As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors, tables, refrigerator trays, mobile phone screen protectors, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.
TEMPERED OR HEAT-STRENGTHENED GLASS
The most common type of safety glass is tempered glass, which is made by heating pre-cut panels of glass to about 650 C (1200 F), then cooling them rapidly through a process called ‘quenching.’
By cooling the outer surfaces of the panel more quickly than the center, quenching puts the surfaces and edges of the glass in compression and the center of the glass in tension.
In addition to making tempered glass four to five times stronger than conventional annealed glass, reheating and rapid quenching dramatically changes the break characteristics of the glass.
Consequently, when tempered glass is broken, it shatters into thousands of tiny pebbles—this practically eliminates the danger of human injury caused by sharp edges and flying shards.
THAT’S WHY WE HAVE TO DO
THE QUALITY CONTROL
Fragmentation testing is required to determine that the glass product meets the minimum breakage particle requirements as specified for thermally toughened glass.
The destructive test and the fragmentation due to breakage is the way to define the stress and safety level of the tempered glass. The fragmentation shows several things from the stresses, e.g. the stress level and the uniformity of the stresses.
Manual Fragmentation Analysis
Fragmentation analysis is a proven way to confirm the safety level of glass. In essence, when thermally strengthened glass fractures into smaller pieces, it is less dangerous. Depending on the glass thickness, several standards define the minimum number of fragments required for safety.
- Fragment counting should occur between 3 to 5 minutes after fracturing the glass. This is important because after a longer period of time, secondary cracks start to form.
- The counting area should be 50 x 50 mm, ± 1 mm.
- The chosen counting area should have the least number of fragments.
- All particles wholly contained within the area should be counted as one fragment.
- Particles which are partially within the area should be counted as 1/2 a fragment.
The tempered glass fragmentation test gives good insight into the quality of the tempering process.