Session D4: Materials Strength III: High-Rate Material Strength

Modelling of shear band interaction in 1D torsion

When two shear bands are being formed at a close distance from each other, they interact, and further development of one of them may be quenched down. As a result, there should be a minimum distance between shear bands. In the literature there are at least three analytical models for this minimum distance, but generally, prediction of these models do not agree with each other or with test results. Recently we developed a 1D numerical scheme to predict the formation of shear bands in a torsion test of a thin walled pipe. We validated our code by reproducing results from the pioneering experiments of Marchant and Duffy, and then used it to investigate the mechanics of shear localization and shear band formation. We describe our shear band code in a separate publication, and here we use it as a tool to investigate the interaction between two neighbouring shear bands during the process of their formation. We trigger the formation of the shear bands by specifying two perturbations on the initial strength. We vary the perturbations in terms of their amplitude and/or their width. Usually, the stronger perturbation triggers a faster developing shear band, which prevails and quenches the development of the other shear band. We change the distance between the perturbations and find that up to a certain distance one of the shear bands becomes fully developed, and the other stays only partially developed. Beyond this distance the two shear bands are both fully developed.   

Study of the strength of molybdenum under high pressure using electromagnetically applied compression-shear ramp loading

MAPS (Magnetically Applied Pressure Shear) is a new technique that has the potential to study material strength under mega-bar pressures. By applying a mixed-mode pressure-shear loading and measuring the resultant material responses, the technique provides explicit and direct information on material strength under high pressure. In order to apply sufficient shear traction to the test sample, the driver must have substantial strength. Molybdenum was selected for this reason along with its good electrical conductivity. In this work, the mechanical behavior of molybdenum under MAPS loading was studied. To understand the experimental data, a viscoplasticity model with tension-compression asymmetry was also developed. Through a combination of experimental characterization, model development, and numerical simulation, many unique insights were gained on the inelastic behavior of molybdenum such as the effects of strength on the interplay between longitudinal and shear stresses, potential interaction between the magnetic field and molybdenum strength, and the possible tension-compression asymmetry of the inelastic material response.   

Properties and behavior of diamond ablators

Diamond is an attractive ablator for laser loading experiments as it is efficient in converting laser energy to pressure, it transmits multi-kV x-rays such as are used for in-situ diffraction measurements, and it is readily available as single crystals, which do not produce diffraction rings that could obscure signals from a polycrystalline sample. However, radiation hydrodynamics simulations with standard models do not match the detailed velocity histories in ramp-loading experiments. Experimental measurements at the Omega laser showed that the (110) orientation exhibits much less elastic relaxation following the initial yield than did (100). Stress-density relations deduced from these experiments were consistent with the results obtained previously on thinner samples by Bradley et al (PRL 102, 075503, 2009), indicating that time-dependence in plastic flow had little effect on these time scales. The effect of dissipation, ignored in the characteristics analysis of ramp experiments, was assessed by analyzing simulated data, and was found to be negligible for diamond. Significant differences were found between equations of state in the several-megabar pressure regime, requiring quite different strength models to reproduce the stress-density relation.   

The effect of temperature and microstructure on dynamic yield behavior of fcc aluminum and bcc tantalum

This talk will compare the dynamic strength of fcc aluminum and bcc tantalum at a range of temperatures under extreme loading conditions to investigate the influence of temperature on the early stages of yielding and plastic flow in materials of distinct crystal structure. Laser-driven shock experiments have been performed on aluminum and tantalum foils at initial temperatures extending from approximately 120 K - 800 K at strain rates reaching 10$^{7}$ s$^{-1}$. In the case of aluminum, time-resolved velocimetry measurements reveal an anomalous, marked increase in yield strength with initial temperature, whereas the yield strength of tantalum is seen to remain nearly constant over the same temperature range. These results are consistent with prior work at lower strain rates, and discussion will explore various deformation mechanisms and how they interact and compete to determine yield behavior.   

Elastic precursor shock waves in tantalum at very high strain rates

We have obtained data from micron-thick tantalum films using our ultrafast laser shock platform. By measuring free surface velocity time histories at breakout, and shock wave arrival times at different film thicknesses, we have been able to estimate the dependence of particle and shock velocities on propagation distances and strain rates. We will show how elastic precursor shock waves depend on strain rate in the regime up to and above 10$^{9}$ s$^{-1}$. We find that while elastic amplitudes are very large at very early times decay occurs rapidly as propagation distance increases. Finally we will consider the prospects for using these data to obtain the dynamic strength of tantalum at these very high strain rates. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344 with Laboratory directed Research and Development funding (12ERD042).   

Structural Stability of Zirconium at High P-T Conditions investigated by combined x-ray diffraction and ultrasonic measurements

Advances in high pressure experimental techniques are continuously providing opportunities to study and gain deeper insight into behavior of materials subjected to extreme P---T conditions. Over the last 60 years, many of the emerging high pressure experimental techniques are coupled with xray probes, and in particular with large scale x-ray sources at multiuser national facilities. One of the newer capabilities at beamline 16BM---B of High Pressure Collaborative Access Team (HPCAT) at Advanced Photon Source allows for simultaneous xray diffraction, xray radiography, and ultrasonic velocity using a large volume Paris-Edinburgh press. Structural stability of group IV---B (Ti, Zr, and Hf) transition metals at high P---T has been widely investigated. As a result of these studies, it is known that Zr, in particular, undergoes a structural phase change from the ductile $\alpha$ (HCP) phase to the more brittle $\omega$ (open hexagonal) phase. Using combined ultrasonic and x---ray measurements, we are able to better define onset of structural transition in Zr. In addition, these better establish correlation between elastic moduli and resulting structural stability. Some aspects of the HPCAT capability will be discussed, as well as measurements recently performed on zirconium (Zr).