Ultrasonic Transients Research Articles

Laser-induced ultrasonic waveform derivation and transition from a point to a homogeneous illumination of a plate

Ultrasound modeling, being an established practice, is used to study the fundamentals of light-matter interactions. Although much has been published on the matter, pressure and thermal expansion induction mechanisms in laser ultrasonics have rarely been combined, as they should, in a single ultrasonic source while the effects of its size variation have only been shown to a limited extent. In the paper, we unite these light-matter interaction mechanisms, with inclusion of lateral optical forces, into a single laser-stimulated source as it is observed in nature. With a laser pulse as a manipulable source, we simulate the multifaceted workings of light-matter interactions by exposing the distinct transients originating from different source localities as generated by different induction mechanisms. We also present a transition of simulated ultrasonic waveforms in the epicentral point on the surface of a solid plate opposite from the source while it is expanded from a point to a quasi-limitless extent for pressure and thermal expansion generation regimes. The model utilizes geometric probability theory together with Huygens’ superposition principle and temporal convolutions to construct the desired waveforms out of individual Green’s functions. We show how the ultrasound generation regimes stem out of a single source and how its size together with energy and momentum transfers during the light-matter interactions affect the induced ultrasonic transients.

An ultrasonic transducer transient compensator design based on a simplified Variable Structure Control algorithm

A non-linear control method, known as Variable Structure Control (VSC), is employed to reduce the duration of ultrasonic (US) transducer transients. A physically realizable system using a simplified form of the VSC algorithm is proposed for standard piezoelectric transducers and simulated. Results indicate a VSC-controlled transmitter reduces the transient duration to less than a carrier wave cycle. Applications include high capacity ultrasound communication and localization systems.

Optical and optoacoustic measurements of the absorption properties of spherical gold nanoparticles within a highly scattering medium

In the present paper the requirements for optical parameter characterization of absorbing materials located within a highly scattering medium has been addressed. The measurement scheme incorporates the optoacoustic technique where a single acoustic transducer is used to detect ultrasonic transients generated from laser irradiation. The absorbing medium is based on different concentrations of spherical gold nanoparticles (SGNP’s), these are currently being considered as non-toxic targeted optical contrast agents for both medical imaging and cancer therapeutics. In this paper we present results which demonstrate the two main advantages the optoacoustic technique has over other measurement schemes. These are the possibility to obtain information on the position and dimensions of absorbing bodies using a time of flight analysis (TOF) and secondly, the higher sensitivity of the optoacoustics compared to optical transmission techniques. The former advantage is of particular interest for imaging applications and the latter for detection and characterization of absorbing materials surrounded by high levels of high scattering mediums. We present for the first time the characterization of SGNP within a highly scattering medium. To further demonstrate the feasibility of the optoacoustic technique, the scattering coefficient of the surrounding medium has also been characterized.

Laser ultrasonic diagnostics of residual stress

Ultrasonic NDE is one of the most promising methods for non-destructive diagnostics of residual stresses. However the relative change of sound velocity, which is directly proportional to applied stress, is extremely small. An initial stress of 100 MPa produces the result of δV/V∼10 −4. Therefore measurements must be performed with high precision. The required accuracy can be achieved with laser-exited ultrasonic transients. Radiation from a Nd-YAG laser (pulse duration 7 ns, pulse energy 100 μJ) was absorbed by the surface of the sample. The exited ultrasonic transients resembled the form of laser pulses. A specially designed optoacoustic transducer was used both for the excitation and detecting of the ultrasonic pulses. The wide frequency band of the piezodetector made it possible to achieve the time-of-flight measurements with an accuracy of about 0.5 ns. This technique was used for measuring of plane residual stress in welds and for in-depth testing of subsurface residual stresses in metals. Plane stress distribution for welded metallic plates of different thicknesses (2–8 mm) and the subsurface stress distribution for titanium and nickel alloys were obtained. The results of conventional testing are in good agreement with the laser ultrasonic method.

The applicability of a material-treatment laser pulse in non-destructive evaluations

A practical optodynamic study was performed to determine the usability of different lengths of laser pulses for the generation of ultrasonic transients in a solid material. The aim of the study was to evaluate the possibility of a dual use for a laser pulse—for laser material processing, on the one hand, and for the ultrasonic wave generation on the other—with both processes being combined on the same production line. The propagation of the laser-generated ultrasonic waves is evaluated by detecting and measuring with a PID-controlled stabilized interferometer. Thus, both systems provided the basic tools, the generation and detection of ultrasonic waves, for an ultrasonic, laser-based, non-destructive material evaluation. The ultrasonic transients generated by ‘classical’ nanosecond laser pulses were compared with the transients generated by industrial laser pulses with a duration of a few tenths of a microsecond. The experimental results are compared with the results of a time-of-flight analysis that also involved part of a mode-conversion analysis for both regimes in a layered material structure. The differences between the two waveforms were assessed in terms of their visibility, wavelength and resolution. The limit values were calculated and estimated for the laser-pulse parameters, when such pulses are intended for use in an ultrasonic, laser-based, non-destructive evaluation. The possibility of using an industrial marking laser for laser ultrasound generation is thus demonstrated.

Focused array transducer for two-dimensional optoacoustic tomography.

Optoacoustic (OA) imaging utilizes short laser pulses to create acoustic sources in tissue and time resolved detection of generated pressure profiles for image reconstruction. The ultrasonic transients provide information on the distribution of optical absorption coefficient that can be useful for early cancer diagnostics. In this work a new design of wide-band array transducer is developed and tested. The array consists of 32 focused piezo-elements made of PVDF slabs imposed on a cylindrical surface. A single array element response to an OA signal coming from arbitrarily located point source is investigated theoretically and experimentally. The measured signals correspond well to numerically calculated ones. Focal zone maps of the elements with aperture angles 30 degrees and 60 degrees are presented and discussed; the resolution in direction perpendicular to the imaging plane is determined. Point spread function of the whole array is calculated using experimentally obtained signals from the sources located at different distances from the array. Backprojection algorithm is employed for reconstruction of the optoacoustic images. It is shown that the spatial resolution of the images yielded by the proposed array increases significantly compared to previous transducer designs.

Synthetic aperture focusing techniques in time and frequency domains for photoacoustic imaging

Synthetic aperture focusing techniques (SAFT) have been applied to B-scan images of artificial defects produced by a scanning photoacoustic probe operating under water. The probe consisted of a fibre-optic laser beam delivery system combined with a polymer (PVDF) transducer mounted at the tip of the probe for ultrasonic reception. This system had a broadband frequency response in the 1-10 MHz range. Such an integral probe was designed to optically transmit and receive near on-axis ultrasonic transients. A frequency domain synthetic aperture method was successfully applied using phantom samples to produce refined 2D images from initial B-scans. In this paper, the phantom consisted of a black nylon rectangular sample with a 1.0 mm circular defect at a depth of 5.9 mm from the surface. A comparison was made of conventional B-scan images with SAFT images. Results showed that synthetic focusing apertures, in time or frequency domains, offer better signal-to-noise ratios with improved capabilities in lateral resolution.

501 レーザーを用いた無色透明材料の音速測定に関する研究

A pulse laser has been used to generate ultrasonic transients in samples of the laser ultrasonic nondestructive inspection. However, it is impossible to excite ultrasonic wave in transparent materials by a pulse laser. In this study, the surface of a transparent ceramic specimen is painted black in order to absorb the laser energy. As a result, heating by pulse laser, the specimen with a black surface band makes the pulse wave. By using the pulse wave, the sound speeds in a transparent ceramic specimen are measured. The experimental results were identical with the results by the ultrasonic pulse method.

Backward mode detection of laser-induced wide-band ultrasonic transients with optoacoustic transducer

Time-resolved piezoelectric detection of wide-band ultrasonic transients induced by laser pulses in absorbing medium was studied. An optoacoustic transducer was developed for measuring the profiles of ultrasonic transients propagating in backward direction out of the laser-irradiated medium. For this purpose, an optical fiber for delivery of laser pulses to the surface of absorbing medium and a wide-band lithium niobate acoustic transducer were incorporated in one compact system, optoacoustic front surface transducer (OAFST). The transducer possesses temporal resolution (rise time) of 3.5 ns, low effective thermal noise pressure (10 Pa), and high sensitivity of piezoelectric detection (10 μV/Pa) over the ultrasonic frequency range from 1 to 100 MHz. Nd:YAG laser pulses at 355 nm were employed to generate distribution of acoustic sources in water solutions of potassium chromate with various concentrations. A temporal course of ultrasonic transients launched into an optically and acoustically transparent medium, coupled to the absorbing medium, was studied. Ultrasonic profiles experimentally measured at the site of laser irradiation were compared with profiles calculated using theoretical model. Experimental curves were in a good agreement with theoretical profiles. The backward detection mode permitted accurate reconstruction of the axial distribution of heat deposition in the laser-irradiated homogeneous or layered medium from the measured profiles. OAFST may become a useful modality for optoacoustic imaging in biological tissues and nondestructive evaluation of industrial materials.

Differential fibre-optic sensingof laser-generated ultrasound

A differential fibre-optic device has been developed for noncontact detection of laser-generated ultrasound. Using a 5 mW He-Ne laser as the source, laser beam delivery on to the sample of the surface utilised a singlemode fibre and lens combination. When photoacoustic ultrasonic transients propagated across the surface, the incident spot was perturbed. With backscattered light monitored by a fibre-optic bundle incorporating differential photodetection. Symmetric and asymmetric Lamb wave modes have been observed in a 50 mu m aluminium sheet, as have longitudinal and Rayleigh waves in a 30 mm-thick sample. >

Air-coupled piezoelectric detection of laser-generated ultrasound

A pulsed laser has been used to generate ultrasonic transients in samples of metal and fiber-reinforced polymer composite material. These have been detected using an air-coupled piezoelectric transducer. It is demonstrated that such a transduction system can be used for longitudinal waves in bulk material, Rayleigh waves at solid surfaces and Lamb waves in thin plates.

Characterisation of Adhesive Bonds from Inspection by Laser-Generated Ultrasound

Ultrasonic transients with high frequency components have been used to interrogate the bond region of lap-bonded joints. With through-transmission measurements, based on a non-contact laser techniq...

Wavefields near transverse cusp caustics produced by reflecting ultrasonic transients and tone bursts from curved surfaces

Abstract : Ultrasonic wavefields reflected from curved surfaces were studied in the vicinity of caustics, Acoustical and optical transverse cusp diffraction catastrophes produced by reflections from a curved metal surface in water were imaged by displaying the amplitude or intensity in an observation plane transverse to the general direction of propagation. The optical image was used to locate the cusp point in the observation plane. Acoustical diffraction patterns for sine waves, described by the Pearcey function, were calculated with the parameters determined by the experimental setup leaving no adjustable scaling parameters. The calculated and experimental acoustical diffraction patterns showed good agreement near the cusp point. The acoustical diffraction pattern showed the expected mirror symmetry about an axis. The pattern was shown to scale properly with frequency. The transverse cusp caustic separates space into a region with three rays and a region with one ray. Inside the caustic there are three rays, on the curve two of the rays merge and disappear leaving one ray outside. Transient signals reflected from curved surfaces exhibited the merging and disappearance of rays on the caustic. Relative arrival times for signals in calculated and recorded time traces agree well. The relation to the wavefront parameters of the temporal orientation of the travel time surface is discussed. The general shape of the travel time surface is that of the swallow tail caustic surface.

Aberrated foci in the time and frequency domains and experiments with acoustical diffraction catastrophes.

Properties of wave fields at and near foci are considered for situations in which the amplitude is limited by the shape of an initial wave front rather than its aperture or initial dimensions. Such aberrated foci are relevant to high-frequency scattering from curved surfaces and to understanding the response of source and receiver arrays. Caustics classified by generic catastrophes are considered such as produced by random phase perturbations or by aberrations of imaging systems [M. V. Berry and C. Upstill, Prog. Opt. 18, 258 (1980)]. Experiments were carried out in which ultrasonic transients and tone bursts were reflected from a curved metal sheet in water such that the resulting caustic surface was a cusp which opens up generally transverse to the propagation direction of the reflected wave front [C. K. Frederickson, Ph.D. dissertation, Washington State University (1991)]. The results illustrate several more general properties of transient responses and diffraction patterns near weak foci. For example, the singular part of the travel time surface describing the merging of echoes has a shape like that of the next higher cuspoid catastrophe: a swallowtail catastrophe. Conditions for neglecting the effects of apertures on catastrophe diffraction patterns are examined. [Work supported by ONR.]

Ultrasonic surface modes generated by laser pulses on duraluminium

Studies of ultrasonic transients generated by pulses of laser ultraviolet light on plates of various thickness are presented. The propagation of the first train carrying most of the elastic energy was investigated. The experimental method used resulted in a precise determination of propagation velocities of the elastic signal. A comparison of the experimental and computed results led to the conclusion that the signal corresponded to the lowest antisymmetric Lamb mode. The mode group velocity was equal to the Rayleigh wave velocity, c R, for reduced wave numbers greater than 6.5. The group velocity became different from c R for smaller reduced wave numbers. It is suggested that the experimental method may be used for precise determination of Poisson ratio for metallic samples.

QUADRATURE IMAGING INTERFEROMETRY

A new imaging interferometer system is described which has been designed with industrial applications in mind. Based on CCD image array detection and Digital signal processing of interferograms in a quadrature optical system, high sensitivity (≈ 1nm) detection of ultrasonic transients and fast data analysis can be achieved. Details of the system are discussed and the results of initial feasibility tests with clean and speckle pattern data presented

Optical sensing of in-plane ultrasonic transients

An optical sensor has been developed, based on the differential laser Doppler anemometer, for the detection of in-plane ultrasonic transients. Ultrasonic pulse propagation around a 90° corner has been investigated in an aluminum block. By generating ultrasonic pulses on diametrically opposite sides of the detector a polarity inversion of the ultrasonic pulses detected is demonstrated. The results obtained are as expected for the case of in-plane displacement measurements.

Mechanisms of pulsed photoacoustic generation

The mechanisms by which ultrasonic transients may be generated by pulsed laser irradiation are reviewed, with particular reference to the wave forms resulting in both fluid and solid media. Specific examples of experimental results are presented that demonstrate that a pulsed ultraviolet source is a convenient means for ultrasonic generation in both liquids and solids.

A pulsed photoacoustic investigation of ultrasonic mode conversion

Experiments have been undertaken into the generation and detection of ultrasonic transients in metals over a 40-MHz bandwidth, using pulsed-laser generation and interferometric detection. Longitudinal, shear, and surface waves have been investigated, and their form compared favorably to theory. The techique has been used to study the interaction of surface and bulk waves with slots in the surface of a plate. It has been shown that significant mode conversion occurred from surface to bulk waves and vice versa. The significance of this result to nondestructive testing is discussed.

Elastic waveforms using laser generation and electromagnetic acoustic transducer detection

A Q-switched, frequency-doubled ruby laser has been used to generate wide bandwidth ultrasonic transients in aluminum and steel plates. Both longitudinal and shear waves were generated, resultant waveforms being dependent upon the irradiation conditions. Various designs of electromagnetic acoustic transducers were then used to detect these waveforms at the opposite side of the plate. The result is a method of ultrasonic inspection with potential in noncontact applications.