Robotic-based terahertz imaging for nondestructive testing of a PVC pipe cap

The detection and localization of damage in a polyvinyl chloride (PVC) pipe cap were experimentally investigated by performing pulsed terahertz time-domain spectroscopy and using a six-degree-of-freedom (DOF) robotic arm. This study focuses on the development of a 3D imaging system to monitor the thickness of and detect defects in 3D structures. The damage and thickness of the PVC pipe cap were monitored by designing the reflection geometry of the terahertz scanner based on optical components, such as beam splitter and focal lens. Additionally, the PVC pipe cap was controlled with a six-DOF robot arm for path planning. The corresponding predicted value for the thickness of the sample was computed as a function of at least one wall thickness parameter of the pipe that uses a captured terahertz wave because these waves are partially reflected when the medium is changed. The defects were simulated using a PVC pipe cap (inner diameter: 89 mm, outer diameter: 100 mm), and the specimen was mounted on a six-DOF robot arm to inspect the entire area of the PVC pipe cap. The reflected signals were analyzed to determine the thickness, defectiveness, and locations of the defects through signal processing in the time and frequency domains. Consequently, we can detect and visualize the various types of defects that are simulated in a PVC pipe cap. In addition, the validity of the structural health monitoring method for 3D structures using terahertz waves was successfully verified.