Experimental and Numerical Investigation of the Bending Strength of Glass
Annealed glass has a high compression resistance but it is fragile and its tensile strength is low due to the random distribution of surface flaws and impurities, which induce cracks without prior warning and each crack is regarded as a failure. This phenomenon is dependent on surface micro-defects in the glass due to inclusions within the glass or to scratches caused by normal use and by the shaping process. Classical four point bending tests are not suitable for glass plates, because of the test’s configuration, the maximum stress is reached on the plate edges where, due to the cutting process, the defects are more likely than on the internal part of the plate. A satisfactory way of overcoming this uncertainty is the use of coaxial double ring tests in which the maximum stress is reached approximately in the centre of the glass plate, far from the edges so that the ultimate strength is not influenced by the defectiveness of the cutting edges. This test provides good results when a uniform and equibiaxial state of stress in the core of the specimen is induced because the geometric non-linearities are less significant in this condition. The EN 1288-2 European standard proposes applying an additional overpressure during the load phase, to compensate the second-order effect, especially in the case of big specimens where the non-linearities are more significant. The application of the overpressure makes the test more difficult to perform and furthermore some authors have proved that the induced stress state is not uniform and equibiaxial. In this paper the first results of a bending strength investigation on glass plates are shown. This investigation consists of a numerical non linear analysis in which the geometric non linearity is considered. At the same time an experimental investigation is carried out using coaxial double ring tests in order to compare the results of the numerical analysis with experimental tests using glass plates with different thicknesses. This experimental campaign will provide new and useful information on how to perform an easy testing method to evaluate glass bending strength without the use of overpressure.