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Probabilistic Simulation of Fatigue Processes in a High-Strength Aluminum Alloy

John A. Blume Distinguished Lecture
Anthony R. Ingraffea
Wednesday, January 19, 2011 - 4:15 pm
Huang Center, Room 300 - Mackenzie
Most of the fatigue life of high-strength aluminum components in aircraft and some wind turbine blades is spent in the microstructurally small length-scale. Therein cracks are small relative to the grain size (microns) of the material, and their evolution is controlled by the geometry and property distributions in the microstructure. Although the mechanics, and fracture mechanics, at this scale are similar to those describing the life of, say, a steel frame building, geometry and property distributions are much more complex and stochastic.
 
Microstructurally small fatigue crack formation includes stages of incubation, nucleation, and propagation. In AA 7075-T651, the fracture of Al7Cu2Fe constituent particles is the major incubation source. It has been observed that only a small percentage of these particles crack in a highly stressed volume. The first part of the work addresses the identification of the particles prone to cracking and the prediction of particle cracking frequency. For the incubation stage, good agreement is found between the predicted frequency of cracking and preliminary validation experiments.
 
An estimate of particle cracking frequency is important for simulating the next stage: nucleation of cracks into the grain containing a cracked particle. It is hypothesized that nucleation can be predicted by computing a non-local nucleation metric near the crack front. The hypothesis is tested by employing a combination of experimentation and finite element modeling in which various slip-based and energy-based nucleation metrics are tested for validity. For the nucleation stage, it is found that a continuum crystal plasticity model and a non-local nucleation metric can be used to predict the nucleation event.
 
 
Professor Ingraffea’s research concentrates on computer simulation and physical testing of complex fracturing processes.  He and his students have performed pioneering research in interactive computer graphics and realistic representation methods in computational fracture mechanics.  He has authored with his students and research associates over 250 papers, and he is Director of the Cornell Fracture Group (www.cfg.cornell.edu). Since 1977, he led research projects from the NSF, NASA, Nichols Research, AFOSR, FAA, Kodak, U. S. Army Engineer Waterways Experiment Station, U.S. Dept. of Transportation, IBM, Schlumberger, Gas Technology Institute, Sandia National Laboratories, the Association of Iron and Steel Engineers, General Dynamics, Boeing, Caterpillar Tractor, DARPA, and Northrop Grumman.  For his research achievements in hydraulic fracturing he has won the International Association for Computer Methods and Advances in Geomechanics "1994 Significant Paper Award", 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. In 2006, he won ASTM’s George Irwin Award for outstanding research in fracture mechanics, and in 2009 was named a Fellow of the International Congress on Fracture.
 
Professor Ingraffea has received numerous awards for his outstanding teaching at Cornell, and was named a Weiss Presidential Teaching Fellow at Cornell in 2005. He was PI on the NSF sponsored Synthesis Education Coalition and Co-PI on a grant to create TechCity, a science museum interactive display on the process of engineering.Ingraffea has implemented engineering mini-courses at the Manhattan Center for Science and Mathematics and Project High Jump, minority serving middle/high schools in New York City and Chicago, respectively. In 2008, he won the Richard J. Almeida Award given each year to an individual whose dedication and contribution to High Jump have been extraordinary.