Simulation of Ultrasonic Waves Generated by Laser and Evaluation of Laser Parameters Effects on Ultrasonic Waves (Co-supervised by Dr. Badrossamay)
Year: Sept. 2010- Oct. 2012.
Ultrasound waves are mechanical waves which their frequency is higher than 20 kHz. These waves are generated in different ways. The most common ways are Mechanical, Piezoelectric and Magnetostriction. The most important of their deficiencies can be mentioned to the contact and the sample must be completely available. Therefore, it needs to have a non-contact method which uses in inaccessible and hot areas, for example. The technique to improve this problem is laser ultrasonics technique (LUT). Laser ultrasonics includes those technique which use lasers to generate and to detect ultrasonic waves in materials. This technique allows remote testing of materials in environment where other ultrasonic testing technique might yield unsatisfactory results. This technique allows inspecting large structures. The generated ultrasound wave form is affected by features of laser pulse (wavelength, pulse duration, intensity). Since the laser beam is used to generate and receive ultrasound signals, it doesn’t need to any mechanical contact. The Only need to generate an ultrasound wave in a sample is accessible of laser beam. Moreover, another advantage of laser-ultrasonics technique is to generate ultrasonic waves simultaneously. In general, mechanisms for generation of ultrasound wave depend on the power density of laser are classified into two mechanisms: Thermoelastic and Ablation mechanisms.
The main objective of this dissertation is to investigate the laser ultrasonics technique in order to achieve a deeper understanding of the process as a non-destructive method for inspecting materials. To achieve the goal, laser generated ultrasonic waves are simulated in a virtual environment (software) in terms of temperature and displacement. In the present study, the temperature dependence of the material and laser parameters (rise time and beam spot size of laser) on the ultrasonic waves that are generated by a pulsed laser are investigated in an aluminum plate by finite element method. At first, the transient temperature field can be precisely calculated by using the finite element method. Heat flux is applied on the surface of sample by a subroutine which called DFlux. Meanwhile, the heat flux has Gaussian type. Then, laser generated surface acoustic wave forms are calculated in an aluminium plate. In this simulation the sequential field coupling is used. Simulation results show that the laser parameters have a significant influence on the ultrasound waves and will be able to be utilized to choose best experimental parameters of laser. Next, ablation mechanism is simulated and the results of interest are obtained. In this mechanism the surface pressure resulting from plasma formation serves as a source for ultrasonic wave generation. In continuous, the results are compared with experimental results. Finally, the available experimental systems are introduced and evaluation of surface and internal defects using time of flight diffraction procedure (TOFD) is presented and a new method for evaluation of internal defects in the sample will be offered. To evaluate the accuracy of the results, the results of the simulation are compared with experimental results.
Keywords: ultrasonic waves, laser-generated ultrasound, thermoelastic mechanism, ablation mechanism, finite element method.