Lock-in Thermography Imaging Of Solids With Arbitrary Corner Geometries Based On Photothermal Radiometry

Lock-in Thermography Imaging(LITI)is a photothermal non-destructive evaluation technology based on photothermal radiometry(PTR)that extends from the original single-point detection to array imaging detection.Compared with the traditional PTR,LITI not only inherits the advantages of non-contact,fast response and high sensitivity,but also greatly improves the detection speed,which is of significance in both scientific research and practical applications.Two key problems are involved in LITI technology.One is the lock-in function of CCD imaging device,which is used to extract amplitude and phase information in image in the case of low signal-to-noise ratio(i.e.,weak signal),and the second is the establishment of theoretical temperature field of the tested sample that is excited photothermally,which is the basis for the quantitative detection of properties and defects of materials.The spatial distribution of the photothermally excited temperature field is closely related to the geometry and shape of the material and is a complex function of the shape of the material's structural dimensions.So far,the main research has been focused on simple planar structural samples.In this paper,a lock-in thermogrophy imaging detection technique of non-planar angled structures is proposed.The main work includes:1)The theoretical model of the photothermal temperature field distribution of an angle structure is established,and the effect of each parameter on the temperature field is discussed.By using the Green's function method,the mathematical expression of the temperature field distribution of the angled structure is established in the cylindrical coordinate system and numerical simulations are carried out.The influences of thermal diffusivity of the material,geometric angle of the sample and modulation frequency of the incident light are analyzed;2)A lock-in thermography imaging detection system was built,and the theoretical thermal wave field of the angled structure was verified by experiments.The lock-in imaging function of an infrared camera was realized with mathematical calculation,from which both amplitude and phase imaging of the photothermal field were obtained;3)The experimental results were fitted to the theoretical model,and further modification of the theoretical model was conducted based on the deviation between the theory and experiments,and thermal diffusivity of the material was obtained,which is in good agreement with the literature value.This proposed method provides feasibility of photothermal lock-in imaging detection in non-destructive,fast and quantitative detection for non-planar structures.