The brittle to ductile transition in a soda–lime–silica glass

The brittle to ductile transition (BDT) occurring in a standard window glass around Tg has been studied both theoretically and experimentally by introducing artificial cracks in glass specimens tested in bending at different temperatures with different displacement rates. The transition results from the competition between the loading and the relaxation kinetics in the vicinity of the most critical flaws. The results show that the higher the loading rate the higher the transition temperature range where the behavior turns from elastic (brittle) to viscoelastic. This study therefore examines the BDT, which corresponds to a change in the mechanical behavior from elastic to viscoelastic and is relevant to the general problem of the crack tip singularity in viscoelastic solids. In order to get insight into this complicated problem, the temperature dependence of Youngʹs modulus, shear viscosity, and relaxation kinetics were first characterized. A simple model was further developed to address the BDT more quantitatively, using both crack tip stress, and stress intensity factor as key parameters. Depending on the imposed strain-rate between 10−5 and 10−3 s−1, the BDT ranges from 490°C to 560°C. The model allows for a good description of the behavior, accounting either for the strain-rate dependence or the temperature sensitivity. Furthermore, the increase in the apparent fracture toughness usually reported in most glasses or glass-containing ceramics near the glass transition temperature is also predicted by the model.