The various nondestructive testing methods are practically used for flaw detection at material state, welding inspection in the production stage and crack detection in the maintenance stage. The technologies in the normal temperature and the high temperature were established for these nondestructive testing methods. However, the technologies under the very low temperature (-196°C) have not been practically established yet.
Three techniques, namely Ultrasonic Testing (UT), Electromagnetic Acoustic Testing (EMAT), and Eddy-Current Testing (ECT), have the possibility of realization for nondestructive testing under the very low temperature.
The influence of the boundary noise of ECT is so large when the component elements of the welding material and the base metal material are different. Only the defefct in the vicinity of the surface can be detected. The application of ECT is very difficult from the above characteristics.
EMAT will be able to use even at the low temperature. But, the resolving power is low because the low frequency wave is used. And, the operability of the sensor is difficult for the magnetized material.
As for the Ultrasonic testing, the determination of the defect size is possible.
So we developed the inspection technology under the very low temperature (-196°C) using the Ultrasonic testing.
