Thermal Opening Technique for Nondestructive Evaluation of Closed Cracks

This paper describes the “thermal opening technique” for enhancing the ultrasonic response of a tightly closed crack. In this approach, thermal stress is generated by cooling the part locally to reduce the crack closure stress in stainless steel plate, which improves sensitivity to detect the scattering wave from the crack surface; this has however not been successful by the usual ultrasonic inspection technique without introducing the thermal stress. It has been observed that the enhancement of the scattering wave continues for several hours after removing the thermal stress. This valuable and unique characteristic of the scattering wave is related to the change in the contact condition between the mating crack surfaces. Finally, a method is proposed for the evaluation of the depth of tightly closed crack from the difference in the time-of-flight between the scattering wave and the corner echo. The quantitative characteristic of the method is also verified experimentally.