Open-cell metal foams are hierarchical structural-material systems that have applications as light-weight impact absorbers, noise insulators, biomedical implants, and heat sinks, to name a few. This talk will highlight two recent studies in which we investigate the effect of grain structure on compressive mechanical response of open-cell metal foam using both numerical modeling and in-situ grain-scale characterization at the Advanced Photon Source (APS). In the numerical study, multiple polycrystalline instantiations (overlaid on a foam volume derived from X-ray tomography) are simulated using crystal-plasticity finite-element modeling to quantify grain-size effect on the global compressive response of investment-cast aluminum foam. The high-fidelity numerical framework captures the deformation mechanisms across multiple length scales and is able to predict the inhomogeneous grain-to-continuum compressive response in the foams. Also, by incorporating grain-boundary strengthening and free-surface softening mechanisms, the framework accounts simultaneously for the Hall-Petch effect in polycrystalline alloys and the effect of unconstrained slip-based deformation at the strut free surfaces. Results from the numerical simulations provide new insights into the mechanical behavior of open-cell metal foams and are used to enhance the Gibson-Ashby model for predicting plastic collapse strength. In a parallel effort, 3D grain and precipitate structures are characterized for an open-cell aluminum foam using synchrotron characterization techniques. X-ray tomography and high-energy X-ray diffraction microscopy (HEDM) data were collected in-situ at interrupted loading intervals during compression. A novel scanning strategy developed at the APS 1-ID beamline enabled complete characterization of a 6%-dense foam sample that was four times larger than the X-ray beam width. A data-analysis procedure was developed to track grains through large strut displacement and deformation. The 3D precipitate maps were used to correlate ligament failure to precipitate distributions. The methods and procedures developed for both studies can be applied to other low-density structures (e.g., AM lattices) and enable new possibilities for investigating the micromechanical failure mechanisms of open-cell metal foams and lattices.
Dr. Ashley Spear is an Associate Professor of Mechanical Engineering and an Adjunct Professor of Materials Science and Engineering at the University of Utah. She directs the Multiscale Mechanics & Materials Laboratory, where her group specializes in integrating experiments, physics-based modeling, and data science to examine three-dimensional deformation, fatigue, and fracture in structural materials. Spear received her B.S. in Architectural Engineering from the University of Wyoming and her Ph.D. in Civil Engineering from Cornell University. She has received numerous teaching awards, the Young Investigator Award from the Air Force Office of Scientific Research, the TMS Early Career Faculty Fellow Award, the ASTM International Additive Manufacturing Young Professional Award, and the National Science Foundation CAREER award.