Strength and fracture behaviour of annealed and tempered float glass

Glass is a material that is strong in compression but has widely varying strength in tension and fractures uncontrollably. The results that are reported in the literature vary widely. Reliable data about the strength of annealed and tempered float glass are scarce. 4 point bending tests have been conducted on annealed and fully tempered float glass panels of two different sizes. The tests were done with the panels in a standing position rather than the more common lying position. The results show that the design strength of tempered glass is much less than commonly assumed. Failure in both types of glass starts at defects in the tensile zone. The amount of crack branching correlates well with the strength of the glass. In tempered glass cracks first grow from the defect that initiates failure. The decohesion of the tempered glass into little fragments follows after the panel has already failed.