Local thermal treatment for opening the closed cracks

Nondestructive evaluation of crack size is an important practical issue. Usual techniques of evaluation are sometimes inadequate for sensitively detecting the cracks in actual structures, which is mainly because of the fact of crack closure. However, the techniques can be successfully applied if the crack is open. The present paper describes a new approach to open the closed cracks, which is called the 'thermal opening technique'. The technique is based on the principle of introducing tensile thermal stress in the cracked part by the use of local cooling or heating. The effectiveness of the technique has been demonstrated here through the ultrasonic testing of a closed crack.