W.R. Thissell

Micromechanics of spall and damage in tantalum

The authors conducted a series of plate impact experiments using an 80-mm launcher to study dynamic void initiation, linkup, and spall in tantalum. The tests ranged in peak shock pressures so that the effect of peak pressure on the transition from void initiation, incipient spall, and full spall could be studied. Wave profiles were measured using a velocity interferometry system (VISAR), and targets were recovered using {open_quotes}soft{close_quotes} recovery techniques. The authors utilized scanning electron microscopy, metallographic cross-sections, and plateau etching techniques to obtain quantitative information concerning damage evolution in tantalum under spall conditions. The data (wave profiles and micrographs) are analyzed in terms of a new theory and model of dynamic damage cluster growth.

Void Coalescence Model for Ductile Damage

A model for void coalescence for high strain rate ductile damage in metals is presented. The basic mechanism is void linking through an instability in the intervoid ligament. The formation probability of void clusters is calculated, as a function of cluster size, imposed stress, and strain. A wave speed limiting is applied to the cluster size enhancement of cluster growth. Due to lack of space, model formulas are merely described and not derived.

Quantitative description of damage evolution in ductile fracture of tantalum

Dynamic ductile fracture has been studied through incipient spallation experiments on two grades of tantalum. A commercially pure Ta material incipiently spalled at 252 m/s, a highly pure Ta material incipiently spalled at 246 m/s, and a highly pure Ta material preshocked at 250 m/s and incipiently spalled at 246 m/s were used. Microstructural parameters of the fracture process such as porosity, void-size distributions, and void aspect ratios have been quantified using image analysis and optical profilometry techniques. The commercially pure Ta, the highly pure Ta preshocked prior to spall, and the annealed high-purity Ta exhibited 27, 16.6, and 5.5 pct porosity, respectively. The void-size distribution observed in all three tests was adequately represented by either a log-normal or a linear combination of a log-normal and a Weibull distribution function. At least 80 pct of the aspect ratios observed in all three tests were adequately represented by a gamma distribution function.

Microstructure of depleted uranium under uniaxial strain conditions

Uranium samples of two different purities were used for spall strength measurements. Samples of depleted uranium were taken from very high purity material (38 ppm of carbon) and from material containing 280 ppm carbon. Experimental conditions were chosen to effectively arrest the microstructural damage at two places in the development to full spall separation. Samples were soft recovered and characterized with respect to the microstructure and the form of damage. This allowed determination of the dependence of spall mechanisms on stress level, stress state, and sample purity. This information is used in developing a model to predict the mode of fracture.

Roughness of Ductile Damage Paths in Shock Loaded Tantalum

The damage in the “spall plane” of tantalum incipiently spalled in a gas gun or from HE loading consists of voids and/or regions of localized shear through which a long, continuous path can be traced. The geometry of this path has been analyzed using wavelets to show that it constitutes an affine fractal with a well defined “roughness”, i. e it scales statistically with the scaling factor in the shock direction being a power of the transverse scaling factor.

Spall wave-profile and shock-recovery experiments on depleted uranium

Depleted Uranium of two different purity levels has been studied to determine spall strength under shock wave loading. A high purity material with approximately 30 ppm of carbon impurities was shock compressed to two different stress levels, 37 and 53 kbar. The second material studied was uranium with about 300 ppm of carbon impurities. This material was shock loaded to three different final stress level, 37, 53, and 81 kbar. Two experimental techniques were used in this work. First, time-resolved free surface particle velocity measurements were done using a VISAR velocity interferometer. The second experimental technique used was soft recovery of samples after shock loading. These two experimental techniques will be briefly described here and VISAR results will be shown. Results of the spall recovery experiments and subsequent metallurgical analyses are described in another paper in these proceedings.

Modeling Incipient Copper Damage Data from the Tensile Hopkinson Bar and Gas Gun

Ductile damage in copper has been created using a split tensile Hopkinson pressure bar. Precise momentum trapping has made it possible to arrest the damage after a short tensile pulse before complete fracture. This process has been modeled with a void nucleation and growth law. 2D calculations have been performed to compare with final porosity and void number density data. The tensile bar damage modeling has been supplemented with modeling of incipient spallation of copper in a plate impact gas gun experiment. These two experiments differ widely in the (negative) pressure level and modeling both their porosity results will permit the creation of a nucleation model that spans the resulting wide pressure range.

Spallation studies on shock loaded uranium

Several spallation experiments have been performed on uranium using gas gun driven normal plate impacts with, VISAR instrumentation and soft recovery. The shock pressures achieved were 81, 53, and 37 kbar. This paper will focus on modeling the free surface particle velocity trace U with of 300 ppm carbon using the 1 d characteristics code CHARADE. The spallation model involves the growth and coalescence of brittle cracks. Metallographical examination of recovered samples and details of the experimental apparatus are discussed in separate papers.

Spallation studies on shock loaded uranium Los Alamos report LAUR-97-3169

Several spallation experiments have been performed on uranium using gas gun driven normal plate impacts with VISAR instrumentation and soft recovery. The shock pressures achieved were 81, 53, and 37 kbar. This paper will focus on modeling the free surface particle velocity trace U with of 300 ppm carbon using the 1 d characteristics code CHARADE. The spallation model involves the growth and coalescence of brittle cracks. Metallographical examination of recovered samples and details of the experimental apparatus are discussed in separate papers.