Session L1: ME.3 Inelastic Deformation, Fracture, and Spall III

Experimental observation and theoretical modeling of the effect of magnetic field on the strength of molybdenum under ramp wave compression

A new experimental technique has been developed at Sandia National Labs to study the dynamic material strength at high pressures using ``magnetically applied pressure shear (MAPS)'' ramp waves. 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. It was observed that an imposed magnetic field of around 10 Tesla induced some annealing on molybdenum. Furthermore, when subjected directly to magnetohydrodynamic loading as encountered for the driver material, molybdenum exhibited an apparently stiff response and did not show a discernible elastic plastic transition. To better understand the experiments and establish a predictive capability for molybdenum, a tentative strength model that incorporates the possible magnetic effects including magnetic diffusion, Joule heating, and the coupling between the magnetic field and material strength has been developed. Experimental observations and the model will be discussed.   

Elastic-Plastic Deformation of Molybdenum Single Crystals Shocked along $\mathbf <$100$\mathbf >$

To examine and understand elastic-plastic deformation in shocked Molybdenum (Mo), single crystal samples (99.99$\%$ purity) were shocked along $<$100$>$ orientation. The peak longitudinal stress in our experiments was $\sim$12 GPa and sample thicknesses ranged between 0.23 and 2.3 mm. The Mo samples were backed by c-axis sapphire optical windows, and wave profiles were measured at the Mo/sapphire interface using laser interferometry (VISAR). A two-wave structure, expected from elastic-plastic deformation, was observed in all cases. Elastic wave amplitudes ranged between 2.9 and 4.2 GPa with an average value of 3.6 GPa. The scatter observed in the elastic wave amplitudes, though somewhat high, is comparable to that observed in a previous work on single crystals of tungsten, another bcc metal (T. E. Michaels, Ph. D. thesis, WSU). Measured wave profiles showed stress relaxation behind the elastic wave front for samples of 0.46 mm or larger in thickness. No obvious correlation could be established between the measured elastic wave amplitudes and the sample thicknesses examined. Relationship of the present results to slip systems postulated for Mo will be discussed. This work is supported by DOE/NNSA.   

Effect of shock wave duration on dynamic failure of Tungsten Heavy Alloy

It has been well established that dynamic fracture or spall is a complex process strongly influenced by both microstructure and the loading profile imparted to the specimen. Having previously considered ductile materials with damage and deformation kinetics that are slow relative to the shock wave, here we consider a brittle material with damage and deformation kinetics that are fast relative to the shock wave. The present study elucidates the effect of loading profile on the fundamental mechanisms of brittle fracture in brittle tungsten heavy alloy (WHA) specimens. Spall experiments are performed on with two significantly distinct shock pulse durations and accompanying unloading rates. Detailed fractographic analyses of the damage in the spalled WHA samples as a function of shock-wave profile of comparable peak stress is presented. For both profiles, it is observed that the failure in WHA is by brittle trans-particle crack growth with additional energy dissipation through crack branching in the more brittle tungsten particles. We also observe that for the 15.4 GPa peak shock stress, the wave profile does not influence the spall strength significantly. This is believed to be directly linked with the relative insensitivity of WHA to time dependent processes.   

Study of lead behavior features at shock-loading and further unloading

The determination of mass, size, shape and velocity of particles, which appear as a result of microcumulative ejection from the metal surface at the moment of shock wave outlet, is a very complicated and multivariate problem. The presentation object is to value lead melting area boundaries and to measure velocity of particle cloud ejected from the surface under shock loading. The free surface and ejecting particle cloud velocities were simultaneously recorded using the method of heterodyne-interferometer (PDV). The solid surface velocity of lead is recorded on spectrogram at the shock pressure amplitude of 22~GPa and less; while the particle cloud motion is recorded in front of the surface. The lead melting occurs after shock loading with the pressure of 25~GPa and more, and the further unloading. As it is estimated, the density of melted particle cloud is greater (by an order of magnitude) than the density of particle cloud ejected from the solid surface. The material seems to be in the composite state (liquid-solid phase) at the pressure of 22$\div$25~GPa. It is shown that the maximum velocity of melted particle cloud (3.3-3.8 km/s) is higher than the velocity of particles ejected from the solid surface (2-2.5 km/s).   

Influence of preliminary loading on formation of adiabatic localized shift in copper

It is revealed (for example, by G.T.Gray III, Q.Xue) that formation of strain localization centers is very sensitive to the initial density of defects and peculiarities of the basic microstructure. At the same time, the other experts revealed in their works that the material ability for strain hardening plays an important role in localization of shear bands. Presently this process is still under active study. This paper is devoted to results of investigation of localized shift in as-received copper, and which was subjected to preliminary quasi-isentropic shock loading by the pressure of $\sim$30 GPa. Tests were performed with hat-shaped samples by the SHPB method. The authors present estimation of quantitative characteristics of localized shift in tested materials (shear stress, relative shear strain, shear band width, relative strain rate in shear band). The paper includes data of metallographic investigations of the tested samples. Also the authors made an attempt of numerical simulation of stress field change during formation.   

The Spall Strength and Hugoniot Elastic Limit of Monocrystalline and Polycrystalline Copper near Melting Temperature

In the present work the Hugoniot elastic limit (HEL) and the spall strength of the polycrystalline commercial grade copper and of the copper single crystal of [100] and [111] orientations were determined for the sample temperatures varying from 293 to 1353K, what is some 3K below the copper melting point Tm. The preheated samples in thickness between 0.5 and 2 mm were shock-loaded by the copper plates of 1-mm thickness accelerated up to 300-400-m/s velocity in the 58-mm smooth bore gas gun, or by the aluminum plates of 0.4 mm in thickness accelerated up to $\sim$660 m/s with explosive facilities. The velocity histories of the free rear surface of the loaded samples were recorded with VISAR laser velocimeter. The velocity histories of the samples of polycrystalline copper demonstrate 9-fold growth of the stress at HEL between room and melting temperatures. Unlike the other metals, commercial grade copper maintains a very high spall strength near melting point; it is only twice as low as that of the copper at 0.85 Tm. The copper single crystals of the both orientations also demonstrate substantial spall strength at 0.94 Tm (1273K). But the increase of the stress at HEL with temperature in these samples is much weaker than that found for polycrystalline samples of copper.   

Ductile damage in Taylor and Rod-on-rod impact experiment

At equivalent impact velocity, pressure in Taylor and ROR impact experiment is not the same and this reflects in the resulting condition for ductile damage development. In this work, finite element parametric simulation was performed to investigate pressure wave development as a function of material and target work hardening curve. Using the Bonora damage model, the impact velocity necessary for generating ductile damage in high purity copper was assessed. Taylor and ROR experiments were performed at different equivalent impact velocities and metallographic investigation were performed on impacted samples in order to validate damage model predictions. In addition, the effect of temperature on damage development was also investigated performing impact tests at different reference temperatures.   

Influence of tensile duration on dynamically induced damage evolution of OFHC

Plate impact experiments have been carried out to examine the influence of tensile duration on dynamically induced damage evolution of OFHC. A new double-layer-sample target experimental technique, tentatively termed as damage-frozen technique, is presented in this paper, with which the tensile duration can be controlled. The experimental configuration used in this work permits real-time (VISAR) measurements of the second sample free surface velocity histories and post experiment metallurgical analysis of the soft recovered samples. It is shown that for the same dynamic loading profile but different tensile duration, the free surface velocity profiles are nearly the same, with only differences in the slope between the pull-back minima and spall peak, but pre-spall damage including number, size and distribution of the void are significantly different. Based on these results, the relationship between wave profiles and damage evolution with different tensile duration time is established.