This talk describes a new generation of mechanical testing methods that uses high energy x-ray diffraction to probe the elastic-plastic deformation behavior of every crystal within a deforming metallic aggregate. Traditionally, models for inelastic deformation, fatigue and fracture of engineering alloys have developed hand-in-hand with macroscale mechanical testing and microstructural characterization experiments. Advances in computational capabilities have enabled “larger” simulations capturing behaviors on smaller and smaller size scales. The hope is that the designs of more efficient materials and components can be realized by using models formulated on scales “closer to the physics” of deformation and damage processes. Many forms of image based microstructural characterization can now be extended literally to the atomic scale. However, the creation of high fidelity microscale mechanical tests, which can be used to motivate and calibrate micromechanical models, has been more of a challenge; models formulated at the crystal scale and below are often validated with macroscale stresses and strains. Nano indenter and micromachined pillar experiments are invaluable for extracting material response and mechanical properties during simple loading of surface crystals but extracting the response of a crystal deeply embedded within deforming sample has been an elusive goal. This talk describes the use of high energy, highly penetrating synchrotron x-rays and in situ loading and heating stages to extract the micromechanical response of metallic polycrystals during loading using diffraction. In some experiments, the full stress tensor associated with each individual crystal can be extracted along with subgrain-scale information about plasticity-induced lattice orientation gradients. Much of the work described was developed and conducted at the Cornell High Energy Synchrotron Source (CHESS) but these high energy x-ray diffraction (HEXD) methods have been evolving over the past decade at all 5 of the high energy light sources around the world including the Advanced Photon Source at Argonne National Labs. The mechanical testing / model development interface is highlighted in the talk along with a description of the recent upgrade at CHESS and the creation of the AFRL-sponsored Materials Solution Network at CHESS (MSN-C).
Matthew P. Miller is a Professor in the Sibley School of Mechanical and Aerospace Engineering at Cornell University. He is the Associate Director of the Cornell High Energy Synchrotron Source (CHESS) and the director of the ONR-sponsored Insitm (Integrated Simulation and x-ray Interrogation Tools and training for micromechanics) center at CHESS. Miller’s research focuses on the microscale experimental characterization of the mechanical behavior of structural materials to build new modeling capabilities for processes such as plasticity, fracture and fatigue. Over the past decade, this work has involved the development and use of high energy synchrotron x-ray diffraction (HEXD) experiments to probe the microstructural and micromechanical evolution of structural materials in real time. Providing enhanced support for HEXD experiments and the associated processing and performance simulations on structural materials is the mission of InSitm.