Evaluation of Closed Stress Corrosion Cracks in Ni-Based Alloy Weld Metal Using Subharmonic Phased Array

Abstract

Closed stress corrosion cracks (SCCs) have been generated in Ni-based alloy weld metal in nuclear power plants. The ultrasonic inspection is difficult because of the crack closure. For the application of new inspection methods and training/educating of inspection engineers, realistic closed SCC specimens are required. However, there is no means for forming such SCC specimens in a reasonable amount of time. Here, we present a two-step method. The first step is to form an open SCC in chemical solution. The second step is to close the SCC by generating oxide films between the crack faces in high-temperature pressurized water (HTPW). To verify the crack closure, we used a closed-crack imaging apparatus, the subharmonic phased array for crack evaluation (SPACE). Consequently, we found that parts of the SCC after 1321 h immersion were closed in the HTPW. Thus, we verified the two-step method for forming realistic closed SCC specimens in a reasonable amount of time.

Technical Report on the Status of Nonlinear Ultrasonic Techniques for Non-Destructive Inspection

“…A phased array broadcasts a coherent acoustic energy beam along a certain direction and listens for reflected energy signatures [10] [11]. In the case of SPACE [12] [13] [14], the phased array emits energy at one frequency, the fundamental, and listens for reflections at both the fundamental frequency and half of that original frequency, i.e., the subharmonic. This allows for traditional ultrasonic reflection imaging (using the fundamental) as well as nonlinear imaging (using the subharmonic.)…”

“…As described previously, Bulk-SPACE measurements are ideal for imaging cracks on the opposing side of the phased array, i.e., probing interior surfaces from external access [12] [13] [14]. The probe used for this study is the same as that used for the linear phased array measurements.…”

High-Selectivity Ultrasonic Imaging of Closed Cracks Using Global Preheating and Local Cooling

“…We have verified it in closed fatigue cracks 9,10,1214) and stress corrosion cracks (SCCs). 8,10,1518) However, when short-burst input waves are used to obtain a high temporal resolution, not only closed cracks but also linear scatterers appear in SA images, since the spectra of the waves scattered at linear scatterers leak in the band-width of superharmonic and subharmonic waves. This results in ghosts that degrade the selectivity of closed cracks in SA images.…”

High-selectivity imaging of closed cracks using elastic waves with thermal stress induced by global preheating and local cooling

“…Crack depth can be measured by ultrasound if cracks are open, since ultrasound is strongly scattered by the crack tip. 1 However, if cracks are closed because of compressive residual stress 2,3 and/or the oxide films generated between the crack faces, 4 ultrasonic inspection causes the underestimation or nondetection of cracks since ultrasound penetrates through the closed crack.…”

“…We have demonstrated its performance in closed fatigue cracks 5,6,8 and stress corrosion cracks. 4,6,9 However, when short-burst input waves are used to obtain a high temporal resolution, not only closed cracks but also linear scatterers appear in SA images, since the spectra of the waves scattered at linear scatterers leak in the bandwidth of superharmonic and subharmonic waves. This results in ghosts that degrade the selectivity of closed cracks in SA images.…”