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Using Shearography to Find the Flaws PUBLIC ACCESS

Digital Speckle Shearing Interferometry Uses a CCD Camera and Computer Image Processing to Produce the Fringe Patterns that Spell Trouble in Composite Panel and Pipes.

[+] Author Notes

Siew-Lok Toh and Fook-Siong Chau are associate professors in the Department of Mechanical and Production Engineering at the National University of Singapore.

Mechanical Engineering 121(02), 62-63 (Feb 01, 1999) (2 pages) doi:10.1115/1.1999-FEB-7

Abstract

This article highlights warning signs that a composite panel may be delaminating or thinning, but engineers have a powerful nondestructive testing tool in shearography. This is a laser interferometric method developed originally for full-field observation of surface strains of components. Flaws usually induce strain concentrations around them, and shearography can be employed to detect those flaws. A relatively new variation, digital speckle-shearing interferometry (DSSI), uses a charged-coupled device (CCD) camera and computer image processing to capture and process the interferometric fringe patterns. The digital version is faster than conventional shearography, and does not require any film or Fourier filtering. The main advantages of digital speckle shearing interferometer are the ease and speed with which fringe patterns can be obtained.

Article

In Looking for warning signs that a composite panel may be delaminating or thinning, engineers have a powerful nondestructive testing tool in shearography, a laser interferometric method developed originally for full-field observation of surface strains of components. Flaws usually induce strain concentrations around them, and shearography can be employed to detect those flaws.

Conventional shearography uses a speckle-shearing camera, in which a thin glass wedge covers one half of the lens. The object under study is illuminated by laser light, and the camera produces two sheared images that interfere with each other. This random interference is known as a speckle pattern. When the object is deformed, the speckle pattern changes. High-resolution film is exposed before and after the object is deformed. The two speckle patterns interfere to produce a fringe pattern that depicts the surface gradient of the deformed object. The fringe pattern is only visible using a high-pass Fourier filtering technique. Though the images obtained are good, this method is time-consuming.

A relatively new variation, digital speckle-shearing interferometry (DSSI), uses a charged-coupled device (CCD) camera and computer image processing to capture and process the interferometric fringe patterns. The digital version is faster than conventional shearography, and does not require any film or Fourier filtering.

The optical arrangement for DSSI makes use of the Michelson interferometric principle as the shearing device. The reflected beam from the object occurs on the beam splitter, where it is split in two. The split beam is transmitted to the CCD camera, where the parts combine to give interference fringes.

To improve the quality of the fringe patterns, various image processing routines are often incorporated. Phase shifting is a technique used to determine phase distribution in an interferometric fringe pattern. This is done by attaching a mirror to a piezoelectric translator. When the technique is applied to the fringe patterns obtained in DSSI, values of the displacement derivative can be determined quantitatively.

In our experiments, a four-image method is used to determine the phase distribution. Three speckle-pattern images of the object are obtained before deformation, each with a different phase shift. Then, after the object has been deformed, the fourth image is captured. By subtracting the first three images from the fourth image, three fringe patterns corresponding to phase shifts of 0, 21t/3, and 41t/3 are obtained. Subsequently, from these fringe patterns, the phase variation of the deformed object can be calculated.

To test this method on a composite plate with suspected debonds, we used the wet layup technique to fabricate a glass-reinforced polyester (GRP) flat plate of about 270x250 millimeters. To simulate the overlapping of the debonds in a four-ply GRP plate, two square flaws-each made of two plastic sheets with sides 25 mm long and then taped together-were positioned below the first and second plies of the plate to give a 10-mm overlap. The first ply in this case is the one nearer to the illumination surface. The GRP plate was placed in a specially constructed vacuum chamber. Double-exposure shearography was performed; one exposure before and one after a vacuum pressure were applied.

Fringe patterns of overlapping debonds.

Grahic Jump LocationFringe patterns of overlapping debonds.

The fringe patterns of the overlap flaws detected by conventional shearography are shown in Figure 1. Flaw 1 is below the first ply and flaw 2 is below the second ply. Note that the fringes of flaw 1 are much denser than those of flaw 2. This is because, being closer to the illuminated surface, flaw 1 will deform more under vacuum loading. Also note that the fringes of the two flaws interact; the zero-order (outermost) fringe of flaw 1 joins that of flaw 2. This indicates that the displacement of the left edge of flaw 1 is not zero. The results show that the two overlapping flaws tend to produce two sets of fringes with different fringe densities. The outline of the fringe pattern gives the sizes of the flaws which, in this case, differ from the actual sizes by about 5 percent.

To illustrate the use of shearography in the detection of internal thinning in pipes, a 315-mm-long mild steel pipe with a mean diameter of 127 nun and a thickness of 6.5 mm was used. A 15-mm-square defect was machined (using electrodischarge machining) to a depth of 3 mm on the inside of the pipe. An internal pressure of 3 .4 megapascals was applied to the pipe by a hydra ulic pump.

Figure 2 shows the fringe pattern of the square flaw as imaged by conventional shearography. The outline shows the flaw's shape clearly, but due to the shearing effect, it appears elongated. With DSSI (Figure 3) , the defect is clearly revealed in the phase map. The fringe patterns in Figures 2 and 3 correspond to the out-of-plane displacement derivatives of the deformed pipe with shearing along the pipe's longitudinal axis.

Since fringe patterns produced by DSSI are inherently noisy, a smoothing operation performed in a 9x9-pixel window is carried out after subtraction. The computational time to produce one phase map is approximately 10 seconds, using a real- time image processing board (Imaging Technology MVC 150/ 40-PCI board) installed in a Pentium PC running at 133 MHz.

In conventional shearography, a 15-mW heliull1.-neon laser was used as the light source. The image-shearing camera used was a Mamiya RB67 Pro-S with a specially ground wedge of about 1-degree included angle, placed in the iris plane of the lens. High-resolution Agfa Holotest type 10E75 holographic film was used as the recording medium. A double-exposure technique was adopted whereby the film was exposed before and after the loads were applied. The double-exposed films were then developed, after which the shearograms were viewed using the high-pass filter.

Fringe pattern of a pipe with a 15-mm-square by 3-mm-deep flaw using conventional shearography

Grahic Jump LocationFringe pattern of a pipe with a 15-mm-square by 3-mm-deep flaw using conventional shearography

In DSSI, a 10-mW helium-neon laser served as the light source. The resolution of the image processing system is 512x512x8 bits. Before pressure was applied, the CCD camera obtained three images of the speckle patterns of the pipe, with each image having a different phase shift. The fourth image was captured after the pressure had been applied. In the system, real-time subtraction using a pipeline image processor was implemented to obtain the fringe patterns of the test specimen when loaded. The resultant image was displayed at a rate of25 frames per second.

Vacuum stressing has proven to be a effective method for detection of delaminations or debonds in composites. This technique has the advantages of being noncontacting, yielding a fast response, and covering the full field.

Fringe pattern of the same pipe (15-mm-square by 3-mm-deep flaw) using DSSI shearography: (left) fringe pattern of pipe; (right) phase map of fringes.

Grahic Jump LocationFringe pattern of the same pipe (15-mm-square by 3-mm-deep flaw) using DSSI shearography: (left) fringe pattern of pipe; (right) phase map of fringes.

The main advantages of digital speckle shearing interferometer are the ease and speed with which fringe patterns can be obtained. A simple yet effective optical setup coupled to a powerful image processing system forms a useful , on- line nondestructive testing apparatus for industrial applications. The phase-shift technique provides numerical values of the displacement derivative at all points on the surface of the specimen, allowing rapid determination of the location, size, and shape of a defect

Copyright © 1999 by ASME
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