title: Phase-field modeling of thermal fracture and its applications to additive manufacturing
Creators: name: Ruan, Hui
Publisher: name: UNSPECIFIED
Year: 2024

description: Modeling and prediction of fracture processes remain challenging problems in computational mechanics, particularly in a multiphysics environment. In various practical applications, fracture is coupled with other involved physics which in turn severely influences the damage progression inside the material. Thermal fracture is universal in many branches of engineering applications, and is one of the most devastating defects in the metal additive manufacturing process. Due to the interactive physics involved, the computational simulation of such a process is challenging. This thesis is dedicated to understanding the fracture mechanism of such a complex material system, in particular the thermal cracking mechanisms of the additive manufacturing process, and the fracture behaviors of additively manufactured parts. 

This thesis presents a thermodynamically consistent framework for thermo-elastic coupled brittle fracture at small strains using the phase-field model. The coupling mechanisms such as damage-informed thermomechanics and heat conduction, and temperature-dependent fracture properties, as well as different phase-field fracture formulations, are discussed. Numerical examples show that the proposed model is capable of simulating thermal brittle fracture, and the coupling mechanisms are indispensable to the accurate prediction of the thermal fracture process. Moreover, the phase-field model for thermal ductile fracture in thermo-elasto-plastic materials undergoing finite deformation is developed. Thereby the intercoupling mechanisms among elastoplasticity, phase-field crack and heat transfer are considered comprehensively. The finite element implementation of the coupled phase-field model is validated by comparing simulation results of a tensile test of an I-shape specimen, encompassing elastoplasticity, hardening, necking, crack initiation and propagation with experimental results. 

The validated models are further employed to simulate the multiphysics hot cracking phenomenon in additive manufacturing in the context of an interpolated temperature solution, the phenomenological model, and the powder-resolved model of powder bed fusion. Thereby not only the classical thermal strain but also the solidification shrinkage are considered to calculate the thermal stress. Simulation results reveal certain key features of hot cracking and its dependency on process parameters like laser power and scan speed. A higher laser power and a lower scanning speed are favorable for keyhole mode hot cracking while a lower laser power and a higher scanning speed tend to form the conduction mode cracking. These findings provide valuable insights into the fundamental understanding of crack formation mechanisms and process optimization.

Furthermore, a multiscale framework using the cohesive phase-field fracture method is presented to investigate the anisotropic fracture of additively manufactured parts. Herein, the anisotropic properties including anisotropic elasticity and anisotropic fracture resistance are considered, with both effects on crack patterns studied separately and combined. The orientation-dependent elastic moduli are calculated by the computational homogenization approach, while the stress-based spectral decomposition method of stress and strain energy is adopted as a result of anisotropic elasticity. A direction-dependent structural tensor which relates to the printing process is introduced to the phase-field fracture model to include the anisotropic fracture toughness. The simulation results show that it is necessary to consider both anisotropic elasticity and anisotropic fracture properties to accurately capture the fracture behaviors of additively manufactured parts.