Nondestructive Testing Of Curved Surfaces Based On Photothermal Lock-in Thermography Imaging

Photothermal lock-in thermography(LIT)imaging detection technology is a cutting edge detection method,which combines the advantages of photothermal radiometry(PTR)technology,lock-in technology and imaging technology.It is,therefore,of great significance and practical values in the field of non-destructive evaluation.In addition to the advantages of high sensitivity,non-contact,non-destructive,and fast responses of PTR technology,LIT technology detect both the amplitude and phase information of the thermal wave field in a large area simultaneously in real time by the use of pixelated array detection,which greatly enhances the detection speed when compared with the point by point detection in the traditional PTR technology.Meanwhile,the LIT signal has two signal channels of amplitude and phase,which can provide the surface and internal structural features of the sample conveniently when compared with the single amplitude signal only in traditional infrared thermography.Therefore,it is of great importance to conduct the study on the LIT technology and its applications in various fields such as scientific research,industrial production,and biomedical inspection.But so far,all the researches and applications of LIT is mainly focused on the samples of planar structures.In this thesis,a non-destructive testing technology for curved samples based on photothermal lock-in imaging technology was proposed and experimentally demonstrated.Specifically,the photothermal LIT detection for curved cylindrical samples is investigated,which includes the establishment of a theoretical model of photothermally excited thermal-wave field on the curved surface,numerical simulation and analysis,and the experimental demonstration of the photothermal LIT technique.The main content includes:(1)Firstly,based on the Green’s function theory,a theoretical model of the thermal wave field distribution of a three-dimensional cylindrical sample irradiated by an intensity-modulated laser was established.The influence of thermal diffusivity of the material,the modulation frequency of the excitation light source and the radius of curved cylindrical sample are numerically simulated and analyzed.The effects of the relative orientation of the excitation light source and thermal imager and the Lambertian cosine effect of the infrared radiation on the curved surface were discussed.(2)Secondly,an experimental laser photothermal LIT detection system for detecting cylindrical curved structures was established.The curved sample was continuously irradiated by a periodically modulated excitation light source.The thermal images of the curved sample surface at different times were collected by using a thermal imager;The averaging and normalization were performed on the acquired multiple thermal image signals by programming,and followed by the parallel signal processing for the temperature signals that are recorded in the time-sequential thermal images for each pixel using a dual-channel digital lock-in algorithm.Finally,both the LIT amplitude and phase images of the curved surfaces can be obtained.In this thesis,AISI 304 stainless steel cylindrical samples with different diameters were used for experimental demonstration.Combining the thermal wave field distribution of curved sample and the experimental modification of Lambert cosine scattering effect,the experimental data in both amplitude image and phase image were theoretically fitted.The fitting results show that the experimental and theoretical curves are highly consistent.The LIT study of cylindrical curved samples presented in this thesis provides a non-destructive,rapid and real-time quantitative tool for the detection of thermophysical material properties and structural characteristics of cylindrical curved samples,and also provides an important method for the measurements of physical properties and defects detection of other types of curved samples.