Multi-scale Numerical Simulations for Crack Propagation in NiTi Shape Memory Alloys by Molecular Dynamics-based Cohesive Zone Model

The multi-scale modeling combined with the cohesive zone model (CZM) and the molecular dynamics (MD) method were preformed to simulate the crack propagation in NiTi shape memory alloys (SMAs).The metallographic microscope and image processing technology were employed to achieve a quantitative grain size distribution of NiTi alloys so as to provide experimental data for molecular dynamics modeling at the atomic scale.Considering the size effect of molecular dynamics model on material properties,a reasonable modeling size was provided by taking into account three characteristic dimensions from the perspective of macro,meso,and micro scales according to the Buckingham π theorem.Then,the corresponding MD simulation on deformation and fracture behavior was investigated to derive a parameterized traction-separation (T-S) law,and then it was embedded into cohesive elements of finite element software.Thus,the crack propagation behavior in NiTi alloys was reproduced by the finite element method (FEM).The experimental results show that the predicted initiation fracture toughness is in good agreement with experimental data.In addition,it is found that the dynamics initiation fracture toughness increases with decreasing grain size and increasing loading velocity.