William W. Predebon

The role of plasticity as a limiting factor in the compressive failure of high strength ceramics

Abstract The behavior of aluminum oxide under compressive loading is investigated over a wide range in strain rate and degrees of confinement. It is shown that plastic flow can be generated in Al 2 O 3 at all strain rates if confinement is sufficient to prevent premature failure via microfracture. Moreover, plastic flow is itself a source of microfracture, and the threshold for multiple slip apparently constitutes the practical ultimate strength for the ceramic. Thus, for sufficiently fine-grained alumina tested under optimum conditions, no confinement is required to generate plastic flow, at which stress the material fails via dislocation-induced general microfracture.

Micromechanisms of deformation in high-purity hot-pressed alumina

Abstract A high-strength aluminum oxide was produced by vacuum hot pressing high-purity, submicron-size alumina powders. The uniaxial compressive fracture strength was strongly strain-rate sensitive and varied from 5.5 GPa at 10 −4 s −1 to 8.3 GPa at 10 3 s −1 . A Hugoniot elastic limit of about 11.9 GPa was determined from flyer plate impact tests. The deformation/fracture process was examined using both uniaxial stress and uniaxial strain conditions. Under a uniaxial stress condition, microplasticity was observed in the form of aligned dislocations that appeared similar to shear bands in metals. Under a uniaxial strain condition, extensive dislocation activity, grain boundary microcracking and occasional twins were observed. Based on the experimental results and microscopic observations, possible mechanisms responsible for the observed high strength and high strain-rate sensitivity in this alumina are discussed.

Inclusion of evolutionary damage measures in Eulerian wavecodes

Most continuum descriptions of damage evolution generally require history-dependent material variables. The Lagrangian formulation of the continuum equations is the natural coordinate system for tracking material-history quantities. In numerical simulations of dynamic events such as in penetration and perforation of target plates by projectiles, the Lagrangian mesh can become severely compressed and distorted which effectively terminates advancing the solution in time. On the other hand, the Eulerian formulation, with its fixed coordinate system, does not suffer from mesh distortion. However, Eulerian descriptions usually follow only what crosses cell boundaries, and instead of computing the time history of material particles, they describe the average instantaneous state of a material in a computational zone. This paper describes the inclusion and evaluation of the history of equivalent plastic strain, which is a representative measure of damage, as an internal state variable within an Eulerian numerical framework. The formulation and method of advection of the equivalent plastic strain are described, and the results for two example problems are discussed and comparisons are made with the results of Lagrangian calculations.

Computational modeling of explosive-filled cylinders

Abstract A time-dependent, two-dimensional, finite-difference code can be used to model fragmenting cylinders. Strictly hydrodynamic treatment of the casing material generally overpredicts the final fragment velocity. A more definitive final fragment velocity is predicted when the casing material is treated as an elastic-plastic material, but the final fragment velocities occur at unrealistically high cylindrical expansion ratios. To remove some of these objections and, at the same time, model the casing motion more realistically, a gas leakage model has been developed to simulate explosive gas leakage around fragments after casing breakup. Comparisons have been made between code calculations and experimental data. The experimental data include different length-to-diameter ratios, natural and discrete fragmenting cylinders, different charge-to-casing mass ratios, and different initiation postures. The gas leakage model predicts definitive final fragment velocities in excellent agreement with the experimental data.

The response of a high purity alumina to plate‐impact testing

Alumina disks which were vacuum hot pressed from a 99.99% pure Al2O3 powder were subjected to flyer‐plate impact testing. VISAR techniques were used to measure rear surface velocities. The Hugoniot elastic limit (HEL) for this alumina was 11.9 GPa. At a precompression of three times the HEL, a remarkably high spall strength of 1.2 GPa was observed. However, a negligible spall strength was found when the alumina was shocked to approximately 1.3 times the HEL. These results indicate that the spall strength of pure polycrystalline alumina goes through a transition, first decreasing in value near the HEL and then increasing again above the HEL. In other flyer‐plate impact tests, manganin stress gauges were used to measure the decay of the HEL with specimen thickness. The HELL for this alumina decreases slightly when the thickness of the specimen was increased but stabilized for specimens thicker than about 9 mm.

Strain-rate effects in high-purity alumina

Ultrahigh-strength alumina specimens made from disks produced by vacuum hot pressing high-purity alumina powders were subjected to uniaxial compressive loads at a range of strain rates. It was observed that the material exhibited a failure strength far superior to commercially available alumina. The failure strength was strongly strain-rate dependent and varied from 5.5 GPa at 10-4 s-1 to 8.3 GPa at 103 s-1. Microscopic studies on the fragments of the specimens deformed under uniaxial strain revealed extensive twinning and dislocation activity. Based on the experimental results and microscopic observations, the factors and mechanism responsible for the observed high compressive strength are discussed.

Dynamic launch process of performed fragments

It is shown through numerical simulations that the gap between performed fragments closes during explosive launch, trapping the detonation products until radial expansion of the fragments is sufficient to separate the fragments. Circumferential strains from the numerical calculations are in good agreement with the plastic strains from recovered fragments. Additionally, considerable insights into the dynamics of the explosive launch process are obtained by comparing spall failure and nucleation and growth of ductile voids in the recovered fragments against the numerical simulations.

Deformation and fracture in directionally solidified Co-CoAl eutectic

The effect of growth defects known as lamellar terminations on the yielding and fracture behaviour of Co-CoAl eutectic single crystals was studied using tensile tests and finite-element modelling. The yield strength and strain to fracture were found to decrease with increasing termination density. Observations of deformed surfaces and serial sectioning experiments on fractured tensile specimens revealed that crack initiation during the fracture process was enhanced by the presence of lamellar terminations. The fracture surfaces were found to have a staircase-type appearance, which indicated that the final fracture process was discontinuous with a step-wise propagation from one CoAl lamella to adjacent CoAl lamellae. A computer simulation was conducted to determine the stress distributions about lamellar terminations in model microstructures, since the experimental results suggested that the lamellar terminations behaved as stress concentrations in the microstructure. The finite-element calculation confirmed that lamellar terminations can influence the yielding process; the stress at which the first slip system was activated was found to decrease with increasing termination density.

An investigation of incipient fracture in shock‐loaded lamellar cobalt‐aluminum eutectic

Lamellar cobalt‐aluminum eutectic provides in in‐situ composite in which shock induced fracture may be studied. The lamellae consist of alternating layers of the two constituent phases of the eutectic. Using the Hugoniot equations‐of‐state for each constituent phase, the eutectic is modeled using a two dimensional finite‐difference code for the case of an initially planar pulse travelilng parallel to the interphase boundary. The as‐grown eutectic is not a perfectly lamellar structure, but rather, it contains terminations and branching of the lamellae. The effects of terminations and branchings on incipient dynamic fracture of the eutectic are considered and compared to the case without these imperfections. Individual layers of the eutectic are coupled through boundary interaction with two extreme cases, perfect bonding and perfect lubrication, considered.

An investigation of shock-induced fracture in a lamellar eutectic two-phase metal alloy☆

Abstract A preliminary study of the nature of dynamic fracture in a bi-phase lamellar eutectic metal is made by a finite-difference computer code simulation. Through the simulation, the mode and location of incipient fracture are predicted and compared to experimental results. The ease where an initially planar shock pulse traveling parallel to the direction of the lamellae is considered. Incipient fracture is predicted through the use of the cumulative damage spall model, based on a maximum principle stress criterion for the damage threshold. Results of the simulation show that incipient fracture occurs in the intermetallic CoAl phase, and along the interphase boundary. Dynamic fracture experiments with soft recovery of the lamellar cobalt-aluminum eutectic using a bi-crystal have been performed. The experimental results indicate that incipient dynamic fracture occurs throughout the CoAl phase and along the interphase boundary at approximately the stress level predicted. Thus agreement between the experimental results and the simulation was achieved.