Ultrasonic Surface Waves Research Articles

Thermo-optical modulation of ultrasonic surface waves for NDE

The well-known thermo-elastic effect of laser irradiation can be exploited to produce strong localized stresses when an expanded, long pulse, low-intensity laser beam is used to irradiate the specimen. These stresses will produce a parametric modulation of the received ultrasonic signals, that is somewhat similar to the acousto-elastic effect often used in nonlinear ultrasonic studies. It is shown in this paper that otherwise hidden small cracks in fatigue-damaged aluminum and titanium specimens can be readily detected by exploiting this optically induced thermo-elastic modulation during ultrasonic surface wave inspection since they are susceptible to crack closure and therefore exhibit strong parametric modulation. The temporal and spatial variations of the ultrasonic signals due to laser irradiation were evaluated numerically and experimentally. Based on these results, the direct temperature modulation of the ultrasonic velocity can be separated from the thermo-elastic stress modulation present only in cracked specimens. It was found that this method can be used to selectively increase the sensitivity of ultrasonic flaw detection to small fatigue cracks by more than one order of magnitude.

Study of porosity and stiffness measurement of nanoporous aerogel films with wideband ultrasonic surface waves

Nanoporous aerogel films are of great scientific and commercial interest because of their outstanding potential for application to microelectronic interconnect, associated with low dielectric constant. Of the parameters which describe such aerogel films, density/porosity and stiffness are two of the most critical, but are difficult to measure. This paper shows how measurement of the dispersion of laser-generated surface acoustic wavepackets travelling on submicron-thick aerogel films on silicon substrates allows the density and Young's modulus to be extracted reliably. Particular attention is paid to accuracy of measurements and sensitivity to input data; and a method for extracting measurements on very thin films (<300 nm) is presented.

Tunable multiple-scattering system

To study the propagation of multiply scattered waves, it is useful to have a medium in which the scattering properties can be easily controlled. Here, we describe such a system; it involves the propagation of ultrasonic surface waves in a medium with an aligned, disordered pattern of grooves. Waves propagating parallel to the grooves see a homogeneous medium; waves propagating perpendicular to the grooves are strongly scattered. By varying the source–receiver distance and orientation with respect to the grooves, we are able to map out the transition from ballistic to diffusive propagation. In addition, by using an optical detection system we are able to measure the wave motion inside the scattering medium. We measure the angle-dependent macroscopic properties of the medium, such as the group velocity as well as the mean-free path and the diffusion constant in the strong-scattering regime.

Investigation of the microstructure of cast iron by laser ultrasonic surface wave spectroscopy

We have used an all-laser surface wave ultrasonic method to investigate cast iron samples with a variety of morphologies of included graphite precipitation. The relation between ultrasonic attenuation and the microstructure of cast iron is discussed. The frequency dependence of ultrasonic attenuation was obtained in a broad frequency band including the three characteristic regions of scattering. The scatterer size of the samples could be evaluated from the measurement, and good agreement was found between the average scatterer size estimated from our ultrasonic data and micrographic data. The demonstrated sensitivity of the ultrasonic propagation parameters to the microstructural properties of cast iron illustrates the strong potential of this technique for non-destructive microstructural characterization of polycrystalline materials.

Quantitative flaw reconstruction from ultrasonic surface wavefields measured by electronic speckle pattern interferometry.

A new method for imaging flaws in plate and shell structures is presented. The method employs two-dimensional ultrasonic surface wave data obtained by optical electronic speckle pattern interferometry (ESPI) techniques. In the imaging method, the measured out-of-plane displacement field associated with an externally excited ultrasonic Lamb wave is processed to obtain the spatial frequency domain spectrum of the wavefield. A free space Green's function is then deconvolved from the wavefield to obtain quantitative images of effective scattering sources. Because the strength of these effective sources is directly dependent on local variations in sample thickness and material properties, these images provide a direct map of internal inhomogeneities. Simulation results show that the method accurately images flaws for a wide range of sizes and material contrast ratios. These results also demonstrate that flaw features much smaller than an acoustic wavelength can be imaged, consistent with the theoretical capability of the imaging method to employ scattered evanescent waves. Reconstructions are also obtained from ultrasonic Lamb wave displacement fields recorded by ESPI in a flawed aluminum plate. These reconstructions indicate that the present method has potential for imaging flaws in complex structures for which ESPI wavefield measurements cannot be straightforwardly interpreted.

Detection of Surface-Breaking Cracks by Laser-Based Ultrasonics

Surface breaking cracks in a structure can be ultrasonically detected by using Rayleigh or Lamb waves. Either reflected echoes or transmitted signals may be monitored in the pulse-echo or pitch-catch modes of operation. Pulse-echo and pitch-catch techniques can also be used with laser ultrasonics where a high-power pulsed laser is used to generate ultrasound thermoelastically. Laser-based ultrasonic (LBU) techniques provide a number of advantages over conventional ultrasonic methods. However, small flaws give rise to weak reflections or small changes in the amplitude of transmitted signals. These small variations are often too weak to be detected with existing laser detectors. In this paper we provide the theoretical background for an alternate approach for ultrasonicetection of small surface-breaking cracks using laser-based techniques - the Scanning Laser Source (SLS) technique. This approach does not monitor the interaction of a well-established ultrasonic surface wave with a flaw, but rather monitors the changes in the generated ultrasonic signal as the laser source passes over a defect. Changes in amplitude and frequency of the generated ultrasound are observed which result from the changed constraints under which the ultrasound is generated over uniform versus defective surface areas. These changes are quite readily detectable using existing laser detectors even for very small flaws. To analyze the ultrasound generated when the laser beam is right above the defect, the locally important effects of heat absorption and diffusion must be carefully considered.

Multiple scattering attenuation and anisotropy of ultrasonic surface waves

Multiple scattering of waves induces bulk effects such as attenuation and anisotropy that are important in seismology, optics, medical imaging, and other fields involving propagation in disordered media. Measurements are reported of ultrasonic surface wave propagation in a strong-scattering, quasiperiodic medium consisting of a grooved surface of aluminum. Using noncontacting optical methods we have tracked the evolution of surface wave pulses within the scattering medium. Waves propagating parallel to the grooves propagate nearly attenuation and dispersion free, whereas waves propagating normal to the grooves are dispersed and exponentially attenuated with distance as energy is transferred from the direct pulse into the multiple-scattering coda. We measure this attenuation length and show that there is, in addition, a scattering induced anisotropy in the phase velocity. [This work was partially supported by the U.S. Army Research Office under Grant No. DAAG55-98-1-0070.]

Laser ultrasonic surface wave inspection of alumina ceramics of varying density

In this paper, the surface acoustic wave velocity results acquired from the inspection of specially manufactured and characterised alumina ceramic materials are presented. Ultrasonic velocity data of alumina-based ceramics in the range 60–100% theoretical density was generated utilising non-contacting laser-ultrasonic measurements based on laser generation and detection of surface acoustic waves with the objective of creating a routine technique for industrial advanced alumina inspection. With linear fitting the surface acoustic wave velocity data serves as a calibration graph for using laser ultrasonics for routine monitoring of alumina. A second laser ultrasonic technique based on the laser generation and foil transducer detection of surface acoustic waves was used to validate the surface acoustic wave velocities measured by the laser generation/detection technique.

Characterisation of anisotropic elastic constants of continuous alumina fibre reinforced aluminium matrix composite processed by medium pressure infiltration

The anisotropic elastic properties of an Al metal matrix composite unidirectionally reinforced with continuous α-alumina fibres fabricated, by a medium pressure infiltration technique, has been characterised using ultrasonic bulk and surface waves, tensile tests and microstructural analysis. The ultrasonic data are analysed under the continuum mechanics approximation. Elastic anisotropy is studied by varying the direction of propagation of ultrasonic waves. An optimisation process is used to recover all the volumic effective elastic moduli from bulk ultrasonic velocities. Two local (sub-surface) shear effective moduli are determined from the surface waves velocities. To discuss the elastic anisotropy of the material in relation with its microstructure and the manufacturing process, microstructural observations are correlated to the global and local ultrasonic evaluations and to the mechanical characterisation conducted in and out of symmetry axis.

Remote laser generation of narrow-band surface waves through optical fibers

This paper demonstrates the use of a fiberoptic bundle for flexible, compact, remote, and noncontact laser generation of surface ultrasonic waves in materials. The bundle is able to deliver Nd:YAG pulsed light with a 60% delivery efficiency up to an average energy of 55 mJ/pulse for a pulse duration on the order of 10 ns and a pulse repetition rate of 20 Hz without signs of fiber damage. Details of the bundle construction and surface preparation are given, and pulsed light delivery tests performed with single tapered fibers are discussed. The high-power light delivery capabilities of the bundle are demonstrated for the generation of narrow-band surface waves in a Carbon/PEEK composite laminate by a spatial modulation technique that employs a periodic transmission mask. Single laser pulse ultrasonic tonebursts are clearly detectable using a small aperture piezoelectric transducer while ensuring thermoelastic generation conditions. The theory of narrow-band generation of surface acoustic waves is improved by accounting for the strength nonuniformity of the illumination sources. In addition, the effect of the number of illumination sources on the bandwidth of the generated surface wave is assessed experimentally, and excellent agreement is shown with the theoretical results predicted by the improved model.

Thermoelastic effects on surface acoustic wave propagation

The effect of thermal stresses on ultrasonic surface wave propagation was investigated. Quasi-static and dynamic thermal stresses and deformations were considered in materials with and without fatigue cracks. The perturbation of ultrasonic surface waves due to these thermal effects were studied both theoretically and experimentally. A long-duration infrared laser pulse was used to irradiate a series of aluminum and titanium specimens with fatigue cracks of known sizes between 0.5 and 1.0 mm. Preliminary numerical results from finite element simulation were found to be in good agreement with the experimentally observed laser-induced modulations of the ultrasonic surface wave. In particularly, both the numerical and experimental results indicated the same characteristic transient behaviors of the thermo-optical modulations for cracked and intact materials. The different behaviors suggest a new promising method for ultrasonic nondestructive evaluation, which cannot only effectively distinguish fatigue cracks from other artifact scatters, but potentially can also provide a method for quantitative evaluation of crack features. [This effort was sponsored by the Defense Advanced Research Project Agency (DARPA) Multidisciplinary University Research Initiatiative (MURI), under Air Force Office of Scientific Research Grant No. F49620-96-1-0442.]

Multiple scattering attenuation and anisotropy of ultrasonic surface waves

Multiple scattering of waves induces bulk effects such as attenuation and anisotropy that are important in seismology, optics, medical imaging, and other fields involving propagation in disordered media. We report here measurements of ultrasonic surface wave propagation in a strong-scattering, quasiperiodic medium consisting of a grooved surface of aluminum. Using noncontacting optical methods we have tracked the evolution of surface wave pulses within the scattering medium. Waves propagating parallel to the grooves propagate nearly attenuation and dispersion free, whereas waves propagating normal to the grooves are dispersed and exponentially attenuated with distance as energy is transferred from the direct pulse into the multiple-scattering coda. We measure this attenuation length and show that there is, in addition, a scattering induced anisotropy in the phase velocity.

Ultrasonic surface waves above a doubly periodic grating

Ultrasonic surface waves above a layer indented by a doubly periodic set of holes having a square-shaped cross section are studied experimentally. Measurements are compared with predictions obtained from a modal model inspired by the theory of the diffraction of electromagnetic waves. Predictions obtained with the modal model and a simplified model where the periodic structure is replaced by an impedance plane are compared.

The intermediate regime of light diffraction by ultrasound in planar gyrotropic optical waveguides in an external electric field

A theoretical investigation is carried out for an intermediate regime of optoacoustic interaction, close to the Bragg one, by ultasonic Rayleigh surface waves in gyrotropic planar waveguides in an external electric field. A system of equations of associated waves is presented which describes the planar optoacoustic interaction in intermediate, Raman—Nath, and Bragg regimes of diffraction by ultrasonic surface waves in gyrotropic crystals with electrically induced anisotropy. It is shown that the intermediate (transition) regime of optoacoustic interaction, which is characterized by an angular selectivity and by the presence of several diffraction maxima, is the basic regime of diffraction. In this case diffracted light is generally elliptically polarized with an ellipticity and polarization azimuth that depend on the external electric field intensity.

Quantitative flaw reconstruction from ultrasonic surface wave fields measured by laser interferometry

A quantitative imaging method for ultrasonic nondestructive testing of structures is presented. The method employs two-dimensional surface-vibration data that is obtained by laser interferometry methods such as holography and digital phase-stepping shearography. In this method, a surface wave field generated by a narrow-band contacting or noncontacting ultrasonic transducer is Fourier transformed to obtain a spatial-frequency domain representation of the wave field. The appropriate Green’s function is then deconvolved from the wave field to obtain quantitative images of effective scattering sources. Since scattering strength is directly proportional to the local contrast of the structure, these images provide a direct map of internal flaws in structures such as plates and shells. The method is valid for strongly scattering inhomogeneities, and can employ scattered evanescent waves to obtain resolution of flaws much smaller than a wavelength. Simulated and experimental results illustrate the efficacy and quality of the imaging method.

Mechanical and tribological properties of hard aluminium–carbon multilayer films prepared by the Laser-Arc technique

The method of laser-induced vacuum arc (Laser-Arc) combines the good controllability of pulsed laser deposition with the high efficiency of a vacuum arc technique. One advantage of this technique is the reduction of droplets, allowing the deposition of high-quality amorphous carbon films. These hydrogen-free films, with a very high hardness, exhibit excellent wear resistance and low friction. Multilayer films consisting of aluminium and carbon were deposited on a metallic substrate (90MnV8). A Rockwell C tester was used to assess and the adhesion of the coatings to the substrate. Young moduli of around 420 GPa were measured by the non-destructive ultrasonic surface wave method (US-SAW). The coefficient of friction was investigated by using two different fretting wear tests with the same load but different frequencies and stroke lengths, and values between 0.1 and 0.25 were recorded against steel. The coatings showed an increase in adhesion and a decrease in the tribological properties with increasing aluminium content.

Enhanced ultrasonic detection of fatigue cracks by laser-induced crack closure

Fatigue cracks are usually initiated by small geometrical irregularities or material inhomogeneities that give rise to sharp local stress concentrations. In the early stages of fatigue, small cracks often remain hidden from conventional ultrasonic detection by stronger scattering from the very same structural imperfection that produced them in the first place. A new experimental method was developed to selectively increase the sensitivity of ultrasonic echographic techniques for such hidden fatigue cracks by exploiting one of their most characteristic features, their susceptibility for closure under compressive stress. Thermo-optical modulation by pulsed infrared laser irradiation was introduced to produce a temporary compressive thermal stress on the surface of the specimen. The resulting dynamic closure of microcracks was detected by a high-frequency ultrasonic surface wave technique. It is demonstrated that this method can be used to effectively distinguish fatigue cracks from other structural imperfections present in the material.

Ultrasonic surface waves above rectangular-groove gratings

A modal model, initially developed to modelize diffraction of electromagnetic waves by rectangular-groove gratings, is used to describe ultrasonic surface waves above the same structures in air. A simple analytical formulation which provides results comparable to the modal method is presented. Measurements performed on ultrasonic surface waves are compared to predictions obtained with the simplified formulation and the modal model.

Cavitation and capillary wave from a parametric decay scheme

Acoustic cavitation in liquids and capillary waves which cause ultrasonic atomization are considered to be related to the 1/2 subharmonic of ultrasonic surface waves via parametric decay instability. The cavitation bubble oscillation mode is an asymmetric surface bubble oscillation that does not easily emit ultrasonic waves into water and accumulates acoustic energy from longitudinal waves. The capillary waves accumulate acoustic energy in a surface of liquid, enough to be used not only for atomization but also the separation of water-ethanol solution. These were confirmed by experiments using ultrasonic longitudinal waves in water.

Ultrasonic surface waves for studying the properties of thin films

The change of the spreading velocity of ultrasonic surface waves by thin film deposition is described by an approximation of the nonlinear equations of motion. The effect of residual stress in deposited films on the spreading velocity is considered. As an example for practical applications, the velocity change of surface waves after TiN-film deposition on steel bars is investigated.