Laser Ultrasonic Attenuation Characteristics Of The Heating Process Of The Heat-driven Smp Materials

In this paper, the finite element method (FEM) is applied to simulate the physical processes, which include the generation and propagation of the surface acoustic wave (SAW) in the SMP(Shape Memory polymer composite material) material on the condition of the room and rising temperature respectively. Then, waveforms and its propagation characteristics of SAWs excited by thermoelastic laser in the SMP material are studied by computer simulation combining with experiment.From the excitation mechanisms of laser ultrasound, the physical properties of SAWs and the detection methods used in laser-generated SAWs are introduced in a common manner. Subsequently, two main analytical models of the laser-generated ultrasound are given, constructed by line source irradiating on the sample at room temperature and rapid rising temperature respectively. The features of each model are also analyzed. All these works are preparations for the further theoretical study.Without considering viscosity modulus, in order to simulate the laser-generated SAWs, the finite element models of the line source irradiating on. the plate of SMP material is established by the elastic theory of plane strain. Compared with the results calculated by means of fitting viscosity modulus, the causes of amplitude attenuation of laser-generated Rayleigh waves with increasing propagation length are analyzed in this paper. Then, combined with the spatial distribution characteristics of laser pulse energy density and isotropic viscoelastic characteristics of the SMP material under varied temperatures, based on the thermoviscoelasticity of plane strain, the finite element method is employed to simulate the interactions of the laser-generated Rayleigh waves with the material, while the area of SMP material between excite point and detect point is heated to produce phase transformation. More important of all, the attenuation characteristics of the SAWs are studied by using FEM method when heating material is taken into account. Finally, the excitation and detection systems according to the acoustic principle are set up. After serials of ultrasound pulses frequencies are multiplicated, SAWs signals excited by ultraviolet line source can be detected by PVDF (polyvinylindene fluoride) transducer at the same point of the sample surface. The de-noised experimental results are consistent with simulation analysis, which demonstrates that the more viscous material becomes, the less elastic it will reflect, along with the temperature rising. When the temperature is about to reach glass transition temperature, the SAWs almost disappear. Thus, we can obtain that elastic behavior can represent physical characteristics of viscosity effectively.Ultrasound attenuation plays a vital role in the evaluation of ultrasonic properties and nondestructive testing technology. The results of this paper may provide theoretical and experimental references for SAWs attenuation measurement using laser ultrasound, and accelerate the development and application of laser ultrasonic nondestructive testing technology for the new composite materials.