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Received Nov 23, 2017; Accepted Mar 8, 2018
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1. Introduction
Various cracks that occur in structures cause appearance degradation as well as decreased strength of the structures and may lead to casualties and property damages if proper maintenance work is not done beforehand. Therefore, early detection and repair of cracks in structures is a very basic and important procedure to prevent social loss. The research using image processing is the most representative study regarding the determination of size of cracks in structures, and its measurement accuracy is also high [1–3]. Meanwhile, the research on the crack depth measurement mainly deals with nondestructive testing (NDT), which is largely divided into contact and noncontact testing methods. However, the crack depth measurement technology requires complicated test equipment and experimental procedures, and the variety of crack shapes causes accuracy problems in field applications.
Nondestructive testing, which is an inspection method related to the measurement of crack depth in structures, generally refers to the inspection technique that allows materials or components to be examined without changing their original shapes or functions, and it also refers to all inspection methods to examine the properties, conditions, and internal structures of a product without disassembling or destroying it. Nondestructive testing methods include radiographic testing (RT), ultrasonic testing (UT), magnetic practice testing (MT), penetrant testing (PT), eddy current testing (ET), leak testing (LT), and infrared thermography (IRT), and so on, and the demand is fast growing with the development of high-value-added industries [4]. Among the various nondestructive techniques that can measure the depth of cracks, the noncontact testing typically includes eddy current testing [5–7] and infrared thermography [8–10], and it is the inspection method using the characteristics that the size and depth of the crack affect the formation of the temperature distribution of an object. However, the eddy current testing has a disadvantage in that a probe must be located very close to the surface of the object and is very sensitive to the surface roughness. In addition, the infrared thermography has a problem in that the influence of absorption scattering must be removed...