مدل سازي توليد امواج فراصوتي به وسيله ليزر و بررسي اثر پارامترهاي ليزر بر امواج توليد شده

چكيده انگليسي :

110 Simulation of Ultrasonic Waves Generated by Laser and Evaluation of Laser Parameters Effects on Ultrasonic Waves Peyman Soltani p soltani@me iut ac ir 18th September 2012 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language Farsi Supervisors Mohammad Mashayekhi Mohsen Badrossamay mashayekhi@cc iut ac ir Abstract 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

سلطاني، پيمان

مدل سازي توليد امواج فراصوتي به وسيله ليزر و بررسي اثر پارامترهاي ليزر بر امواج توليد شده

مكانيك ترموالاستيك , مكانيزم برش , روش اجزاي محدود