A Read Consulting glass expert witness performed a glass failure analysis of a glass crack driven by thermal stresses. The failed bowl cracked in a microwave oven. Often failures with known causes can be analyzed to lend insight as to the cause of  an "unknown" glass fracture. This fracture has many of the aspects of an impact failure. However, the crack never reaches terminal velocity (i.e. there is no mist hackle generated). This glass failure initiated at a damage site on the bottom, bearing surface of the bowl. The bearing surface is the surface that would be in contact with a flat surface when the bowl was set down.  On the left is an upside down photograph of the bowl. The location of the origin is indicated. On the right is a photograph of the fracture origin on the fracture surface. This glass failure started where there was previous mechanical damage to the bottom of the bowl. The thermal stresses due to the microwave heating were high enough to cause failure as a result of the pre-existing damage. Absence of mist hackle is a key factor in this failure.

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.

Glass Expert Witness Discusses Tempered Glass Fracture Surface

California failure analysis expert witness examines two distinct fracture surfaces for tempered glass failures.
According to the tempered glass expert witness, the surfaces of tempered glass are under compressive stress. This stress is balanced by an "equivalent" tensile stress in the center of the glass. Tempered glass failures are really the end result of self destruction. The mid-plane tensile stresses take control of any defect that enters the center of the glass pane and drives the crack to the point that the original piece becomes a large number of almost cubic diced pieces. , under normal conditions, the fracture surface consists of two sets of Wallner lines separated by a central zone of mist hackle (upper  photo-micro-graph). The fracture surface for a tempered glass piece that failed as a result of bending is distinct. The fracture surface shows a shift of the central mist hackle toward the convex side of the bend. In this case, the Wallner  lines have essentially disappeared (lower photo-micro-graph).

Glass Expert Witness Services

Read Consulting is a California failure analysis laboratory that provides a complete line of glass expert witness services. These include glass failure analysis, window damage assessment,  patent expert witness support and stress testing. In addition Read Consulting also offers glass expert witness support in all areas of glass and ceramic machining and processing.  Process and device patent expertise is also provided.  

Glass Expert Witness Discusses Window Failure

Glass failure analysis expert discusses interesting window failure, that of a piece of antique window glass. The window expert performed a failure analysis that determined the failure initiated at crush damage from the scribing operation, and it started as a bending crack. It then transformed into a "thermal crack". That is the crack front was driven by thermally generated stresses ( either residual stresses or temperature differential generated stresses). Termal cracks tend to wander, and after this crack initiatiated it wandered and almost created a semi-circle (crack growth is indicated by the arrows in the above photographs). This window failure is interesting because this glass was made in the 1930's, and it was ground and polished. Present flat glass (i.e. float glass) is solidified on liquid tin and does not require glass machining (i.e.grinding and polishing). Also, modern float glass processing is better controlled than the previous roll forming processes.

Glass Failure Analysis Of Bottle Defect

California Glass Expert Witness was assigned to determine the cause of failure of wine bottles at the manufacturing facility. During processing the bottles had a higher than normal failure rate. A root cause failure analysis was performed by the glass failure analysis expert witness. He inspected a large number of whole bottles and removed those with visible defects. In addition failure analysis was performed on those bottles that failed in manufacturing. Above left is a photomicrograph of a "crush" or "bruise" found on one of the unbroken bottles. This type of defect was found on several bottles. This bottle defect appears to be a result of impact damage. Curiously, there is an open surface bubble (i.e. seed) in the vacinity of the bruise. Upper right is a photomicrograph of the fracture surface of one of the failed bottles. The origin of this failure is at the same location on the bottle as the bruises found on whole bottles. In addition, this failure started at a small diameter bruise. The repetitive nature of the location and the nature of the defect indicates that this manufacturing defect is caused by the glass handling machinery either at the bottle manufacturing facility or at the bottling plant. Also, this damage is severe enough to cause bottle failure.