A Small Speck Causes Big Problems: Identifying Spontaneous Glass Fracture

03.08.16   Thomas L. Bane, PE | More by this Author

A Small Speck Causes Big Problems: Identifying Spontaneous Glass Fracture

Imagine the frustration of a high rise owner who finds tempered glass windows shattering into thousands of pieces with no idea why. A misguided bird, perhaps? SME loves a tough a challenge, and gladly answered the call to take a look.

Upon examining the pane, it was easy to find where the break started:

Here is a closer look:

Our team made a few helpful observations. First, the glass broke into pieces that generally have three or four sides. But where the break starts, there are two six-sided shards right next to each other (called cat’s eyes). Lastly, between these shards, we spotted a small speck:

Under magnification, that small speck identifies an impurity in the glass:

That speck is nickel sulfide which formed a crystal in the molten glass during manufacturing. At high temperatures, nickel sulfide creates a small, dense crystal lattice. It makes a larger, less dense lattice structure at lower temperatures. When the manufacturer quenched the glass, they locked the nickel sulfide crystal into its dense lattice. Then, as the nickel sulfide was exposed to sunlight streaming through the window pane, the crystal began to reconfigure itself. It transitioned over many years into its lower temperature lattice, which made it expand and create a small crack in the glass.

Normal, annealed glass can have a small crack and be just fine, but the core of tempered glass is under tension. This tension causes the crack to propagate throughout the rest of the glass, causing spontaneous glass fracture.

Unfortunately, glass manufacturers have not been able to remove all the impurities from the manufacturing process. Nickel sulfide issues tend to be more common in certain batches of glass than others, and have become an understood risk of tempered glass.

If you have glass breaking in your building, SME can help you check for nickel sulfide. In some cases, the glass manufacturer may work with you to come up with a solution for replacement.

For more information, contact Tom Bane.

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