Development of improved reflective fringe pattern technique for subsurface flaw detection on objects with specularly reflective surfaces

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.