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|Title: ||Development of improved reflective fringe pattern technique for subsurface flaw detection on objects with specularly reflective surfaces|
|Other Titles: ||Fan guang tiao wen tu an ji shu de gai jin fa zhan ji zai biao mian fan guang wu jian di ceng que xian tan ce de ying yong|
|Authors: ||Chan, Wan Yin (陳韻賢)|
|Department: ||Department of Manufacturing Engineering and Engineering Management|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2010|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Reflective materials -- Nondestructive testing.|
Reflective materials -- Optical properties.
|Notes: ||CityU Call Number: TA417.43 .C45 2010|
226 leaves : ill. (some col.) 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2010.
Includes bibliographical references (leaves 198-211)
|Abstract: ||Concerns for the safety of aging structures have called for new/improved
nondestructive testing (NDT) methods, as the currently available methods are inadequate
to meet various inspection requirements. Optical techniques have recently attracted
considerable interest due to their merits of being full-field, non-contacting and allowing
speedy detection of defects in any materials (including metals, non-metals, composite
materials, biological tissues, etc.).
Among the existing optical techniques, shearography has a desirable feature of
detecting defects by means of stressing, since structural failures are generally caused by
excessive stress level. Shearography measures materials’ response to stresses and reveals
material defects by identifying defect-induced deformation anomalies. When a test object
containing a flaw is loaded, stress concentration at the vicinity of the defect is induced
and hence the flaw can be detected by shearography. However, shearography relies on the
formation of the random interference of light scattered from a diffused object surface and
thus, it is not applicable to objects with specularly reflective surface.
Recently a reflective three-dimensional computer-vision method is developed for
NDT of specularly reflective objects. In the setup, a computer-generated fringe pattern
displayed on a computer monitor is placed in front of the test object, whose specularly
reflective surface behaves as a mirror, thus producing a mirror image of the fringe pattern.
When the object is stressed, the object surface will be deformed causing a distortion in the
fringe image. When the method is applied to nondestructive flaw detection, the phase
distributions of the two fringe images (before and after deformation) are separately determined. The difference of the two phase distributions measures the surface
deformation (surface slope change). A surface or subsurface defect will cause an anomaly
in the deformation and thus can be revealed. The method is simple, robust and applicable
in industrial environments. However, the method lacks the desirable sensitivity that is
limited by the bit-depth of the monitor used to display the fringe pattern.
In this thesis, an improved technique is developed, which overcomes the
sensitivity limitation of the former method. The improved technique eliminates the use of
the computer-generated and monitor-display fringe pattern. Instead, an optical
interference fringe pattern having an analog resolution is produced by a two-point source
of coherent light; hence, the fringe phase determination is no longer subjected to the
depth resolution of the computer-monitor. With the use of higher gray-level image
digitizers, the sensitivity of the fringe phase determination would be proportionally
increased. The principle and the instrumentation of the improved technique are presented.
Experimental verification of the applicability of the improved technique in crack
detection as well as detection of debonds in laminated materials are demonstrated. It
should be noted that the flaw detection mechanism is based on the response of the flaw to
stress. Should the applied stress during test be similar to the service stress, flaws that are
critical and detrimental to the service life of the object would be revealed, and cosmetic
flaws that do not undermine the structural integrity of the test object can be ignored. This
would minimize false rejections during inspection.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b3947881|
|Appears in Collections:||MEEM - Doctor of Philosophy |
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