Kenneth L. Telschow

Direct imaging of traveling Lamb waves in plates using photorefractive dynamic holography

Anisotropic stiffness properties of sheet materials can be determined by measuring the propagation of Lamb waves in different directions, but this typically requires multiple positioning of a suitable transducer at several points or scanning over the area of the sample plate. A laser imaging approach is presented that utilizes the adaptive property of photorefractive materials to produce a real-time measurement of the antisymmetric Lamb traveling wave displacement and phase in all planar directions simultaneously without scanning. Continuous excitation and lock-in methodology is employed, enabling the data to be recorded and displayed by a video camera. Analysis of the image produces a direct quantitative determination of the phase velocity in all directions showing plate stiffness anisotropy in the plane. The method is applicable to materials that scatter light diffusely and provides quantitative imaging of the dynamic surface motion exhibited by traveling elastic waves. A description is given of this im...

Photorefractive optical lock-in vibration spectral measurement.

An optical photorefractive frequency-domain method is described for measuring displacement amplitude and phase of vibrating surfaces. The method is applicable to diffusely scattering surfaces and usable in either a point-detection or imaging configuration. The method utilizes an optical lock-in approach to measure phase modulation of light scattered from continuously vibrating surfaces. Picometer displacement sensitivities have been demonstrated over a frequency range of 100 Hz to greater than 100 kHz. The response of the spectral method is independent of the vibration frequency above the photorefractive cutoff frequency. Two methods are described that produce a readout beam intensity that is a direct function of the vibration amplitude suitable for imaging.

Photorefractive interferometers for ultrasonic measurements on paper.

Photorefractive interferometers have been employed for the detection of ultrasound in metals and composites since 1991 [1–4]. Instances of laser-generated ultrasound and laser-based detection in paper were reported in 1996 [5]. More recently, bismuth silicon oxide (BSO) photorefractive interferometers were adapted to detect ultrasound in paper [6]. In this article we discuss BSO and GaAs photorefractive detection of ultrasound on different paper grades and present the resulting waveforms. Compared to contact piezoelectric transducer methods, laser interferometry offers signifcant advantages. One of these is that it is a noncontact technique. This is especially important for on-line application to lightweight papers which could be marked or damaged by contact transducers. Broadband ultrasonic laser generation matched with the broadband sensitivity of laser interferometers is another beneft. This is important for obtaining narrow pulses in nondispersive time-of-fight determinations and for measuring the phase velocity of dispersive modes over a wide frequency band. Also, laser ultrasonic techniques provide a measure of bending stiffness through the analysis of low frequency A0 waves.

Method and apparatus for detecting internal structures of bulk objects using acoustic imaging

Apparatus for producing an acoustic image of an object according to the present invention may comprise an excitation source for vibrating the object to produce at least one acoustic wave therein. The acoustic wave results in the formation of at least one surface displacement on the surface of the object. A light source produces an optical object wavefront and an optical reference wavefront and directs the optical object wavefront toward the surface of the object to produce a modulated optical object wavefront. A modulator operatively associated with the optical reference wavefront modulates the optical reference wavefront in synchronization with the acoustic wave to produce a modulated optical reference wavefront. A sensing medium positioned to receive the modulated optical object wavefront and the modulated optical reference wavefront combines the modulated optical object and reference wavefronts to produce an image related to the surface displacement on the surface of the object. A detector detects the image related to the surface displacement produced by the sensing medium. A processing system operatively associated with the detector constructs an acoustic image of interior features of the object based on the phase and amplitude of the surface displacement on the surface of the object.

Off-axis propagation of ultrasonic guided waves in thin orthotropic layers: theoretical analysis and dynamic holographic imaging measurement

The elastic properties of many materials in sheet or plate form can be approximated with orthotropic symmetry. In many sheet material manufacturing industries (e.g., the paper industry), manufacturers desire knowledge of certain anisotropic elastic properties in the sheet for handling and quality issues. Ultrasonic wave propagation in plate materials forms a method to determine the anisotropic elastic properties in a nondestructive manner. This work explores exact and approximate analysis methods of ultrasonic guided wave propagation in thin layers, explicitly dealing with orthotropic symmetry and propagation off-axis with respect to the manufacturing direction. Recent advances in full-field ultrasonic imaging methods, based on dynamic holography, allow simultaneous measurement of the plate wave motion in all planar directions within a single image. Results from this laser ultrasonic imaging approach are presented that record the lowest anti-symmetric (flexural) mode wavefront in a single image without scanning. Specific numerical predictions for flexural wave propagation in two distinctly different types of paper are presented and compared with direct imaging measurements. Very good agreement is obtained for the lowest anti-symmetric plate mode using paper properties independently determined by a third party. Complete determination of the elastic modulus tensor for orthotropic layers requires measurement of other modes in addition to the lowest anti-symmetric. Theoretical predictions are presented for other guided wave modes [extensional (S), flexural (A), and shear-horizontal (SH)] in orthotropic plates with emphasis on propagation in all planar directions. It is shown that there are significant changes in the dispersion characterization of these modes at certain frequencies (including off-axis mode coupling) that can be exploited to measure additional in-plane elastic moduli of thin layers. At present, the sensitivity of the imaging measurement approach limits experimental investigation to relatively large amplitudes easily produced by flexural wave motion (>0.1 nm). Extension of the measurement range and application to other plate wave modes are in progress and shall be reported in future work.

Real-Time Measurement of Material Elastic Properties in a High Gamma Irradiation Environment

This paper describes the first noncontact elastic vibration measurements of an object in a high gamma radiation field. Using a laser-coupled resonant ultrasound technique, the vibration modes of an Inconel hollow capped cylinder were measured as the gamma radiation field was increased to 104 Gy/h. This measurement technique allowed shifts in the resonant frequency of the sample’s vibration modes to be tracked over a 170-h period. The vibration mode frequencies changed in a manner consistent with the temperature dependence of the elastic stiffness coefficients of the material. These results demonstrate the efficacy of the laser approach for real-time resonant ultrasound measurements in this severely hostile nuclear environment.

Imaging anisotropic elastic properties of an orthotropic paper sheet using photorefractive dynamic holography

An important material property in the paper industry is the anisotropic stiffness distribution due to the fibrous microstructure of paper and to processing procedures. Ultrasonic methods offer a means of determining the stiffness of sheets of paper from the anisotropic propagation characteristics of elastic Lamb waves along the machine direction and the cross direction. That is, along and perpendicular to the direction of paper production. Currently, piezoelectric ultrasonic methods are employed in the industry to measure the elastic polar diagram of paper through multiple contacting measurements made in all directions. This paper describes a new approach utilizing the INEEL Laser Ultrasonic Camera to provide a complete image of the elastic waves traveling in all directions in the plane of the paper sheet. This approach is based on optical dynamic holographic methods that record the out of plane ultrasonic motion over the entire paper surface simultaneously without scanning. The full-field imaging technique offers great potential for increasing the speed of the measurement and it ultimately provides a substantial amount of information concerning local property variations and flaws in the paper. This report shows the success of the method and the manner in which it yields the elastic polar diagram for the paper from the dispersive flexural or antisymmetric Lamb wave.

Fabry-Perot Laser Ultrasonic Elastic Anisotropy Measurements on a Moving Paper Web

On-line measurement of material properties is a goal of many manufactures for production improvement and quality control. In the paper industry, the elastic stiffness of the paper web needs to be accurately measured and controlled online. Currently, contact ultrasonic methods are employed to measure the elastic constants of paper offline. Piezoelectric transducers are placed in contact with the paper surface to generate and detect plate wave modes, known as Lamb wave modes, in the paper in both the machine and cross directions. Laser ultrasonics offers a new approach to making elastic wave measurements that does not require contacting the paper web. A pulsed laser generates ultrasound in the paper by delivering a high intensity short impulse to the paper through thermoelastic expansion. An optical interferometer is used to demodulate the small phase modulation that the elastic wave produces on a continuous probe laser beam. Difficulty in applying this approach to online measurements of rapidly moving pape...

Mathematical Modeling of Laser Ablation in Liquids with Application to Laser Ultrasonics

The use of laser ablation as a means of generating ultrasonic waves in liquid metals is studied in this paper. A mathematical model for predicting the onset of ablation is developed, as is a model of the ablation process based on steady state, one-dimensional gas dynamics in which the vapor phase is treated as an ideal gas. The results of this model are then used in a quasi-two-dimensional model of laser ablation that accounts for the spatial distribution of intensity in the laser beam. Model predictions are compared with experiments conducted on liquid mercury and excellent agreement is obtained. Based on these results, a simplified model is developed that shows excellent agreement with both the theory and the experiments.

Vibration modal analysis using all-optical photorefractive processing

A new experimental method for vibration modal analysis based on all-optical photorefractive processing is presented. The method utilizes an optical lock-in approach to measure phase variations in light scattered from optically rough, continuously vibrating surfaces. In this four-wave mixing technique, all-optical processing refers to mixing the object beam containing the frequency modulation due to vibration with a single frequency modulated pump beam in the photorefractive medium that processes the modulated signals. This allows for simple detection of the conjugate wavefront image at a CCD. The conjugate intensity is shown to be a function of the first-order ordinary Bessel function and linearly dependent on the vibration displacement induced phase (delta) , for (delta) equals 4(pi) (xi) /(lambda) << 1 where (xi) is the vibration displacement and (lambda) is the source wavelength. Furthermore, the results demonstrate the unique capabilities of the optical lock-in vibration detection technique to measure vibration signals with very narrow bandwidth (< 1 Hz) and high displacement sensitivity (sub-Angstrom). This narrow bandwidth detection can be achieved over a wide frequency range from the photorefractive response limit to the reciprocal of the photoinduced carrier recombination time. The technique is applied to determine the modal characteristics of a rigidly clamped circular disc from 10 kHz to 100 kHz.