High-Selectivity Ultrasonic Phased Array Imaging of Closed Cracks in Coarse-Grained Material Using Global Preheating and Local Cooling

摘要

In nuclear power plants, crack depth measurement is indispensable to ensure their safety and reliability. To this end, there are two difficulties that are crack closure and coarse grains. Crack closure causes the underestimation or miss-detection because the ultrasound transmits through the closed cracks. Coarse grains cause strong linear ultrasonic scatterings and thereby the crack tip responses can be hidden because crack tip responses are generally weak. To overcome the crack closure, we have developed two closed-crack imaging methods. The first is subharmonic phased array for crack evaluation (SPACE), based on subharmonic generation and phased array algorithm with frequency filtering. [1] The second is global preheating and local cooling (GPLC)[2-4] that temporarily opens closed cracks by the application of tensile thermal stress. GPLC is a very simple but still powerful. For coarse grains, many studies have been done. [5-9] However, the previous studies have dealt with only linear defects. On the other hand, we have developed a load difference phased array (LDPA)[10]. LDPA is a method of subtracting phased array images between different external loads. The fundamental performance was demonstrated in the fatigue crack specimen in terms of improving the selectivity of the closed crack for the notch response, which is a linear scattering source away from the crack tip. However, it has yet to be verified in actual coarse grained materials. The objective of this study is to demonstrate that the combination of GPLC and LDP A is useful in accurately measuring closed crack depth in coarse-grained material.