Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2000-07-24
2002-06-04
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S579000
Reexamination Certificate
active
06397680
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ultrasonic apparatus and method based on resonance frequencies for determining unknown parameters in a multilayer structure, such as thickness, elastic properties of individual layers, or bonding strength between layers. The method is of particular interest when the thickness of the multilayer structure is less than two millimeters or when the ultrasound is generated by a laser.
2. Description of Prior Art
Ultrasonic measurement techniques generally refers to the principle of 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. Conventional ultrasonic devices have been developed which involves the use of transducers, including piezoelectric and electromagnetic acoustic transducers (EMATs). Another ultrasonic device is based on laser-ultrasonics, wherein one laser with a short pulse is used for generation and another one, long pulse or continuous, coupled to an optical interferometer is used for detection. Either laser may be coupled 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 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).
To determine material properties from detected signal, usually the time of flight (TOF) method is used, which consists in measuring the time that the ultrasonic pulse takes to travel from the point of generation to the point of detection. This travel time relates to the travel distance and ultrasonic velocity which depends on some elastic properties, and knowing one parameter allows to get the other. For thickness measurement or properties in the thickness direction, the detection is made on the opposite side of the object aligned with the generation point, or on the same side and close to the generation point. This conventional method for determining properties of an object is limited to materials having a thickness which is relatively large compared to the wavelength of the ultrasonic pulse used. Otherwise, ultrasonic waves reflected from the front and back surfaces of a thin object (<2 mm) overlap and interfere in the time domain, precluding any measurement of time-of-flight. The generation of an ultrasonic pulse with smaller wavelength or high frequency content for cases of thin specimen may be limited by availability and costs associated with such an instrumentation.
The situation is even worst for multilayer structures comprising several layers of different materials with a finite bonding strength between layers. For multilayer structure with thick enough layers with respect to the ultrasonic wavelength, direct application of the TOF method is possible provided that a proper identification of reflected pulses is made with each layer. Recent examples are found in the U.S. Pat. No. 5,866,819 (Albu et al.) and U.S. Pat. No. 5,974,886 (Carroll et al.). For thinner multilayer structures, more refined techniques have been developed in recent years still analyzing the detected signal in the time domain. Examples of methods used are reduction of pulse duration with a deconvolution technique as found in the U.S. Pat. No. 5,723,791 (Koch et al.), comparison of detected waveform with a set of reference waveforms as in the U.S. Pat. No. 5,038,615 (Trulson et al.) and analyzing the interfering ultrasonic tone-bursts (narrowband signal, typically of about ten cycles in duration) with selected frequencies as in U.S. Pat. No. 5,608,165 (Mozurkewich).
The ultrasonic spectroscopy method is an alternative for this purpose. Closely related with the latter application, constructive or destructive interference between reflected tone-bursts at a particular frequency arising from propagation of an ultrasonic wave between the boundaries of the object corresponds to an ultrasonic resonance. For example, an ultrasonic resonance can arise in a plate when an ultrasonic wave propagates in the thickness direction and that the plate thickness corresponds to an integral multiple of half the wavelength of the ultrasonic wave. A longitudinal resonance refers to an ultrasonic resonance of a longitudinal wave, i.e., of an elastic wave polarized in the propagation direction. A shear resonance refers to an ultrasonic resonance of a shear wave, i.e., of an elastic wave polarized in a direction perpendicular to the propagation direction. In this method, a tone-burst is generated with a selected frequency, the signal detected is digitized and analyzed into the frequency domain typically by using the Fast Fourier Transform to get amplitude or phase information. By sweeping the frequency of the generated tone-burst and processing the each detected signal over the range of interest, the frequency response is obtained to identify resonances. Examples of applications using the frequency response but without relying on resonances are found in the U.S. Pat. No. 5,305,239 (Kinra), U.S. Pat. No. 5,663,502 (Nagashima et al.). Examples of those using the resonance information are found in U.S. Pat. Nos. 4,305,294 (Vasile et al.), 5,062,296 (Migliori), and more recently, U.S. Pat. No. 5,591,913 (Tucker). This method has the limitation of a long acquisition time as well as processing time, even with very recent improvements as in the U.S. Pat. No. 6,023,975 (Willis).
A different ultrasonic spectroscopy method is to launch a wideband pulse either with a transducer or with a radiation source like a laser and to analyze the single interaction signal comprising several reflected pulses or echoes into the frequency domain. The interference between overlapping echoes produces resonance dips or peaks in the amplitude of the frequency response. This method has been used with success, the overlapping of echoes can be very high, and the resonances are usually sharp enough to get a very good estimate of material properties, even with a moderately high level of attenuation. However, as in the previous method, this is mostly used for single plate or sheet of one material. A recent example from one of the inventors, measuring longitudinal as well as shear resonances with laser-ultrasonics to get material properties including texture in metal sheets, can be found in the U.S. Pat. No. 6,057,927 (Levesque et al.). For multilayer structure, an intricate coupling between resonance modes occurs which requires a more careful analysis to get any material properties. With the presence of a fluid between layers, the resonance modes are mainly those of individual layers in vacuum and an inversion algorithm has been successfully applied to recover some properties. For adhesively bonded layers, the resonance modes of the individual layers are strongly coupled and important frequency shifts are observed. The sensitivity of the ultrasonic signal to adhesive bond strength and the possibility of its determination is found in the U.S. Pat. No. 5,408,881 (Piche et Levesque), from one of the inventors. It is generally found that a model for propagation of ultrasound is essential to determine any unknown parameter from a multilayer structure.
It is an object of the present invention to alleviate the afore-mentioned disadvantages of the prior art.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method for determining layer thickness or other parameters of a multilayer structure, comprising the steps of producing an ultrasonic pulse in the multilayer structure for interaction therewith over a range of frequencies t
Choquet Marc
Levesque Daniel
Massabki Maroun
(Marks & Clerk)
Miller Rose M.
National Research Council of Canada
Williams Hezron
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