| Hypersonic aircraft safety is tightly related with structural health status of thermal protection material,and damage detection is a prerequisite for ensuring the structural integrity of the material.Laser ultrasonic technology is an emerging non-destructive testing technology that combines optical and ultrasonic testing methods.It is widely used for damage detection of new materials due to its advantages of non-contact,frequency bandwidth,rich waveform and long-distance operation.The purpose of this paper is to study a damage detection technology of thermal protection material based on laser ultrasonic.The thickness detection and internal defect positioning were realized by processing and analyzing the detected echo signals.First of all,the material model was established based on ANSYS software,and the propagation process of laser ultrasound inside the material were studied by thermal stress coupling analysis.Secondly,a hybrid improved laser ultrasonic signal processing and envelope extraction algorithm based on wavelet denoising,Chebyshev bandpass filtering and Morlet continuous wavelet transform was proposed.It could realize noise reduction of laser ultrasonic echo signals,intercepting narrowband signals and extracting signal envelope amplitude spectrum.The results show that the signal envelope extracted by the algorithm has higher smoothness and resolution.In addition,in order to make signal processing results more visible,this paper designed a visualization software system based on Matlab GUI.The system has good display effect and high real-time performance.At last,the formula for calculating ultrasonic wave velocity at different temperatures was fitted by studying the influence of temperature on signal.Based on the built-in laser ultrasonic nondestructive testing system,thickness detection,internal defect detection and positioning experiment of thermal protection materials were carried out respectively.The experiment proves that the technology has high detection accuracy.In addition,two-dimensional reconstruction of material section was realized based on the cubic spline interpolation algorithm.By comparing with ultrasonic phased array B-scan imaging,the unique advantages of two techniques for material damage detection were analyzed.According to above theoretical and experimental research,the signal processing and envelope extraction algorithm proposed has good detection and localization effects,which proves that the technology based on laser ultrasonic for thermal protection material damage detection is feasible. |
