Research On Defects Quantification In Infrared Thermography Based On Thermal Wave Theory

With the development of science and technology,the quality and safety of mechanical equipment and components have been increasingly drawn much attention.For the aspects of the safe operation of equipment,product quality control and ensuring the equipment work safely and reliably,infrared thermography nondestructive testing technology plays an indispensable role.However,the judgment of defects is mostly about qualitative evaluation at present,and the degree of quantitative is not high,the theoretical research is mainly focused on the regular defects of plate structure,quantitatively analyzes the connection between defect depth and temperature distribution of specimen's surface.For irregular defects,complex materials and components,there is still lack of the relevant theoretical research.Therefore,this thesis mainly studies the mechanism of the interaction between the thermal wave and the defective specimen,and reconstructs the shape of the defect by simulation and experiments.In this thesis,infrared thermal wave nondestructive testing mainly based on thermal wave theory is used to quantify sub-surface defects.The reconstruction method of defect shape is studied by the theoretical inference,emulation analysis and test inspection.It focus on using the phase difference to characterize the defect depth,then reconstruct the defect shape according to the depth information by inversion,finally simulation and experiment show that the method can achieve the purpose of reconstructing defect shape.The main contents of the thesis are as follows:(1)Study the basic theory of infrared thermal wave nondestructive testing system,and discuss the influence of the geometric characteristics of the defect on the propagation,reflection and scattering of the heat wave in the specimen,that is the mechanism of the transient heat wave generated after the thermal excitation on the surface of the specimen and the internal structure and interface of the test piece and the characterization after the action.(2)According to the theory of heat transfer,using the COMSOL multi-physics simulation software,the triangular,rectangular and step-type defects in the two-dimensional model are simulated by sinusoidal excitation and pulse excitation,we establish a two-dimensional heat conduction mathematical model of temperature field distribution.Through the relationship between the phase difference and the depth of the defect the depth information of the defect is inversed.The defect shape is then determined by combining the phase difference profiles at different heat source points and the defect shapes reconstructed using the envelope.At the same time,the effect of volume-depth ratio of defects on the results in the detection process is analyzed.(3)On the basis of the pulsed eddy current thermal wave nondestructive testing system,the aluminum plate specimen with triangular,rectangular and stepped defects inside is tested.To eliminate the interference of high frequency noise,filter and the polynomial fitting process are used in the temperature-time thermal response data of the pixel locates near the heat source.Then the time domain signal is transformed into frequency domain by Fourier transform.The phase difference of the low frequency band is taken as the characteristic quantity of the defect depth,obtain the depth information corresponding to the position of the heat source,then reconstruct the defect shape with envelope.It can be seen from the experimental results that the proposed method can achieve the purpose of reconstructing the defect shape.