Session U1: ME.3 Inelastic Deformation, Fracture, and Spall IX

Interpreting the shock response of porous oxide systems

Oxide powders subjected to varying levels of shock loading can exhibit a complex response that differs significantly from that which is commonly observed in metals. As much of the early model development for particulates has been focused on metallic systems, the current state of the art in compaction and equation of state modeling is often unable to capture the wide range of compression responses observed in porous oxides. Specifically, the possibility of polymorphic phase transformations requires additional considerations in the development of compaction and equation of state models for these systems. In the present work, the shock response of several porous oxide systems is critically examined with respect to the equilibrium phase boundaries to identify the existence and extent of transformations under shock loading, and the influence of intrinsic and extrinsic properties on the onset of transformation.   

Dynamic yield and tensile strengths of spark plasma sintered alumina

Fully dense alumina samples with 0.6-$\mu $ grain size were produced from alumina powder using Spark Plasma Sintering and tested in two types of VISAR-instrumented planar impact tests. . In the tests of the first type the samples of 0.28 to 6-mm thickness were loaded by 1-mm tungsten impactors accelerated up to velocity of about 1 km/s. These tests were aimed to study of the Hugoniot elastic limit (HEL) of the SPS-processed alumina and the decay of the elastic precursor wave with the propagation distance. In the second type of the tests the samples of $\sim$ 3-mm thickness were loaded by 1-mm copper impactors accelerated up to velocities 100-1000 m/s was. These tests were aimed to the study of the dynamic tensile (spall) strength of the alumina. The data on the decay of the elastic precursor wave allow determining the rates of the irreversible (inelastic) strains in the SPS-processed alumina at the initial stages of the shock-induced plastic deformation and, thus, to derive some conclusions concerning the mechanisms responsible of the deformation. The data on the tensile fracture of the alumina demonstrate a monotonous decline of the spall strength with the amplitude of the loading stress pulse.   

Impact Simulations for a Pre-Stressed Ceramic

Applying a pre-stress to a ceramic using confinement increases the initial strength and ductility and helps suppress tensile failure. This confinement is not necessarily hydrostatic, so deviatoric pre-stresses may also be introduced. Although experimental testing of confined ceramic is becoming more common, numerical modeling of ceramic response in these configurations has been limited. We have developed a methodology to examine shrink-fit configurations using ALE3D. Experiments are conducted to examine the effects of ceramic confinement when impacted with projectile rods, and we are modeling these experiments using this methodology and the Johnson-Holmquist-Beissel model.   

Low Pressure Shock Hugoniot of Silica Aerogel

We present measurements on the low pressure shock Hugoniot of silica aerogel. Previous measurements of the shock Hugoniot of silica aerogel give a Hugoniot that when extrapolated to lo pressures give unphysical results. In this study, we used photonic Doppler velocimetry to measure the shock transit time in silica aerogel. The new shock Hugoniot data show a turnover in the shock Hugoniot of silica aerogel at low pressures. The turnover resolves the unphysical behavior in the shock Hugoniot. \\\\ This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Deformation Behaviour of Coarse Grain Alumina under Shock Loading

To develop better understanding of the shock wave induced deformation behavior of coarse grain alumina ceramics, and for measurement of its Hugoniot Elastic Limit (HEL), in-situ and recovery gas gun experiments have been carried out on coarse grain alumina (grain size $\sim$ 10 $\mu $m), prepared in the form of discs (\textgreater 99.9{\%} TMD) by pressure-less sintering of alpha alumina powder at 1583 K. The HEL value of 1.9 GPa has been determined from the kink in the pressure history recorded using piezoresistance gauge and also from the free surface velocity history of the sample shocked to 9 GPa. The nano-indentation measurements on the alumina samples shocked to 6.5 GPa showed hardness value 15{\%} lower than 21.3 GPa for unshocked alumina, and strong Indentation Size Effect (ISE); the hardness value was still lower and the ISE was stronger for the sample shocked to 12 GPa. The XRD measurements showed reduced particle size and increased microstrains in the shocked alumina fragments. SEM, FESEM and TEM measurements on shock treated samples showed presence of grain localized micro- and nano-scale deformations, micro-cleavages, grain-boundary microcracks, extensive shear induced deformations, and localized micro-fractures, etc. These observations led to the development of a qualitative model for the damage initiation and its subsequent growth mechanisms in shocked alumina. The work performed in collaboration with K.D. Joshi of BARC and A.K. Mukhopadhyay of CGCRI.