Edge Strength Testing of Thin Glasses




Following a brief history of various tests for measuring edge strength, we present the theory and application of four-point vertical bend test (VBT). The latter is similar to a standard four-point bend test except that the specimen is turned on its edge so as to permit application of uniform tension to a majority of the edge flaws. The article cautions against lateral buckling of the VBT specimen, resulting in failure initiation from the “compression” edge, if its dimensions are not optimized. We provide guidelines for designing the width and thickness of the VBT specimen relative to its length, which obviates the onset of lateral buckling. Three simple inspections to verify the validity of VBT include the shape of load versus deflection curve, location of failure origin, and fracture pattern. Two examples, one involving edge quality of an aluminosilicate glass and the other involving factors that affect edge strength of automotive glass, help illustrate the application of VBT.