The Woodruff School

SUMMARY

Microstructure determines fracture toughness of materials through the activation of different fracture mechanisms. To tailor the fracture toughness through microstructure design, it is important to establish relations between microstructure and fracture toughness. To this end, systematic characterization of microstructures, explicit tracking of crack propagation process and realistic representation of deformation and fracture at different length scales are required. A cohesive finite element methods (CFEM) based multi-scale framework is proposed for analysing the effect of microstructural heterogeneity, phase morphology, texture, constituent behavior and interfacial bonding strength on fracture toughness. The approach uses the J-integral to calculate the initiation/propagation fracture toughness, allowing explicit representation of realistic microstructures and fundamental fracture mechanisms. Both brittle and ductile materials can be analyzed using this framework. For two-phase Al2O3/TiB2 ceramics, the propagation fracture toughness can be improved through fine microstructure size scale, rounded reinforcement morphology and appropriately weak bonding strength. These microstructure characteristics can promote interface bonding and discourage particle cracking induced the catastrophic failure. Based on the CFEM results, a semi-empirical model is developed to provide a quantitative relation between the propagation toughness and statistical measures of microstructure, fracture mechanisms, constituent and interfacial properties. The analytical model provides deeper insights into the fracture process as it quantitatively predicts the proportion of each fracture mechanism in the heterogeneous microstructure. It is found that an optimal level of interfacial stiffness is needed to maximize the fracture toughness. The ability of the CFEM framework will be extended to 3D with crystalline plasticity in constitutive response modeling. The crystallographic information will be incorporated by reconstruction of image-based 3D microstructure. Direct correlations will be made between applied loading, response, microscopic damage, properties, and strength. These conclusions can be used in the selection of materials and the design of new materials with tailored properties.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Yan Li

TIME:	Wednesday, May 23, 2012, 2:00 p.m.

PLACE:	MRDC Building, 4211

TITLE:	Prediction of Material Fracture Toughness as a Function of Microstructure

COMMITTEE:	Dr. Min Zhou, Chair (ME/MSE) Dr. David L. McDowell (ME/MSE) Dr. Rick Neu (ME/MSE) Dr. Ting Zhu (ME) Dr. Donald Shih (Boeing) Dr. Shuman Xia (ME)

SUMMARY Microstructure determines fracture toughness of materials through the activation of different fracture mechanisms. To tailor the fracture toughness through microstructure design, it is important to establish relations between microstructure and fracture toughness. To this end, systematic characterization of microstructures, explicit tracking of crack propagation process and realistic representation of deformation and fracture at different length scales are required. A cohesive finite element methods (CFEM) based multi-scale framework is proposed for analysing the effect of microstructural heterogeneity, phase morphology, texture, constituent behavior and interfacial bonding strength on fracture toughness. The approach uses the J-integral to calculate the initiation/propagation fracture toughness, allowing explicit representation of realistic microstructures and fundamental fracture mechanisms. Both brittle and ductile materials can be analyzed using this framework. For two-phase Al2O3/TiB2 ceramics, the propagation fracture toughness can be improved through fine microstructure size scale, rounded reinforcement morphology and appropriately weak bonding strength. These microstructure characteristics can promote interface bonding and discourage particle cracking induced the catastrophic failure. Based on the CFEM results, a semi-empirical model is developed to provide a quantitative relation between the propagation toughness and statistical measures of microstructure, fracture mechanisms, constituent and interfacial properties. The analytical model provides deeper insights into the fracture process as it quantitatively predicts the proportion of each fracture mechanism in the heterogeneous microstructure. It is found that an optimal level of interfacial stiffness is needed to maximize the fracture toughness. The ability of the CFEM framework will be extended to 3D with crystalline plasticity in constitutive response modeling. The crystallographic information will be incorporated by reconstruction of image-based 3D microstructure. Direct correlations will be made between applied loading, response, microscopic damage, properties, and strength. These conclusions can be used in the selection of materials and the design of new materials with tailored properties.