Process for producing high strength alumina

A vacuum hot pressed alumina material having small, isometric grains; a uniform distribution thereof; relatively low, predominantly transgranular porosity; and a density approaching the theoretical density of pure alumina produced by vacuum hot pressing alumina powder which contains at least 98.0% alumina, is substantially free of any sintering aids or any other additives, and has a median particle size less than about 3 microns, in a vacuum hot press operated at a temperature of at least about 1350° C. and a pressure of at least 28 MPa (3500 PSI) for a sintering period of at least 1.5 hours. The vacuum hot pressed alumina material also has compressive strength, flexural strength, impact strength, and wear resistance superior to that for most conventional sintered alumina materials.https://digitalcommons.mtu.edu/patents/1072/thumbnail.jp

High strength alumina and process for producing same

A vacuum hot pressed alumina material having small, isometric grains; a uniform distribution thereof; relatively low, predominantly transgranular porosity; and a density approaching the theoretical density of pure alumina produced by vacuum hot pressing alumina powder which contains at least 98.0% alumina, is substantially free of any sintering aids or any other additives, and has a median particle size less than about 3 microns, in a vacuum hot press operated at a temperature of at least about 1350° C. and a pressure of at least 28 MPa (3500 PSI) for a sintering period of at least 1.5 hours. The vacuum hot pressed alumina material also has compressive strength, flexural strength, impact strength, and wear resistance superior to that for most conventional sintered alumina materials.https://digitalcommons.mtu.edu/patents/1075/thumbnail.jp

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

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. © 1982

A generalized analysis of thermal and mechanical loads in inertial confinement reactors

An analysis of thermal and mechanical loads acting on an inertial confinement fusion (or fusion/fission) reactor is presented. It is shown that, as a result of the pulsed mode of operation, quasi-steady-state temperatures and stresses can be separated into static and dynamic components. Numerical results are presented for a specific design of a spherical fusion/fission reactor with a lithium-wetted wall. For the purpose of scaling, materials selection, and fatigue damage assessment, approximate formulas for temperatures and stresses are also given. © 1980 Taylor & Francis Group, LLC

Computational modeling of explosive-filled cylinders

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. © 1985

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

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. © 1995 TMS

Testing of High‐Strength Ceramics with the Split Hopkinson Pressure Bar

The split Hopkinson pressure bar was used to study the failure of a high‐strength alumina at strain rates on the order of 103 s‐1. There appears to be a critical strain rate above which the traditional method of using the transmitter bar signal to calculate the stress in the specimen is no longer valid. To compute the correct stress in the specimen it was necessary to use the strain signal from a gage mounted directly on the specimen. Copyright © 1993, Wiley Blackwell. All rights reserve