Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session E4: Inelastic Deformation II: Spall-Fragmentation II |
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Chair: Roger Minich, Lawrence Livermore National Laboratory Room: Hyatt Regency Constellation E |
Monday, August 1, 2005 3:30PM - 3:45PM |
E4.00001: Solutions in Spall and Fragmentation of Solids Dennis Grady After the Hugoniot elastic limit, spall is probably the next most familiar of failure mechanisms to the shock wave scientist. Fragmentation in the spall process is not commonly considered. This fact is probably due to the nature of the typical one-dimensional spall experiment, and the types of diagnostics commonly employed. Many spall events in application lead to intense fragmentation of the participating bodies, and the character of this fragmentation is often of paramount interest. In the present study several analytic solutions addressing both spall and fragmentation in solids are pursued. Various spall criteria are examined including critical stress, impulse, energy and amplitude of the Tuler-Butcher integral. Relationships among spall stress, characteristic fragment size, and dynamics of the spall event are derived. Analysis results are compared with available spall and fragmentation data for several materials. [Preview Abstract] |
Monday, August 1, 2005 3:45PM - 4:00PM |
E4.00002: Status of Statistical Modeling for Damage from Nucleation and Growth of Voids T.W. Wright, K.T. Ramesh Spall fracture is often dominated by rapid growth of voids. Recent research on void growth shows that nucleation may be represented as a continuum bifurcation, that the static critical tensile pressure may be calculated from standard constitutive data, and that growth rapidly becomes dominated by inertia. Sensitivity to imperfections arises from microstructural details, which is assumed to be represented by a statistical distribution of local critical stresses. This set of ideas, when coupled with a history of tensile pressure, is sufficient to generate the early history of porosity and the expected distribution of void sizes. One key result is that a higher rate of loading will nucleate voids at more potential sites, with higher pressure for a given porosity, and a more uniform distribution of smaller voids. Another prediction is that over a few decades of volumetric strain rate, the pressure at a fixed critical void volume fraction will vary as that strain rate raised to a power. This power law dependence corresponds well to spall data reported in the literature. The theory as developed so far (Molinari and Wright, 2005) will be reviewed and progress toward a physical model for void interaction and pressure release will be presented. A Molinari {\&} TW Wright, A Physical Model for Nucleation and Early Growth of Voids in Ductile Materials under Dynamic Loading, in press, J. Mech. Phys. Solids, 2005 [Preview Abstract] |
Monday, August 1, 2005 4:00PM - 4:15PM |
E4.00003: Void Nucleation and Growth in Shocked Materials K.T. Ramesh, T.W. Wright, A. Molinari Spall fracture and other rapid tensile failures are often dominated by the rapid growth of voids. Recent research on the mechanics of void growth clearly shows that void nucleation may be represented as a bifurcation phenomenon, followed by highly localized plastic flow around the new void. The critical bifurcation stress can be calculated, given the thermomechanical constitutive equation for the material. Nucleation and early growth (limited void volume fraction) have been estimated (Molinari and Wright, JMPS, to appear) by combining the simple dynamical equation for void growth with an assumed distribution for the local critical stress. We extend this to include rate effects in growth and to consider defect distributions in pure polycrystalline metals. A necessary consequence of the defect distribution is the development of loading rate effects (e.g., a higher failure stress is predicted for a higher rate of loading, and a more uniform distribution of smaller voids is developed). Various classes of defects are considered, and the consequences of defect evolution as a result of prior shock loading are investigated. [Preview Abstract] |
Monday, August 1, 2005 4:15PM - 4:30PM |
E4.00004: On the Role of Material Post-Necking Stress-Strain Curve in Simulation of Dynamic Impact Nicola Bonora, Andrew Ruggiero, Joel House, Philip Flater, Robert DeAngelis Still today material modeling is a critical for a wider use of numerical simulation in impact phenomena. The impossibility to reproduce experimental tests has been explained with the need to incorporate other effects in the material formulation, such as damage. Bonora (2004) demonstrated, for smooth and notched bar geometries, that the capability to simulate the strain localization is mainly, if not exclusively, controlled by the provided material plastic flow curve while damage modeling contributes only in determining the material state at which failure occurs. Here, the importance of providing the correct material plastic flow curve in numerical modeling is demonstrated simulating the deformation process in Taylor impact test. The material under investigation is 99.9{\%} half hardened Cu in as-received and annealed conditions. Material has been characterized under static and dynamic conditions and the post-necking response has been identified with as proposed by Ling (1996). Ductile damage has been modeled using the CDM based model proposed by Bonora (1997). An extensive parametric numerical investigation of dynamic tractions, at different strain rates on Hopkinson bar, and Taylor impact test has been performed providing material plastic flow curves differing only in the post-necking regime. [Preview Abstract] |
Monday, August 1, 2005 4:30PM - 4:45PM |
E4.00005: Numerical simulations of dynamic fragmentation in brittle materials Ramesh Raghupathy, Fenghua Zhou, George Gazonas, Jean-Francois Molinari The problem of fragmentation of a contiguous body subjected to intense loading has been under strong scientific scrutiny over the past few decades. Variations in fragment size have important implications in ballistic impact, crash performance, explosive drilling, and even the clustering of galaxies resulting from the big bang theory. In ceramic materials under high stress loads, cracks will initiate at flaws, and potentially propagate catastrophically to cause large-scale fragmentation. Multiple cracks will initiate at seemingly random locations and material failure will occur through a complex stress-wave communication process. In this presentation, a numerical approach relying on the cohesive element approach to cracking is advocated. The proposed model naturally accounts for stress-wave communication and other non-instantaneous cracking processes. The robustness of various cohesive element models in accurately predicting fragment size is discussed at length. In particular, a cohesive zone length scale, which is made explicitly dependent on the loading rate, is proposed to capture the correct amount of energy dissipated during fragmentation. Numerical evidence shows that this new formalism provides a simple and robust replacement of quasi-static predictions of the cohesive zone size. Illustrative examples are given for ceramic materials with various defect populations. [Preview Abstract] |
Monday, August 1, 2005 4:45PM - 5:00PM |
E4.00006: Investigation of Ejecta Production in Tin using Plate Impact Experiments P.A. Rigg, W.W. Anderson, W.T. Buttler, R.T. Olson, R.S. Hixson, D.D. Koller Experiments to investigate ejecta production in shocked tin have been performed using plate impact facilities at Los Alamos National Laboratory. Three primary diagnostics -- piezoelectric pins, Asay foils, and low energy x-ray radiography -- were fielded simultaneously in an attempt to quantify the amount of ejecta produced in tin as the shock wave breaks out of the free surface. Results will be presented comparing and contrasting all three diagnostics methods. Advantages and disadvantages of each method will be discussed. [Preview Abstract] |
Monday, August 1, 2005 5:00PM - 5:15PM |
E4.00007: Liquid breakup under one-dimensional strain Andrew Lloyd, John Borg The fragmentation characteristics of liquid systems at atmospheric pressure has been investigated experimentally and compared to hydrodynamic calculations as well as theoretical predictions. The geometry is a one-dimensional nylon flat plate impacting a flat plate liquid system at a velocity of approximately 0.3 km/s. The experiments were carried out at the Marquette gas gun facility. Hydrocodes were used to investigate early time shock evolution and material deformation. Witness cards were used to assess the drop distributions. The variations in strain rate and viscosity are investigated in order to assess the effects of these variations on final drop size distribution. The drop distributions are compared theoretical models. [Preview Abstract] |
Monday, August 1, 2005 5:15PM - 5:30PM |
E4.00008: Hydrocode Postprocessing Study of Optical Signatures from Fragment Distributions P.K. Swaminathan, J.C. Taylor, K.T. Ramesh, J.H. Molinari, F. Zhou Hypervelocity impact fragments generate optical signatures that provide key information about the shock-induced fragmentation behavior at high strain rates. This is because of a key dependence on fragment temperatures and size distributions which in turn vary according to the thermodynamics of energy partitioning and material behavior under high strain rates. We have carried out CTH calculations of the widely experimented case of spheres on plates to simulate the material response. Fragmentation patterns generated according to different fracture models of response under calculated strain are used to predict optical signatures from the resultant debris cloud. For prediction of optical signatures, several challenges need to be faced including CTH incorporation of accurate temperature dependent equations of state and large strain rate fragmentation models. [Preview Abstract] |
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