A fractographic criterion for subcritical crack-growth boundaries in hot-pressed alumina |
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Authors: | H P Kirchner R M Gruver |
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Affiliation: | (1) Ceramic Finishing Company, P.O. Box 498, 16801 State College, Pennsylvania, USA |
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Abstract: | The percent intergranular fracture (PIF) was measured along radii extending from fracture origins in hot-pressed alumina specimens, fractured at various loading rates and temperatures, and plotted versus estimates of stress intensity factors (K
I) at the various crack lengths. Minima in PIF occur at values ofK
I that are close to the critical stress intensity factors (K
IC) for cleavage on various crystal lattice planes in sapphire. The subcritical crack-growth boundary (K
I=K
IC of the polycrystalline material) occurs near the primary minimum in PIF suggesting that this minimum can be used as a criterion for locating this boundary. In addition, it was noted that the polycrystallineK
IC (4.2 MPa m1/2) is very close to theK
IC for fracture on {¯1 ¯1 2 6} planes which is 4.3 MPa m1/2. These observations suggest that critical crack growth begins when increased fracture energy can no longer be absorbed by cleavage on these planes. There is a secondary minimum atK
I>K
IC that appears to be associated with theK
IC necessary for fracture on combinations of planes selected by the fracture as alternatives to the high fracture-toughness basal plane. |
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