Daniel Cernadas

Measurement of the complex amplitude of transient surface acoustic waves using double-pulsed TV holography and a two-stage spatial Fourier transform method

We present a technique to measure the mechanical complex amplitude, i.e. the mechanical amplitude and phase of vibration, of an ultrasonic plane wavefield of nanometric amplitude that propagates on a surface. Our aim is to detect perturbations of the initially smooth wavefronts that indicate the presence of flaws in the material. We use bursts of surface acoustic waves (SAWs) and a double-pulsed TV holography system that records two correlograms with time separations down to 1.5 µs. The phases of the correlograms are calculated separately using the spatial Fourier transform method (SFTM) and operated on to obtain the phase change between exposures. In the resultant optical phase map, the field of instantaneous displacements of the surface (that comprises several periods of the SAW) acts as a modulated spatial carrier, now related to the mechanical phase and amplitude, that are extracted by applying the SFTM again.

Detection of transient surface acoustic waves of nanometric amplitude with double-pulsed TV holography.

We describe the detection of bursts of surface acoustic waves by a double-pulsed TV holography technique. We describe mathematically the long- and short-wave bursts in the output correlograms and validate theoretical results with experimental images. The use of short-wave bursts permits us to scan the surface and makes it easier to distinguish, for purposes of nondestructive testing, the disturbances produced by flaws.

Non-destructive testing with surface acoustic waves using double-pulse TV holography

In this paper we present the visualization of the out-of-plane displacement fields produced by Rayleigh and Lamb waves on aluminium surfaces, using a double-pulse TV holography technique. This method presents several interesting characteristics, as a possible way of making whole-field remote measurements of instantaneous acoustic displacements, with good immunity from environmental perturbations. We also show examples where different surface and subsurface flaws have been detected, demonstrating the great potential of this technique for non-destructive testing in industrial applications.

Measuring Amplitude and Phase of Vibration With Double-Exposure Stroboscopic TV Holography

We present a technique for the measurement of both amplitude and phase of sinusoidal vibrations with double-exposure stroboscopic TV holography combining optical and mechanical phase shifting. Using double-exposure stroboscopic illumination, there is no need of recording a reference state of the object and, consequently, the measurements can be made “in flight”, with a sensitivity to environmental noise much lower and with a more efficient use of the available laser power than with the former single-exposure techniques developed for the same purpose.

Enhancing the sensitivity to small phase changes in double-exposure stroboscopic television holography

We present a new technique for enhancing the sensitivity of double-exposure stroboscopic television holography (TVH) to detect and measure vibrations of small amplitude. The technique is based in the modulation of the phase of the reference beam in synchronism with the vibration of the measurand and derives from a former technique that we originally contrived for phase evaluation. We propose two variants, characterized by the demodulation process used to generate the secondary correlograms, with different behaviors in terms of the sensitivity to the sign of the measurand and of the ease in detecting the presence and shape of the vibration. We have implemented this new technique in an electronic speckle-pattern interferometer and compared its performance with standard TVH techniques; vibrations with amplitudes as small as 8 nm have been observed with this setup.

Rayleigh Wave Amplitude Field Determination by a Simple Speckle Point Interferometer

A very simple speckle point interferometer of the Michelson type and the method employed to achieve nanometric resolution and repeatability is described. Among the subjects of interest, the selection of the dimensions of the detector compared to the speckle size and fringe separation and the effects of averaging the signal obtained from successive acquisitions under identical conditions are evaluated. An application of the interferometer to the measurement of the amplitude field of a Rayleigh wave train, propagating on the surface of an aluminum slab, with nanometric resolution is presented. The average repeatability obtained is of the order of 2 nm.

A multiplicative analogical moiré phase-shifting Twyman-Green technique for Nd:YAG rod wavefront distortion measurement

Nd:YAG rod wavefront distortion has been quantified in a Twyman-Green interferometer which employs the multiplicative analogical moire phase-shifting method for demodulation. The measuring procedure is based on the superimposition of a transmission grating located outside the interferometric module with a phase modulated high-frequency Twyman-Green spatial carrier. Later, a set of fringe patterns, provided by low-pass filtering of the phase-shifted multiplicative moire patterns obtained, is combined in a generic phase-shifting algorithm (GPSA). The interesting test information is achieved once the previous procedure has been carried out with and without the transparent object located in the measuring area. The wavefront distortion (WFD) is calculated by the subtraction of these results provided by the GPSA, or directly if a differential phase-shifting algorithm (DPSA) is employed. The WFD obtained is in accordance with characteristics of the test object. The comparison of the results obtained with both families of algorithms provides information about the sensitivities to error sources of the DPSA employed. A first approximation to the analysis of significant error sources of DPSAs is presented.

Measurement of beating effects in narrowband multimode Lamb wave displacement fields in aluminum plates by pulsed TV Holography

Narrowband ultrasonic surface acoustic waves are of the greatest current interest for the nondestructive testing of thin-walled members and shell structures like plates, pipes, bridge girders, cans and many others. The measurement and characterization of ultrasonic displacement fields of Lamb waves by pulsed TV holography (TVH) is presented. Narrowband ultrasound is generated in a few millimeters thick aluminum plate by the prismatic coupling block method using a tone-burst excitation signal in the range of 1MHz. At this frequency, the plate supports only a few Lamb wave modes, mainly the A0 and S0 ones. The simultaneous presence of these modes produces a beating clearly detectable as a spatial amplitude modulation. Our self-developed TVH system performs the optical phase evaluation by the Spatial Fourier Transform Method and renders the instantaneous out-of-plane mechanical displacement field along the whole inspected area. From this field, the wavenumber of each Lamb mode can be obtained and, by combining them with the value of the ultrasound frequency and with the Rayleigh-Lamb theoretical frequency spectrum, information about the elastic constants of the specimen material is obtained.

The spatial resolution performance of a fibre-optic reflectometric technique for the automatic detection and measurement of surface cracks

We present an analysis of the spatial resolution achievable with a new fibre-optic reflectometric technique, contrived by the authors for the automatic detection of cracks in the inner sides of steam generator tubes of small diameter (≃20 mm). An analytical expression of the modulation transfer function (MTF) of our measurement system is obtained, from which the resolution limit of the technique is defined. Experimental values of the MTF, measured with a first laboratory prototype, are compared with the theoretical predictions and the range of validity of the theoretical expressions is discussed. Also, some of the results obtained with a second-generation prototype are presented, it being demonstrated that parallel cracks with a spacing of 50 μm between adjacent edges can be resolved in practical inspection tasks.

Measurement of the mechanical amplitude and phase of transient surface acoustic waves using double-pulsed TV holography and the spatial Fourier transform method

We present a technique to calculate the mechanical amplitude and phase of an ultrasonic plane wavefield of nanometric amplitude that propagates on a surface. Our aim is to detect perturbations of the initially smooth wavefronts that indicate the presence of flaws in the material. We use bursts of Rayleigh waves and a double-pulsed TV Holography system that records two correlograms separated down to 1.5 microseconds. The phases of the correlograms are calculated separately using the spatial Fourier transform method (SFTM), and subtracted. In the resultant phase map, the field of instantaneous displacements of the surface (that comprises several periods of the surface acoustic wave) acts as a modulated spatial carrier, now related to the mechanical phase and amplitude, that are extracted applying the SFTM again.