Reliable and accurate nondestructive evaluation of crack size is an important practical issue. Usual techniques of evaluation are sometimes found to be inadequate to accurately predict the size of closed cracks in actual structures. However, the techniques can be applied successfully if the crack is free from crack closure. The present article reports a simple but effective method of thermal treatment which is capable of generating sufficient tensile thermal stress in the cracked part, thereby allowing evaluation of a closed crack without being affected by the problem of crack closure. The treatment is performed by applying the spray of a suitable coolant to a local area of the test surface. The electrical potential drop techniques are considered to verify the enhancement of evaluation sensitivity of closed cracks achieved by the present thermal treatment. The closely coupled direct current potential drop technique is verified to be less sensitive to crack closure than usual alternating current potential drop technique. Both the experiment and simulation firmly establish the reliability as well as suitability of the present thermal treatment for sensitively testing the size of closed surface cracks in a nondestructive way.
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