Relationship between crack resistance and plastic deformation was investigated using various kinds of commercial glass. Crack resistance was evaluated by an applied load initiating crack under Vickers indentation test. The observed cracks were radial cracks with their origins beneath the indenter. It is widely accepted that crack initiation is affected by plastic deformation. The extent of plastic deformation is indicated by Vickers hardness, but crack resistance did not have an apparent correlation with it. Plastic deformation consists of two processes: densification and plastic flow. Then densification was evaluated by recovery of Knoop indentation depth after heat-treatment. An apparent correlation between crack resistance and densification was found, indicating that glass with more densification has higher crack resistance. Densification causes a reduction in the interstitial space of the 3-dimensional network by a decrease in atomic bond angle and distance. If more reduction of the space occurs, the region of glass beneath the indenter deforms more easily to fit the contour of indenter. So it is considered that densification lessens stress concentration, leading to an increase of crack resistance. Densification is the fundamental property to determine crack resistance rather than Vickers hardness.
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