To solve the underestimation of closed-crack depth, we have developed an imaging method, subharmonic phased array for crack evaluation (SPACE). However, a single-array SPACE can image only the vicinity of a transmission focal point (TFP) when the TFP is fixed. In this study, we have developed a confocal SPACE that defines multiple TFPs for imaging closed cracks over a wide area. We demonstrated its usefulness by measuring a stress corrosion crack (SCC). Moreover, we proposed a radarlike display that shows single-focus images with a line indicating the incident direction. By applying it to the SCC specimen, a moving crack response (MCR) was observed with varying incident angles. To analyze this behavior, we performed a simulation using a finite-difference time-domain (FDTD) method with a damped double node (DDN) model. Furthermore, we examined the ratio of the subharmonic to fundamental responses depending on the stress ratio between input wave stress and crack closure stress (σ c ).

To accurately measure closed crack length, we proposed an imaging method using a subharmonic phased array for crack evaluation using surface acoustic waves (SAW SPACE) with water immersion. We applied SAW SPACE to the hole specimen in a fundamental array (FA) image. The hole was imaged with high resolution. Subsequently, SAW SPACE was applied to fatigue crack and stress corrosion crack (SCC) specimens. A fatigue crack was imaged in FA and subharmonic array (SA) images, and the length of this particular fatigue crack measured in the images was almost the same as that measured by optical observation. The SCC was imaged and its length was accurately measured in the SA image, whereas it was underestimated in the FA image and by optical observation. Thus, we demonstrated that SAW SPACE with water immersion is useful for the accurate measurement of closed crack length and for imaging the distribution of open and closed parts of cracks with high resolution.

For ultrasonic imaging closed cracks, unable by traditional ultrasonic techniques, the subharmonic phased array for crack evaluation (SPACE) has been developed. However, a simulation model for optimizing inspection conditions and for scientific understanding of subharmonic waves is required. In this paper, by extending the finite-difference time-domain (FDTD) model with damped double nodes (DDNs) to realistic testing conditions and comparing the simulation with the experiment, the availability of this model is demonstrated.To model a compact tension specimen with a fatigue crack extended from a notch, the DDN model was extended to the vertical closed crack mode. When the tensile stress of incident ultrasonic wave at the closed crack is larger than the compression residual stress T th at the crack, the crack changes to the open state with particle velocity nodes separated to dual nodes. Moreover, to suppress the numerical noise due to the contact vibration, the damping term is introduced.In the experiment with small amplitude incident waves, only the notch response was observed, whereas in the experiment with large amplitude incident waves, the crack response was observed. The latter behavior was reproduced by the simulation when the tensile stress of the incident wave was larger than T th . The experimental image of the crack tip obtained by the SPACE was also reproduced by the simulation, indicating the usefulness of the DDN model.Moreover, in both experiment and simulation, a scattered wave was observed at the crack face between the crack tip and the notch in the subharmonic images. Since only "reflection" takes place on a crack face with no singularity, whereas "scattering" takes place at a crack tip with singularity, the scattering at a crack face with no singularity is surprising. Therefore, we examined the cause of this phenomenon and found that the crack was continuously opened until the crack closure point (CCP) where the tensile stress of incident wave became less than T th . The scattered wave was generated at the CCP as a new singularity formed at the crack face. This is a novel finding and will be useful in evaluation of local compression residual stress at cracks, which plays important role in prediction of lifetime of structures.

For safety operation of nuclear power plants, soundness assurance of structures has been strongly required. In order to evaluate properties of inner defects at plant structures quantitatively, non-destructive inspection using ultrasonic testing (UT) has performed an important role for plant maintenances. At nuclear power plants, there are many structures made of cast austenitic stainless steel (e.g. casings, valve gages, pipes and so on). However, UT has not achieved enough accuracy measurement at cast stainless steels due to the noise from large grains. In order to overcome the problem, we have developed comprehensively analyzable phased array ultrasonic testing (PAUT) system. We have been noticing that dependency of echo intensity from defect is different from grain noises when PAUT conditions (for example, ultrasonic incident angles and focal depths) were continuously changed. Analyzing the tendency of echoes from comprehensive PAUT conditions, defect echoes could be distinguished from the noises. Meanwhile, in order to minimize the inspection time on-site, we have developed the algorithms and the full matrix capture (FMC) data acquisition system. In this paper, the authors confirmed the detectability of the PAUT system applying cast austenitic stainless steel (316 stainless steel) specimens which have sand-blasted surface and 3 slits which made by electric discharge machining (EDM).