Finding the strength of glass

Abstract

Over the years, glass has been increasingly used as a structural building material. Structural calculations on glass are conservative because the strength is quite uncertain. Weibull plots of the strength of glass can range from 20 MPa to 150 MPa. This is caused by flaws, which are not visibile to the human eye and therefore it is difficult to predict glass strength. Because of this uncertainty, in practice, 20 MPa is assumed and a high safety factor is applied. Therefore, more research on glass' strength is important. This thesis focusses on gaining a better understanding of the biaxial strength of glass. This was achieved by performing a literature study and conducting strength tests and microscopic observations. A ring on ring test was chosen for the tensile biaxial strength testing. 53 cruciform plates were tested of which 5 with a rosette strain gauge. It was found that the ring on ring formula, that gives the biaxial tensile stress within the inner loading ring, deviates around $20\%$ from the strain measurements. This can be attributed to assumptions that were made to derive the formula. Therefore, the formula should be further explored with more strain measurements and, if necessary, finite element modelling . Additionally, there was a variability in the strain measurements, which might have been caused by friction. Moreover, 30 out of the 53 tested plates showed circumferential cracking at the support ring, which is not mentioned in literature. For these tests, recommended ratios from biaxial testing on ceramics were applied. 10 tests of glass plates with different ratios resulted in no circumferential cracking at the support ring. It was concluded that the recommended ratios should be changed for the application on glass. A traditional fractographic analysis of glass consists of a fracture pattern analysis by eye and a fracture surface analysis by eye and by microscope. In this research, it was suggested to also study the fracture pattern by microscope. By using a transmitted light microscope with Z axis builder software a multifcous image was obtained representing all flaws and fracture marks throughout the thickness of the glass plates. This turned out to be a valuable addition, because this image revealed fracture marks that would otherwise have been overlooked. These can be used as a quick guide to find the fracture origin and give new insights on the crack propagation. In future research, it is recommended to optimise the ring on ring test so that certainty is acquired about the strength results. With a fractographic analysis, different flaw types can be characterised. It is expected that when these flaw types are related to the strength data, more sensible Weibull plots can be made. This information can then be used as an input for a better glass strength assessment and improvement.