Research On Laser Ultrasonic Testing Method And Technology For Mechanical Properties Of Metallic Materials

Based on the laser ultrasonic technology,the elastic constants,residual stresses and surface cracks of the metal materials are measured by experiment,and the equivalent mechanical model of the finite element analysis is established.The corresponding finite element analysis for the laser ultrasonic nondestructive testing of metallic materials is carried out.The main contents are as follows:Firstly,the laser ultrasonic nondestructive testing system has been developed independently.The system has the advantages of high precision,good stability,no damage,no contact,adapt to all kinds of harsh environment and so on,which provide a sound basis for future research of laser ultrasonic experiments.The experimental system is mainly composed of the excitation system,the receiving system and the prestressed loading device.Nd:YAG laser(Beamtech Dawa-100)emits high energy laser pulse with Gaussian profile.A cylindrical lens is employed to convert the laser to a line source with line width adjustable.The measurement system consists of a photoelectric detector(Thorlabs DetlOA/M),a laser Doppler vibrometer(Sdptop LV-S01),a digital oscilloscope(Tektronix Dpo4102,and a computer.The oscilloscope collects signal from photoelectric detector and laser Doppler vibrometer.The prestressed loading device can realize the restraint of the sample and the synchronous loading at the time of detection.Secondly,the equivalent load method is proposed and applied in the finite element analysis for simulation the propagation of Rayleigh surface wave and longitudinal wave.The pulsed laser is equivalent to the surface load in time and space domain to meet the Gaussian profile.The relationship between the physical parameters of the laser and the load is established by the correction factor.The method does not need to deal with the thermo-mechanical coupling problem.The elastic modulus and Poisson's ratio of metallic materials are measured by laser ultrasonic method,and the elastic constants are obtained by measuring the propagation velocities of the laser induced ultrasonic longitudinal wave and surface wave.In order to eliminate the measurement error and the time delay of the experimental system,the linear fitting method of experimental data is applied.Finally,the nonlinear relationship between the detection angle and the surface wave velocity of the laser ultrasonic detection of residual stress is discussed,and the concept of the optimal detection path along the direction of the principal stress is proposed.The residual stress and surface micro crack is measured by laser ultrasonic technique.The acoustoelastic constant of the material is obtained by prestressed loading method.On the basis of known acoustoelastic constant,the welding residual stress is measured by the laser ultrasonic technique.The finite element method is used to simulate the generation and propagation of surface wave in the residual stress state.The laser ultrasonic technique applied in detection the micro crack is investigated and the influence of the geometrical dimension of the crack on the reflected wave and the transmitted wave is discussed.The results in this paper will provide the theoretical and experimental evidence for the testing mechanical properties of metal materials by laser ultrasonic.It is also helpful for the further development and application of nondestructive testing and evaluation using laser ultrasonic technology.