The spatial resolution performance of a fibre-optic reflectometric technique for the automatic detection and measurement of surface cracks

Abstract

We present an analysis of the spatial resolution achievable with a new fibre-optic reflectometric technique, contrived by the authors for the automatic detection of cracks in the inner sides of steam generator tubes of small diameter (similar or equal to 20 mm). An analytical expression of the modulation transfer function (MTF) of our measurement system is obtained, from which the resolution limit of the technique is defined. Experimental values of the MTF, measured with a first laboratory prototype, are compared with the theoretical predictions and the range of validity of the theoretical expressions is discussed. Also, some of the results obtained with a second-generation prototype are presented, it being demonstrated that parallel cracks with a spacing of 50 um between adjacent edges can be resolved in practical inspection tasks.