Have you ever seen a piece of glass take an impact without immediately shattering? Or perhaps noticed how the glass used in cars or the screens on our smartphones can withstand impacts and accidental drops better than ordinary glass? This is part of the remarkable nature of a material widely used across various industries: tempered glass. Let’s take a closer look, UNDIRA friends.

At its core, tempered glass is made from materials similar to those used in conventional glass, primarily silica or silicon dioxide (SiO₂), combined with other oxides to achieve specific compositions and properties. However, the real advantage of tempered glass lies in the way its fracture behavior is engineered.

In brittle materials such as conventional glass, once a crack begins to form, it can propagate rapidly because there is little to stop it from spreading. In tempered glass, however, the surface is placed under compressive stress, which helps prevent cracks from initiating and limits their propagation.

During the manufacturing process, the glass is first cut to the required size and shape before entering the tempering stage. At this stage, the glass is heated to an appropriate temperature, typically around 650–700 °C. It is then rapidly and controllably cooled, generally using jets of air. This is the stage where the material’s mechanical characteristics begin to change significantly.

The surface of the glass cools more quickly than its interior. As a result, the surface hardens first while the interior remains at a higher temperature. As the interior subsequently cools and contracts, a distribution of residual stresses develops throughout the glass.

Once the tempering process is complete, the surface of the glass is placed under compressive stress, while the interior experiences tensile stress. This combination of opposing stresses is one of the key secrets behind the strength of high-quality tempered glass. The compressive stress at the surface makes it more difficult for cracks to initiate and propagate, allowing the glass to withstand impacts better than conventional glass.

However, this does not mean that tempered glass cannot break. A sufficiently strong load or impact can eventually overcome the compressive stress at the surface. When a crack manages to penetrate the compressed surface layer and disrupts the balance of residual stresses within the material, the elastic energy stored in the glass can be released rapidly.

As a result, cracks can propagate extremely quickly in multiple directions, causing the tempered glass to fragment into numerous relatively small pieces. This characteristic is one of the reasons why tempered glass is widely used in applications where safety is an important consideration. In addition, the tempering process allows the glass to better withstand relatively significant temperature changes, although it still has limitations when exposed to extreme thermal conditions.

Despite its impressive durability, tempered glass also has an important limitation that manufacturers need to consider. Once the tempering process is complete, the glass can no longer be drilled, bent, or cut without risking damage to its internal stress balance. Such modifications can create stress concentrations that trigger cracks, potentially causing the glass to shatter unexpectedly and increasing production costs.

The cooling process must also be carefully controlled. Uneven or improper cooling across the surface of tempered glass can create a non-uniform distribution of internal stresses. This may result in visible patterns or spots on the glass surface, a phenomenon commonly known as quench marks.

The story of tempered glass teaches us an important lesson: a material’s properties are determined not only by what it is made of, but also by how it is processed. From its chemical composition to the precise control of heating and cooling, every stage plays a role in determining how the material ultimately behaves under stress.

Behind the vehicles, machines, aircraft, medical devices, and countless other technologies we use every day, there is always a science of materials and engineering working quietly in the background. At the Mechanical Engineering Study Program of Universitas Dian Nusantara (UNDIRA), you will learn how metals, glass, polymers, and other materials can be engineered to achieve specific properties and meet the demands of modern technology.

Sometimes, curiosity about how something works can be the first step toward becoming an engineer. So, why not explore that curiosity and discover the fascinating world of materials with the Mechanical Engineering Study Program at Universitas Dian Nusantara (UNDIRA)?