Scanning Laser Ultrasound Study Of Reversible/irreversible Changes In Photoinduced Crack Closure

The time-domain laser ultrasonic technique is used to study closing response of real microcracks under photo-thermal modulation and local plastic/elastic microscopic changes of cracks caused by crack closure in this paper.The pulse point laser source is used as ultrasound excitation source,and the light deflection method is used to detect ultrasound.First,the TOFD method is used to distinguish various modal ultrasonic signals excited by laser on sample by moving the sample.Subsequently,the excitation source and detection source are scanned in three areas of the crack on sample(start of crack,middle of crack and tip of crack).In each detection area,we experiment as follows: Frist,irradiate a beam of continuous heating laser with constant power from 0 m W on the crack to close the crack;then,fix the heating laser and move the excitation-detection light source to scan in this area so that we can obtain the scanning results of the transmitted surface wave and mode converted wave signals in this area when the crack is heated under the heating optical power and cooled after the heating light closes;finally,the heating optical power is gradually increased in increments of 10 m W each time and we repeat the scanning step in this area until the heating light is increased to 300 m W.The entire experimental step is repeated three times in each crack area.The relationship between peak-peak value of transimission signal and heating optical power was acquired in three areas when the crack is heating and cooling.The response of transmitted ultrasonic signal to the local reversible/irreversible changes of the microscopic irregularities on the crack surface during the process of photo-induced crack closure was studied.The change trend of peak-peak value of transmitted signal detected when the crack is heated in three photo-thermal loading cycles at three areas of crack was measured and combined with previous experimental research to analyze and summarize the closed state of crack in each area.Finally,the change trend of transmitted surface wave signal arrival time detected when the crack is heated in the first photothermal loading cycle in three areas of crack.The changes of crack morphology and structure and the propagation path of transmitted surface waves during the irreversible closure of cracks in three areas are simply analyzed.The results show that transmitted surface wave signal arrival time in the scanning area is inconsistent with the cracks in the three areas without heating,and the signal arrival time decreases as the heating optical power continues to increase.When the heating optical power is increased to a certain power,the signal arrival time of all positions in the scanning area is almost the same.As the heating optical power is further increased,the signal arrival time may be delayed,which may be caused by the increase in the heating optical power that causes the propagation speed of surface waves in the material to become slower.