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Digital approaches for determination of fringe order and principal stress direction in photoelastic coatings are presented. The relationships between the intensity values and the fringe orders of two wavelengths, and relevant theory and explicit conditions for directly determining the principal stress angles in the range z/2 are derived. The scheme developed allows for the automatic determination of the fringe orders with or without zero-order fringes in the fringe patterns. Test results are reported.

The feasibility of using holographic interferometry for the nondestructive testing of arc and flame sprayed coatings is studied by using thermal stressing technique. The detection of bonding flaws in several specimens has been demonstrated. The location and shape of flaws detected are closed related to the artificial defects. By employing fringe control technique, thermal stressing method can be more effective for flaw detection. With thermalpiastic recording and vision system, the size and location of flaw can be visualized and evaluated in a few minutes.INTRODUCT I ON Thermal spraying has been widely used in the power, automobile, and aerospace industriest . Components coated by arc or flame spraying can provide some specific physical and/or chemical and/or mechanical properties which can not be achieved by using a single material. Usually these properties are found to be in the need to counteract the operation of a specific wear mechanism, high temperature, or corrosive fluids. Because of the variety of production parameters ,and the complexity of physical and metallurgical processes occurred during the spraying operation, the quality of coating can not always be guaranteed. Therefore a reliable test is required to assure the quality of coated components for service use.Generally the required quality standard of coating depends on the bonding flaws between substrate and coating, cracks and pores in coating, and the deviation from the required coating thickness. Perhaps the most serious problem encountered in the spraying procedure is the absence of a good bonding. Since conventional ultrasonic testing is affected by surface roughness and geometry2, it is not suitable to test coarse grained material like arc-or flame-sprayed coating.Optical holographic interferometry provides a very powerful technique for visually measuring wave-length type displacement of all or part of the surface of an arbitrary body3. This technique can be used for nondestructive testing if the presence of a flaw results in anomalous of the surface of the components being tested when it is slightly stressed. This anomalous deformation is usually indicated by 484 / SPIE Vol. 1553 Laser Interferometry IV: Computer-Aidedlnterferometry (1991) O-8194-0681-3/92/$4.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/12/2015 Terms of Use: http://spiedl.org/terms a change in the pattern of interference fringes relative to the pattern formed when a flaw-free component is stressed in the same manner4. Various approaches, such as mechanical stressing, thermal stressing, pressure stressing, and vibration stressing, have been successfully used for nondestructive testing, depending on the structural configuration, the material being used, and the type of anomaly suspected. Comparatively thermal stressing technique may be the simplest and most rapid approach to apply. In this paper the application of thermal stressing technique to evaluate the bonding flaws in arc or flame-sprayed coating nondestructively is presented. HOLOGRAPHIC THERMAL ...

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