Speaker
Description
Active thermography is a non-destructive inspection technique widely used for various structures such as automobiles, aircrafts, and buildings. Although this is a convenient and non-contact inspection method, one practical limitation is that its inspection area is restricted by the heated area (or the size of the heat source); thus, its inspection efficiency cannot be sufficient when applying this method to large-sized objects. Large-area inspections typically require a large-sized heat source or multiple inspections with changing the heating area. To improve the efficiency and accuracy of active thermography inspection for large-sized objects, this study focuses on an inspection method with periodically scanning the heat source. In this method, a heat source is continuously scanned to expand the heating/inspecting area. In addition, to enhance inspection capability, we attempted to use "periodic" scanning of the heat source and phase image transformation of the thermal data. The periodic scanning induces periodic temperature changes on the heated surface. Subsequently, the temperature change (temperature–time data) is transformed to phase data using Fourier transform. The non-destructive inspection technique using the phase data (or phase images) is well-known as lock-in thermography technique and demonstrates higher inspection capabilities than the thermal-image-based inspection. Therefore, in the current study, using phase data at the same frequency as the scanning cycle should improve inspection capability. Experiments for a large-sized carbon fiber reinforced plastic (CFRP) specimen with artificial defects were performed. The specimen was heated using a scanning halogen line heater and surface temperature during heating was monitored by an infrared camera. The experimental results showed that the entire area of the specimen was periodically heated. Furthermore, defect detection capability in the phase image at the same frequency as the scanning frequency (obtained by applying Fourie transformation to the thermal data) was improved than that in the raw thermal images. In addition, the experimentally observed phase contrasts in the defective regions were compared with the theoretical phase contrasts calculated based on the theory of lock-in or pulse phase thermography method. The experimental results were well agreed with the theoretically estimated phase contrasts. These results show that the proposed method is effective for applying lock-in thermography inspection for large areas and thus it is promising for efficient inspection technique for large-sized structures.
