Dynamic fracture

Structural materials often comprise two phases — one brittle, the other ductile. This motif is employed in materials ranging from metal matrix composites to low carbon steels. For the material to be tough overall, the ductile phase must catch and arrest any (dynamic) cracks originating within the brittle phase. For instance, in low carbon steels at low temperatures, brittle transgranular fracture ensues when the alpha-ferrite grains fail to arrest cracks originating within brittle carbides located at the grain boundaries. Thus, the quasi-static toughness of the material at the macro-scale depends on the outcome of a dynamic process at the micro-scale.

Setup for dynamic cleavage test; material 1 is brittle, material 2 is ductile.
Setup for dynamic cleavage test; material 1 is brittle, material 2 is ductile.

I investigated this phenomenon — of dynamic crack growth from a brittle material into a ductile one — using a combination of continuum mechanics and numerical simulation (molecular dynamics). The simulation setup is shown above. To perform the simulations in a clean, controlled way, I developed a new set of interatomic potentials for which the material ductility (in the Rice sense) can be tuned independently of the elastic constants or the Griffith fracture energy. (Code to generate these potentials is available in my GitHub repository.)

We found that dynamics does not appear to affect fracture in these bi-material systems: if a material is quasi-statically ductile, then it will also be dynamically ductile. One example is shown below, where a dynamic crack originating within a brittle material blunts and arrests within a (quasi-statically) ductile material.

Blunting of a dynamic crack in a ductile material
Blunting of a dynamic crack in a ductile material

Publications

V. P. Rajan and W. A. Curtin. Crack Tip Blunting and Cleavage Under Dynamic Conditions. Journal of the Mechanics and Physics of Solids, 90:18-28, 2016.

V. P. Rajan, D. H. Warner, and W. A. Curtin. An Interatomic Pair Potential with Tunable Intrinsic Ductility. Modelling and Simulation in Materials Science and Engineering, 24:025005, 2016.