Laser Ultrasonic Monitoring Of Photoinduced Microcrack Closure

Since the dynamic change of the crack in the process of closure has not been studied,the closure process of the crack will be studied by experiment in this paper.First,An experimental device using laser ultrasonic method to detect cracks was set up.The ultrasonic generated by point pulsed laser was detected using optical deflection method.The crack was irradiated by a continuous heating beam and closed gradually by increasing the power of the heating beam.Different modes of ultrasonic generated on a black glass sample were distinguished by TOFD method.Then,at the same position of the crack,the power of the heating beam was increased gradually and the transmitted surface acoustic wave and longitudinal wave mode converted by surface acoustic wave was detected at each heating power both when the crack was heated and cooled after heating.The whole process is repeated three times and the relationship of the three time between the peak-peak value and heating power was acquired.Experiment results show the great difference of the peak-peak values between heating and without heating of the crack.Also,the significantly difference of the experiment result between the first time and the last two times whose results are very similar was observed.The change of the peak-peak value when the crack was cooled after heating of the last two times is obviously smaller than the first time.The experiment results indicate that irreversible modification could happen to the crack during heating and the modification becomes unnoticeable after several times of heating because the crack become more stable.Finally,by moving the sample,the excitation,detection and heating of the light source are simultaneously moved along the crack.Increased the heating power gradually and detected the transmitted surface acoustic wave and mode converted wave at each heating power both when the crack was heated and cooled after heating at different positions on the crack.The relationship between peak peak values of the singals and heating power at different position on the crack was acquired by this experiment and then the relationship when irreversible change of the crack happened was discussed and summarized according to the results obtained in this experiment and some other research which had been obtained before.Based on the relationship acquired in the experiment,a possible change of the crack during the closure process when the crack was heated was discussed.The change of ultrasonic signals’ amplitude when the crack was closing and separating were monitored and the relationship between peak-peak values of the signals and time was obtained.Some phenomena which had never been observed from the quasi-static experiment were observed.The research of this paper can provide useful information for the further study of nonlinear laser ultrasonic crack detection method provide a new idea to systematically explain the behavior and change of moving crack.