13 research outputs found
Entropy-Dominated Dissipation in Sapphire Shock-Compressed up to 400 GPa (4 Mbar)
Sapphire (single-crystal Al2O3) is a representative Earth material and is
used as a window and/or anvil in shock experiments. Pressure, for example, at
the core-mantle boundary is about 130 gigapascals (GPa). Defects induced by
100-GPa shock waves cause sapphire to become opaque, which precludes measuring
temperature with thermal radiance. We have measured wave profiles of sapphire
crystals with several crystallographic orientations at shock pressures of 16,
23, and 86 GPa. At 23 GPa plastic-shock rise times are generally quite long
(~100 ns) and their values depend sensitively on the direction of shock
propagation in the crystal lattice. The long rise times are probably caused by
the high strength of inter-atomic interactions in the ordered three-dimensional
sapphire lattice. Our wave profiles and recent theoretical and laser-driven
experimental results imply that sapphire disorders without significant shock
heating up to about 400 GPa, above which Al2O3 is amorphous and must heat. This
picture suggests that the characteristic shape of shock compression curves of
many Earth materials at 100 GPa pressures is caused by a combination of entropy
and temperature.Comment: 12 pages, 4 figure
A simple model for dynamic shear failure of stainless steel
We propose a simple model to reproduce ductile failure by
shear localization in simulations of perforations tests. The model
incorporates a positive feedback process of shear strain localization which
results in a catastrophic decrease of flow stress. We use the model for
perforation tests with 304L stainless steel. It succeeds in reproducing
perforation thresholds as well as qualitative features of the perforation
process, including shear band formation in some of the projectiles
Measuring dynamic strength at low plastic strains using a hat-shaped specimen
In the standard derivation of the stress-strain curve from a Split Hopkinson Pressure Bar (SHPB) test, the initial region of the stress-strain curve, at low strains, does not reflect the real strength of the material. The measurement in this initial region is affected by reverberations in the specimen, and the standard assumption of stress equilibrium does not hold. For typical specimen dimensions and striker velocities, our SS304L specimens reach strains of 20–50%, but the equilibrium required for the analysis is achieved only above strains of 5–10%. Therefore, in calibrating a constitutive model, e.g. the Johnson-Cook (JC) model, the free parameter of the model that expresses the material's initial strength cannot be fixed correctly from the experimental data. While conducting 2D simulations of SHPB tests with hat-shaped specimens, we have found that the strain-gauge signals are sensitive to the behavior at low plastic strains. We have used this information as a complementary test for the calibration of a JC model at the low strain region. Using 2D simulations, we demonstrate the particular stress fields in the deforming hat-specimen as well. These simulations prove to be a powerful tool in the calibration procedure
More on the penetration of yawed rods
One of the most complex processes, in the field of
terminal ballistics, is that of yawed impact of long rods. In spite of many
experimental observations, and some analytical modeling, a clear picture of
this issue is still lacking. In order to gain some insight into the
operating mechanisms, we developed a simple engineering model which
considers the yawed rod as a series of small disks. We then define the
effective length and diameter of the rod by considering those disks which
are going to hit the initial crater which is opened by the impact. We also
performed a series of 3D numerical simulations with various L/D tungsten
alloy rods impacting a steel target, at yaws in the full range of 0-.
We analyzed the results of these simulations in terms of the normalized
penetration (P/D), where D is the rod diameter, and looked for systematic
trends in the results for the various rods. The agreement between our model
predictions and both experimental data and simulation results is quite good.
Based on this agreement we can highlight some new features of the
penetration process of yawed rods
Eliminating the thermal softening of dynamically loaded specimens in the Kolsky bar system by multi-step loading
We performed series of multi-step loading tests in our Kolsky bar system, and demonstrated that the thermal softening in strong aluminum alloys can be eliminated by multi-step loading. We showed that there is a significant difference in their stress-strain curves, compared with the result of a single shot test, due to adiabatic heating. The tests were carried out using our interferometry-based system, where the bar velocities are measured directly rather than the strains. The optical technique has several advantages over traditional strain gauge measurements; it is non-intervening, highly repeatable, and more accurate at low strains, thus allowing good estimation of the dynamic yield point in these experiments
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Hepatitis C virus infects and perturbs liver stem cells
Hepatitis C virus (HCV) is the leading cause of death from liver disease. How HCV infection causes lasting liver damage and increases cancer risk remains unclear. Here, we identify bipotent liver stem cells as novel targets for HCV infection, and their erroneous differentiation as the potential cause of impaired liver regeneration and cancer development. We show 3D organoids generated from liver stem cells from actively HCV-infected individuals carry replicating virus and maintain low-grade infection over months. Organoids can be infected with a primary HCV isolate. Virus-inclusive single-cell RNA sequencing uncovered transcriptional reprogramming in HCV+ cells supporting hepatocytic differentiation, cancer stem cell development, and viral replication while stem cell proliferation and interferon signaling are disrupted. Our data add a new pathogenesis mechanism-infection of liver stem cells-to the biology of HCV infection that may explain progressive liver damage and enhanced cancer risk through an altered stem cell state.ImportanceThe hepatitis C virus (HCV) causes liver disease, affecting millions. Even though we have effective antivirals that cure HCV, they cannot stop terminal liver disease. We used an adult stem cell-derived liver organoid system to understand how HCV infection leads to the progression of terminal liver disease. Here, we show that HCV maintains low-grade infections in liver organoids for the first time. HCV infection in liver organoids leads to transcriptional reprogramming causing cancer cell development and altered immune response. Our finding shows how HCV infection in liver organoids mimics HCV infection and patient pathogenesis. These results reveal that HCV infection in liver organoids contributes to liver disease progression