MSE Colloquium: Yanfei Gao, Intergranular Strain Evolution near Fatigue Crack Tips in Polycrystalline Metals

Associate Professor, Department of Materials Science and Engineering, University of Tennessee, Knoxville

Abstract

Despite significant advances in material lifetime studies, previous works on fracture and fatigue typically rely on ex situ microstructural characterizations and crack growth monitoring with replica techniques. The in situ, nondestructive, neutron and synchrotron X-ray diffraction techniques, together with the multiscale simulations, will provide us a linkage between the stress analyses (top-down point of view) to the failure mechanisms on inter- and intra-granular scales (bottom-up point of view). A two-scale model is developed to predict the lattice strain evolution near fatigue crack tips in polycrystalline materials. An irreversible, hysteretic cohesive interface model is developed to simulate a steady fatigue crack, which allows us to generate the stress/strain distribution and history near the fatigue crack tip. It is found that the general deformation characteristics near a fatigue crack tip include a plastic zone in front of the crack tip and a plastic wake left behind, where the cyclic loading and fatigue crack growth lead to a compressive strain. The continuum deformation history is used as inputs for the micromechanical analysis of lattice strain evolution using the slip-based crystal plasticity model, thus making a mechanistic connection between macro- and micro-strains. Crystal plasticity simulations motivate us to develop a Taylor-type analysis of the intergranular strains and their decay during cyclic loading. By considering the intergranular damage, which leads to vanishing intergranular strains as damage proceeds, we find an excellent agreement between predicted and measured lattice strains in the entire field. Thus the intergranular damage near fatigue crack tip is concluded to be responsible for fatigue crack growth.

1. R.I. Barabash, Y.F. Gao, Y. Sun, S. Lee, H. Choo, P.K. Liaw, D.W. Brown, G.E. Ice, "Neutron and x-ray diffraction studies and cohesive interface model of the fatigue crack deformation behavior." Phil. Mag. Letters, 88, 553-565 (2008).
2. L.L. Zheng, Y.F. Gao, S.Y. Lee, R.I. Barabash, J.H. Lee, P.K. Liaw, "Intergranular strain evolution near fatigue crack tips in polycrystalline metals." J. Mech. Phys. Solids, 59, 2307-2322 (2011).
3. S.Y. Huang, D.W. Brown, B. Clausen, Z.K. Teng, Y.F. Gao, P.K. Liaw, "In situ neutron-diffraction studies on the creep behavior of a ferritic superalloy." Metall. Mater. Trans. A, 43, 1497-1508 (2012).
4. W. Wu, S.Y. Lee, A.M. Paradowska, Y.F. Gao, P.K. Liaw, "Twinning-detwinning behavior during fatigue-crack propagation in a wrought magnesium alloy AZ31B." Mater. Sci. Eng. A, 556, 278-286 (2012).
5. L.L. Zheng, Y.F. Gao, Y.D. Wang, A.D. Stoica, X.L. Wang, "Grain orientation dependence of lattice strains and intergranular damage rates in polycrystals under cyclic loading." Scripta Mate. In press.

Bio

Dr. Gao is an Associate Professor of the Department of Materials Science and Engineering, University of Tennessee, and a Joint Faculty in Materials Science and Technology Division, Oak Ridge National Laboratory. His research group focuses on modeling and simulation of plasticity at small length scales, thin-film growth, contact and friction, and constitutive behavior of amorphous alloys, among many others. Most of these projects are motivated from his colleagues’ experiments at UT and ORNL. Dr. Gao holds a B.S. in Engineering Mechanics and a B.S. in Computer Science from Tsinghua University in 1999, and a Ph.D. in Mechanical and Aerospace Engineering from Princeton University in 2003. His dissertation is on theoretical study and phase-field simulations of self-assembly in thin films and surfaces. Prior to joining UT/ORNL in August 2005, he worked as a post-doctoral research associate in solid mechanics group, Brown University, on surface plasticity and adhesion, microstructural evolution, and structural integrity. He is author or co-author of 58 research papers in mechanics of materials, and has made over 60 presentations (~30 invited) at national/international conferences and academic institutions.