Thermally Curved Glass - Research and Standardisation of the Four-Point Bending Test
Research has shown that the general approach to determine the bending strength of thermally curved glass with the aid of the four-point bending test for flat glass according to EN 1288-3 is applicable. At the moment, however, there is no statutory method available for directly extrapolating the breaking strain from the breaking load without the use of expensive strain gauges. Such a method is developed by means of an experimental and numerical study in a research project between the Bundesverband Flachglas e.V. (Federal Flat Glass Association) and the Fachverband Konstruktiver Glasbau e.V. (Professional Association Structural Glass Constructions), the Institute of Building Construction at the Technische Universität Dresden and four glass bending specialists. Moreover, it will provide the basis for testing cylindrically curved glass of any bending radius. The project partners have already submitted a proposal for a new standard to the Deutsches Institut für Normung e.V. (German Institute for Standardisation). The paper gives the theoretical background for determining the strength of flat glass according to EN 1288‑3 and quantifies the additional effects for the testing of thermally curved glass. Furthermore, the testing method of the executed experimental study on thermally curved glass is described. The range of parameters for the tested specimens included the glass thickness from 3 mm to 12 mm and the bending radii from 400 mm to 2000 mm. We recorded the strain at selected points at the tensioned side and the input force for each specimen. The measured relation between strain and load validates a numerical model, which is able to depict the actual two-dimensional stress distribution for thermally curved glass and includes non-linear aspects. The model allows for the determination of correction factors. With those, the bending strength of thermally curved glass can be calculated by measuring merely the input force during the four-point bending tests. The correction factors are summarised in charts depending on the radius and the thickness of the glass sample.