Time-of-flight method for sizing cracks through fluid-filled gaps in structures

#This invention relates generally to non-destructive examination of material, such as metal, for voids, flaws, cracks and other defects that can be detrimental to the integrity of the material. Specifically, the invention relates to the ultrasonic inspection of nuclear and non-nuclear components at operating plants and facilities.

#Ultrasonic characterization of cracks in materials is at least a two-step process: 1) detection and location; and 2) sizing in absolute or relative terms.

#U.S. Pat. No. 5,118,464 discloses a method for detecting the presence of material flaws, such as a crack 8 (see FIG. 1 herein), behind normally opaque barriers, such as fluid-filled gap 6 between structures 2 and 4. In this method transducer 10 is excited to emit a longitudinal ultrasonic wave which is coupled to structure 2 via couplant 12, e.g., normal water in a nuclear reactor. At the correct frequency the emitted wave bridges the gap and enters structure 4, where it is reflected by crack 8. The return path of the reflected wave also bridges the gap and the wave impinges on the transducer 10, where it is detected as a "pulse echo" signal.

#The method and apparatus of the invention will be described in detail with reference to the drawings, wherein:

#FIG. 1 is a schematic representation of conventional ultrasound interrogation of two structures separated by a gap;

#FIG. 2 is a schematic representation of a conventional time-of-flight diffraction technique used in sizing cracks;

#The time-of-flight method for sizing a surface-connected crack 8' across a fluid-filled gap 32 in accordance with the invention is depicted in FIG. 3. The crack 8' extends from the back surface 18 of wall 36 to crack edge 24.

#As seen in FIG. 3, gap 32 of width d separates an inner wall 34 of thickness t.sub.1 from an outer wall 36 of thickness t.sub.2. Gap 32 is opaque to ultrasound, except for resonant frequencies for standing waves in the gap, as described in U.S. Pat. No. 5,118,464. Therefore, if the emitter at location 20 and the detector at location 30 are separated by a distance s which is of the order of the crack height h, the resonance condition in the gap is much more likely to occur than if the two transducers are separated by the distance 2S, which is of the order of (t.sub.1 +t.sub.2). The detector and emitter are located at respective distances S and (S+s) from the known crack plane 26. The emitter and detector could be reversed with respect to the crack plane, and/or with respect to each other, without changing the basic principle of operation and without departing from the scope of the invention.

#The emitter at location 20 is coupled to inner surface 16 by surrounding water 12. The emitted ultrasonic waves enter inner wall 34 at an angle of refraction .theta. which is determined by the type of metal making up wall 34 and the type of transducer employed. For sizing surface-connected cracks, the emitter is of the focused type. The angle .theta. may be assumed to be a known value for the transducer mode of operation. For the sake of definiteness, the preferred longitudinal-wave mode is described, although similar considerations apply to shear waves.