Title: MULTI-SCALE COMPUTATIONAL SIMULATION OF FATIGUE CRACKING PROCESSES IN ALUMINUM ALLOYS

Author: ANTHONY R. INGRAFFEA, Dwight C. Baum Professor of Engineering - Cornell University.

ANTHONY R. INGRAFFEA Photo Dr. Ingraffea spent two years as a structural engineer with the Grumman Aerospace Corporation and two years as a county engineer with the Peace Corps in Venezuela before he began doctoral studies. He has taught structural mechanics, finite element methods,and fracture mechanics at Cornell since 1977.

Dr. Ingraffea's research concentrates on computer simulation and physical testing of complex fracturing processes. He and his students performed pioneering research in the use of interactive computer graphics in computational mechanics. He has authored with his students over 200 papers in these areas. He has been a principal investigator on over $35M in R&D projects from the NSF, NASA Langley, Nichols Research, NASA Glenn, AFOSR, FAA, Kodak, U. S. Army Engineer Waterways Experiment Station, U.S. Dept. of Transportation, IBM, Schlumberger, Digital Equipment Corporation, the Gas Research Institute, Sandia National Laboratories, the Association of Iron and Steel Engineers, General Dynamics, Boeing, Caterpillar Tractor, and Northrop Grumman Aerospace.

Professor Ingraffea was a member of the first group of Presidential Young Investigators named by the National Science Foundation in 1984. For his research achievements he has won the International Association for Computer Methods and Advances in Geomechanics "1994 Significant Paper Award" for one of five most significant papers in the category of Computational/Analytical Applications in the past 20 years, and he has twice won the National Research Council/U.S. National Committee for Rock Mechanics Award for Research in Rock Mechanics (1978, 1991). His group won a NASA Group Achievement Award in 1996, and a NASA Aviation Safety Turning Goals into Reality Award in 1999 for its work on the aging aircraft problem. He became a Fellow of the American Society of Civil Engineers in 1991. 

Professor Ingraffea has received numerous awards his outstanding teaching at Cornell  Most recently, he recieved the first Society of Women Engineer's Professor of the Year Award in 1997, the 2001 Daniel Luzar '29 Excellence in Teaching Award from the College of Engineering, and, in 2005, was named Weiss Presidential Teaching Fellow at Cornell University.  He has been a leader in the use of workstations and information technology in engineering education, with grants from the NSF, U.S. Department of Education, Digital Equipment Corporation, Sun Microsystems, and Hewlett-Packard in these areas. He organized and was the first Director of the NSF-supported, $15M Synthesis National Engineering Education Coalition, a team of eight diverse engineering colleges. Synthesis developed, implemented, and assessed innovative programs and technologies to improve the quality of undergraduate engineering education and to attract and graduate larger numbers of women and under-represented minority engineers. He is Cornell Co-PI on a NASA/NYS/AT&T sponsored project to develop an Advanced Interactive Discovery Environment for collaborative distance design in engineering education, teaming with faculty from aerospace, mechanics, and civil engineering from Cornell and Syracuse universities.

He was named Co-Editor-in-Chief of Engineering Fracture Mechanics in 2005, and received the ASTM Irwin Award for meritorious contributions to the practice of fracture mechanics in 2006.

Abstract:

We are developing physics-based models for simulating incubation, nucleation and propagation of fatigue cracks in aluminum alloys. Our models are part of a DARPA-funded, broad-team project on structural integrity prognosis. The salient features of our approach are:

  1. The use of statistically representative, realistic microstructures as a starting point for our simulations. Using unique microstructure builder tools, we assemble three-dimensional digital material representations from actual microstructural observations. These contain realistic morphologies, textures, particle distributions, etc. Constituents are assigned statistically representative distributions of properties such as yield strengths and toughnesses.
  2. The use of polycrystal plasticity models to compute accurately stress and strain fields in polycrystals using the finite element method. In polycrystalline metals, the grain structure and phenomena occurring on the grain scale, such as interactions between grains and particles and crystallographic slip, strongly influence the fatigue behavior of the materials. Statistically realistic 3D microstructures are directly simulated in order to investigate the effect of elasto-plastic response within the microstructure on the fatigue behavior.
  3. The use of an explicit geometric representational approach in a multi-scale methodology. At each length scale, fatigue crack precursors, such as grain boundary or particle decohesion, are represented geometrically in the finite element model, and allowed to evolve through changes in the underlying geometric and mesh models. The need for concomitant quantitative experimental data on microstructural damage processes (particle fracture, debonding, etc) becomes apparent.

We will report on progress in development, verification, and validation of our probabilistic simulation models, and show example simulations specific to aluminum alloy 7075-T651. This microstructure has two distinct micro-phases: grains and constituent particles. Fatigue crack incubation is a stochastic process, whereby through-cracks develop in some of the iron-bearing (Al7Cu2Fe) particles. We will describe a fracture mechanics-based, probabilistic criterion for fatigue crack incubation, where the probability of particle cracking depends on the particle geometry, adjacent grain texture, and stress fields inside and surrounding the particle. In this study, the incubation criterion is being validated for nine microstructure samples, each with a single incubated crack, observed on a notch surface of a double edge-notched (DEN) specimen. For each sample, the experimentally recorded grain textures, microstructure geometries, and strain fields at the time of incubation are being replicated in a finite element model. A highly detailed, parallel finite element analysis is then run on the model to determine the stress fields inside the particle in its uncracked state. The incubation criterion is validated by showing that it accurately predicts particle cracking for all nine combinations of geometries, textures, and resulting stress fields.