|
Optical non-destructive testing have been broadly used for flaw analysis in engineering structures. The valuable of these techniques has been their easiness to obtain full field information. Likewise, with them one can determine with high precision micro—displacements due to different types of loads. Holographic, moire, and speckle interferometry are the most common optical non-destructive techniques. Particular experimental arrangements can be built to measure in- and out-of-plane displacements. Commonly, in-plane speckle and moire grating techniques use collimated symmetrical beams for target illuminating. However, when the target illumination beams are spherical, a series of errors in the interpretation of the interference fringes appears. Among these errors, under this kind of illumination, are those due to light arising out-of-plane displacement components. These components are of supreme importance when the objects to be analyzed are of considerable size. In this paper, the importance of shape and out-of-plane displacements as factors that introduce error in the interpretation of a phase map obtained from a grating moire and in-plane ESPI interferometers, are evaluated. The analysis is done utilizing the uncertainty of the sensitivity vector varying as a position function on the surface under study. The effect of a slight out-ofplane displacements and surface shape upon the change of the sensitivity vector are numerically analyzed. This numerical analysis shows that serious measurement error can be obtained when these factors are neglected. It shows that the possible errors in the measurement from the ESPI and grating moire interferometers using non-collimated object illumination can be estimate before doing the measurement. A general function of correction in phase map interpretation for any in-plane sensitive interferometric scheme, in spherical illumination, is proposed. Since moire interferometry uses a grating specimen recorded on a mirror-like and prepared with a fine diffraction grating (typically ~12OO lines per millimeter). Gratings are commonly made of photoresist. However, if we want to analyze engineering structures in spherical illumination, in order to have a surface without pre-prepared under polishing, the surface roughness is an important parameter to be characterized. Then, additionally in this work first we analyze the fringe visibility of moire during the fabrication of specimen gratings as a function ofthe surface object roughness.
|
