Material Point Method For Thermal Responses Of Metals By Ultrashort Pulse Laser Irradiation

Ultrafast laser interaction with metals could be generally described by the two-temperature model(TTM).So far,the TTM has been largely solved by the mesh-based numerical methods.,e.g.,finite difference and finite element methods.However,in addition to the electron-lattice non-equilibrium thermal state,the ultrashort pulse laser irradiation could cause the large deformation of the targets,and even fracture.As a result,the use of mesh-based methods to solve the TTM suffers from the mesh distortion.As a mesh-less method,the material point method(MPM)combines the advantages of the Lagrangian and Eulerian methods,and has been shown to be well suitable for simulating problems involving large deformations and material discontinuity.Recently,the MPM has been used to study the Fourier heat conduction processes.The main work of this thesis is to develop one MPM for simulating the thermal response of the metal irradiated by the ultrashort pulse laser.Firstly,the weak formulations of TTM electron and lattice energy equations are established.The electron and lattice are discretized to material points,and the discrete forms of the control equations are constructed.Then,by using the finite element shape functions,the particle information is mapped to the background grid nodes for solving the equation,and the grid information is mapped back to particle to update temperatures at material points.The MPM algorithm and finite difference method have been used to simulate the thermal response of a thin gold film irradiated by the ultrashort pulse laser in the one and two dimensions.The results show that,the electron and lattice temperature calculated by the MPM and the finite difference method agree with each other,which verifies the validity of the MPM algorithm.In addition,the convergence and stability of the proposed MPM have been verified via the simulations with various time steps,mesh spacings,point numbers per grid cell.As compare with the mesh-based method,the proposed MPM requires no need for treating the adiabatic boundary condition because it is automatically satisfied in the MPM procedure.Therefore,the MPM algorithm is superior to the mesh-based methods for thermornechanical response of the metallic targets heated by ultrashort pulse lasers.Finally,to accelerate the multi-dimensional computation,the OpenMP parallel version of proposed MPM algorithm has been developed via background grid region decomposition method and demonstrated by representative examples.The results show that the developed parallel shceme can effectively improve the computational efficiency.