A new ultrasonic evaluation method has been developed for the quantitative characterization of smaller tight cracks under no load condition. In this approach, the ultrasonic testing is considered as an inverse problem to determine simultaneously the crack size and the extent of crack closure. The present paper is on the analysis of the influence of wall thickness on the evaluation of closed surface cracks from the inaccessible side of the wall. Based on experimental observation, the evaluation algorithm has been extended for the characterization of cracks with different wall thickness from the measured angle beam response. Evaluated results of both open and tightly closed small cracks with different wall thicknesses verify the method as a powerful tool for detecting and characterizing small surface cracks quantitatively.
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