Longitudinal Wave Precursor Signal from an Optically Penetrating Thermoelastic Laser Source

The thermoelastic laser ultrasonic source depends on the optical absorption of energy at the sample surface to produce a volumetric expansion. This paper presents the results of calculations and measurements on the effects of optical penetration of the laser beam into the sample and the elastic waveforms produced. A central result is prediction of a sharp longitudinal waveform that precedes the main waveform and is very similar to that observed with an ablative source (normal point force). The shape of this precursor signal is strongly dependent on the optical penetration depth of the material. A basic explanation of the origin of the precursor signal is given in terms of a one-dimensional model using point sources imbedded within the material. Experimental measurements on a material with a substantial optical penetration depth directly confirm calculations using 2-D integral transform techniques by taking into account the temperature variation with depth

Interaction of laser generated ultrasonic waves with wedge-shaped samples

Wedge-shaped samples can be used as a model of acoustic interactions with samples ranging from ocean wedges, to angled defects such as rolling contact fatigue, to thickness measurements of samples with non-parallel faces. We present work on laser generated ultrasonic waves on metal samples; one can measure the dominant Rayleigh-wave mode, but longitudinal and shear waves are also generated. We present calculations, models, and measurements giving the dependence of the arrival times and amplitudes of these modes on the wedge apex angle and the separation of generation and detection points, and hence give a measure of the wedge characteristics

A New Concept for a Laser-Based Ultrasonic Phased Array Receiver Using Photo-emf Detection

The virtues of laser-based ultrasound [1], LBU, in general, and phased-array generation and detection in particular, have been appreciated for many years. The ability to improve the spatial resolution of an imaging system, coupled with the potential reduction in local laser intensity at a given location on a component to avoid surface damage while still realizing enhanced performance, represent but two motivating factors that have driven the community to seek methods by which to realize phased-array processing. There has been much activity in demonstrating that phased-array generation of ultrasound can lead to an enhanced directivity of the ultrasound as well as to a decrease (or, increase) in the bandwidth of the generated ultrasound (if desired), be it in the bulk or along the surface of components. Examples of such phased-array generationtechniques (either in the thermoelastic or ablative regimes) include illumination of several discrete spots or locus of points with pulsed lasers [1] on the surface of a workpiece (simultaneously or sequentially), be it a line, an annular ring, or a plurality of spots — or illumination of a scanning pattern of lines along the surface of a sample, the so-called phase-velocity scanning technique [2]. By extension of the phased-array generation concept, one is led to consider the notion of laser-based, phased-array detectionof ultrasound [3]. By reciprocity, this can lead to a receiver of higher resolution relative to a single location for the optical sensing of the ultrasound, as well as to a reduction in the local laser fluence required to achieve a given spatial performance. Moreover, one can, in principle, combine the two modes of phased-array excitation and detection to realize even greater resolution capabilities, which one may refer to as “product processing.” In this case, one has, in essence, a focusing transmitter and an imaging detector, both functioning in concert

Non Destructive Determination Of Elastic Moduli By Two Dimensional Fourier Transformation And Laser Ultrasonic Technique

Broadband laser ultrasonics and two dimensional Fourier transformation are used to characterize the properties of varieties of foils and plates. Laser ultrasonics generation is achieved by use of a pulsed laser which deposits pulsed laser energy on the surface of the specimen. The displacement amplitude of the resulting broadband ultrasonic modes are monitored using a two wave mixing photo-refractive interferometer. By applying a two dimensional Fourier transformation to the detected spatial and temporal displacement waveforms, the images of density of state (DOS) for the excited ultrasounds are obtained. Results are presented for a 150 um thick paper sample, a 52.8 um stainless steel foil and a 1.27 mm thick aluminum plate. The DOS image demonstrates the ability to measure the properties of each generated ultrasonic modes and provides a direct, non destructive, measure of elastic moduli of the tested specimensComment: Laser ultrasonics, Fourier transformation, NDT&E, Lamb waves, plate mode

Phase sensitive absolute amplitude detection of surface vibrations using homodyne interferometry without active stabilization

A detection scheme for obtaining phase and absolute amplitude information of surface vibrations on microacoustic components using homodyne laser interferometry is described. The scheme does not require active stabilization of the optical path length of the interferometer. The detection setup is realized in a homodyneMichelson interferometer configuration, and selected measurements on a 374 MHz surface acoustic wave fan-shaped filter and two different piezoelectrically actuated micromechanical resonators are presented to demonstrate the performance of the instrument. With the current detection electronics, the interferometer is capable of detecting out-of-plane surface vibrations up to 2 GHz with a lateral resolution of better than 1 μm and with a minimum detectable vibration amplitude of ∼1 pm.Peer reviewe

Narrow-Band Hybrid Pulsed Laser/EMAT System for Non-Contact Ultrasonic Inspection Using Angled Shear Waves

Conventional ultrasonic testing (UT) using angled shear waves to locate and size potentially critical cracks and flaws in power generation and refinery equipment has become a widely utilized industrial tool. Because this technique uses piezoelectric transducers it requires intimate surface contact and fluid couplants. Therefore, conventional UT has the important drawback that it is difficult to use on surfaces at elevated temperature and, as a result, may require costly plant shut downs to implement. The development of non-contact techniques for angled shear wave UT would represent a significant improvement in the ability to test hot vessels and pipes

Lamb wave near field enhancements for surface breaking defects in plates

Near field surface wave ultrasonic enhancements have previously been used to detect surface breaking defects in thick samples using Rayleigh waves. Here, we present analogous surface wave enhancements for Lamb waves propagating in plates. By tracking frequency intensities in selected regions of time-frequency representations, we observe frequency enhancement in the near field, due to constructive interference of the incident wave mode with those reflected and mode converted at the defect. This is explained using two test models; a square based notch and an opening crack, which are used to predict the contribution to the out-of-plane displacement from the reflected and mode converted waves. This method has the potential to provide a reliable method for the near field identification and characterisation of surface breaking defects in plates

Spatially resolved acoustic spectroscopy for rapid imaging of material microstructure and grain orientation

Measuring the grain structure of aerospace materials is very important to understand their mechanical properties and in-service performance. Spatially resolved acoustic spectroscopy is an acoustic technique utilizing surface acoustic waves to map the grain structure of a material. When combined with measurements in multiple acoustic propagation directions, the grain orientation can be obtained by fitting the velocity surface to a model. The new instrument presented here can take thousands of acoustic velocity measurements per second. The spatial and velocity resolution can be adjusted by simple modification to the system; this is discussed in detail by comparison of theoretical expectations with experimental data

Point source in a phononic grating: stop bands give rise to phonon-focusing caustics

We use locally-excited gigahertz surface phonon wavepackets in microscopic line structures of different pitches to reveal profound anisotropy in the radiation pattern of a point source in a grating. Time-domain data obtained by an ultrafast optical imaging technique and by numerical simulations are Fourier transformed to obtain frequency-filtered real-space acoustic field patterns and k-space phononic band structure. The numerically-obtained k-space images are processed to reveal an intriguing double-horn structure in the lowest-order group-velocity surface, which explains the observed non-propagation sectors bounded by caustics, noted at frequencies above the bottom of the first stop band. We account for these phonon-focusing effects, analogous to collimation effects previously observed in two- and three-dimensional lattices, with a simple analytical model of the band structure based on a plane wave expansion. As the frequency is increased, a transition to dominant waveguiding effects along the lines is also documented