Defects in most structures can be generated not only on outside but also on inside or on the back-side during the manufacturing or construction process. Also they cause the growth of defects due to operation of various complex environments and structures will be destroyed eventually. In order to improve the reliability of the structure, the detection and size-estimation of defects should be investigated. In this paper, as an extension of previous studies on surface defects, two-dimensional artificial backside cracks (blind cracks) into paramagnetic material were evaluated by using the same aluminum probe. The potential drop at the defect position is distributed in the n-shape in the case of the back defect, which is different from results of the surface defect (u-shape). The potential drops at the defect position are measured with the largest value. The potential drop ratio (Vcmax/Vs) for the defective position is used as a parameter to predict the thickness (l) of defect position.

Nondestructive testing is a method of inspecting particular target objects without destructing them in industrial sites. Infrared thermal imaging is one of the nondestructive testing techniques. Among them, lock-in infrared thermography technique is a technique to detect a defect by generating a temperature difference of an object using periodic heat waves. This paper deals with the development of lock-in infrared thermography technique by using numerical analysis model for SM45C metal specimens. As a result, the appropriate frequency was determined for defect detection in SM45C metal specimen by using the established thermal behavior mechanism by periodic heat wave.