D5 Mechanical Tests (Flexure and Fiber)

Topic

The mechanical tests for this model cover two distinct geometries of this glass: glass in plate form, using a flexure test, and glass in fiber form, using a direct tensile test. Because each geometry is tested differently, neither shares the exact same testing method as the other.

The flexure test—in either a three-point or four-point configuration—measures the flexural strength of the glass in plate geometry; for this test, the model reuses the same flexure method employed for the same purpose by another model in this set (the one describing the flexure test for a ceramic). Since a brittle glass plate is difficult to grip for a direct tensile test without the gripping mechanism itself causing premature failure, flexure is preferred over straight-line tension—for the exact same reason that motivates the ceramic flexure test. The higher moment of inertia of the specimen's cross-section results in lower maximum elastic deflection under a given load, just as occurs in the reused model.

Glass in fiber form, conversely, is tested via direct tension; this is possible because the fiber possesses a much higher intrinsic strength than the glass plate. According to the scale effect established in another model in this set, the expected strength of the glass depends on its exposed surface area, and a thin fiber has far less exposed surface area than a plate. This reduced exposed surface area makes it less likely that the surface contains any of the most severe surface defects; consequently, the fiber's intrinsic strength is much higher than that of a plate made from the same glass. The fact that this intrinsic strength is so high allows the fiber to undergo a direct tensile test without the specimen's gripping mechanism causing premature failure.

Consequently, this model—utilizing the two glass geometries with which it was initiated—allows each form to be tested in the manner best suited to its specific strength: the plate is tested using the same flexural test adapted from ceramics, as its strength is insufficient to withstand the gripping required for direct tension without failing; conversely, the fiber undergoes a direct tensile test, because the very scale effect that reduces its exposed surface area is precisely what provides the intrinsic strength needed to tolerate that gripping.

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