Surface Skimming Longitudinal Wave Research Articles

Quantitative Ultrafast Magnetoacoustics at Magnetic Metasurfaces.

Femtosecond (fs) time-resolved magneto-optics is applied to investigate laser-excited ultrafast dynamics of one-dimensional nickel gratings on fused silica and silicon substrates for a wide range of periodicities Λ = 400-1500 nm. Multiple surface acoustic modes with frequencies up to a few tens of GHz are generated. Nanoscale acoustic wavelengths Λ/n have been identified as nth-spatial harmonics of Rayleigh surface acoustic wave (SAW) and surface skimming longitudinal wave (SSLW), with acoustic frequencies and lifetimes being in agreement with theoretical calculations. Resonant magnetoelastic excitation of the ferromagnetic resonance (FMR) by SAW's third spatial harmonic, and, most interestingly fingerprints of the parametric resonance at 1/2 SAW frequency have been observed. Numerical solutions of Landau-Lifshitz-Gilbert (LLG) equation magnetoelastically driven by complex polychromatic acoustic fields quantitatively reproduce all resonances at once. Thus, our results provide a solid experimental and theoretical base for a quantitative understanding of ultrafast fs-laser-driven magnetoacoustics and tailoring the magnetic-grating-based metasurfaces at the nanoscale.

The influence of edge waves in local surface skimming longitudinal wave generation using a focused PVDF transducer

Acoustic microscopy is extensively used for high-frequency imaging and material characterization. In a focused ultrasonic transducer, the presence of edge waves from the edge of the transducer is usually considered a disadvantage. For high-frequency imaging applications, the edge waves adversely affect the quality of the image. This paper discusses edge wave's influence on generating a surface wave in bulk metal samples using a limited aperture PVDF transducer. Acoustic microscopy-based defocusing experiments are conducted on aluminum, stainless steel, copper, and brass samples. A detailed wave-path analysis is done to understand the different wave components in a signal as obtained from defocusing experiments. The travel path of each wave component is analytically obtained and compared with the experimental results. The different wave modes observed in the experiments are identified by overlaying the analytical plots on the experimentally obtained B-scans. A good correlation is obtained between the experimental and analytical results. The surface wave velocity of the samples is calculated using the time-resolved method, and the percentage of error in the measurement is estimated. The challenges for using this method in measuring surface wave velocities in samples with bulk longitudinal velocities <5000m/s are also discussed.

Laser ultrasonic monitoring of reversible/irreversible modification of a real crack under photothermal loading

We study the responses of laser-generated acoustic waves to localized reversible/irreversible modifications of microscopic asperities on crack surfaces during crack closure, which is an essential process in nonlinear photoacoustic/photothermal crack detection techniques. Our laser ultrasonics technique involves optical measurement of the transmission and mode conversion of the laser-generated surface acoustic waves caused by the crack. Reversible/irreversible modifications of asperities can be achieved via non-contact photothermal loading of the crack. Three photothermal loading cycles were realized in individual succession and were monitored using the laser ultrasonics technique at various experimental locations along the crack. In our experiments, each photothermal loading cycle includes multiple successive subcycles, in which the material is first heated and then cooled to its equilibrium temperature, thereby initiating local closing, followed by opening of the crack. Each subcycle is monitored twice using the laser ultrasonics technique, once each at the end of heating and cooling. Furthermore, each successive subcycle is accomplished at a higher power of heating laser than that of the previous subcycle. Significant differences in the peak-to-peak amplitude of the surface skimming longitudinal acoustic wave, which is excited by mode conversion of the Rayleigh wave by the crack, are revealed during the first cycle of photothermal loading. These differences clearly indicate a partial irreversibility of the mechanical processes occurring in the crack surfaces during subcycles of the first cycle. In a larger temporal scale, irreversible modification of crack surfaces is observed from the significant difference between the experimental results of the first photothermal loading cycle and the subsequent two cycles, whereas a reversible response of the crack surface to thermoelastic loading is observed from the similarity of the measurements accumulated during the two subsequent cycles.

Ultrafast magnetoelastic probing of surface acoustic transients

We generate in-plane magnetoelastic waves in nickel films using the all-optical transient grating technique. When performed on amorphous glass substrates, two dominant magnetoelastic excitations can be resonantly driven by the underlying elastic distortions, the Rayleigh Surface Acoustic Wave and the Surface Skimming Longitudinal Wave. An applied field, oriented in the sample plane, selectively tunes the coupling between magnetic precession and one of the elastic waves, thus demonstrating selective excitation of coexisting, large amplitude magnetoelastic waves. Analytical calculations based on the Green's function approach corroborate the generation of the non-equilibrium surface acoustic transients.

Laser ultrasonics for defect detection and residual stress measurement of friction stir welds

The laser-ultrasonic technique is investigated for defect detection and sizing as well as for residual stress measurement in welds obtained by friction stir welding (FSW). When combined with the Fourier domain synthetic aperture focusing technique, very good performances are achieved for detecting lack of penetration in butt joints, the detection limit coinciding with the conditions of reduced mechanical properties. Also, the detection of kissing bonds seems to be possible in lap joints when probing with ultrasonic frequencies up to 200 MHz. Residual stresses induced by the FSW process can also be probed by laser ultrasonics. The method is based on monitoring the velocity change of the laser-generated surface skimming longitudinal wave, propagating just below the surface and being found much more sensitive to stress. The residual stress profile measured across the weld line is in good agreement with results from a finite element model and from strain gauge measurements.

Measurement of texture in steel by laser-ultrasonic surface waves

A laser-ultrasonic approach based on the propagation of surface skimming longitudinal wave (P-wave) and Rayleigh wave (SAW) is investigated for the measurement of texture in moderately thick steel plates. The angular dependence of the P-wave and SAW velocities presents different patterns, both of amplitude of a few % attributed to texture. Moreover, the difference of the P-wave and SAW profiles is shown to be very robust against small path length changes or other detrimental effects. For measurements on large plates, a solution based on the rotation of a generation line using an axicon lens is tested. Texture variations across the width of steel plates and the influence of residual stress are discussed.

Pulsed Rayleigh wave scattered at a surface crack

This paper investigates Rayleigh wave interaction with machined slots on flat aluminium blocks to simulate surface breaking cracks. Using a finite element method, Rayleigh wave scattering by narrow slots of varied depth ranging from 0.5 mm to 20 mm is calculated. Pulsed wideband Rayleigh waves with a centre frequency of 590 kHz and −6 dB bandwidth of 520 kHz is considered. Reflection and transmission coefficients are calculated and compare well with the published literature. We and other workers have reported enhancement of the measured amplitude or particle velocity of an apparent Rayleigh wave close to a surface defect. In this paper, it is found that the predicted enhancement of in-plane components of particle velocities close to a crack is significantly higher than that of the out-of-plane components of particle velocities which appears to be mainly due to the mode-converted surface skimming longitudinal wave from the crack that has mainly in-plane components near the sample surface. The enhancement of the in-plane particle velocity will be observed regardless of the type of in-plane sensitive ultrasonic detector used. The explanation of the discrepancy of the reflection and transmission coefficients obtained by pulsed and narrow band or pseudo continuous Rayleigh waves is discussed. The later-arriving Rayleigh waves from reverberation along the inside of the crack surface are observed, as has been previously reported by other workers, and this may also be used to gauge slot depth.

Numerical simulation of laser-generated surface acoustic waves in the transparent coating on a substrate by the finite element method

The optimum finite element model in the system consisting of a transparent coating and an opaque substrate is established based on the analysis of two important parameters: meshing size and time step, and the stability of solution. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field are obtained. According to the thermoelastic theory, this temperature gradient field can be taken as a buried bulk source to generate ultrasonic wave. The surface acoustic waves (SAWs) are obtained. The influence of the coating thickness on the SAWs is analyzed. The model provides a useful tool for the determination of modes which are generated by a laser source in transparent coating on opaque substrate. The surface skimming longitudinal wave exists for the multiple oscillations and it charges from unipolar waveforms to dipolar.

Numerical simulation of laser-generated ultrasound by the finite element method

The results obtained from the finite element model of laser-generated ultrasound are presented in terms of temperature and displacement. According to thermoelastic theory, considering the temperature dependence of the thermophysical parameters of the material, the transient temperature field can be precisely calculated by using the finite element method; then, laser-generated surface acoustic wave forms are calculated in Al plates of various thicknesses. The elastic waves excited by a pulsed laser in a thin plate are typical Lamb waves, and the numerical results demonstrate that the surface vibration is mainly determined by the lower frequency components of the symmetric mode s0 and antisymmetric mode a0 of the lowest order in very thin plate materials. It is also indicated that, when the sample thickness increases, both the higher frequency components of the lower Lamb wave modes and the higher order Lamb wave modes should be considered. In a relatively thicker plate, the numerical model can still capture the significant details of the transient features taking place in the plates. The surface skimming longitudinal and shear waves are relatively stronger in the near field, distorting the observed wave form of the Rayleigh wave. These effects must be considered when the quantitative near field is applied to determining elastic parameters or residual stresses.

Local surface skimming longitudinal wave velocity and residual stress mapping.

Local variation in surface skimming longitudinal wave (SSLW) velocity has been measured using a scanning acoustic microscope. A very narrow width electrical impulse has been used to excite the transducer of the acoustic lens. This permits the separation of the SSLW signal from the direct reflected signal in the time domain. A simple method of measuring the time delay between the directly reflected signal and the SSLW signal at two defocuses has been utilized for the local measurement of SSLW velocity. The variation in the SSLW velocity measured over an area of the sample is scaled and presented as an image. The method has been implemented to image the variation of the SSLW velocity around a crack tip in a sample of Ti-6Al-4V. Since the SSLW velocity is known to change linearly with the stress, the SSLW velocity image is considered as a representation of the image of stress around the crack tip. Local stress variation in the same region of the crack tip is directly measured using x-ray diffraction. The SSLW velocity image is compared with the x-ray diffraction stress image. The contrast in the two images, spatial resolution, and the penetration depth into the sample of acoustic waves and x rays are discussed.

Stress measurement using critically refracted longitudinal waves

Critically refracted longitudinal waves (LCR) offer considerable advantage in achieving nondestructive measurement of stress in engineering components and structures. Of the various wave types, the LCR is most sensitive to stress and least sensitive to texture. Also called the surface skimming longitudinal wave (SSLW), the LCR wave skims along just underneath the surface of a material at bulk wave speeds. Further, there is experimental evidence of frequency dependence where the higher frequencies concentrate their energy more toward the surface than do the lower frequency waves. This effect, which has not been fully explained theoretically, enables stress gradient evaluation below the surface. Further, there is observed scatter in the travel times found at low stress levels. At higher levels of stress, the data often become very linear. The specific nature of the LCR wave, acoustoelasticity, LCR probes and beam patterns, and methods for its excitation, are described. Probe frequencies from 1.0 to 20 MHz have been used. System travel-time repeatability is 0.01% or better and stress resolution of ±21 MPa (3 ksi) is obtained in typical applications. Tests showing the frequency dependence and the scatter observed at low levels will be discussed. Several applications described using the LCR technique for stress measurements include pressure vessels, piping, welds, titanium turbine engine blades, aluminum plate and a turbine rotor. Future applications and research using the technique will be mentioned.

Scanning Acoustic Microscopy and x-ray Diffraction Investigation of Near Crack Tip Stresses

ABSTRACTScanning Acoustic Microscopy has been used to measure and map the Rayleigh wave velocity and the Surface Skimming Longitudinal wave velocities near a crack tip in a sample of Ti-6AI-4V. X-ray diffraction measurements have been performed to map the stress in the same region of the sample. The differences in the contrast between the two acoustic velocity images and their sensitivity to stress are examined. Similarities between x-ray stress images and acoustic velocity images are discussed.

Analysis of interference patterns in acoustic photomicrographs

Images from the Scanning Acoustic Microscope (SAM) have been used previously to study fiber orientations in opaque matrices. SAM images are superficially similar to reflected light images. They also frequently exhibit interference patterns which can be used for quantitative analysis. These patterns arise at discontinuities near or intersecting the specimen surface, and result from interference between acoustic waves reflecting from subsurface boundaries and from the specimen surface. For the case of fibers intersecting a planar surface, the position of the interference fringes gives the orientation of the fiber with respect to the surface, and the shape of the fringes indicates any surface irregularities. Thus, if the fiber shape and orientation are known and if the waves are determined to be longitudinal, then fringe spacings can be used to calculate the velocity and attenuation of the longitudinal acoustic wave in the matrix surrounding the fiber.This technique was applied to graphite-epoxy composites, imaged in the SAM at a frequency of 1.2 GHz. First it was determined that other waves, such as shear waves, surface-skimming longitudinal waves, and surface acoustic waves (Rayleigh waves) were not present in the composite. Their addition to the interference pattern would have disrupted the interpretation of the object’s shape and surface features from the longitudinal fringes and complicated the solutions for wave reflections.