Materials are often intentionally or unintentionally subjected to loads, or stresses, during service. There is evidence that these stresses may not be uniformly distributed in materials due the presence of clustering of atomic defects in the material. These defects are responsible for facilitating or "carrying" the normal permanent or plastic deformation in materials that results from the external stresses. However, this redistribution of the stress can lead to long range internal stresses (LRIS) in materials. It is critical to characterize and understand the details and origin of long range internal stresses that lead to backstresses. This will be researched by the Principal Investigator and his students using, new cutting-edge, x-ray microbeam characterization techniques at the Advanced Photon Source (synchrotron) at Argonne National Laboratory. Learning the details of the origin and the magnitude of the backstresses can lead to a new understanding of important performance aspects of materials. This particularly includes fatigue and metal forming. Fatigue is one of the most common catastrophic-failure mechanisms in structural materials. Better understanding of fatigue would lead to enhanced structural-materials performance. Understanding backstresses in metal forming operations could lead to enormous savings in the metal-forming manufacturing sector. The PI will also work with outreach programs to high schools in south-central Los Angeles.
Backstresses or long range internal stresses (LRIS) have been, in the past, widely suggested to exist in plastically deformed crystalline materials. Elevated stresses can be present in regions of elevated dislocation density or dislocation heterogeneities in the deformed microstructures. The existence of long range internal stress is especially important for the understanding of cyclic deformation but also monotonic deformation. X-ray microbeam diffraction experiments will be performed by the principal investigator and students using a synchrotron that are able to determine the strain within dislocation cell walls and interiors for the first time. This work includes an attempt to measure the full elastic strain tensor in the cell interiors and cell walls. This will also be the first experiment ever performed of this type. The PI will also perform in-situ microbeam experiments and measure the lattice parameter while the specimen is under external load. The PI and his students will examine severely plastically deformed (SPD) materials, where relatively high LRIS has been suggested to be present. These experiments will be performed by the PI and graduate students at the Advanced Photon Source at Argonne National Laboratory. Our understanding of plastic deformation, in general, is expected to be enhanced. This project will understand backstresses that are important to fatigue, the cause for most structural material failures, and also spring back in metal forming. Understanding LRIS in metal forming operations could lead to enormous savings in the manufacturing sector. The PI will also work with outreach programs to high schools in south-central Los Angeles.
