SUBJECT: Ph.D. Dissertation Defense
BY: Ushasi Roy
TIME: Friday, December 13, 2019, 1:00 p.m.
PLACE: MRDC Building, 3515
TITLE: Microstructure Sensitive Multiscale Modeling of Fracture in Polycrystalline metals
COMMITTEE: Dr. Min Zhou, Chair (ME)
Dr. David McDowell (ME)
Dr. Ting Zhu (ME)
Dr. Antonia Antoniou (ME)
Dr. Yavari Arash (CE)


Systematic exploration of the correlation between macroscale properties and microstructure attributes provides decision support to design new material microstructures with tailored properties. Currently no systematic approach exists that correlate overall fracture behavior of ductile polycrystalline materials with their microstructure attributes such as grain size, grain orientation distribution functions, grain boundary misorientation distribution etc. With an aim of establishing correlation between fracture toughness and microstructure attributes of ductile polycrystalline metals, a cohesive finite element method (CFEM) based multiscale computational framework is developed. The framework uses fully resolved 2D and 2.5D microstructures and explicitly models crack propagation through the grains and along the grain boundaries. Fracture resistance is measured in terms of JIC, KIC, and crack growth resistance curves. The framework captures the competitions between (a) intergranular and transgranular mechanisms of fracture, and (b) plastic deformation and crack growth. Using this computational framework, the effects of grain boundary behavior and crystallographic texture on fracture of polycrystalline bcc Mo are studied. Fracture resistance tends to increase with increasing skewness in the grain size distribution, decreasing density of weaker grain boundaries, and increasing density of favorably oriented primary slip systems. The use of multiple statistically equivalent instantiations of microstructure facilitated characterising the stochasticity in fracture toughness evaluation. Further analyses led to development of mathematical relations to quantify the trends. Finally the 2D model is extended to 2.5D. The 2.5D model introduced plastic dissipation in intergranular fracture. This led to the conclusion that the transgranular and intergranular proportions of crack propagation needs to be optimized for maximum fracture resistance. The overall methodology has the potential to be used in materials design exercises for developing novel microstructures with tailored properties.