Patent Number: 051184648
Section: summary

This invention relates to nondestructive examination of material, such as metal, for voids, flaws, cracks, and the like that can be detrimental to the continuity and integrity of materials. More particularly, a method and apparatus for non-destructive examination is set forth in which the interrogating ultrasound bridges gaps--such as those gaps found between closely spaced manufactured parts. In the disclosure, sound is incident on a first material, bridges a manufactured gap to become incident upon a second material to be tested, is reflected at defects in the second material, returns across the manufactured gap, and is thereafter analyzed. BACKGROUND OF THE INVENTION Ultrasound has been used since the 1940's to non-destructively inspect a wide variety of materials for flaws, phase constitution, dimension measurement, grain structure and integrity. In particular, modern nondestructive examination (NDE) methods typically utilize sonic energy in the megahertz range to penetrate and image the inner body of metals, as well as their outer surfaces, taking advantage of their acoustical properties in locating discontinuities that reflect or scatter acoustical waves. The reflective property of voids, flaws, cracks, etc., that could be detrimental to the continuity and integrity of the material is the basis of NDE methodology. The frequency used is determined by the type of material and technique employed; for steel it is in the range of 1-10 megahertz with 2.25 to 5 megahertz the preferred range set by the propagation and attenuation characteristics of various steels. Other frequencies are used for zircaloy, titanium, aluminum and composite materials as dictated by their particular acoustic properties. Typically, ultrasonic waves generated by a piezoelectric crystal transducer, common and known to the art, are introduced via a coupling fluid, such as water or acoustical grease, at the surface of the metal to be inspected. As the waves propagate in the bulk of the material they may impinge on some type of discontinuity affecting the acoustical impedance of the medium. It is well known in the science of acoustics that this impingement produces reflections and transmissions that compete against each other, depending on various factors such as flaw size and shape, angle of incidence, and magnitude of the change in impedance. In case of a gas gap (usually and naturally filled with air) change of impedance is so abrupt and large that virtually all of the incident sound waves are reflected at the interface. Very little sonic energy traverses such a gap, and inspection of material beyond an air gap is never considered in NDE practice. Thus, in many applications important to nuclear plant component inspections NDE effectiveness is limited by the presence of gaps that shield important joints and zones from inspection. An example is that of the control rod drive housing to stub-tube attachment weld and heat affected zone, known to be subject to cracking. Referring to FIG. 1A, a reactor vessel V is shown in partial section to display a core C. Core C contains control rods, whose drive housings H extend through the bottom of the vessel V through stub-tubes T. Those familiar with the nuclear industry will recognize that FIG. 1A is a boiling water reactor operating under a standard pressure in the range of 1200 pounds. Further, the vessel is in the range of 120 feet in height, 30 feet in diameter, and contains radioactive material contained in fuel rods as the natural result of the nuclear reaction. The preferred nondestructive examination is directed at the stub-tube environment which is at the bottom head of vessel V. Referring to FIG. 1B, bottom head 14 of the vessel V is illustrated at stub-tube T placed within a recessed aperture 15 of vessel V. Typically, the inside of vessel V can be clad with stainless steel 16. Connection of the stub-tube T to the cladding 16 and vessel V at bottom head 14 occurs at weld L. Similarly, connection of the control rod drive housing H occurs at the top of the stub-tube T. It is required that the alignment of the control rod drive housing H within the stub-tube T be precise. Consequently, the stub-tube T and the control rod drive housing define a gap G therebetween. This gap G enables the verticality of the control rod drive housing H to be maintained during the placement of welds J. Welds J and L, and the heat-affected zones adjacent to the welds are subject to certain conditions of metal cracking. Specifically, these zones have proven to be candidate zones for the metallic cracking defect known as intergranular stress corrosion cracking (IGSCC). Simply stated, the conditions of metallic tension, stagnation of water flow, and oxygen concentration cause crack propagation along the granular boundaries of the metal. This phenomenon is known to occur within and adjacent to welds L and J. Before this disclosure, gap G prevented ultrasound examination of cracking in stub-tube T from the interior of control rod drive housing H. SUMMARY OF THE PRIOR ART The detection of gaps in the ultrasonic non-destructive examination of materials is known. See Applications of Ultrasonic Interference Spectroscopy to Materials and Flaw Characterization by B. G. Yee et al., "Materials Evaluation," August 1975. The detection of the gap has primarily been used either for measurement of the thickness of the materials, location of the faults in laminations, determining dimension of a gap, or other measurements all related to the gap itself. It has not been suggested by the prior art to examine utilizing ultrasound transmitted through and bridging the gaps to nondestructively test materials on the other side of gaps. Gaps and their properties in transmitting and reflecting sound are understood. See J. and H. Krautkramer, "Ultrasonic Testing of Materials," 4th Edition, Springer-Verlag, New York 1990, pp. 18-23. Again, testing through the gaps to inspect materials on the other side of such gaps (see FIG. 1B at G) has not been set forth. SUMMARY OF THE INVENTION An improved apparatus and method for ultrasonic inspection of materials through barriers such as gaps in manufactured parts is disclosed. As in normal ultrasonic detection, a transducer sends a signal through a couplant fluid into the solid material to be inspected. Typically, a located discontinuity, such as a crack or other flaw, gives a reflecting echo. A transducer receives and transduces the reflected echo for electronic display of the acoustically reflected results. Analysis of the display and, hence, the time and character of echo received, results in nondestructive inspection and analysis for flaws and cracks. The improvement herein is directed to enabling such ultrasonic testing to bridge gaps, such as intentionally formed gaps in composite structures having a first structure for originally receiving and transmitting sound separated by the gap from another structure to be inspected. Preferably, the gap is flooded with a gas having a predictable and optimum speed of sound relative to the material of the first and second structures. Sound is propagated to the first structure in a wave packet that is transmitted through the couplant fluid. The sound is generated in a wave packet having a width at least twice the dimension of the gap to be bridged. The wave packet has a contained frequency having a wavelength (relative to the speed of sound of the gas flooding the gap) to create a constructively interfering standing wave within the gap. The sound propagated to the gas-filled gap has a standing wavelength, which is a half-integer with respect to the gap dimension. Sound passes through the first structure, creates a standing wave at the gas-filled gap, enters and acoustically interrogates the second structure for flaws and reflects. Reflected ultrasound from the interrogated second structure again bridges across the gap as a constructively interfering standing wave, passes through the primary structure and then through the couplant fluid to a transducer for detection and analysis of the received ultrasound. A disclosure of an analytic method coupled with the disclosed apparatus and process is made to enable analysis of a given geometry of gap, any given gas flooding the gap, any given interrogating wave packet, including the spectral power density and bandwidth, for bridging gaps having a given range of dimensions (usually 2 to 10 mils) between primary structures and nearby secondary structures to be interrogated, as well as other parameters that may be encountered in the use of the method. An example of a preferred inspection across the gap between a control rod drive housing for interrogation of a stub-tube structure within a nuclear reactor is disclosed.