Characterization of defects in thick walled pressure vessels using acoustical holography

Acoustical holographic imaging techniques are being used to characterize more precisely imperfections in thick-wall pressure vessels. The acceptance or rejection of indications in pressure vessels not only depends on the size, location and orientation of the indication but also on the nature of the indication. This new technique is used to define more accurately indications detected by accepted ultrasonic inspection techniques. One of the major advantages of acoustical holography over conventional ultrasonic evaluation techniques is that a calibration standard (block) is not required. The information obtained holographically is not related to an equivalent or reference reflector in a test standard. The reflecting surfaces of an indication are characterized directly from the reconstructed holographic information. The results of several A.H. evaluations conducted on pressure vessel components are compared to conventional U.T. test data and destructive test data. Destructive test results were obtained as the indications were removed in the shops and from actual sections of the pressure vessel which were removed and destructively analyzed in the laboratory. The difficulty of interpreting reconstructed acoustical holographic images is a major problem which must be corrected before this technique can become a widely accepted evaluation tool. The use of computerized reconstruction and analysis methods will play a key role in future A.H. systems. (author)