Tempered Glass Expert Describes Two Tempered Glass Fracture Surfaces

Glass failure analysis expert witness compares two typical tempered glass fractures. The lower photomicrograph shows a typical thermally tempered glass fracture surface . This piece is from a fully tempered glass sheet. That is; this glass has a surface compression stress of 10,000 psi or more. In addition, the compressive layer on each face is 20% of the thickness. This fracture surface has two sets of Wallner lines separated by mist hackle. The upper photomicrograph shows a representative chemically strengthened glass fracture surface. This one is of a 0.05" thick piece of chemically tempered glass. In this case the compressive layer is approximately 10 microns thick and the compressive stress is approximately the same as thermally tempered glass. The chemically tempered compressive layer is too thin to dominate the fracture. The Wallner lines on this fracture surface are similar to those one would find on the fracture surface of annealed glass.