Relaxation and nanomechanical studies of the vickers residual stress field in glass

Sammanfattning: The Vickers residual stress field (VRSF) in soda-lime glass results from the elastic-plastic contact event that takes place when a Vickers diamond pyramid is loaded onto the surface of the material in an indentation cycle. The importance of elastic-plastic indentation lies in the contact damage that it gives rise to in the surface of the glass. Since such surface flaws can be characterised, with respect to shape and size, they offer the opportunity to study naturally occurring flaws in glass and brittle materials in general. The residual stress field is not passive; rather it exerts a crack opening force on the associated crack system during subsequent strength testing of a Vickers-indented sample through a residual stress field coefficient, c. Besides the strength-controlling properties, the elastic-plastic contact residual stress field is also important as a region where the influence of mechanical excitation on material properties such as hardness, H, and elastic modulus, E, can be studied.This thesis concerns studies that were made to characterise the Vickers residual stress field by first measuring the magnitude and distribution of stresses around it, using nanoindentation with a cube corner tip. With a Berkovich tip in nanoindentation, experiments were conducted in the VRSF to study the dependence of hardness, H and elastic modulus, E, on stresses in soda-lime glass: a strong E dependence on stress was observed, while H was not affected unless the stresses were high. In the process, a method was developed to determine the true contact area during elastic-plastic nanoindentation when the Oliver-Pharr method is used for the data analysis.The observed elastic modulus dependence on stress was then utilised in a study where it was shown that the VRSF responds differently to relaxation annealing on either side of the glass transition temperature. This result was then used to explain strength recovery trends in annealed Vickers-indented glass specimens.