Why did my glass break? The study of glass fractography.

Why did my glass break? The study of glass fractography.

Thanks to my professors George Quinn and Jim Varner for a great course on glass fractography. The short course is one week long, and is held at Alfred University in Alfred, NY. George wrote the bible on this subject. It is entitled “Fractography of Ceramics and Glass”.

Glass fractography is used to answer a question I am often asked, “Why did my glass break?”

By viewing the glass edge under a microscope, one can see much about how the fracture occurred, including the fracture origin, the direction of crack propagation, the amount of energy involved, and much more.

Fracture features such as mirror, mist and hackle are preserved in the glass edge and can be observed. And when you zoom in and see what is really going on inside this transparent material, the sights can be quite beautiful.

The Fracture Origin

Glass fractures tell a story about the forces and objects that broke them and the quality of the glass that broke. The center of the fracture is the origin. The origin is the place to look when unlocking the sectors of the glass breakage.

Fracture origins have different appearances. When nickel sulfide inclusions break tempered glass, the inclusion can actually be seen at the fracture origin in the “butterfly pattern”.

When impact breaks glass, a starburst pattern is common, with radial lines converging at the fracture origin. A close-up look here can reveal the hardness and sharpness of the projectile that broke the glass.

Cracks in glass tell us about the stress in the glass

Glass creates cracks in order to relieve stress. The more cracks that occur during breakage, the more stress was present in the glass at the time of the break.

Stress levels in the glass: “Built In”

Glass, such as tempered or heat strengthened glass has a permanent stress level permanently held in the glass. These stresses serve purposes: In tempered glass the high stresses cause the glass to break in small cubes which are considered to be safer in the event of human contact. Heat strengthened glass also has “built-in” stresses, but these stresses are lower than tempered glass. The purpose of heat strengthening is to enhance the ability of the glass to resist thermal loads.

Stress levels in the glass: Breakage induced by loads applied to the glass

If a load (force) applied to the glass exceeds the ability of the glass to endure the load, the glass breaks. In such instances, the stresses within the glass are created by forces applied to the glass. Simple enough, but what are these loads?

Wind loads.

Here the wind is the culprit. It can be a steady wind, called a uniform load or a gusty wind, called a dynamic load. Usually window glass can handle these loads, so if glass breaks due to the wind, there is much to study including the actual weather and the strength of the glass. Sometimes the nasty weather, such as a hurricane, carries projectiles with it, so what appears to be breakage due to the wind is better described as impact breakage.

Impact loads.

Here a projectile or object hits the glass. The weight, speed, hardness and sharpness of the projectile affects its ability to break the glass. The glass itself may have material properties that make it more or less susceptible to impact breakage.

Thermal loads.

Bright sunlight can heat the glass, especially if the glass has an inherent color or certain coatings. But glass is melted from sand under much higher heat than the sun can provide, so why does sunlight break glass? The answer is in the thermal expansion of the glass. When heated, glass grows a small amount but with great force. When it cools, it shrinks a small amount but with great force.

Internal loads.

These loads are applied by impurities in the glass. The most famous one is the nickel sulfide inclusion. This tiny ball, made of nickel and sulfur, is in the glass from the beginning but it only causes trouble when the glass is heated and then cooled during the tempering process. When the glass is heated, the inclusion shrinks. But when the glass is cooled, the inclusion tries to grow again but it is locked in. After a time of pushing on the glass that surrounds it, it finally breaks the glass.

Key Takeaway

Glass fractography examines broken glass to identify why it failed. It reveals stress levels, failure modes, and material properties of the glass. This information helps to prevent future breakage through better design and practices. It helps architects, engineers, and forensic experts understand how glass performs and fails. The fractured face’s unique pattern reveals the explosive moment when molecular bonds of the glass are ripped apart under stress.