Acoustical Holographic Transverse Wave Scanning Technique for Imaging Flaws in Thick-Walled Pressure Vessels
Transverse (i.e., shear) wave scanning configuration used in acoustical holography for imaging internal flaws or voids in thick-walled pressure vessels are discussed. Simultaneous source-receiver scanning configuration was employed. Because of the severe off-axis nature of the shear waves, large third order aberrations are present in the image. Methods of reducing and effectively eliminating these aberrations are discussed.
The severe resolution limitations of the traditional nondestructive testing techniques to image radial planar flaws deep in reactor pressure vessels has been alleviated by the use of T-wave scanned holography techniques with aberration compensation procedures. The image resolution capability exceeds all other NDT methods at depths greater than a few cm.
One of the major advantages of using transverse waves in nondestructive testing is the gain in resolution due to the fact that shear waves are approximately one-half the wavelength of longitudinal waves in given solid at a given frequency. Furthermore, many internal cracks, voids, etc. have geometrical orientations which require slant angle insonification and viewing for detection. The vertical or radial flaws can then be viewed from either side at approximately 45 degrees. The apparent flaw height or projection in the image is usually always less than the true height as a result of the viewing angle.
The flaw images appear two-dimensional as a result of the large difference between the construction and reconstruction wavelengths. However, the hologram does contain the complete depth information and this is easily extracted by focusing on different planes in the image field. Different depths within the image field are brought into focus by adjusting the effective position of the reconstruction light source through the use of a lens. The lateral and longitudinal dimensions of the flaws are then calculated using the conventional image location and magnification equations.
The resolution and aberration correction capabilities have been demonstrated by experiment.
KeywordsReactor Pressure Vessel Image Flaw Slant Angle Holographic Image Pressure Cycling
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