# Cavity Detection in Steel-Pipe Culverts Using Infrared Thermography +
## 2 Literature Review
Although the majority of the earlier IRT methods (including PT and LT and their variants), relied on optical stimuli such as photographic flashes and halogen lamps, a considerable portion of recent studies has attempted to use nonoptical heating sources, such as induction heating, radio frequency, and microwave. Despite the success of optically excited LT in resolving some problems of optical-based PT such as poor signal-to-noise ratio (SNR) and the impact of reflections and surface features on thermograms, it has some drawbacks such as sensitivity to the thermal boundaries within the specimen [12; 37]. Thermographic inspection with nonoptical heating sources can address some of these problems. For example, using ultrasonic excitation can yield better spatial resolution due to the possibility of selective heating of flaws; for a comprehensive discussion refer to [12]. Many studies have shown the usefulness of ultrasound-excited thermography in applications such as detection of loose rivets and cracks in aerospace structures [37], microcrack detection in concrete materials [38], void detection in concrete-filled steel tubes [39], delamination detection and estimation of its depth in multi-layer concrete structures [40]. Another way of heating a test object is to employ eddy currents which are generated according to [PERSON]'s law of induction. Eddy current-based techniques have been used widely for NDT [41], but their main limitation is that they cannot be used for inspection of nonconducting materials as they require electrical loops to flow within the test object. Some works (e.g., [42]) have attempted to use this method for low conductivity materials such as carbon fiber-reinforced plastic composite (CFRP), but they faced some difficulties. The eddy current excitation has also been used for thermography inspection [43; 44]. As expected, the comparative analyses [44] support the fact that this method is not suitable for nonmetallic (i.e., low conductivity) materials. Traditional PT methods with short thermal pulse stimuli are not effective in detecting in-depth defects. They also require flash lamps which are expensive and need specialized equipment to interface with the data acquisition system. These shortcomings can be overcome by long pulse thermography (LPT) in which a long pulse excitation (e.g., a halogen lamp) is used. Despite a long history [45], it seems the capabilities of this approach have been less explored. In a recent study [46], the performance of PT and LPT was compared for three different metals (aluminum, mild steel, and stainless steel) and CFRP. The author reported that LPT was more effective in detecting defects in materials with low thermal conductivity while the conventional flash excitation technique (PT) performed well with both low and high thermal conductivity materials. But the PT thermal images were noisier and of lower contrast, and the LPT performance for detection of defects in mild steel and stainless materials were as good as the PT's. In another work [47], the power of LPT for determining in-depth defects in basalt fiber-reinforced epoxy laminate were examined and compared with PT. This work also showed that LPT is better than PT in terms of contrast and intensity. More recently, the performance of LPT in detecting anomalies in metals and composites has also been examined against eddy current PT (ECPT) in [44]. Results showed that LPT is more effective for CFRP whereas ECPT is more successful for metallic materials. Quality of thermal images, specifically those generated by PT, is often undesirable due to the factors such as the presence of noise and heterogeneity in distributions of thermal stimulation. As the quality of thermograms directly affects the success of IRT-based inspection, the enhancement of thermal images is of critical importance.