The present invention relates to the field of nondestructive testing, and, in particular, to the field of speckle-shearing interferometry, or shearography.
Nondestructive testing of aircraft structures, such as honeycombs, has been performed by techniques which include tap testing (i.e. using an inspector""s ear to judge the presence of a skin-to-core disbond), impedance bond testing, and pulsed-echo ultrasonics. While effective, these prior art methods are either subjective, require considerable recalibration with changes in skin thickness, provide subjective interpretation, or are slow. Shearography has been found to be a more preferred method of nondestructive testing.
Shearography was first introduced by Hung and Taylor, in xe2x80x9cMeasurement of slopes of structural deflections by speckle-shearing interferometryxe2x80x9d, Experimental Mechanics, vol. 14, pages 281-285 (1974), and by Leendertz and Butters, in xe2x80x9cAn image-shearing speckle-pattern interferometer for measuring bending momentsxe2x80x9d, J. Phys. E.: Sci. Instrum., vol. 6, pages 1107-1110 (1973). Shearography essentially comprises the formation of an image comprising two laterally-displaced images of the same object.
Shearography is a full-field optical speckle interferometric procedure which is capable of measuring small deformations of a surface. These deformations can be produced by several mechanisms, and are measured for several purposes. Deformation mechanisms include, among others, vacuum, pressure, microwave, thermal, vibration, and ultrasonic excitation. Purposes for measurement include stress analysis, vibration measurement, acoustic and elastic wave visualization, and other non-destructive testing (NDT) including detection of flaws in a test object.
The first practical apparatus for performing shearography electronically was introduced in 1987, and was based on the technology described in U.S. Pat. No. 4,887,899, the disclosure of which is incorporated by reference herein. The form of shearography described in the above-cited patent, using birefringent optics as a means of generating a sheared image, provided the first high-resolution, real-time shearography system which could produce images of disbonds due to out-of-plane deformations caused by vacuum stress of the skin of the structure being tested.
The first portable vacuum stress shearography instruments used video subtraction to provide rapid imaging of flaws. While this technology was a major advance in nondestructive testing, enabling one to perform tests at a rate of 150 square feet per hour, as compared with 3-8 square feet per hour using prior methods, the sensitivity of the technology to defects was limited. Subtraction shearography provides images of flaws only when the out-of-plane deformation exceeds one-half wavelength in the shear offset distance, the distance between the sheared images. The portable subtraction shearography devices of the prior art have found important uses in the aerospace field, in the inspection of composite honeycomb structures including radomes, composite fuselages, wheel doors, close-out panels, and many other components.
Portable vacuum stress shearography cameras are subject to several types of vibration, noise, and mechanical instabilities that considerably degrade the image quality and the ability to detect defects in aerospace structures. Not only do these sources of noise have to be eliminated with techniques that do not add weight and size to the portable instrument, but any method used to increase sensitivity must be made to run as fast as possible to prevent degradation of the images.
One method of increasing defect sensitivity is the use of phase-stepping shearography, introduced by Nakadate in 1985. While Nakadate demonstrated the use of phase-stepping and the increased defect sensitivity provided by phase map presentations, the software and hardware available at that time required ten minutes to yield the phase map image, far too slow for use in a practical nondestructive testing instrument. In addition, the phase step process requires capturing consecutive video frames and considerable arithmetic manipulation of between four and ten images to calculate the phase map. The level of noise rapidly increases as the time to perform the calculations increases, dramatically reducing sensitivity.
A basic setup of an electronic shearography system is as follows. Coherent laser light is spread out to illuminate a portion of the object""s surface, e.g. one square foot. The light reflects from the surface, passes through an optical shearing mechanism, and then enters a CCD camera. The surface is then deformed by one of the aforementioned mechanisms, such as heat. As the surface expands slightly due to the applied heat, the deformation of the surface is viewed, in real time, on a video monitor. This deformation is usually not visible to the eye because it is on the order of the wavelength of the laser light being used, i.e. approximately 250 nm. Deformation of the surface often shows direct evidence of a subsurface flaw.
Shearography has evolved over the years from a film-based to a video-based (electronic, or analog) system, and finally to systems which store the images in a computer, in digital form. In its film-based form, shearography is typically limited to an optics laboratory. In its electronic or digital form, if the system is made compact, shearography can be removed from the laboratory, and used in real-world settings. It can survive environmental factors such as slight heat and vibration fluctuations due to its particular optical setup.
As implied above, there are two fundamental modes in which shearography can be used, namely, speckle correlation fringe formation due to subtraction, and phase map formation due to phase stepping. As already noted, phase stepping is beneficial since it results in increased signal-to-noise ratio (SNR), increased displacement resolution (resulting in increased flaw detection sensitivity), quantitative rather than qualitative results, and other factors. Either of the two modes (fringes or phase stepping) can be used with either of the three bases (film, electronic, digital), creating six combinations. Then there are more than seven known optical shearing mechanisms, and more than six known phase-stepping methods. The number of possible system implementations is hence quite high. Furthermore, the phase-stepping algorithm, of which there are at least ten, the computational method of implementing the algorithm, and the consequent speed with which the algorithm is executed are all important. These directly affect the accuracy and SNR of the final measurements as well as the rate at which a user can view the results and make slight changes. Finally, the entire optical system is either used as a research tool in an optics laboratory on a vibration-isolation table, or it must be packaged such that it can be taken into the field and used with several excitation mechanisms in order to find flaws effectively, efficiently, and conveniently.
The present invention provides a high-speed phase-stepping shearography system which requires less than about one second to produce an image, and which demonstrates remarkable stability, sensitivity, and image quality. Compared to the prior art, the present invention improves defect sensitivity by a factor of 50, enabling a portable shearography system to compete with widely-accepted ultrasonic systems, but at a speed of operation which is about 50 times faster.
The system of the present invention includes four subsystems, namely a shearography head, an enclosure, an excitation mechanism, and a computational subsystem. The shearography head preferably uses a Michelson interferometer to generate the sheared images, and can receive laser light either from an external laser, coupled by an optical fiber, or from a laser diode which is internal to the head. The amount of phase stepping is automatically adjusted by moving one of the mirrors of the Michelson interferometer, through the use of a piezoceramic disk which is controlled by a voltage determined by a computer. The amount of shearing is adjusted by manually tilting a different mirror in the Michelson interferometer.
The enclosure includes, at a minimum, a casing having a transparent window to permit laser illumination and formation of an image of the test object. In a preferred embodiment, the shearography head is mounted on the enclosure, the enclosure having a hole which cooperates with a similar hole in the shearography head, such that laser light can pass from the head, to the object, and back into the head, through the cooperation of various mirrors. The enclosure also includes means for stabilizing the system relative to the object being tested.
The excitation mechanism can be a vacuum, or it can be a thermal or vibration system. In the preferred embodiment, all three excitation mechanisms (vacuum, thermal, and vibration) are built into the same enclosure, so that any or all of these mechanisms can be used without modifying the apparatus. The preferred source of the vacuum is an external blower which is connected, through suitable holes in the enclosure, to the interior of the enclosure. The preferred source of thermal excitation is a grid of thin heated wires disposed near the transparent window of the enclosure, so as to be in a position to heat the test object. The preferred source of vibrational excitation includes a shaker/stinger/plunger arrangement which is built into the enclosure.
The computational subsystem includes a programmed computer which is connected, through appropriate analog-to-digital and digital-to-analog converters, to the components described above. In the preferred embodiment, the computer directs a four-step algorithm which captures and stores images of the test object at four different positions of the mirror, when the object is both in the deformed and undeformed states. Comparison of the stored images, coupled with application of a smoothing algorithm, yields a pattern that can be viewed, essentially in real time, on a video display.
The present invention therefore has the primary object of providing an apparatus for performing real-time digital shearography.
The invention has the further object of providing the benefits of the phase-stepping technique in real-time digital shearography for increased sensitivity and signal-to-noise ratio as well as quantitative rather than qualitative data.
The invention has the further object of providing real-time shearography, using a portable unit.
The invention has the further object of providing a shearography system which produces a high-resolution image in real time, and which can conveniently be used in commercial or industrial environments.
The invention has the further object of providing an integrated, compact digital shearography system, wherein a plurality of mechanisms for excitation are present in a single housing.
The invention has the further object of enhancing the efficiency of nondestructive testing of objects.
The invention has the further object of making it easier to search for defects in structures where interior access is difficult or impractical, such as in aircraft, space vehicles, boats, and civil-engineered structures.
The reader skilled in the art will recognize other objects and advantages of the present invention, from a reading of the following brief description of the drawings, the detailed description of the invention, and the appended claims.