Ultrafast Laser Induced Thermo-elasto-plastodynamics in Polycrystalline Metals
Abstract
A comprehensive thermo-elasto-plastodynamic model of the laser-material interaction in polycrystalline metals in response to ultrafast laser heating is formulated. Lacking a fundamental understanding for the non-ablation and non-melting ablation processes and the underlying mechanism that governs coupled thermal-mechanical generation impedes the broader application of ultrashort lasers. The transport dynamics established in the dissertation describes the initial plasma plumes as the result of the photoelectric and thermionic emissions of electrons. The formulation admits finite electron and lattice energy transport speeds and incorporates energy losses to electrons emission and thermoelastic and thermoplastic generations. Elastic-plastic constitutive laws are incorporated to describe the complex elasto-plastodynamic cyclic behaviors attributed to the rapid thermal processing and metallic characteristics of the Bauschinger’ effect. A staggered-grid finite difference scheme is time-integrated to resolve the coupled field responses using a one-dimensional formulation and an axisymmetric model.
The balance equations considered in the research obey the principle of energy conservation and follow the characteristic time scales associated with optical energy absorption, particle emission, and electron and lattice relaxations. Electron energy transport in polycrystalline metals is comprehensively investigated by considering temperature-dependent thermophysical properties and the grain size effects due to surface and grain boundary scatterings. Numerical results obtained for the non-ablation of a gold film under ultrafast optical heating is favorably examined against published experimental data for model validation. Parametric studies considering target thickness, grain size, and optical parameters indicate the impact of these parameters on energy transport, electrons emission, and coupled thermal-mechanical response. Crack initiation is investigated by
considering the propagation of stress waves generated by non-ablation heating. The novel concept of power density is adopted as the energy rate-based criterion to evaluate ablation mass removal and ablation depth as functions of laser fluence. The thermo-elasto-plastodynamic formulation is feasible for describing non-ablation thermoelastic generation and for exploring the correlation between the incident laser pulse and ablation and damage in polycrystalline metals.
Citation
Mao, Xu (2018). Ultrafast Laser Induced Thermo-elasto-plastodynamics in Polycrystalline Metals. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192055.