A SENSITIVE ULTRASONIC APPROACH TO NDE OF TIGHTLY CLOSED SMALL CRACKS

A sensitive ultrasonic technique for the quantitative nondestructive e valuation of small closed cracks is developed The use of an oblique lo ngitudinal wave with small angle of incidence upon the specimen surfac e is emphasized. Firstly. ultrasonic testing is performed with open cr acks in order to predict the optimum angle of evaluation, and rm empir ical calibration equation is then derived for the same as a basis to d eal with the analysis of closed cracks. From the measurement on real f atigue cracks, the parameters of interest, specifically the crack size and the closure stress, are determined simultaneously by analyzing th e inverse problem. The method developed here is fully automated and co mputer-controlled. Evaluated results are found to be in good conformit y with the destructive measurements, and the corresponding superiority of the method is realized. especially for tightly closed small fatigu e cracks, when compared with the method using a beam of longitudinal w ave incident normally and/or with a large angle.