New research exposes strength beryllium at extreme conditions

New research exposes strength of beryllium at extreme conditions

7:45 AM, 12th August 2015
New research exposes strength of beryllium at extreme conditions
Marc Henry de Frahan is the lead author of the paper, “Experimental and Numerical Investigations of Beryllium Strength Models Using the Rayleigh-Taylor Instability.”

LIVERMORE, US: Until recently, there were very little experimental data about the behaviour of beryllium (Be) at very high pressures and strain rates, with existing material models predicting very different behaviours in these regimes. In a successful example of international research collaboration, a team of scientists from Lawrence Livermore National Laboratory (LLNL) and the Russian Federal Nuclear Center-All-Russian Research Institute of Experimental Physics (RFNC-VNIIEF) changed this field of knowledge.

In a paper published on the cover of the Journal of Applied Physics, the team showed that at extreme conditions, beryllium has very little strength and most models over-predict its material strength.

“This finding has important implications for scientists working with technology where beryllium is subject to extreme pressures and strain-rates,” said Marc Henry de Frahan, lead author of the paper. Henry de Frahan began conducting this research as a summer student with LLNL’s NIF and Photon Science directorate and is now a graduate student at the University of Michigan.

“Since Be presents its own unique challenges and the Russians had an experimental capability and experience with this technique, we decided to form a collaboration with them in 2009,” said co-author Rob Cavallo, a physicist in LLNL’s  design physics Division.

The purpose of the experiments was to put Be into regions of stress and strain-rate that are difficult to access with focused experiments by using a technique originally developed at Los Alamos National Laboratory in the early 1970s. The technique has since been used extensively by RFNC-VNIIEF over the past few decades.

The technique involves setting off a piece of high explosives (HE) near the Be. On the side of the Be facing the HE, the team imposed a sinusoidal ripple pattern designed by co-author Jon Belof. When the expanding HE products load up against the target, the target accelerates. Since there is a low density gas pushing against a higher density metal, the interface is Rayleigh-Taylor unstable and the ripples grow in amplitude as the target accelerates.

If the target has no strength, the ripples will grow indefinitely and become turbulent at some point. However, since the Be does have strength, the ripple growth is limited by the strength of the material itself. The main diagnostic for the experiments is an x-ray image from the side of the target showing the height of the ripples at some time after the HE loading has occurred. The other diagnostic is velocimetry of the target showing its acceleration profile.

Using this technique allowed the team to reach pressures of about 50 GPa (500,000 atmospheres) and strain rates near 1,000,000/s (a rate of 1/s under tension means a piece of material would double its length in 1 second – 1,000,000/s indicates a million-fold increase).

“We wanted to determine how well these models would work for Be when the Be is loaded far away from the phase space where they were originally fit,” said Cavallo.

The end result was that only the new relaxation model, designed by co-author Olga Ignotova, came close to matching the data. The challenge for the models is that they are based on the assumption that the material response is largely a combination of the equation of state and plastic flow. However, Be is known to be susceptible to material failure and damage.

© Lawrence Livermore National Laboratory News



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