Ask an Expert – Safety Glass Failure Analysis by Wade Lanning Ph.D.
A glass expert explains the differences between different kinds of safety glass, how they are engineered for safety, and how they fail.
Thanks to its optical properties, glass occupies a unique niche in both technology and art. However, glass has an Achille’s heel: Glass tends to shatter when it fails. Once a cracking starts, it can grow as fast as the speed of sound leaving behind sharp fragments.
But not all glass is the same. When an expert performs a failure analysis on glass, they must account for the type of glass. Safety glass is engineered to be shatter-resistant and less likely to hurt people if it does fracture. Here, we will discuss common types of safety glass, how they fracture, and how an expert can analyze the pieces in a failure analysis.
Sharp Float Glass Shards
Most glass sheets are float glass, which gets its name because it is made by floating a layer of glass on top of of molten metal. When hot glass is cooled, shrinkage of the glass creates stress in the material. These residual stresses can cause the glass to fracture suddenly. To remove residual stresses, float glasses are annealed, or heated then slow-cooled. Most building windows are made out of one or more layers of annealed glass.
When annealed glass fractures, the cracks start at a flaw or damage then grow driven by stress in material. The resulting long, sharp fragments can inflict severe, even fatal, lacerations. So it is best to avoid using annealed glass anywhere strength and safety are critical.
The fragmentation pattern of glass can be helpful to an expert performing a failure analysis. The fracture surfaces of glass fragments also have delicate features that an expert can use to understand how the failure started. It is important to never put broken pieces of glass (or any fractured material) back together because this can destroy evidence on the fracture surfaces. Pieces of broken glass must be handled with care both for your own safety and to preserve evidence.
Laminating Glass for Toughness
Glass is actually quite strong – it takes significant force to break it. The problem is that glass has low fracture toughness, meaning that cracks can grow easily once they start at a scratch, chip, or flaw. When an expert performs a failure analysis on broken glass, they use clues on the broken pieces to find where cracking started.
Laminated safety glass is made by joining two or more layers of glass together with a tough plastic film. Even if laminated glass cracks, the plastic film holds the pieces together, greatly reducing the danger posed by the fragments. Thanks to the plastic film, laminated glass also retains some ability to keep doing its job even after it cracks. Laminated glass is used in situations where it needs to carry a load or act as a protective barrier even after it fractures. Examples include car windshields, glass walkways, or armored vehicles.
When laminated glass fractures, most of the pieces remain adhered to the plastic film, and the sheet retains some strength thanks to the toughness of the plastic film. The plastic film also helps preserve the arrangement of glass fragments. A glass expert can use the patterns in a fractured laminated glass sheet to identify how and why the sheet failed.
Tempering Glass for Graceful Failure
Tempered glass is a type of safety glass that takes two problems with glass – residual stress and poor fracture toughness – and combines them into a safety feature. Tempered glass is heat-treated so that residual stresses squeeze the surface of the glass in compression. The residual stresses act as a barrier to crack growth and make the glass about four times stronger than annealed glass. If a crack does start in tempered glass, the residual stresses cause the entire sheet to spontaneously shatter into small, blunt fragments that are less dangerous than shards of annealed glass. Tempered glass is used in shower doors, phone booths, refrigerator shelves, display cases, and other applications where extra strength is required and annealed glass would pose a laceration hazard.
Because tempered glass shatters into tiny fragments, it is extremely difficult to perform a failure analyses by examining the pieces. Instead, an expert can evaluate the surroundings for likely sources of damage that caused the failure, such as contact with hard materials like metal or sand, especially on the edges of the glass that are unprotected by the residual stress. The fragments themselves can sometimes be helpful in determining if the glass was properly heat-treated, contained defects, or was overloaded during failure.
Broken Glass? Call an Expert
There are many other varieties of glass besides annealed, tempered, and laminated glass. Glass can be both laminated and tempered, chemically strengthened, coated, and more. The kind of glass and how it was made influence how it fractures. A materials expert can use that information to analyze the pieces and determine why the failure occurred.
Click on the photo below to learn more about the author, Wade Lanning Ph.D.