Ductile damage model with void coalescence

A general model for ductile damage in metals is presented. It includes damage induced by shear stress as well as damage caused by volumetric tension. Spallation is included as a special case. Strain induced damage is also treated. Void nucleation and growth are included and give rise to strain rate effects. Strain rate effects also arise in the model through elastic release wave propagation between damage centers. Underlying physics of the model is the nucleation, growth, and coalescence of voids in a plastically flowing solid. Implementation of the model in hydrocodes is discussed

Spallation modeling in tantalum

A gas gun plate impact spallation experiment has been performed on commercial purity rolled tantalum. The shock pressure achieved was about 7 Gpa and was sufficient to induce incipient spallation. The particle velocity was measured at the free surface of the spalled plate, and the spalled sample was recovered and examined metallographically using image analysis. The quantitative image analysis results are being used to develop a damage model. The model is micromechanically based and involves novel void growth and coalescence processes. The 1D characteristics code CHARADE has been used in a preliminary simulation of the VISAR free surface particle velocity record. Implications for ductile damage modeling will be discussed

Damage evolution and clustering in shock loaded tantalum

Two grades of tantalum were shock loaded by plate impact and recovered. The loading conditions were varied to study the damage evolution in te materials from incipient to full spallation. The authors performed quantitative image analysis and optical profilometry on the recovered specimens. Statistical analyses are shown of the void sizes, void clustering, and void linking in the two material grades

Shock wave plasticity in Mo at 293K and 1673K

The shock wave plasticity of Mo is extracted from two VISAR wave profiles; of about 110 kbar strength at 293 K and of about 120 kbar strength at 1673 K. The Wallace weak shock analysis is used to obtain the plastic strain and deviatoric stress, and the normal stress and volumetric strain, through the shock rise from the velocity profile data. The Wallace analysis uses the steady wave assumption for the plastic portion of the shock rise, a plausible evolution for the precursor portion, a thermoelastic model, and the mechanical equations of motion. Comparison of the high and low temperature results is of interest in assessing the mechanisms of plastic flow. In the results, the (von Mises equivalent) peak deviatoric stresses are 12.8 kbar and 20.3 kbar, for the hot and cold Mo, respectively, while the peak plastic strain rate of the hot Mo is about 2.6 times that of the cold Mo. These values rule out thermal activation. In addition, they are not consistent with a simple phonon viscosity linear in the temperature. Additional effects are needed to explain the results, e.g. evolution of the mobile dislocation density

Percolation wave propagation, and void link-up effects in ductile fracture

Ce papier décrit la croissance et le contour de l'endommagement ductile basé sur la croissance et la coalescence de vides. La influence de la taille des fissures et de la vitesse des ondes sur la croissance de l'endommagement est pris en compte. La porosité de l'écaillage est calculer avec la theorie de le percolateur.This work investigates the time evolution and spatial morphology of ductile damage based on void growth and coalescence. The size enhancement of damage cluster growth, as well as wave speed limiting of growth, are treated microscopically. Simplified 2D plane strain simulations using individual voids are done with uniaxial strain and explained with a probabilistic theory. At low strain rates, fracture occurs by long, localized cracks. At high strain rates, widespread, random damage breaks the system

Constitutive relation for 6061T6 aluminum under shock loading conditions

We publish for the first time a constitutive relation of Wallace and Straub based on plastic flow data extracted from 6061T6 Al shocks by Wallace. The data involves strain rates up to 10/sup 7/ and shear stresses up to 5kbar. We interpret the relation in terms of a dislocation-kinetics model

Thermoelastic-plastic flow and ductile fracture in solids

The main aim of these lectures is to develop the theory of elastic-plastic flow in solids, with application to a particular class of processes, namely those in which the dissipation of plastic work cannot be neglected. Examples of such processes are highspeed impact phenomena and cratering, shock compression, often shock release as well, and explosive deformation and welding. An important part of the development is to include the anisotropic elastic properties of a solid, and for this reason the theory applies to solids in general. On the other hand, it is possible that some of the results will have limited validity for nonmetals. The secondary aim of the lectures is to develop a general framework for ductile fracture, and describe the current practice in the field. Our theory is incremental in nature and suitable for integrating along a process in small timesteps as is done in ''hydrodynamic'' computer programs. A notational conflict arose, because in continuum mechanics the extensive quantities are taken per unit mass, while in thermodynamics they are usually per unit volume. The continuum mechanics normalization, i.e., per unit mass, is used throughout, with the result that uncommon factors of density show up in the thermodynamic equations

Plastic flow in weak shock waves in uranium

Measurements of the particle velocity in weak shock waves in metals are available for a number of materials. These measurements use the laser interferometer or VISAR technique in conjunction with a plate impact experiment. These measurements are important for determining the elastic -- plastic behavior of materials at high strain rates. Strain rates up to 10{sup 7}/s are measurable with this technique, while more conventional mechanical testing machines, such as the Hopkinson bar, achieve rates only up to about 10{sup 4}/s. In this paper, the VISAR measurements of Grady on uranium are analyzed using the weak shock analysis of Wallace to extract the plastic and total strains, the deviatoric and total stresses, and the plastic strain rates. A brief error analysis of the results will be given. 7 refs

The rate dependence of the saturation flow stress of Cu and 1100 Al

The strain-rate dependence of the saturation flow stress of OFHC Cu and 1100 Al from 10{sup {minus}3}s{sup {minus}1} to nearly to 10{sup 12}s{sup {minus}1} is examined. The flow stress above 10{sup 9}s{sup {minus}1} is estimated using Wallace's theory of overdriven shocks in metals. A transition to the power-law behavior {Psi} {approximately} {tau}{sub s}{sup 5} occurs at a strain rate of order 10{sup 5}s{sup {minus}1}. 10 refs., 2 figs