The present invention relates generally to an apparatus and method of non-destructive evaluation of materials, especially composite materials, and more particularly, to an apparatus and method for ultrasonically measuring the porosity of a composite material by accessing only one side of the composite material.
In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (xe2x80x9cNDExe2x80x9d) methods are required to assess the structural integrity of composite structures; for example, to detect inclusions, delaminations and porosities. Conventional NDE methods, however, are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and systems have been proposed to assess the structural integrity of composite structures. One method to generate and detect ultrasound using lasers is disclosed in U.S. Pat. No. 5,608,166, issued Mar. 4, 1997, to Monchalin et al. (the xe2x80x9c""166 Patentxe2x80x9d). The ""166 Patent discloses the use of a first modulated, pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage associated with this approach is that the first pulsed laser beam must be modulated.
Fiber-reinforced polymer-matrix composite materials have been proved of great interest in various fields for their high strength and low weight. In the aircraft industry in particular, the use of these materials increases steadily. However, polymer-matrix composites also must be inspected before installation for the presence of defects such as delaminations, inclusions, and porosity.
Porosity is the presence of voids created in composites during manufacturing. Measurement of ultrasonic attenuation provides a measurement of porosity level. Ultrasonic techniques were found to be the best technique to assess porosity level in composites. Unfortunately, it is not always possible to use a self-referencing technique exploiting two different echoes in the signal, like two back-wall echoes. This is because in some cases, the porosity is so high that barely one echo is observed. Porosity measurements must rely on only one echo. Therefore, unlike delamination and inclusions, the ultrasonic response to porosity is frequency and amplitude dependent. This characteristic means that each ultrasonic system has a different response to a given porosity level. Therefore, each ultrasonic system must be calibrated relative to certain porosity levels.
Two different techniques are used to measure porosity: reflected through transmission ultrasound (RTTU) and through transmission ultrasound (TTU). These methods compare the amplitude of an ultrasonic wave that has propagated through a sample with the amplitude of an ultrasonic wave that has propagated through a reference sample. The ultrasonic echo used is the one reflected from a steel or glass plate in the RTTU technique. FIG. 1 depicts this operation. Alternatively, in the TTU technique, ultrasonic echo is the echo coming directly from the piezoelectric emitter. FIG. 2 illustrates the TTU Technique
Unfortunately, these techniques require an ultrasonic couplant, such as water and access to both sides of each composite panels. Ultrasonic refraction at the water/composite interface changes the ultrasonic wave direction when the piezoelectric emitter is not normal to the sample surface. Porosity must be evaluated at all points over the sample surface. The ultrasonic system must be moved at several positions over the sample surface to form a tight grid of ultrasonic measurements. The grid spacing must be smaller than the size of a critical porosity defect. The normalcy requirement makes the scanning of samples having curved surfaces expensive and slow. Moreover, when the two sides of a composite part are not accessible, such as for a closed box, porosity evaluations for such parts simply cannot be made using these techniques.
Therefore, a need has arisen for a method and system that permit making porosity measurements and evaluations of composite materials, which corrects the problems identified above, including the problems of needing water or similar couplant or access to both sides of the composite material.
Moreover, there is the need for a method and system that permit making porosity measurements and evaluations of composite materials without the need to modulate a pulsed laser beam or other similar problems such as those existing with the ""166 Patent.
The present invention provides a method and system for ultrasonically measuring the porosity in a composite material by accessing only one side of the composite material that substantially eliminates or reduces disadvantages and problems associated with previously developed ultrasonic systems and methods.
The present invention provides a method and system to measure porosity in composite material by ultrasonic techniques, such as laser ultrasound, which method only requires access to one side of the composite material. The method and system evaluate ultrasonic attenuation that correlates with porosity, by comparing the amplitude of the sample material back-wall echo to the amplitude of the back-wall echo of a reference sample. The method and system further involves normalizing the signal amplitude according to a reference echo (for laser-ultrasound, the surface echo) to eliminate amplitude variations caused by phenomena not related to porosity.
According to another aspect of the invention, there is provided a method and system for ultrasonically measuring the porosity in a sample composite material by accessing only one side of the sample composite material. The method includes the steps of measuring a sample ultrasonic signal from the sample composite material, normalizing the sample ultrasonic signal relative to the surface echo of the sample composite material, and isolating a sample back-wall echo signal from the sample ultrasonic signal. A sample frequency spectrum of said sample back-wall ultrasonic signal is then determined. Next, the method and system include the steps of measuring a reference ultrasonic signal from a reference composite material, normalizing said reference ultrasonic signal relative to the surface echo of the reference composite material and isolating a reference back-wall echo signal from said sample ultrasonic signal. A reference frequency spectrum of said reference back-wall ultrasonic signal is then determined. The invention further includes deriving the ultrasonic attenuation of the sample ultrasonic signal as the ratio of the sample frequency spectrum to the reference frequency spectrum over a predetermined frequency range. Comparing the derived ultrasonic attenuation to predetermined attenuation standards permits evaluating the porosity of the sampled composite material.
A technical feature of the present invention is the use of the back-wall echo amplitude of a composite material and comparing it to the back-wall echo amplitude measured from a reference composite material. The present invention normalizes all ultrasonic signals (including reference sample echo) relative to a reference echo (in the case of laser-ultrasound, the surface echo) to eliminate amplitude variations caused by any effects not directly related to porosity
Another technical feature of the present invention is the use of spatial averaging of the ultrasonic traces to easily identify the position of the back wall echo. The present invention includes calculating the full frequency spectra of the sample back wall echo and of the reference back wall echo using windowing techniques. The ratio of these two spectra provides the ultrasonic attenuation as a function of frequency. The present invention uses the ultrasonic attenuation at a given frequency to evaluate porosity relatively to standards established previously for the material inspected, or to use the frequency dependency of ultrasonic attenuation to correlate with porosity level.