STUDIES OF THE BRITTLE BEHAVIOR OF CERAMIC MATERIALS. Quarterly Report No. 1

Technical Report · Fri May 31 04:00:00 EDT 1963

OSTI ID:4698529

Parikh, N M; Rostoker, W eds

Investigations on the relation between failure strengths and strain rates for Al indicate no appreciable strain rate effect at room temperature, but data at elevated temperatures indicate that a significant effect exists. The results of the effects of nonuniform stress fields upon the fracture characteristics of Al/sub 2/O/sub 3/ show that two different specimens have almost the same mean failure stress, although the stress gradients are widely different, having a ratio of about 4 to 1. This suggests that there is no stress gradient effect per se on failure stress and that the material is governed by volumetric flaw distribution. A continuum theory was developed for calculation of force constants for charged dislocations in MgO crystals. Similarity between activation energies of the internal and steady state creep in Al/sub 2/O/sub 3/ suggests that the latter is grain boundary controlled at low stress (3600 psi) and temperature (below 1300 c- C) levels. This effect, combined with an apparent insensitivity of the activation energies of the processes to atmosphere, suggests grain boundary control through impurity domination. Tests on surface and fracture energy of Al/sub 2/O/sub 3/, Mg0, and glass show that the crack branching criteria yield approximately the correct reversible surface energy for vaiues of K close to unity. Fracture mechanics for MgO are also discussed. MgO doped with Al, Fe, or Cr increased in microhardness with increasing amounts of impurity. Modulus of rupture for the same systems was found to decrease or remain essentially constant with increasing impurity content. On the other hand, MgO doped with Zr, Sr, or Ti decreased in microhardness with increases in the added impurity. The tensile strength of polycrystalline hot- pressed MgO was also measured. The tensile strength seemed to be insensiti ve to heat treatment or surface condition. It was found that porosity does not affect the strength of Al/sub 2/O/sub 3/ as much as grain size, intergranular pore size, and intergranular pore spacing. (P.C.H.)

• Research Organization: Illinois Inst. of Tech., Chicago. Armour Research Foundation
• NSA Number: NSA-17-025770
• OSTI ID: 4698529
• Report Number(s): ARF-B6012-1
• Country of Publication: United States
• Language: English

Related Subjects

ALUMINUM
ALUMINUM OXIDES
BRITTLENESS
CERAMICS
CHROMIUM
COMPACTING
CONTROL
CRACKS
CREEP
CRYSTALS
DEFECTS
DEFORMATION
DISLOCATIONS
DISTRIBUTION
EXCITATION
FAILURES
GLASS
GRAIN BOUNDARIES
GRAIN SIZE
HARDNESS
HOT WORKING
IMPURITIES
IRON
LATTICES
MAGNESIUM OXIDES
MATERIALS TESTING
MEASURED VALUES
METALS, CERAMICS, AND OTHER MATERIALS
NUMERICALS
POROSITY
POWDERS
QUANTITY RATIO
STRESSES
SURFACES
TEMPERATURE
TENSILE PROPERTIES
TRACE AMOUNTS