Glass Failure Analysis: Windshield

A failure analysis was performed on a light rail windshield failure. In this there was a pop and the operator was showered with glass. The glass failure analysis expert witness determined that there were two leads to the windshield defogger that were in proximity and because they were improperly wired, arcing occurred between them. The arcing generated heat and pressure that caused the glass to crack and spall locally. Upper left is a photograph of the damaged region of the windshield. Upper right is a closeup of the leads that arced. One can see that the lead ends are melted.

Failure Analysis Expert Examines Bottle Failure

Glass failure analysis expert witness prepared numerous bottle failures of "off the shelf" 12 oz beer bottles by over pressuring them with a hydraulic pump. These tests were needed to generate bottle failures of a known origin in order to examine the crack patterns, the fracture surfaces and the failure origins. Numerous bottles were broken, and even though the locations of the failure origins varied (i.e. some were on the body and others on the heel), they all had identical characteristics. They initiated as a single crack parallel to the bottle axis. Also, the origin was always on the outer surface of the bottle and located at a minor external glass defect. In addition, the failure initiated as a fracture mirror. The fracture mirror was surrounded by mist hackle. The failure shown here initiated at the heel of the bottle. an overview of the failed bottle is shown in the two upper left photographs. The initial crack is parallel to the bottle axis, and it wraps around the heel. At both ends of the initial straight crack the crack fans out. On the upper right is a 40X photomicrograph of the failure origin. The mirror fracture and mist hackle border are clearly visible. The minor defect at the origin is just the weakest part of thisstandard bottle. This bottle failed at 410 psi. This was not a defective bottle. This would not be a product liability case.

High Pressure Bottle Failure Analysis Tests

Glass failure analysis requires looking at failed bottles and determining the cause of failure. The failure analysis procedure includes carefully examining the crack pattern and the failure origin. It also helps to have failures with known causes to confirm the conclusions. In this case bottles were purposly broken using hydraulic pressure. Their performance was documented for future use. Numerous bottles were tested, and all showed similar failure characteristics. All bottle failures were caused by "hoop stress"; thus they started as a straight crack parallel to the axis of the bottle and then branched out. Microscopically, all the over pressure glass bottle failures originated as a slow growing crack and formed a distinct fracture mirror surrounded by mist hackle. Upper left is a photograph of a representative over pressure failure. This particular failure originated on the body of the bottle. Others originated on the heel, but all had the same characteristics. Upper right is a 40X photomicrograph the origin of the failure. It consists of a slow growth fracture mirror surrounded by mist hackle. The small arrows indicate the crack travel direction. During falure analysis, a glass failure analysis expert can compare the unknown glass failure to this example to aid in determining the cause of the failure.

Nickel Sulfide Particles in Tempered Glass

Glass failure analysis expert witness performs a failure analysis on a tempered glass window. The failure was initiated by a nickel sulfide, NiS particle. The glass failure analysis expert was able to retrieve both halves of the failure origin. One side of the failure origin contained the protruding NiS particle. On the other side of the failure origin there was a hole out of which the NiS particle had pulled out when the two halves of the failure origin were separated. Above are two photographs of the two halves of the failure origin retrieved during the failure analysis. These photomicrographs were taken at 100X magnification using Nomarski interference contrast optics. This is a manufacturing defect. The NiS entered the glass during the melting operation.

Glass Expert Analyzes WIndow Damage

Glass failure analysis expert analyzes scratched windows on new construction. There have been numerous occasions where a glass expert has been brought in the determine the cause of window scratching. This is often on new construction that has a stucco exterior. Post construction window claening is critical because of adhered construction materials. Removal of these materials is difficult because of their adhesion. Therefore, cleaning crews should not use blades to remove debris. In many cases abrasive debris such as stucco or cement can be captured by the blade and scratch the glass as the blade is being drawn across the surface. Abrasives can also be captured by dry cloths and pads and can also cause window scratching. The photographs above show the resulting window scratches. Upper left is a sample of cloth related window scratching, and upper right is a sample of metal blade related window scratches. These are window cleaning related and are not manufacturing defects. Only experienced window cleaners should be assigned to clean windows on new construction.

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.

GLass Expert Discusses Chill Cracks

Glass failure analysis expert witness discusses a common glass bottle manufacturing defect. After the bottle has been formed it is removed hot from the clam shell tool. If it encounters any localized cooling at this stage, it can form a manufacturing defect called a "chill crack". Thermal shock cause the cooled area to shrink, and this causes localized tensile stresses. This event can create a small crack that can cause later failure of the bottle. Above are two photos of the same chill crack. Upper left is a 20X photomicrograph, and upper right is a 40X photomicrograph. To start, this crack is less than 0.1" long, and it has grown to be approximately 0.7" long. The bottling line has had numerous failures from this lot of bottles, and the cause has been traced back to this type of chill crack.

GLass Failure Analysis Expert Discusses Wire Glass

Safety glass expert has examined numerous broken wire glass doors and windows. In the past wire glass has been mistaken to be safety glass; however, it is just as dangerous as standard annealed window glass. The wire is bonded between two pieces of semi-soft glass at high temperature. When this "sandwich" cools to room temperature, stresses are created that weaken the glass. The only advantage of wire glass is that the wire will hold the glass in place during a fire, and this will help reduce the spreading rate of a fire. Above left is a photograph of the remainder of a wire glass door through which a person had pushed his hand through when pounding on the door. The glass failure analysis determined that this fracture pattern is similar to an equivalent annealed glass failure. The wire did not prevent this glass failure. Above right is a photograph of a wire strand that pulled out of the glass at the time of the failure. One can see that its bond to the glass was very poor. a wire glass expert would attest to the fact that wire glass is not safety glass.

GLass Expert Discusses Glass Bottle Strength

Glass failure analysis expert performed bottle strength tests to determine the strength of a cylindrical glass bottle. The bottle is approximately 0.85" in diameter and 2.35" tall. The glass thickness is approximately 0.040" thick. It was hypothesized that a female could squeeze the bottle hard enough to crush it. Glass strength was measured by compressing the bottle both radially and axially. Upper left photograph shows the radial "squeeze test. The human hand is soft; therefore the test bottle was compressed between soft vinyl plastic tubing. In this orientation the bottle could withstand a minimum og 240 pounds force. The upper right photograph shows the "axial" strength test. Again, no hard surface touches the vial. Axially the bottle can withstand a minimum of 130 pounds. Even though the bottle may have many surface defects, it is strong because it is under compressive stresses. Because glass fails in tension, the cylindrical shape insures the highest strength. Because the bottle has no flat surfaces, it can withstand very high forces. No average female could generate sufficent forces with their hand to crush this bottle.

Manufacturing Defect on Tempered Glass Window

Glass failure analysis expert witness performed a root cause failure analysis on window defects found on a recently built custom home. The upper left photograph shows the defect from the building exterior. The window defect resembles a dust or dirt streak and was not detected until the post construction window cleannig. In all cases, the manufacturing defect is in the center of the window, is parallel to the long side and it looks like a streak or scuff. The upper right photograph is a 200X photomicrograph of a portion of the window defect. It is composed of an array of small glass particles strongly adhered to the glass surface. It is believed that these particles were generated during the window sizing process and were a contaminant on the horizontal rollers used in the glass temper process. During glass tempering they came into contact with the hot glass surface and welded to it. This is a manufacturing defect. It is not a construction defect. This defect is often blamed for window scratching from window cleaning operations. However, these particles were visibly obvious and could not be dislodged. Therefore, these particles don't fit the popular window damage model proposed by many window cleaning experts.

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.

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.

Glass Failure Expert Witness Analyzes Failed Wine Bottle

Glass expert performed a root cause failure analysis of a broken wine bottle. The bottle had broken at the neck into three pieces (Upper left photograph). One piece still contained the cork with the cork screw still in place. In this case the the cork screw was off center and had exited the cork when it was being "screwed" in. The side of the cork screw crushed the interior wall of the bottle neck, and this initiated the failure. The upper right photograph is of the re-assembled top of the bottle. In it one can see the exposed edge of the cork screw and the "crush" defect on the interior of the bottle neck. With this type of cork screw the cork is pried out with a lever force. In this case, this force was sufficient to cause failure and injuryto the person opening the bottle. With this type of cork screw the user must be careful to center it, or he could be injured.

This glass failure analysis showed that the failure was a result of operator error. There was no manufacturing defect.

Glass Failure Analysis Expert Discusses Window Damage

Window damage expert examines window scratching from a metal blade (see above photo). A California glass failure analysis expert was asked to evaluate window damage at a newly constructed home. The windows at this site were scratched during cleaning with a six inch metal blade. The blade traps abrasive dirt and drags it across the glass. Because the dirt is the same hardness as the glass, the window is scratched. The scratches are in a linear array, and their visibility is enhanced by direct sunlight. Glass expert reccomends that windows be cleaned per GANA reccomendations. GANA reccomends that blades never be used to clean windows.