Tempered Glass Expert Discusses Tempered Glass Fracture Surfaces

Fracture Surface of Thermally
Tempered Glass

Fracture Surface of Chemically
 Tempered Glass

The Glass Expert Witness at Read Consulting have performed hundreds of glass failure analysis on tempered glass pieces. These include both thermally tempered and chemically tempered glass.


Thermally tempered glass has compressive stress on its surfaces. The thickness pf the compressive layer is a function of the over all glass thickness; this thickness is 20% of  the glass thickness. In addition, the resulting fracture surface of thermally tempered glass has distinct characteristics. There are two sets of Wallner lines (which indicate the crack travel direction) separated by a band of mist hackle. In addition, near the surfaces there are parallel markings indicating the surface compressive stresses.

Chemically tempered glass also has its surfaces in compression. For this glass, the compressive layer is very tin; it is on the order of 10µ's. In addition, this thickness is not a function of the glass thickness. It is dictated by the chemical tempering  process. Because the compressive layer is so thin, there are no distinct markings on the fracture surface. Often the fracture surface is featureless, or it will have a single set of Wallner lines indicating the direction of travel.

Touch Screen Failure Analysis Failure Analysis of Chemically Tempered Glass

Fracture Surface of Chemically Tempered Glass
Taken at the origin Mag 30 X

The glass failure analysis experts at Read Consulting failure analysis labs have over 40 years of experience working with chemically tempered glass (such as gorilla glass). Chemical tempering uses ion exchange techniques to create surface compressive stresses. Unlike thermally tempered glass, the thickness of the compressive layer is not a function of the glass thickness, and it is extremely thin (approximately 10µ). Typically chemically tempered glass is used on most touch screen computers and touch screen phones. The glass on these devices is too thin (less than 1mm) to be thermally tempered. Chemical tempering makes the glass stronger than annealed glass in bending, but it is equally vulnerable to cracking from impact. Above is a failure origin from a hand held digital controller. This glass was less than 1mm thick  This failure was caused by point impact. This failure is no different than what one would find if an annealed piece of glass had been subjected to the same sort of point damage.

Safety Glass Expert Performs Tempered Glass Analysis

Glass failure analysis often requires understanding of safety glass. Normal window glass is in the annealed state. Therefore, it will break into large sharp shards that can injure a person who breaks through it. In order to reduce the probability of failure and the possibility of injury two types of safety glass have been developed. The first is laminated safety glass, and the other is tempered safety glass (i.e. surface compression stress >10,000 psi). Automobile windshields and hurricane resistance windows are made from laminated safety glass. These are special applications, and laminated safety glass is appropriate, even though it is more expensive. In most other safety glass applications, thermally tempered safety glass is used. These windows are safer because they are stronger and because they break into small "blocks" when they fail. These two factors significantly reduce but do not eliminate the probability of injury to anyone that breaks through the glass. There are two prevalent techniques to evaluate the level of temper in glass. One is to measure the surface compression stress with a grazing angle surface profilometer (GASP); the other is to break the tempered glass per UL guidelines and measure the area of the largest remaining pieces. The photographs above show the two methods. Upper left is a photograph of the fringe pattern of a tempered 9" X 13" X 4 mm thick glass oven door taken through a Strainoptic Technologies GASP. This image shows the compressive surface stress to be -20,000 psi. Upper right is a photograph of the same door after the UL fragmentation test (i.e. using a center punch to penetrate the compressive layer). One can see that the resulting pieces are extremely small. In general, the pieces are aqlmost cubic.

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.