is therefore necessary to have means of characterization adapted to the scale and nature of the deposited materials. In this context, the dispersion of ultrasonic surface waves induced by a micrometric layer was studied on an amorphous substrate (fused silica) coated with a layer of ormosil using a sol-gel process. Our ormosil material is a silica-PDMS mixture with a variable polydimethylsiloxane (PDMS) content. The design and implementation of Surface Acoustic Wave InterDigital Transducers (SAW-IDT) have enabled quasi-monochromatic Rayleigh-type SAW to be generated and the dispersion phenomenon to be studied over a wide frequency range. Young's modulus and Poisson's ratio of coatings were estimated using an inverse method.

Ultrasonic reflectometry in air: errors of sample positioning.

The reflection coefficient of ultrasonic waves propagating in air and interacting with a plane surface of a solid is considered. The simulation of dependence of the reflection coefficient on errors of sample positioning is performed using a finite beam model along with an angular spectrum method, and next the results are validated experimentally. The simulations show that for the considered range of geometrical parameters, the role of the wave divergence for the reflection coefficient in air is insignificant. The important consequences of errors of the sample positioning are that the shift of the sample influences mostly the phase, while the errors of inclination of the sample mainly affect the magnitude of the reflection coefficient. The experiments confirm simulation results pointing out the necessity of high precision of measurements for ultrasonic reflectometry in air. The results can be used for assessment of precision, calibration, and reduction of errors in applications of the reflectometry tests.