Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
1999-07-27
2001-01-23
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C702S040000, C372S028000
Reexamination Certificate
active
06176135
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to the field of ultrasonic laser testing, and more particularly a method and system for optimizing the frequency content of laser-generated ultrasonic waves for target inspection.
BACKGROUND OF THE INVENTION
The use of advanced composite structures has experienced tremendous growth in the aerospace, automotive and other commercial industries. Non-destructive evaluation (NDE) methods are often employed to detect inclusions, delaminations and porosities in an effort to ascertain the structural integrity of the composite structures. One method of NDE is laser ultrasound.
Laser ultrasound involves the use of lasers for generation and detection of ultrasound in materials such as composites. The technique offers the potential of rapid, non-contact inspection. Typically, a laser source produces ultrasonic surface displacements on the surface of a remote target. A second probe laser beam can detect the ultrasonic surface displacement on the surface of the remote target. Collection optics and instrumentation can then be used to process the probe laser beam and output data representing the ultrasonic surface displacements on the surface of the target.
FIG. 1
illustrates a conventional laser ultrasound inspection method.
FIG. 1
employs pulse laser
10
to inspect an object, such as aircraft
16
. Pulse laser
10
emits generation pulse
12
which is directed towards scanning mirror
14
. Scanning mirror
14
steers generation pulse
12
to inspect aircraft
16
. Various lenses for focusing may also be employed. A CO
2
pulse laser may be employed as pulse laser
10
. Pulse laser
10
deposits generation pulse
12
over a certain depth in an area on aircraft
16
. Generation pulse
12
is converted to heat and causes expansion of an inspected area on aircraft
16
. The expansion of the inspected area on aircraft
16
generates laser-generated ultrasonic waves.
The frequency content of the laser-generated ultrasonic waves contributes to the success of laser ultrasound as an NDE method. Higher frequencies yield better spatial resolution. However, for certain materials such as composites, high frequencies are attenuated more rapidly than lower frequencies. The thicker and more attenuative the component to be inspected, the lower the ultrasonic frequency content must be to avoid large attenuation. Consequently, with conventional methods the resolution and accuracy of defect detection tends to be limited for more attenuative materials.
SUMMARY OF THE INVENTION
In light of the above, a need exists for a system and method that generates a desired frequency content in laser-generated ultrasonic waves used for NDE. The present invention provides a system and method for ultrasonic laser inspection that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods for ultrasonic laser inspection.
More specifically, the present invention provides a method for generating a desired acoustic frequency content in a laser-generated ultrasonic wave emitted from a remote target in response to a generation laser pulse. The method includes generating the generation laser pulse using a laser source. An optimal wavelength &lgr;
0
for the laser pulse is determined using a computer. The optimal wavelength &lgr;
0
is determined from material-specific, empirically calculated data stored in a storage device that is accessible to the computer. An optimal laser pulse is generated by shifting the generation laser pulse to the optimal wavelength &lgr;
0
. The optimal laser pulse is directed to the remote target to generate the laser-generated ultrasonic wave with the desired frequency content.
The present invention provides an important technical advantage in that a laser-generated ultrasonic wave can be generated with a desired frequency content. Thus, for certain materials that require a specific range for frequencies to adequately inspect the material, an optimal optical penetration depth &mgr;
0
can be determined. Based on the necessary optimal optical penetration depth &mgr;
0
, an optimal wavelength &lgr;
0
for the optimal laser pulse can be determined. In turn, the optimal wavelength S
0
to generate the optimal laser pulse wavelength that products the desired range of frequencies in the laser-generated ultrasonic wave is obtained by tuning the source laser wavelength or by shifting the generation pulse laser wavelength using a shifting device such as an optical parametric oscillator. Therefore, depending on the thickness of the material or material composition, the desired frequencies can be generated to produce the best resolution for inspection. Additionally, the attenuation of the ultrasound can be controlled allowing a user to optimize their inspection techniques for the defects to be searched for. Furthermore, by understanding the attenuation characteristics of the ultrasound generated in the target, the scanning technique can be optimized based on these characteristics to reduce or eliminate over sampling and therefore increase the speed and efficiency of the inspection.
REFERENCES:
patent: 4633715 (1987-01-01), Monchalin
patent: 4683750 (1987-08-01), Kino et al.
patent: 5814730 (1998-09-01), Brodeur et al.
patent: 5982482 (1999-11-01), Nelson et al.
Bauco Anthony S.
Dubois Marc
Filkins Robert J.
Lorraine Peter W.
Venchiarutti Barbara
Gray Cary Ware and Freidenrich LLP
Moller Richard A.
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