Multiscale 3D fracture simulation integrating tomographic characterization

Ductile fracture of engineering alloys is a multi-stage process involving nucleation, growth and coalescence of voids across different length scales of microstructure (micron size primary particles and submicron size secondary particles) during the plastic deformation. Generally, primary inclusions debond from the metal matrix and form primary voids under small plastic deformation. When the localized deformation between primary voids is intensified, void linkage can be triggered by nucleation of a secondary population of particles on the intervoid ligament. In these multi-stage and multi-scale processes, shear coalescence of secondary voids (microvoids) plays an important role in the fracture process [1]. However it remains elusive. For high toughness alloys (Ti-modified 4330 steel), the JIC fracture test has been carried out. The CT (compact tension) specimen used in the experiment is shown in [2]. Using an advanced experimental technique, three dimensional microstructure in the fracture process zone and fracture surface both were reconstructed through microtomography [2]. It can be seen from the reconstruction that high toughness alloys typically consist of metal matrix and two populations of distributed precipitates: Primary particles (TiN) usually on the order of micron and secondary particles (TiC) on the order of submicron. The experiment also showed that even under mode I loading in the far-field, the fracture process is mixed-mode caused by multiscale microstructure, resulting in a zig-zag fracture surface. Through parametric study, we first clarify the mechanisms which lead to zig-zag fracture pattern observed in the experiment. A numerical simulation of the fracture process of Mod 4330 steel is then performed in the same context of plane strain, quasi-static and small scale yielding as the experiment[2]. The finite element mesh used for modeling the experiment is given in at the right of Figure 1. The fracture patterns are shown at the left of Figure 1. We put the experiment and the simulation under the same length scale for comparison. It is obvious that the zig-zag fracture profile observed in experiments is reproduced. In the simulation, shear band between voids and primary voids both are observed in the similar place shown in the experiment.