What are the surface stresses for different types of glass？
Flat glass is divided into tempered glass, semi-tempered glass, and annealed glass based on their surface stresses. According to U.S. ASTM C1048-1997b standard, the ranges of surface stresses for each type of glass are: Tempered glass >69MPa (10,000psi), Semi-tempered glass: 24MPa (3500psi) ~ 52MPa (7500psi). China’s national standard, “Tempered and Semi-tempered Glass Used in Curtain Walls” has also specified the stress requirements for each type: Semi-tempered glass shall have stress between 24-60 MPa, and tempered glass > 90MPa.
(1) Tempered Glass
Traditionally, tempered glass is tested using impact method. GB9963 and GB9656 require the number of particles of the shattered glass within 50mm x 50mm area has to be more than 40. Test results show that within the stress range of tempered glass, the amount of particles is correlated to the value of surface stress. A number of 40 particles correspond to approximately 82MPa of surface stress. Whenever it’s possible, high-rise buildings should choose homogeneously treated tempered glass.
(2) Semi-tempered Glass
Semi-tempered glass is also known as “heat-strengthened glass” in China. With its good wind-resistance properties, this type of glass is widely used in architectural applications. In case that it is damaged in natural disasters, this type of glass tends to stay in the window frames and not to self-explode. ASTM requires heat-strengthened glass to have a stress range of 24-52Mpa, and the tempered glass to have a stress value greater than 69MPa. Glass with the value of stress between 51-69MPa is considered defects. It can neither be used as tempered glass, nor as heat-strengthen glass.
(3) Tempered Safety Glass
Experience has shown that when tempered glass is used as safety glass, it is desired to have a surface stress exceeding 105MPa. An example often referred to is the auto glass. Although the particle counts do meet the relevant requirements when the stress is less than 105Mpa, the strength of the glass is barely satisfactory.
(4）Hot Bent Glass
Hot bent glass is made of annealed glass. It requires the glass to have internal stress as low as possible to prevent breakage due to excessive stress being unevenly distributed in the glass. The bent portion generally has a stress concentration with surface stress at approximately 15MPa which will easily cause the glass to shatter. To solve this problem on curved glass, the surface stress should be maintained below 7MPa.