Determination of the bending strength of glass via four point bending: correction of factor kₑ in EN1288-3

EN1288-3 (Glass in building—Determination of the bending strength of glass—Part 3: test with specimen supported at two points (four point bending), 2000) describes the four point bending test for the determination of glass strength. The failure stress is calculated from the vertical force of the testing machine. To determine the difference between the stress calculated from linear 1st order theory and actual glass stress, a factor kₑ is given. This factor is derived from a 1994 publication (Blank in Glass Sci Technol 67:9–15, 1994) that uses a very simplified finite element model. The present paper will show that for tests with high deformation, which becomes more relevant for smaller glass thickness and chemically strengthened glass, these results differ vastly from the actual behaviour of the glass in the four point bending test as it is performed in most industry and lab settings. With the corrected curves for kₑ, increases in the characteristic edge strength of more than 50% compared to the current version of EN1288-3 can be derived from the four point bending test in some cases.