A clear understanding of failure of metallic materials under extreme conditions of pressure and strain rates, such as those induced by shock loading, is extremely important for Stockpile Stewardship. In particular, spall failure induced by the superposition of release waves of shocks reflected from free surfaces has been studied intensively, as it is a complicated physical process where microstructure can play an important role. In particular, recent work by the PI as well as national laboratory researchers has elucidated the role that intrinsic defects, e.g., grain boundaries, can play as damage localization sites in metallic materials. The variability of the local strength and spatial distribution of these defects, in turn, plays a key role on the statistical nature of the spall damage processes. Previous work by the PI on the role of crystallography and 3-D geometry of these defects on their tendency for spall damage localization has shown that large mismatches of properties both along the shock direction and perpendicular to the grain boundary have a strong correlation with damage localization at a particular site. Another important result is from scientists at Los Alamos National Laboratory showing that the shape of the shock pulse makes a significant difference on spall damage. A key question that arises from these facts is the following: is damage localization at particular microstructural sites due to early nucleation of damage, e.g., due to lower local strengths, that lead to more time for damage evolution, due to faster growth kinetics once damage has nucleated or both? The work proposed here is centered on answering this question via carefully designed experiments coupled with modeling. Data on threshold pressures for damage nucleation and on kinetics of damage growth will be gathered by performing experiments on carefully prepared samples that reflect the main characteristics of a microstructural weak link in chosen metallic systems, particularly in terms of mismatch of across grain boundaries induced by crystallography along with material anisotropy. Experiments will be performed in samples with identical characteristics, in this case bicrystals grown from the melt with controlled crystal orientations in each grain. Samples will be tested using flyer plate impacts driven by a light gas gun in two complementary ways: identical samples at increasing pressures and constant pulse duration and tests under at constant pressures and increasing pulse durations. The goal of the first battery of experiments is to determine a damage stress threshold, while the second set will allow quantifying the kinetics of damage growth. Careful characterization experiments will be conducted in recovered samples: density and resistivity measurements will be performed before and after testing to quantify defect densities as functions of shock pressure and pulse duration, and correlated to direct damage characterization using serial sectioning via mechanical polishing and focused ion beam techniques, combined with electron backscattered diffraction (EBSD) and Transmission Electron Microscopy (TEM) to characterize the damage. The results will allow quantifying the range of parameters that favors damage nucleation and growth. Then, 3-D models will be created to evaluate the relative contributions of damage nucleation versus growth kinetics on the damage evolution. The information obtained from the experiment and modeling will allow a more complete understanding of the role of local microstructure on the kinetics of nucleation and growth of spall damage, which in turn will be of immediate direct benefit to the formulation of macroscopic, engineering models of damage evolution for stockpile stewardship.
|Effective start/end date||4/1/13 → 12/31/17|
- DOE: National Nuclear Security Administration (NNSA): $388,800.00