Atomistically informed void nucleation in ductile fracture modeling
Creators: name: Zhao, Qianqian
Publisher: name: No Publisher Supplied
Year: 2024

description: Ductile structural metals fail through nucleation, growth, and coalescence of voids.While the basic mechanisms of this process have been understood for many decades, a quantitative theory for predicting the void nucleation rate is lacking.In this thesis, molecular dynamics (MD) simulations of spherical $heta$ particle debonding within an aluminum matrix are used to identify the microscale mechanisms of nucleation, develop governing equations for the nucleation rate, and then construct a nucleation constitutive model suitable for implementation in the finite element method. First, simulations were conducted to examine the early stages of void formation and nanocrack growth at the particle-matrix interface across a range of temperature and hydrostatic stress values. Crack growth without dislocation activity was identified to obey lattice-trapped fracture dynamics, obeying a thermally activated rate law with an Arrhenius activation energy of $1.37~m eV$ and an activation area of $1.17~mAA^2$.%In constant, crack growth in the presence of dislocation activity was significantly faster than lattice trapped fracture.Next, a comprehensive set of several hundred MD simulations exploring stress states spanning a range of stress triaxialities and Lode parameter values was used to determine the critical conditions for void nucleation. %Simulation results were compiled and mined in order to reveal the governing physics for void nucleation.Void nucleation is observed when the local normal stress at the matrix-particle interface reaches a critical value of 8.14 GPa. When plastic deformation occurs in the matrix before nucleation, additional plastic stress concentrations are observed due to plastic strain accumulation around the particle. The findings reveal that this plastic stress concentration factor increases roughly linearly with equivalent plastic strain, independent of stress triaxiality or Lode parameter. These results contrast with existing literature, showing a much stronger influence of plastic strain than predicted by continuum plasticity theory. The discrepancy is attributed to nucleation being driven by localized stress hotspots resulting from heterogeneous plastic strain, whereas continuum theory is based on homogenized plastic strain fields. Using the results from these studies, a new nucleation model is developed that is suitable for ductile fracture modeling.%The well-established Gurson-Tvergaard-Needleman model is employed with a custom nucleation model that captures the influence of stress state, plastic strain, and particle size on the nucleation rate.Unlike traditional nucleation models, our model takes as input the size distribution of the particle population in the material and material properties for the crack nucleation rate at the particle interfaces.The behavior of the model is explored in a MATLAB script and a preliminary implementation in the ABAQUS finite element code.Overall, the study concludes that void nucleation is predominantly stress-driven and significantly influenced by plastic strain.