It’s common knowledge that glass can be a brittle material, yet it’s used extensively by architects and specifiers, particularly in the design and construction of large skyscrapers. Understanding the strength of glass enables it to be used in a wide range of applications.
The qualities of glass make it a unique material. Anyone who has ever smashed a glass bottle on the floor or kicked a football through a window will realise this. In contrast to most other common building materials, such as metal, plastic or wood, glass exhibits brittle fracture. This can be both spectacular and alarming, yet we are able to manufacture, process and design glass to be used in a vast number of applications that require safety, security and strength.
There are several factors that can affect the strength of glass or lead to premature failure, ranging from glass size, mechanical loading rate and ceramic inks, yet an understanding of each can help prevent glass failure and see it used successfully in a variety of applications.
Crack tips – and the effect of environment
In glass, if the tensile stress is high enough cracks in the material may start to grow and the crack may keep growing until the glass fractures completely. This is unlike metals, where fracture can be prevented by combining crack tip shielding with crack tip blunting, which helps reduce the concentration of stress at the crack tip and makes metals much tougher.
Glass can withstand loads applied at rapid rates better than if they were applied over a longer period of time. The faster the mechanical loading, the stronger the glass will be. The reason for this is because glass can be prone to chemical corrosion.
Glass corrosion is a chemical reaction caused by water, but it can only occur in the presence of large enough cracks and high enough stress. It can lead to chemical attack at the crack tip and the very slow growth of cracks which lead to delayed fracture, this may appear after a long period of time. The longer the glass is under stress during testing, the more time there is available for corrosion to take place.
The influence of temperature
Temperature change also plays a significant role in the chance of breakages in service. When glass is subjected to rapid temperature changes the thermal shock may be severe enough to cause fracture. This can happen when the centre of the glass becomes hotter than the outside surfaces, which generates tension in the cold surface and may ultimately lead to failure.
Similarly, when glass is exposed to solar radiation or indoor heating it absorbs a portion of this heat, resulting in its temperature rising and the material expanding: in this case the edges of the glass may remain cold, giving rise to tensile stress.
Thermal fractures frequently start from the edge of the glass and at 90 degrees to it. For this reason, it is often difficult to identify a thermal fracture without deglazing the system, as the evidence can be hidden within the rebate of the frame.
Printing on glass
Although a popular choice for architects when designing attractive and striking buildings, glass printed with ceramic inks may be weaker than unprinted glass. Careful attention must be paid to the ink firing schedules to ensure that the ink does not contain voids or unfired frit. The effect can be exacerbated if there are layers of ink on top of one another – as the chances of finding a defect depend on how much ink is present.
Fortunately, there are a number of things that can be done to dramatically improve the performance of glass and ensure that it’s robust enough to be used in the application it’s intended for.
The strength of glass is predominantly governed by its surface condition, so good housekeeping is essential. For example, ensuring glass handling equipment is kept free from scoring debris is a simple yet effective method in avoiding glass breakages during manufacturing. Similarly, when transporting glass, it’s important to ensure it’s stacked with uncontaminated interleavant materials and the relative movement of plates during transit is kept to a minimum.
Thermal strengthening – also called toughening or tempering – is also a common method used to improve the strength of glass and is a particularly effective method in preventing thermal breakages. During this process, the glass is heated to about 650 °C, and then rapidly quenched with air jets.
The air cools the surface quickly, while the core takes longer to reach an ambient temperature. As the core cools, compressive stress develops on the surface, which is balanced by tension in the body of the glass. The high compressive surface stresses give the toughened safety glass its increased strength – typically, up to 3-4 times stronger than ordinary glass – and increased resistance to temperature differences.
While careful consideration should always be given to the strength and resistance properties of glass, a knowledge of its intended use can help the manufacturer, processor and designer to extract the most from this highly versatile material, and continue to use it in a safe and secure manner.
It is this knowledge that enables glass to be used extensively in the design and construction of beautiful buildings, furniture and other transparent structures, demonstrating that any challenges can be overcome.