Understanding the evolution of damage and deformation due to spall at grain bulks can provide a basis for connecting micro- to macroscale failure behavior in metals undergoing shocks. Bicrystal samples were shock loaded using flyer-plate impacts with pressures \~ 3 to 5 GPa. Pulse duration as well as crystal orientation along the shock direction were varied for a fixed boundary misorientation to determine thresholds for void nucleation and coalescence as functions of these parameters. Samples were soft recovered and cross-sectioned to perform damage characterization using electron backscattering diffraction (EBSD) and Scanning Electron Microscopy (SEM) to gather information on damage characteristics at and around the GBs, with emphasis on void interactions and lattice rotation around boundary and bulk voids. The EBSD and SEM experimental data collected were compared to results from simulations of individual and interacting voids performed using finite element crystal plasticity models, to understand local plasticity effects. In particular, simulations were performed using two approaches. The first one corresponds to the use a crystal plasticity combined with a Gurson-Tvergaard-Needleman (GTN) models, using a flow rule based on normality as well as a continuous damage mechanics framework to couple the models. This was used to model the overall samples and correlate the presence of damage measured experimentally in the recovered specimens to predictions of the distribution of void fraction in the simulated specimens. The second approach involves the simulation of the growth of a single transgranular void, either spherical and octahedral voids (the latter observed experimentally in spalled samples), using crystal plasticity models to obtain the distribution of lattice rotation around voids as a function of time and position, using two different boundary conditions for he driving force, i.e., either imposed tractions or displacements. The resulting lattice rotation fields predicted by this approach were compared to the experimental measurements obtained via EBSD. Results indicate that for models based on crystal plasticity, material anisotropy was not enough to insure void growth in the well-known octahedral shape, regardless of the initial geometry of the “seed” void, while experiments show that voids tend to remain octahedral under the conditions used to run the experiments. Additional characterization efforts using conventional (2-D) and advanced (3-D) EBSD at selected sites, using focused ion beam for serial sectioning, indicate the presence of more complex lattice rotation gradients around single voids than previously reported. Furthermore, plastic strain and orientation gradients predicted around spherical and octahedral voids show differences with respect to theory and with experiments.


Experimental Measurements and Modeling of Lattice Rotation Around Inter- and Transgranular Spall Voids in Shocked Copper Samples

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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