Investigating the strength effects of drilling in tempered glass

Abstract

The tempering process introduces a residual stress field to the glass that only allows a drilling depth of approximately \({20}{\%}\) of the thickness (compressive zone) before fragmentation will be initiated. The present paper investigates post-drilled holes in tempered glass that can be used for a novel assembling technique leading to an increased accuracy in the connection and avoiding custom made solutions with holes drilled prior to tempering. The partly drilling is first studied experimentally measuring the change in strains (stresses) on the surface close to the hole as function of drilling depth. These data are then further used to validate a finite element model capable of describing the redistribution of residual stresses around the hole for several geometric variations such as depth, diameter, rounding and inclination. Above all, the numerical study shows an increase in the apparent strength (The term “apparent strength” is used for the strength originating from both the material strength and the residual stress state) of the hole. Furthermore, it is seen that drilling into the compressive zone only, will not lead to any tensile stresses at the surface of the hole. As the drilling process will damage the glass around and inside the hole, hence decreasing the strength, an etching technique is finally given to post-improve the quality of the hole surface locally. After 6 h of local etching with ammonium hydrogen fluoride \((\hbox {NH}_{4}\hbox {HF}_2)\), an increase in strength of about \({310}\,{\%}\) is observed. Increasing the etching time further, with the given acid and concentration, is shown not to yield any further (significant) increase in the strength.