Date: June 2019
Type: BSc
Student(s): Pascall Qvistgaard Christensen
Daniel Gunge Johansen Christoffersen Eriksen
Supervisor(s): Jens H. Nielsen
Luisa Giuliani
Others: VELUX (for sponsoring the glass specimens)
El'Sec A/S (for sponsoring additional material needed)
The strength parameters: surface residual stress, inherent and apparent strength of respectively tempered and float glass, subjected to varying temperatures below glass transition temperature, have been examined. This is done both prior to and after a heating process. Surface residual stresses have been measured with a Scattered Light Polariscope. Apparent strengths are calculated by Finite Element Method, using failure loads from Coaxial strength tests. Inherent and apparent strengths prior to heating have been determined by having a reference series which have not been heated.
It was found that the tested tempered glass being heated for 2 hours at temperatures of 350°C yielded relaxation of residual stresses. Additionally, it was found that increasing the temperature up to 450 °C resulted in constantly greater relaxation in residual stresses. For tempered glass being heated at lower temperatures for longer time, the changes in surface residual stresses are not as significant.
Furthermore, it was found when heating for 2 hours that an increase in temperature, had evidence of statistical significance for increased inherent strength, thus being quite remarkable at the highest temperatures for both the tested tempered and float glass.
There was not found any conclusive evidence that residual stresses caused greater healing of surface flaws, as float and tempered glass seemingly gained inherent strength evenly. Apparent strength of tempered glass, did not experience any significant changes after being heated, whereas float glass gained apparent strength evenly with the inherent strength, as residual stresses for float glass already being incredibly low prior to heating. Eventually, evidence of statistical significance, for surface residual stress variation in tempered glass, being more isotropic as the temperature increased, grew rapidly with the increasing temperature. Thus, it can be concluded that the tested specimens in most cases experienced changes in strength parameters due to the test conditions.