Assessing Already Broken Tempered Glass

Glass Expert at Read Consulting is asked to assess whether a broken window or shower door was fully tempered. One method is to select the ten largest "crack free" pieces and determine if these can fit into a 10 square inch area. In the photo above, a 3" X 3" (approximately 10 square inches) "frame" is placed over the broken glass. In this example there are many more than 10 pieces in the opening. Before breaking this glass door, the temper stress was measured to be 16,000 psi using a Strainoptics grazing angle surface profilometer  (GASP). However, it is important to note that there are cases where the fully tempered glass will not fragment properly. The larger pieces result from the stresses applied to break the tempered glass door or tempered glass window.  

Failure Analysis Lab Analyzes Shower Door Failure

Installed Shower Door

Top Roller  at Stop

Disassembled Roller

Fracture Surface at Origin

Tempered glass shower doors have been modernized. Recent designs have the doors suspended by rollers that ride on a pipe. In certain cases the door stop is on the top. In this case, the roller is stopped by a rubber bumper. Several of these doors had failed during use, and the glass experts at Read Consulting were asked to perform a failure analysis and suggest solutions to the glass fracture problem.

The two top photographs illustrate how these types of doors are installed. Basically they are hung on two rollers that are bolted to the tempered glass doors using holes cut into the glass. At the end of travel, these doors are stopped when one roller hits a rubber bumper. There is some cushion from the rubber bumper; however, this is not enough.

Glass failure analysis was performed on a representative failed door. In this case, the failure origin was clamped in one of the two rollers. The failure initiated on the interior wall of one of the through holes in the glass. It is believed that the failure forces are created by torsional moments generated because the top of the door is stopped, but the bottom of the door is not. When the door is stopped at the top, the unrestrained bottom creates a large rotation moment. This moment puts high forces on the walls holes in the glass. These forces act on the ground hole walls to cause failure.

The two lower photographs document the glass failure analysis. It is believed that the stopping mechanism must not generate twisting moments. One solution is to put the rubber stopper in the center of the door, not at the top. Another solution is to slowly reduce the door speed (i.e. create drag) before it hits the stop at the top.

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.

Tempered Glass Oven Door Thermal Shock Test Failures

 Glass experts at Read Consulting  performed a glass failure analysis on several fully tempered glass oven doors that failed in an unexpected manner during thermal shock testing. 100% of these doors failed the glass thermal shock test. The subject tempered glass doors were painted
with a 20µ thick picture frame on one side (top left photo). Using a Strainoptics grazing angle surface profilometer (GASP), the temper stress for all the doors tested (i.e. before thermal shock testing); all doors  measured to have surface compression stresses in excess of 13,000 psi. In the upper right photograph is a typical GASP output for these doors. Therefore one would expect the failure mode to be "dicing".In fact, all the failures initiated with long thermal
 cracks. Only in the later stages of failure did portions of the doors break into small pieces (i.e. dice). A typical failure is shown in the lower left photo. A typical failure origin is shown in the lower right photograph.         The failures initiated under the paint. This indicates that, during thermal shock, tensile stresses were created at the paint-glass interface that overcame the tempered glass compressive surface stresses and initiated a crack. These painted tempered glass oven doors had a 100% failure rate; Identical unpainted doors had a 100% survival rate.

Glass Failure Analysis: Tempered Glass Pot Cover

California glass expert performs a root cause failure analysis on a failed tempered glass pot cover. The pot cover failed "spontaneously" in the users home, and the handle with captured glass was submitted to the California failure analysis lab for evaluation. The failure was caused by the interaction of the handle hardware (i.e. cup washer) with the tempered glass. Crush damage from the cup washer that caused the failure. The series of photo micro graphs above demonstrate the findings. The lesson to be learned here is: there should be no glass-to-metal contact with tempered glass. This type of contact raises the probability of failure. In the case of tempered glass the failure appears to be spontaneous.    

California Glass Expert Examines the Edge of Tempered Glass

California glass expert uses a grazing angle surface profilometer (GASP) to study the compressive stresses at the edge of a tempered glass oven door. The edge effects during the air quench step of the thermal temper process are said to lower the surface stress to zero near the edge of the glass article. In fact, fragmentation tests of tempered glass pieces yield a "picture frame" around the edge of the part. Fir this reason, a GASP was used to examine surface stress at the edge. Upper left is an image of the GASP interference pattern in the center of a tempered glass toaster oven door. The surface compressive stress measures 17,000 psi. Upper right is a similar image measured near the edge of the part. In this region the fringes curve, and they show the surface stress going to zero. This is important because any damage to the glass in this region of reduced surface compression stress is more likely to cause failure. 

Tempered Glass Failure: Nickel Sulfide Particle

California failure analysis expert examines glass door failure to determine the root cause of the failure. The subject tempered glass door failed spontaneously, and because the glass was verticle, the broken pane remained intact. Read Consulting failure analysis laboratory was able to obtain the origin and perform a root cause failure analysis. The attached 25X Nomarski photomicrograph demonstrates that the glass failure originates at a nickel sulfide (NiS) particle. Once the glass failure has been initiated, the failure is driven by the residual stresses in the thermally tempered glass.  Glass fracture analysis shows the Wallner lines initiating at and expanding from a NiS particle. NiS particles are common in float glass, but they are only a problem in thermally tempered glass. Read Consulting Laboratory performs similar failure analyses on metals, plastics, ceramics and glass on a regular basis 

California Glass Expert Discusses Nickel Sulfide in Tempered Glass

In glass failure analysis one uses evidence such as the the attached photograph to demonstrate that a nickel sulfide particle can initiate tempered glass failure. The NiS particle is at the center of expanding Wallner lines. The Wallner lines are used by the glass failure analysis expert to track the direction of the crack growth. In this case the Wallner lines are spreading from a single point; thus, this is the failure origin and the NiS particle is the cause of the tempered glass failure. The California glass expert has seen numerous examples of this type of glass failure.

WIndow Defect Analysis

Glass failure analysis expert witness has analyzed numerous cases of window damage. Often a metal blade is used by the window cleaners. In many cases the window damage is found on new construction. GANA has specific window cleaniong instructions that forbid the use of metal blades. The concern is that the blades pick up abrasive particles that get between the blade and the window glass and cause glass scratching when the blade is moved across its surface. In the case of scratched tempered glass, there is a consern that glass particles have attached themselves to the window surface during the tempering process. Later cleaning will dislodge these particles and they will cause scratching. The california failure analysis expert has examined unscratched clean windows that have streaks that were shown to be adhered glass particles. This observation proves two things:

1. Cleaning tempered windows with adhered particles can be done without scratching. Use of a blade during cleaning of tempered glass will cause scratching and the abrasive is not necessarily adhered glass. It can also be dirt or abrasive materials such as concrete or stucco.

2. Microscopic examination can be used to determine if there are glass particles stuck to the surface of tempered glass.

Above are two microphotographs of a tempered glass surface found to have adhered glass. Upper left is 25X magnification and upper right is a 200X magnification. This window was cleaned before the pattern of adhered glass was discovered. Tempered glass window cleaning defects can be avoided even when there are adhered glass particles. On ther other hand, it is extremely risky to use metal blades for window cleaning if window scratches are a problem.

GLass Expert Discusses Nickel Sulfide Spontaneous Failures

Glass failure analysis expert witness demonstrates that in the case of the spontaneous failure of tempered glass, the tempered glass failure is caused by the NiS particle. The failed glass door was taken apart to reveal the NiS particle. At this point the Wallner lines surrounding the NiS particle were located. The Wallner lines domonstrate that the NiS particle is in the center of the failure origin, and must be the cause of failure. The upper left photograph is a 20X photomicrograph of the NiS particle and its surroundings taken using Nomarski optics. Upper right is the same photomicrograph upon which arrows have been placed to show the crack travel directions as indicated by the Wallner lines. One can see that the NiS particle is at the center of the origin, and is the cause of the failure.

Glass Expert Discusses Termpered Glass Cookware

Modern Pyrex glass cookware is now made from tempered soda lime glass. As a result there is a potential that the tempered glass failure will create dangerous projectiles. In one case it was found that if the temper glass stresses are not symetrical, flying glass pieces will occur. In the present case, tempered glass cookware lids were caused to fail; the resulting failure was completely contained by the stainless rim. The rim performs two functions. It protects the vulnerable edges of the tempered glass lids from glass damage. In addition, it appears that it contains any glass particles that could become projectiles.

Glass Expert Discusses Visibility of NiS Particles

Tempered glass failure analysis is mainly difficult because the panel disintegrates into a very large number of semi identical particles; therefore, the failure origin is impossible to locate. In several cases Read Consulting has had the opportunity to examine several failed (caused by Nickel Sulfide, NiS, particles) tempered glass panels that have remained intact. In some cases the windows were laminated; in the others, the windows remained verticle and stayed interlocked. Once the origin is located and preserved, an eyeloop or low power microscope is all that is required to see the NiS particle. The upper left photograph is a macrophotograph of an inplace failed laminated window. In this case the NiS particle is visible to the naked eye. It is a sphericle particle on the fracture surface. Upper right is an 8X photo-micrograph of the particle after the window has been taken apart. This root cause failure analysis does not require scanning electron microscope. This type of tempered glass fracture analysis merely needs an intact failed window.

Glass Expert Discusses Exploding Tempered Glass

Classic Symetrical Temper Stresses

Three Photomicrographs of

Poor Temper Fracture Patterns

There has been some concern about glass oven wear made from tempered soda lime glass exploding while in use. A Glass failure analysis expert witness performed fractography on a thick glass jar that had been "tempered". Because this is a prototype piece, the temper was not done correctly. When the glass jar surface was abraded prior to thermal shock testing, it exploded violently. Recent tests by this glass expert on properly tempered glass had shown no "explosion". The glass merely cracked in place. Failure analysis of the two types of glass revealed some interesting differences. The non-exploding tempered glass showed symetrical stresses in the glass. The photomicrograph on the top left is representative of the stress pattern for standard tempered glass. In this case, the outer surfaces were under approximately the same compression, and the center of the glass was in tension; this is demonstrated by the "mist hackle" in the center of the fracture surface. Also, there are two sets of symetrical Wallner lines on either side of the center mist hackle. In the case of the exploding glass jar, the temper was found to be non symetrical and not uniform. In some regions the fracture surface had the mist hacklemuch nearer one surface; in addition, one surface does not appear to have been in compression. Other areas on this jar showed weak or very weak surface compressive stresses. The other three photomicrographs demonstrate thiese results. At this point the glass failure expert believes that improperly tempered glass oven ware can "explode". This manufacturing defect may be a product liability and could pose some danger to the user.

Tempered Glass Expert Shows Multple Views of Nickel Sulfide Particle

The result of a failure analysis, a glass expert presents several views of a 180µ diameter origin (nickel sulfide particle) of a failed tempered glass door. The upper left photomicrograph is taken with a low power optical microscope and it includes the NiS particle and the initial Wallner lines. Upper right and lower left are taken with a high power optical microscope. with and without Nomarski. Finally lower right is an Scanning electron microscope photomicrograph of the same particle. The inportance here is that the distinct shape and surface of a NiS particle makes it identifiable with an optical microscope.

Failure Analysis Expert shows Nickel Sulfide Particle

Failure analysis expert observed a nickel sulfide particle using Nomarski optics. The various colors show different elevations of the surface around the particle. This particle was at the origin of a failed tempered glass door. This is an interesting result of a glass failure analysis performed on the door.

Failure Analysis Expert Performs Glass Failure Analysis

Glass expert performs a failure analysis on a failed tempered glass door. The spontaneous glass failure was analyzed to determine the cause of failure. This was one of several doors that failed spontaneously on a new high rise. Fortunately the door did not fall apart; thus, the failure origin could be preserved. The upper left photo shows the failure origin (arrow). The origin was taken apart, and at the center was a spherical 180 micron diameter nickel sulfide particle, NiS particle. During the glass failure analysis, a scanning electron microscope, SEM, image of the particle is shown in the upper right. Elemental analysis of the particle showed it to consist of nickel and sulfur. The SEM image also shows initial Wallner lines that are spiral in shape and are leaving the particle. This identifies the NiS particle as the cause of the failure

Glass Failure Expert Looks at Tempered Glass Failure

Glass expert analyzed a broken tempered glass door. He was able to preserve the origin and perform a failure analysis. The spontaneous glass failure started in the center of the window and was caused by a nickel sulfide particle (NiS particle). This failure mechanism is well known and has been documented by others. In this case, the glass expert spent some time documenting the initial crack growth. The upper left photomicrograph shows two sets of Wallner lines spreading away from the general location of the 100 micron diameter spherical nickel sulfide particle. The photomicrograph on the upper right shows a circular crack arrest line surrounding the NiS particle. This circular artifact identifies the NiS particle (a manufacturing defect) as the cause of the failure. In addition, it also indicates that the crack stopped at least once before the glass went to failure. At first the crack started as a round crack due to the stresses from the nickel sulfide particle. Later the residual stresses in the glass took control. The circular crack opened up and turned into the classic sets of Wallner lines found in root cause failure analysis of tempered glass.

Analysis of Failed Glass Baking Dish

Glass failure analysis expert analyzes failed glass baking dish"Pyrex". Traditionally, glass baking dishes have been made from annealed borasilicate glass (i.e. Pyrex glass). Now baking dishes are made from tempered glass. The glass is still called pyrex but it is really tempered glass (soda lime). As a result of this change, the newer dishes are labeled to be safer because they are made from tempered glass. Given this, a person was injured by a failed baking dish that was sold as tempered glass. A failure analysis was performed on the subject dish.
The failed dish is shown above . This dish failed as a result of a blow to the region indicated by the arrow in the figure. The fracture pattern and the size of the resulting pieces proves that this dish is made from annealed glass; therefore, it is not made as advertised. It is not made from tempered glass.