Glass Expert Discusses Tempered Glass Fracture Surfaces

California glass expert witness discusses crack travel directions during tempered glass failure. During tempered glass failure the glass sheet "self destructs" because the internal residual stresses are tensile and drive the failure. Essentially the glass failure results in the glass breaking into tiny cubes. This means that the crack is constantly changing directions. The California failure analysis expert has observed that the crack breaks through and travels in two directions. This is shown in the 20X photomicrograph above. Even though the crack is spreading in two directions, this does not indicate that this piece contains a failure origin. This is an interesting special case where the crack is spreading from a point, but it is not exiting the origin.

Glass Fracture Due to Liquid Pressure

Glass failure analysis expert witness performs a failure analysis on a hollow glass tube that failed due to internal pressure. The glass fracture initiated at a small bruise on the outer surface of the glass tube. The upper left 25X photomicrograph gives an overview of the failure. One can see that the glass fracture analysis determined that the failure initiated at a small bruise. The upper right 100X photomicrograph gives more detail of the origin. This was a manufacturing defect because the bruise was created during manufacturing.

Failure Analysis of Reed Switch Glass Failure

Glass expert performed a root cause failure analysis on failed reed switches. The upper left photograph shows the type of reed switch that was failing. This switch consists of two ferromagnetic "blades" that are enclosed in glass. At either end are leads that pass through glass-to-metal seals. The sealed tube is filled with inert gas which is introduced through a side tube that is heat sealed after the gas has been introduced. The switch is activated magnetically. The glass expert performed a glass failure analysis and determined that the glass envelopes were cracking due to the presence of mechanical damage (i.e. scratches) on the outer surface of the glass. All the cracks initiated at a flaw and then proceeded circumferentially till the two halves of the crack met on the opposite side from the origin. It is believed that the source of the failure stress was thermal stress generated by heating during potting and assembly. WIthout the pre-existing flaw on the outer glass surface, the switch would not have failed. The center photograph shows the initial crack leaving the origin in two directions. These two halves of the crack eventually meet on the side opposite the origin. The upper right photograph shows the surface flaw where the crack originated. These switches failed as a result of a manufacturing defect.

Glass Failure AnalysisExpert Examines Cracked Automobile Windshield

Glass expert performs a glass failure analysis on a cracked automobile windshield. Records show that the windshields on this particular make and model of automobile are cracking at a very high rate. The windshield shown here is the third replacement in two years. In this case, glass failure analysis shows that the crack started at an impact point two inches below the top of the windshield. Initially the windshield crack was driven by bending stresses. Later it was driven by thermally generated stresses (caused the crack to curve). Factors that are of concern are the glass thickness, mounting (i.e. bending) stresses in the windshield, perpendicularity of the windshield and glass lamination stresses. Eventhough the failure was caused by an impact point; this damage should not have caused a large crack to form. The upper left photo is an overview of the crack. In the center is a close up of the impact point. Upper right is a 40X photomicrograph of the crack origon.

Glass Failure Analysis of Tramp Glass

As a part of a product liability case, a glass failure expert was asked to determine if a piece of "tramp glass" found by a consumer (Upper left photo) was put into the bottle after it was opened. There was a companion bottle from the same six pack that also had glass contamination(upper right photo). If the tramp glass in the subject bottle was planted, then the glass in the "sister bottle" was also put in by the consumer. These were "twist off" bottles; therefore, it was decided that if the torque needed to remove the cap on the sister bottle was low, this would indicate the cap had been previously removed by the consumer. The removal torque was measured on the sister bottle, and it was twice that measured on control bottles. Thus, the cap on the sister bottle had not been removed, and the glass contamination entered this bottle during bottle manufacturing or bottle filling. From this it was concluded that the tramp glass in the subject bottle is also a manufacturing defect.

Wine Bottle Failure Analysis

Glass failure analysis expert locates the origin of a failed wine bottle. Thye failure origin is at the top of the bottle on the inside rim. The origin is a crush spot on the inner rim. This was too high to be caused by a cork screw. It is believed that the damage occured before the bottle was purchased. Root cause of the failure is mechanical damage to the top of the bottle on the inner rim of the neck.

GLass Expert Analyzes Wine Bottle Failure

Glass expert analyzed several leaking wine bottles that eventually broke in half. All the bottle failures occured on the heal. Also, during failure analysis, it was discovered that all of the bottle failures were made in one cavity of a multi-cavity bow mold tool. Bottles from the other cavities did not have this problem. These bottles were made from very thick dark glass. It is believed that the failures were initiated as chill cracks. However, more process development work must be done at the factory to correct this manufacturing defect. Two suggestions are to eliminate any drafts in the factory and increase the amount of post mold flame heating on the conveyer. Bottles made from thick glass are more likely the have this glass failure mode.

Pyrex Glass Bakeware

Glass expert discusses recent changes in glass bakeware:
Traditionally glass bakeware was made from Pyrex type glass (i.e. borosilicate) glass. Borosilicate glass has a lower coefficient of thermal expansion than soda lime glass, and therefore, stresses generated by thermal gradients are less. This has changed. Now glass bakeware is made from tempered soda lime glass. Thermal tempering increases the strength of the bakeware by generating a compressive layer on the outer surface of the glass. This stress must be overcome before the surface can experience tensile stresses. Thus, the glass piece is stronger. Recently Read Consulting was asked to perform a failure analysis of a supposed tempered glass baking dish that had broken. The packaging advertised that this tempered glass is safer because it will break into small pieces instead of the large pieces that normal Pyrex glass would make.

The broken dish is shown directly above.

The fracture pattern is not that of tempered glass. Therefore an edge stress meter was used to look at the glass edge on the failed part and on an exemplar. The top left photo is the stress meter measurement taken of the fractured baking dish.

The next photo is of asimilar measurement on a tempered edge. The color fringes indicate that the glass edge is tempered.

Obviously,the subject dish was not tempered.

Video of Tempered Glass Failure

Read Consulting glass failure experts filmed in real-time the failure of a 7' x 1' u-shaped piece of tempered glass. The objective was to demonstrate how rapidly the glass falls apart and to assess the danger to one standing underneath a failed piece of tempered glass.

Tempered glass self destructs once its compressive layer has been penetrated by a defect. In the case as shown by the attached videos, a center punch was used to create a defect which penetrates into the tensile portion of the glass.

Once in the tensile region, the speed of the actual defect (crack) is in excess of 8,000MPH. To the normal eye, the glass piece fails almost instantly. Basically, it self destructs into thousands of small diced pieces as intended. Therefore, one would expect the failed glass to be safe. However, as one can by the videos, these pieces can remain interlocked and can fall as large interlocked "chunks." Naturally these chunks have the potential to injure anyone underneath.

Thus, in this configuration, tempered glass cannot be considered "safety-glass." Showing that there is risk involved with having tempered glass elevated above public access walkways, etc.