E4 Base Mechanical Properties

Topic

The theoretical cohesive strength of this glass—the strength that would result from directly breaking the chemical bonds of its network if that network were perfect—is not the strength it exhibits in practice; the inevitable surface defects it contains reduce its practical strength far below that theoretical cohesive strength. Because the glass contains surface defects rather than a perfect, defect-free network, its practical strength is determined by those defects rather than by the theoretical cohesive strength of its bonds.

This practical strength is calculated precisely from the size of the most severe surface defect present in the glass, using the same Griffith criterion employed—for an equivalent purpose—by a model for a different set of materials: a larger critical defect size results in lower practical strength according to the Griffith criterion, whereas a smaller critical defect size results in higher practical strength, bringing it closer to the theoretical cohesive strength with which this model began.

When the glass surface is subjected to point contact or indentation, it responds according to the same Hertzian elastic contact model used—for the same purpose—by another model in this set, which describes the fundamental mechanical properties of a ceramic: a higher indentation load on the glass surface results in a larger contact area and a deeper indentation, in accordance with the Hertz model.

Thus, this model—characterized by the aforementioned theoretical cohesive and practical strengths—shares the same elastic-brittle framework used to describe the fundamental mechanical properties of a ceramic; the difference is that, unlike in the ceramic, porosity plays no significant role in the practical strength of this glass, as the glass is essentially dense, save for its discrete surface defects. The fact that this same elastic-brittle system also exhibits an isotropic elastic relationship equivalent to the one employed by another model within that same set of distinct materials means that, for elastic purposes, this glass can be treated using the same isotropic formalism already applied there.

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