SEDL / STP / STP844-EB / STP35242S

Subthreshold Indentation Flaws in the Study of Fatigue Properties of Ultrahigh-Strength Glass

Dabbs, TP
Graduate student, School of Physics, University of New South Wales, New South Wales,

Fairbanks, CJ
Guest student and physicist, Center for Materials Science, National Bureau of Standards, Washington, D.C.,

Lawn, BR
Guest student and physicist, Center for Materials Science, National Bureau of Standards, Washington, D.C.,

Pages: 12    Published: Jan 1984

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Abstract

The rate-dependent characteristics of subthreshold indentation flaws in glass are surveyed. In the first part, the kinetics of radical crack initiation within the indentation field are described. It is shown that an incubation time must be exceeded in the contact process for a critical crack nucleus to develop. This incubation time decreases as the contact load and the water content in the environment increase. Even if incubation is not achieved during the contact, delayed pop-in may occur after the contact due to the action of residual stresses. Scanning electron microscopic evidence shows that the radical cracks initiate from precursor shear faults within the deformation zone. In the second part of the presentation, the fatigue properties of specimens with indentation flaws on either side of the threshold are discussed. The subthreshold flaws differ significantly from their postthreshold counterparts in these properties: the applied stresses at failure are higher, the susceptibility to water is stronger, and the scatter in individual data points is wider. These features are discussed in relation to the preceding crack-initiation kinetics. Finally, the implications of the results concerning design criteria for optical fibers are considered.