Date of Award
Doctor of Philosophy (PhD)
Computational Analysis and Modeling
Thermal analysis related to ultrashort-pulsed lasers has been intensely studied in science and engineering communities in recent years, because the pulse duration of ultrashort-pulsed lasers is only the order of sub-picoseconds to femtoseconds, and the lasers have exclusive capabilities in limiting the undesirable spread of the thermal process zone in the heated sample. Studying the thermal deformation induced by ultrashort-pulsed lasers is essential for preventing thermal damage. For the ultrashort-pulsed laser, the thermal damage is different from that caused by the long pulsed lasers and cracks occur after heating.
This dissertation presents a new finite difference method for studying thermal deformation in 3D thin films exposed to ultrashort-pulsed lasers. The method is obtained based on the parabolic two-step model and implicit finite difference schemes on a staggered grid. It accounts for the coupling effect between lattice temperature and strain rate, as well as for the hot electron-blast effect in momentum transfer. In particular, a fourth-order compact scheme is developed for evaluating those stress derivatives in the dynamic equations of motion. The method allows us to avoid non-physical oscillation in the solution.
To test the applicability of the developed numerical scheme, we investigated the temperature rise and thermal deformation in two physical cases: (1) a 3D single-layered thin film; and (2) a 3D double-layered thin film, where the central part of the top surface was irradiated by ultrashort-pulsed lasers. Results show no non-physical oscillations in the solution. Numerical results also show the displacement and stress alterations from negative value to positive value at the center along the z-direction, and along x and y-directions, indicating that the central part of the thin film expands during heating.
Zhang, Suyang, "" (2008). Dissertation. 466.