Biaxially Curved Glass? Determination of Strength using the Coaxial Double Ring Test

The coaxial double ring test is used for determining the surface strength of flat glass as it is described in existing standards. The main idea of this test setup is to generate a defined area of surface tension stress, which is uniform in all directions within the load ring. Thus, the orientation of surface flaws does not influence the test results. Furthermore, the defined area with uniform stress allows a statistical evaluation with regard to the flaw distribution and therefore a prediction for deviating area sizes is possible.

For biaxially curved glass, the generation of such an area of uniform tension is not possible easily. However, for large radii of curvature and low glass thicknesses it is possible to flatten the specimen and to generate an area of uniform tension stress with increasing load. This contribution shows under which geometrical conditions the coaxial double ring test can be used for biaxially curved glass. Taking into account the method of effective stresses, inhomogeneous stress distributions with regard to orientation and intensity can be evaluated thus the range of applicability can be increased. Finally, the determination of fracture stresses for different load levels for the test setup is illustrated.