The Non-destructive Evaluation Of Surface Breaking Crack By Using Surface Acoustic Waves Generated By The Scanning Laser Source Technique

In this paper, the interaction of the ultrasound generated by the Scanning Laser Source (SLS) technique and the surface breaking crack in a aluminum sample is studied by meanings of numerical simulation with the Finite Element Method (FEM) as well as experimental method by building an experimental system for surface breaking crack detection with optical generation and detection. Corresponding mechanism and the characteristical propagation property of the Surface Acoustic Waves (SAWs) are analyzed. The affection of the surface breaking crack on the laser generated surface wave with variety generation position is studied in detail in order to reveal the mechanism of interaction between the surface breaking crack and the laser induced SAWs.The numerical model of ultrasound generation by laser line source in aluminum plate is built, and the process of the detection for a surface breaking crack by using the SLS technique is simulated by the FEM, where the surface breaking crack is represented by a surface notch. Afterward, the "Dual Laser Source" (DLS) technique is put forward, which concentrates on the ultrasound generation and propagation when the laser is irradiating the crack. The orientation and depth of the crack can be obtained by this technique. Then combine the SLS technique and the DLS technique in order to obtain not only the location of the surface breaking crack but also the orientation and the depth of the crack in one experimental loop. At last, corresponding experimental setup is built to verify the numerical result. The experimental result shows good agreement with the numerical result so the correctness of the SLS-DLS technique is verified.On the base of the numerical model provided above, concentrating on changing the orientation and the depth of the crack in the model, the situation of the orientation of the surface breaking crack varying from 5°to 175°and the depth varying from 1mm to 5mm are analyzed respectively. Generations of different modes of ultrasound are explained and the possibility for reverse detection for the orientation and depth of the crack by analyzing the arrival time of different ultrasound modes is studied.The classical SLS technique is improved experimentally. Line source is replaced by point source and 2-D photoacoustic imaging is achieved by utilizing a 2-D scan on the sample surface. Experimental results for the detection of metallic samples with visible fatigue crack and invisible crack obtained by using this scanning laser source system indicate that by using this detection method, not only the location, but also the length of the fatigue crack can be detected. From the result it also can be seen that, this detection method is also capable for invisible fatigue crack detection.Based on the theory of opto-thermal modulation of opto-acoustic fatigue crack detection, combining the advances of the SLS technique, the crack detection system with opto-thermal modulated laser-induced ultrasound is provided. The aluminum sample with fatigue crack is evaluated with method provided previously and method with opto-thermal modulation. The experimental result shows that the opto-thermal modulated detecting method provides much higher detecting sensitivity and N/R ratio than the classical linear detection. The theoretical explain for the experimental result are provided following.Results of this paper may provide theoretical and experimental basis for surface breaking crack detection on metal using laser-generated ultrasound, and may accelerate the development and application of laser ultrasonic nondestructive testing technology.