Concrete structures can develop cracks for various reasons, and the types and causes of these cracks are diverse. To detect structural cracks, various Non-Destructive Testing (NDT) methods are used. IRT is used in various fields for anomaly diagnosis, including detecting mechanical device failures, electrical leaks, and in the aerospace industry. The containment building of a nuclear power plant is a large concrete structure that occurs defects for various reasons, and various studies and NDT are used to detect them. Among them, IRT enables reliable detection in real time without contacting the target, making it easy to detect cracks in large structures where safety is a top priority, such as containment buildings at nuclear power plants.
IRT technology has two modes: passive mode thermography, which utilizes naturally occurring heat, and active mode thermography, which uses transient thermal energy derived from an outside heat source. In this study, halogen lamp, serving as an outside heat source, is employed to deliver transient thermal energy to the cracks. The cracked and non-cracked parts of the concrete structure, when subjected to this thermal energy, develop thermal contrast owing to their different thermal conductivities. The higher the thermal contrast, the more distinct the visibility of cracks in the thermal image. This study investigates the effects of various environmental variables that influence this thermal contrast.
IRT depends on the temperature profile of the target's surface, making it more susceptible to the adverse effects of ambient environmental conditions in contrast to other NDT methods. The environmental variables mainly include irradiation, atmospheric temperature, relative humidity, and wind flow. Among these, irradiation is the most significant environmental factor, as higher solar radiation energy positively affects the possibility of detecting cracks using IRT, in addition to relative humidity and atmospheric temperature. In this study, a chamber system was constructed to control atmospheric temperature, relative humidity, and sunlight to acquire thermal images under different atmospheric temperatures and humidity levels. The variance in temperature difference in the captured thermal images between cracked and uncracked areas, known as thermal contrast, is measured. By determining how thermal contrast varies under different conditions and calculating the thermal contrast gradient as conditions change, this study proposes optimal atmospheric temperature and relative humidity for crack detection using IRT.
The study showed that thermal contrast between cracked and non-cracked areas becomes more pronounced with an increase in relative humidity and atmospheric temperature. The greatest thermal contrast was observed at an atmospheric temperature of 50℃, with the largest rate of increase in thermal contrast occurring between 30℃ and 40℃. Under different relative humidity conditions, the highest thermal contrast was found at 70% relative humidity, and the most significant increase in thermal contrast rate was observed between 30% and 50%. By suggesting these optimal ambient conditions, the study seeks to improve the performance and reliability of IRT in evaluating surface cracks in concrete structures.