Glass Expert Examines a Tempered Glass Window Failure

California failure analysis expert uses glass fractography to perform a glass failure analysis on a tempered glass window failure. The glass fracture analysis showed that the window failure was a result of heating the glass in the center. Due to the heat, the center of the window expanded. This created additional tensile stress on the window edges. Although the window edge was ground and chamfered, there was enough residual damage at the edge that the thermal stresses were able to cause window failure. Upper left shows the failure initiation site on the edge of the window. Upper right is a 40X photo micrograph of the origin on one of the fracture surfaces. The glass fracture analysis showed that the glass failure initiated on the window's edge face and propogated inward until the tensile center of the tempered glass window was reached. At this point the residual tempering stresses caused the entire window to self destruct into small "cubic" pieces. This may be a manufacturing defect because of the severity of the edge damage.

Glass Failure Analysis Expert Discusses Metal Clad Wood WIndows

Glass failure expert re-visits the window reliability of metal clad wood windows. Recently, numerous entities are suing various window manufacturers because of premature window failure (i.e. poor window reliability) of various brands of metal clad wood windows. These window failures are not manufacturer dependant; they are a result of the fact that these are metal clad wood windows. At issue is the fact that water can breach the caulk seal of the metal cladding and saturate the wood. This inturn causes the wood to expand and compromise these seals further. The trapped water cannot escape and as the window heats in the sun, the wood rot occurs (the wood composts), and the window structural integity is compromised (see the photographs above). A degraded window cannot prevent water intrusion. This window damage can happen rapidly (less than 3 to 5 years), and it is hidden from view by the metal cladding. Window reliability is poor and the chance for significant storm damage to a home is very high

Tempered Safety Glass

California glass failure expert discusses the safety aspects of tempered glass;
Tempered glass is safer than standard (i.e. annealed) glass because "dices" into small pieces under certain failure modes. In reality, the overall glass failure is driven by residual stresses put into the glass by the tempering process. That is, tempered glass "destroys" itself. If the glass is impacted "side ways" the failed piece will break into safe small "diced" pieces. And this provides safety to one impacting the glass. However, there is the possibility that the glass can fail without a sideways impact. In this case, there is a possibility the glass sheet can fall intact. In addition, the edges of failed tempered glass are just as sharp as those created when annealed glass fails. The upper left photograph is of a piece of failed tempered glass that stayed "inter locked" after failure. The failure analysis photo on the upper right is an twenty times magnification photomicrograph of the one small piece of the failed tempered glass (arrow). The edges on this piece are not rounded. They have the same sharp edges associated with any failed piece of glass.

Bottom Line: Tempered glass is not always safe. The edges of the resulting pieces are sharp, and the glass pieces can remain interlocked in large sheets.

Metal Clad Window Failure

During a severe storm a three story house suffered severe water damage. This damage was not caused by flooding; it was caused by large amounts of water entering the top floor of the home as a result of openings to the outside. Read Consulting performed a failure analysis of the property and determine the root cause of the damage. The failure analysis included an inspection of the home and a review of photographs taken of the residence immediately after the storm, and the cause of the water intrusion was determined. Severe rot resulted in the formation of openings in the third floor windows. The windows in this house were expensive metal clad wood windows. Failure analysis determined that moisture had reached the wood structure through seal openings in the exterior metal cladding. This caused swelling of the wood and further opening of the seals; thus the amount of water intrusion was excaserbated. In addition, the moisture was "trapped" in the wood and caused accelerated deterioration. In essence, the wet wood would be at temperatures in excess of 125°F during the warm, hot days. This accelerated wood deterioration (i.e. the wood "composted"). In addition, this degradation was not noticeable because the metal cladding covered the wood. Due to rot, the windows became extremely weak and failed during the storm. The upper left photo shows a window that was so rotten that during the storm, the glass pane slipped down and left a 2" gap at the top. The upper right window failed when the bottom part of the top sash fell away creating an opening. These two windows represent the types of failures discovered on the third floor. These allowed large amounts of water into the home and caused severe damage throughout the house.

Glass Failure Due to Edge Grind Damage

Read Consulting glass failure experts analyzed several cracked laminated windows. They were made from two pieces of 1/4 inch thick glass laminated with 0.02" thick PVB. Oneof the glass panes is tinted such that it absorbs 65% of the incoming "heat" from light. It is the tinted piece of the laminate that broke. The failure analysis determined that the crack initiated as a result of tensile stresses and the presence of edge grind damage. It is theorized that the stress was generated due to the fact that the window edges were covered and therefore cooler than the center of the pane. The photo on the left is of the fracture surface showing the origin. It is in the center of the edge face. The photo on the right is of the edge face. It is apparent that the damage initiated from the edge grind