Stress-corrosion and lifetime in glass : influence of environmental conditions

Abstract

The problem of glass strength arises from the increasingly spread use of the material in building. Due to its high transparency, glass has been always used as closure element, but in recent years a tendency of using it for structural purpose is growing rapidly. In addition, glass provides challenging solutions for conservation of archaeological heritage. These reasons require a deep investigation on failure mechanisms of glass and above all on its resistance to tensile stress. Since 1920, linear elastic fracture mechanics allowed understanding the reasons of the low tensile strength values registered in glass. It seems that this is due to the propagation, even very slow, of surface microdefects induced by manufacturing processes. Later, physical theories empirically founded further investigated on the processes of rupture in glass, observing that microdefects may evolve up to structural failure not only for a stress increase but also as consequence of chemical-physical interaction with surrounding environment. Most of prediction models which aim at evaluating a glass element lifetime are based on empirical laws. The parameters of those theories are related to the influence of environment but their values are provided by the literature only within limited ranges. However, the influence of environmental variables is complex and not secondary. In addition, in recent years, glass is being used in building even in non favorable climatic conditions. This latter is a challenging aim, considering the high aesthetic qualities of glass. For these reasons, the present thesis considered necessary to investigate on the microphysics and chemistry which determine the phenomenon of environmental corrosion, and this allowed identifying the physical variables which play the most influent role. Secondarily, an expression of failure time as explicit function of environmental variables is provided, focusing on temperature and relative humidity. Through parameter calibration, on the basis of four-point beam-bending tests, a lifetime curve is obtained for a given environment. This work arises on the one hand from a physical interest and it shows a scientific coherence, considering the strong influence of environmental variables, and on the other hand it is strictly connected to design purposes, since glass is being used in increasingly disparate climatic conditions. For this latter reason, to be able to express failure time as function of environment becomes a non negligible need.