Laser Ultrasonics Technology And Its Applications In Non-destructive Testing On Metal

With the development of industrial automation, there is more and more demand on the non-destructive testing technology. For example, the equipments and productions in manufacturers are wanted to be real-time monitored and detected without any damage on line even in such conditions as high temperature, high pressure, poisonousness. The laser-induced ultrasonic technique, which is based on the generation of ultrasonic by a laser and the detection of stress wave with laser interferometry, and is an ideal combination of laser and ultrasonic for non-destructive testing, has also the advantage of non-contact, and has become very important in the non-destructive testing fields.This technology has been used widely just for its advantages of non-contract operation, non-destructive testing, broad bandwidth , high time resolution, high space resolution, no limit on sample's shape, one laser pulse exciting many kinds of waves and so on.Firstly, this paper introduces the principles of laser ultrasonic generation, thermoelastic excitation theory and the ablation excitation theory. Besides, it briefly presents the other excitation theories in solids. It also analyses and compares the characteristics of bulk acoustic wave and surface acoustic wave, such as directivities and shapes . And then optical detection methods of laser-induced ultrasonic, especially CFPI detection method, are introduced.Based on the principles of laser-induced ultrasonic generation and detection , the structure of the laser ultrasonic detection system is presented, and its availability is testified by several experiments. Using the velocity of bulk acoustic wave and surface acoustic wave in the aluminium sheet measured by this system, elastic constants of aluminium are deduced. The system also test the surface defects of the aluminium sheet. The experimental results show that laser ultrasonic technique is practical and effective.The influences of the characteristics of pulsed laser source and CFPI on the accuracy of detedtions are analysed in detail. The conclusions are as follows: the ultrasonic intensity isproportion to the laser intensity ; the ultrasonic induced by a laser line source has much more advantages than by a laser point source, such as stronger energy, higher SNR, more directivity and shape; the intensity of CFPI's output signal is linear with ultrasonic vibration velocity, and is effected by incidence angle and scattered angle; SNR will increase by reducing CFPI's bandwidth, reducing incidence angle, increasing cavity's length and reflectivity.