Research On Defect Detecting Method For Mechanical Component Based On Thermoelastic Laser Ultrasound

This thesis was intimately related to National Natural Science Fund Project’Study on an new Detective Method for Fatigue Crack in Mechanical Component based on Laser Ultrasonic.’(No.51375434).In this paper,FEM was mainly used to research on the Laser Ultrasound related principle.In this process,the theorical model of ultrasound stimulated by Gaussian pulsed laser was built to analyze the defect interaction with the ultrasound.Furthermore, new laser ultrasound detecting system was designed and the signal processing of the laser ultrasound feature was discussed fundamentally.The main content of this paper are listed as follows:Based on the elastic theory of plane strain,dynamic finite element model is established, aimed to the analysis of the transient stress field change over time in the process of laser irradiation and the numerical simulation of thermal elastic to process.It is concluded that the expansion stress and compression stress follow certain distribution rules and the magnitude of expansive stress is larger than compressive stress.Taking the laser process and the role of temperature change effect into consideration,the process of laser irradiation was analyzed by transient displacement field distribution changed over time, it is concluded that the laser ultrasonic can be built as a multiple modal and in rising and falling stages,the ultrasonic transmission and energy attenuation follow certain rules.FEM was arried out on the laser irradiation crack material situation,the displacement field distribution of the single crack and the crack of the surface of the material after laser irradiation were simulated respectively.It is concluded that for single crack materials, ultrasonic waves were interacted with crack, and part of the waves were blocked or schema transformated.For the grouping cracks materials, sound field near the micro crack was appeared significantly.To explore the laser ultrasound and the interaction, reflex wave and transmission wave model is set up respectively to analyze the numerical analysis results,showing that for the reflected wave signal, surface acoustic wave at the top of the micro cracks occur,being reflected directly transmitted to the receiver; for the transmission wave signal, SAWmode-conversion occured in the top of the micro cracks to produce longitudinal wave being transmissed to the receiver.New material detecting system based on Laser Ultrasound technology was designed. A new threshold de-noising method based on the IMF method was put forward,considering the threshold processing,transformation model and the effects of such factors as the number of iterations for signal processing respectively.Different methods of signal de-noising treatment were compared,it is concluded that this method overcame the difficulities perfectly.The research results can provide the theorical support for the quantitative and qualitative identification of material crack,as well as the acquiring of useful information from the laser ultrasound signals.