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Reflective Fringe Pattern Technique for Surface Evaluation and Nondestructive Testing

This paper presents a novel optical technique for nondestructive testing of objects having specularly or semi-specularly reflective surfaces. In the setup, a computer-generated fringe pattern displayed on a computer-monitor is placed in front of the test object surface (which acts as a mirror) and forms a mirror image of the fringe pattern. The fringe pattern is perturbed according to the surface slope distribution. Analysis of the fringe phase distribution allows surface imperfections to be identified. This technique can also be used for measuring deformation. In the measurement, the fringe phase distributions before and after deformation are separately determined, and the difference of the phase distributions depicts the change of surface slope due to the deformation. For revelation of sub-surface flaws, the principles are based on measuring the surface deformation of a test object and identifying deformation anomalies caused by sub-surface f aws. The present technique is not based on optical interference and thus it is not subjected to the severe limitations of interferometry. In particularly, it can measure large deformation and is much more tolerant to environmental disturbances, thus allowing the application in field/factory environments. One major difference between the present technique and the conventional methods of nondestructive testing is the way flaws are revealed. Traditional methods such as dye penetrant and magnetic particle reveal surface or sub-surface discontinuities, and techniques such as ultrasound and radiography detect internal discontinuities by detecting heterogeneities in materials. Material discontinuities may not be important as they may not have serious influence on the performance of the material/structure. For example, a flaw located in a low stress region is a cosmetic flaw and it will not impair the structural strength. Therefore, the conventional methods provide no direct information about the flaw criticality. In contrast, the present technique measures the response of a flaw to applied stresses, and hence it provides more direct information about the flaw criticality. In the paper, demonstrations of the technique to surface quality inspection and sub-surface flaw detection are presented.

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