Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-07-11
2003-03-18
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S602000, C073S628000, C073S659000
Reexamination Certificate
active
06532821
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of ultrasonics, and more particularly to a method and apparatus for evaluating the physical properties of a sample. The invention can, for example, be used for measuring the diffusion coefficient of ultrasound in a substance, for measuring the absorption coefficient of ultrasound in a substance and for evaluating grain size in a polycrystalline material.
2. Description of Prior Art
Ultrasonic measurement techniques generally involve generating an ultrasonic pulse in an object, and then detecting the signal after propagation in the object to determine its geometrical, microstructural, and physical properties. This technique is advantageous because it is nondestructive and because it can probe the interior of materials. Conventional ultrasonic devices have been developed which involve the use of transducers, including piezoelectric and electromagnetic acoustic transducers (EMATs). Another ultrasonic technique is laser-ultrasonics, wherein one laser with a short pulse is used for generation and another laser coupled to an optical interferometer is used for detection. The detection laser is either a long pulse or continuous laser. Either laser may be coupled to the material under test through an optical fiber for ease of handling.
This approach is advantageous because it does not require either the generation laser or the laser-interferometer detector to be in contact or close to the object. Furthermore, unlike an EMAT or piezoelectric transducer, the generation laser and laser-interferometer are not subject to precise orientation requirements. Details about laser-ultrasonics can be found in C. B. Scruby, L. E. Drain, “Laser-ultrasonics: techniques and applications”, Adam Hilger, Bristol, UK 1990 and J.-P. Monchalin, “Optical detection of ultrasound,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 33, 485 (1986).
The absorption coefficient of ultrasound is one parameter that characterizes the physical properties and the interaction of ultrasound with the microstructure of the material. The variation of the absorption coefficient with temperature and ultrasound frequency can provide information about internal friction, relaxation phenomena, magnetic properties of the material, dislocation density, phase transformations, or specific microstructural structures. The simplest approach to the measurement of the absorption is to observe the free decay in the amplitude of a vibration mode of a sample. Another popular approach is to use a forced vibration where one measures the phase difference between the driving system and the vibration of the sample. Both approaches are limited to samples with specific geometries and each measurement only gives the absorption coefficient at one frequency and one temperature. Performing a full study of absorption as a function of frequency and temperature can be quite time consuming.
Ultrasound absorption can also be estimated from the attenuation of propagating waves. A serious limitation to this approach is the difficulty of separating the attenuation due to absorption mechanisms from that due to other phenomena like diffraction, scattering by grains, or scattering by rough surfaces. In 1987, Willems proposed a reverberation technique in which an ultrasonic pulse is first generated at the surface of a sample, then propagates in the material and, due to the finite size of the sample, fully insonifies the latter with incoherent ultrasound after some time. [H. Willems, “A New Method for the Measurement of Ultrasonic Absorption in Polycrystalline Materials”, in D. O. Thompson, D. E. Chimenti (Eds.), Review of the Progress in Quantitative Nondestructive Evaluation, Vol. 6A, p.473, 1987] The measured decrease of the ultrasound amplitude with time can then be solely attributed to absorption mechanisms. When a short laser pulse is used to generate ultrasound, a very wide range of ultrasonic frequencies can be observed. A single measurement then allows the determination of the ultrasound absorption coefficient at many frequencies at once. The main limitation of this technique is that it is restricted to samples with finite volume, making it inappropriate for online measurement.
In 1985, Guo et al. demonstrated experimentally, using conventional transducers and samples of large dimensions, that when an ultrasonic pulse propagates and is scattered by the various structures in a material, it gives rise to an energy cloud of incoherent vibration, termed “diffuse ultrasound”, and that this energy cloud spreads as governed by the diffusion equation. [C. B. Guo, D. Holler, and K. Goebbels, “Scattering of Ultrasonic Waves in Anisotropic Polycrystalline Metals”, Acustica, vol. 59, p. 112, 1985] The vibration energy not only diffuses but is also absorbed in the material by various mechanisms. By observing the time evolution of the diffuse ultrasound, one can evaluate the diffusion and absorption coefficients of ultrasound. This technique has many advantages over the previous ones. Firstly, it can be applied to large samples. Secondly, it provides the absorption and diffusion coefficients of ultrasound, both parameters being useful for the characterization of the microstructure.
However, because the technique utilizes piezo-electric transducers, it is subject to several limitations. One such limitation is that the piezoelectric transducer must be either in direct contact with the object to be measured, or coupled to that object using some type of bonding material. In addition, delay lines, also called buffer rods, are often used to transport the ultrasound from the object to the transducer. Thus, the ultrasound field to be measured leaks out of the object and into the bond and the transducer where it may be attenuated both by the transducer's, bond's, or delay line's material properties and by the conversion of acoustic energy into electrical energy by the transducer. In practice, this limits the application of the technique to objects which show either a high diffusivity or a high absorption so that the decrease in the sound field energy caused by the sample itself is much larger than the decrease in the sound field energy caused by the transducer and its bond or delay line.
Another limitation of the technique is that the piezoelectric transducer has a relatively narrow bandwidth (or order ±10% to ±50% of the transducer's center frequency). Wideband transduction is usually preferred because a wideband signal can be made narrowband by filtering, whereas a narrowband signal cannot be made wideband.
Yet another limitation of the technique is that the piezoelectric transducer is of relatively large dimensions. Typical dimensions of such transducers may vary from a few mm in diameter to perhaps 25 mm in diameter. Therefore, these transducers cannot spatially resolve the ultrasound field to an accuracy better than about 1 mm in the best cases. This may cause difficulties when measuring the sound field of a small object, or when attempting to measure the spatial dependence of the sound field as a function of position.
In 1990, Weaver proposed an application of the diffusion coefficient of ultrasound to the characterization of the microstructure of materials. [R. L. Weaver, “Ultrasonic Diffuse Field Measurements of Grain Size”, Non-Destructive Testing and Evaluation in Manufacturing and Construction, 1990, p. 425] Theoretical considerations show that the diffusion coefficient behaves like the inverse of the ultrasonic attenuation coefficient due to scattering. This coefficient is related to the grain size of polycrystalline materials. Weaver showed experimentally that the diffusion coefficient of ultrasound is affected by thermal treatment for an AISI 1045 steel, which supports the idea that the technique can be used for the non-destructive measurement of grain size. However, he did not specify a method for obtaining precise grain size measurements estimates from the measured diffusion coefficient. Instead, he plots
Lamouche Guy
Levesque Daniel
Lord Martin
Moreau Andre
(Marks & Clerk)
National Research Council of Canada
Saint-Surin Jacques
Williams Hezron
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