Session B1: ME.4 Strength I

Extracting Strength from Ramp-Release Experiments on Z

Releasing from a compressed state has long been recognized as a sensitive measure of a material's constitutive response. The initial elastic unloading provides insights which can be related to changes in shear stress or, in the context of classic plasticity, to the material's yield surface. Ramp compression and subsequent release experiments on Sandia's Z machine typically consist of a driving aluminum electrode pushing a sample material which is backed by a window. A particle velocity measurement of the sample/window interface provides a ramp-release profile. Under most circumstances, however, the impedance mismatch at this interface results in the measurement of a highly perturbed velocity, particularly at the late times of interest. Wave attenuation, the finite pressure range over which the material elastically unloads, and rate effects additionally complicate the interpretation of the experiment. In an effort to accurately analyze experiments of this type, each of these complications is addressed. The wave interactions are accounted for through the so-called transfer function methodology and involves a coupling of the experimental measurements with numerical simulations. Simulated window velocity measurements are combined with the corresponding \textit{in situ} simulations to define a mapping describing the wave interactions due to the presence of the window. Applying this mapping to the experimentally measured velocity results in an \textit{in situ} sample response which may then be used in a classic Lagrangian analysis from which the strength can be extracted via the self-consistent method. Corrections for attenuation, pressure averaging, and limitations of the analysis due to rate-effects are verified through the use of synthetic data. To date, results on the strength of aluminum to 1.2MBar, beryllium to 1 MBar, and tantalum to over 2MBar have been obtained through this methodology and will be presented.   

Bounds on the Rate Dependent Plastic Flow of Tantalum up to 75 GPa

We report improvements in a general thermodynamics-based velocimetry analysis method designed to extract strength and plastic-flow information from shock and ramp compression experiments. The method allows extraction of thermodynamic histories, including deviatoric stress and plastic strain, including nonsteady rate-dependent features. The improved method includes free-surface corrections for pullback waves, reduced noise sensitivity, and application to pressures of 75 GPa and higher. Specifically, we show results for shock waves in tantalum, including bounds on the plastic flow behavior at strain rates exceeding 1e7/s.The deviatoric stress appears to be almost entirely dependent on strain rate, with very little pressure dependence.The deviatoric stress in the post-shock plateau state appears to be very small at higher pressures, calling into question the value of considering strength as a steady- state pressure-dependent quantity.   

Shear Strength Development in Tantalum Alloys: Effects of Cold Work and Alloying

The response of tantalum and its alloys during shock loading conditions is controlled largely by the motion of dislocations present within the microstructure. This has been attributed to the high Peierls stress reducing the ability of these materials to accommodate strain by the generation of additional dislocation line length. This has manifested itself in the mechanical response as a clear reduction in shear strength behind the shock front, as dislocation motion is considered a stress relief mechanism. However, it has also been shown that this shear strength reduction can itself be reduced, either by prior cold work before shock loading, or via simple alloying such as an addition of 2.5wt{\%} tungsten. In this work, we explore these issues further by investigating the shear strength development in a cold rolled Ta2.5wt{\%}W alloy (50{\%} reduction in thickness) and an annealed Ta-10wt{\%}W alloy. Results are compared to previous work on annealed tantalum and Ta-2.5wt{\%}W, and a 50{\%} cold rolled pure tantalum.   

Shock compression of single crystal and polycrystalline tantalum from 6 -- 23 GPa

A series of plate impact experiments have been performed to produce simultaneous shock loading of both polycrystalline and the three principal orientations of single crystal tantalum ([100], [110] and [111]) to peak stresses between 6 and 23 GPa. Measured free surface wave profiles demonstrate that the shock behaviour varies significantly between the four variants, exhibiting differences in upper and lower elastic limits, shock velocities and peak stress. Initial findings reveal that the [100] orientation exhibits the largest elastic limit. Shock velocity measurements indicate that for all of the materials, and most notably in the [100] orientation, there is a low stress excursion from a linear Us-up plot similar to that previously seen in polycrystalline tantalum. This suggests sensitivities at low stress which require further investigation. The experiments have been simulated using a single crystal plasticity finite element model that accounts for thermally-activated and drag-resisted dislocation motion, and for evolution of the dislocation density. The model is seen to qualitatively describe some of the features described above.