Detection Of The Micro-crack Through Frequency Domain Analysis Of Laser Surface Acoustic Waves And The Development Of Laser Assisted Heating Micro-crack Detection Method

Because of the local stress concentration brought by structural irregulars and material inhomogeneities, micro cracks often appear on the surface or sub-surface of the workpieces. The opening size of micro crack is very small and the crack walls can touch each other easily. In this paper, the detection and identification methods for micro crack are studied theoretically and experimentally, they are based on the characteristics that micro cracks can get partially closed easily.The temperature rise and thermal expansion of micro crack which brought by laser irradiation can be derived by theoretical calculation, it demostrates that the laser irradiation can cause the displacements of crack walls, and the value of displacements are related with the power and action time of laser irradiation. Based on the heat conduction theory and thermal elasticity theory, the temperature rise and thermal expansion processes are also simulated by the finite element method (FEM). At different depths of crack wall, the temperature and displacement change with heating time are studied. The theoretical results can be used to explain the closing process of micro cracks when they are heated by laser irradiation.The laser assisted heating method for micro crack detecting is presented. By scanning the laser exciting source and laser heating source synchronously, the fatigue crack in the engine blade can be detected succesfully by comparing the D-value of heating signals and signals without heating. Then the scanning heating source method is presented, the micro crack in glass sheet can be detected just by scanning the laser heating source. The pitch-catch method, the scanning laser source method and the scanning heating source method are used to test a specimen with micro crack and surfce protuberance, the ability of scanning heating source method for identifying the micro crack is verified by comparing the experimental results.On the base of the classic theory that ultrasonic waves generated by pulsed laser source thermoelasticly, the FEM model of laser generated surface acoustic waves (SAWs) is built, by changing the size of V slot, the impacts on transmitted SAWs by different size of cracks can be studied. The impacts on the transmittivity of SAWs by different closing state of cracks are also studied experimentally.The pure optical detection system that using laser for exciting ultrasound and using differential optical deflection method for detecting ultrasound is built to study the ultrasonic signals corresponding to different generation positions. The contribution of different modes to the full frequency characteristics is analyzed. The frequency characteristics of signals are presented by B-scan plot, by observing the change trend of frequency characteristics, the crack position can be confirmed.The results of this paper may provide theoretical and experimental basis for micro crack detection using laser-generated ultrasound, and may accelerate the development and application of laser ultrasonic nondestructive testing technology.