By combining very large-scale molecular dynamics simulations with time-resolved data from laser experiments of shock wave propagation through specific metals, scientists at the Lawrence Livermore National Laboratory are now able to better understand the evolution of high-strain-rate plasticity.
Plastic deformation of metals results from the motion of a high density of dislocation lines. A strong shock produces an unusual number of dislocations within a metal's crystalline lattice, which changes the metal's mechanical properties such as strength, ductility and resistance to fracture and cracking.
In a paper published in the Sept. 17 edition of the journal Nature Materials, Livermore researchers, in conjunction with scientists from the University of Oxford, have compared and validated strong shock molecular dynamics simulations to dynamic experimental data in metals.
"We calculated the time needed for the metal to generate defects and relax in a strong shock wave," said Eduardo Bringa, LLNL's lead author of the paper. "We came to understand this time interval in terms of the time needed for line defects (dislocations) to move far enough to relax the strain. It was known that the more dislocations that are produced and the more they move, the more the strain is relaxed."
However, the researchers had a surprise: If the dislocations form too rapidly, they become entangled before they can move far enough to relax the strain. In a ramped pressure wave (rather than an abrupt shock), fewer dislocations form, but they are more effective at relieving the strain because they are freer to move.
"Comprehending this kinetic time scale has unified our understanding of how the tremendous transient stresses in shock waves are compatible with our tried and true understanding of material strength in everyday conditions," said Robert Rudd, an LLNL co-author of the paper.
"This provides a powerful tool to explore new regimes in the emerging field of materials science at extreme conditions, such as those expected in experiments planned for NIF," said Bruce Remington, who leads a group developing such experiments for the National Ignition Facility.
A team including several LLNL researchers previously used time-resolved X-ray diffraction to measure the microscopic lattice response and relaxation behind the shock front in a single crystal piece of copper. The shocked copper relaxed in less than one nanosecond and the current simulations reproduce this timescale. Such large-scale simulations were possible, for the first time, due to the extensive computational power of LLNL supercomputers.
Shock compression of condensed matter occurs in a variety of situations including high-speed automobile and aircraft collisions, explosive welding, armor penetration, meteor impacts, interstellar dust dynamics, and inertial confinement fusion. A detailed understanding of the three-dimensional lattice relaxation process during shock compression beyond the elastic limit had not been achieved previously.
"These results will help us understand what to expect during the extreme material deformation experiments, and better design those experiments," Rudd said. "High rate material deformation is important in explosive fragmentation, penetration, collision, and so on, from the prosaic automobile crash, to the kind of penetration scenarios of interest to homeland security.
The Laboratory's defense-related mission requires an understanding of how metals respond to sudden shock waves and subsequent high-strain-rate deformations. To assess materials properties and performance under extreme deformation conditions, researchers work to understand the fundamental origin of deformation and strength and how the resistance against plastic deformation arises from the collective dynamics of lattice dislocations.
"In our planned materials experiments, we intend to deform a solid-state metal at extraordinary pressures and strain rates," said Remington. "Eduardo has shown us a very promising way for interpreting the results.
Concludes Bringa: "The experiments and simulation combination makes a powerful pair for exploring uncharted even unimagined regimes of material dynamics."
Anne Stark | EurekAlert!
Spider silk key to new bone-fixing composite
20.04.2018 | University of Connecticut
Diamond-like carbon is formed differently to what was believed -- machine learning enables development of new model
19.04.2018 | Aalto University
University of Connecticut researchers have created a biodegradable composite made of silk fibers that can be used to repair broken load-bearing bones without the complications sometimes presented by other materials.
Repairing major load-bearing bones such as those in the leg can be a long and uncomfortable process.
Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.
Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...
Novel highly efficient and brilliant gamma-ray source: Based on model calculations, physicists of the Max PIanck Institute for Nuclear Physics in Heidelberg propose a novel method for an efficient high-brilliance gamma-ray source. A giant collimated gamma-ray pulse is generated from the interaction of a dense ultra-relativistic electron beam with a thin solid conductor. Energetic gamma-rays are copiously produced as the electron beam splits into filaments while propagating across the conductor. The resulting gamma-ray energy and flux enable novel experiments in nuclear and fundamental physics.
The typical wavelength of light interacting with an object of the microcosm scales with the size of this object. For atoms, this ranges from visible light to...
Stable joint cartilage can be produced from adult stem cells originating from bone marrow. This is made possible by inducing specific molecular processes occurring during embryonic cartilage formation, as researchers from the University and University Hospital of Basel report in the scientific journal PNAS.
Certain mesenchymal stem/stromal cells from the bone marrow of adults are considered extremely promising for skeletal tissue regeneration. These adult stem...
In the fight against cancer, scientists are developing new drugs to hit tumor cells at so far unused weak points. Such a “sore spot” is the protein complex...
13.04.2018 | Event News
12.04.2018 | Event News
09.04.2018 | Event News
23.04.2018 | Earth Sciences
23.04.2018 | Trade Fair News
23.04.2018 | Information Technology