Analysis of tempered (Toughened) glass

Disadvantages of tempered glass

Spontaneous fracture

Nickel sulphide inclusions, which expand their volume (Area), up to approximately 750 days after production (Invisibly).

Spontaneous breakage is a sudden failure of thermally tempered glasses without any external action. Particular phenomena is known since 1960's. In high-rise buildings, there are some sudden flying glass debris been occurred according to glazed facade industry records.

A reason for spontaneous breakage are small (50um to 500um diameter) that undergo a volume change. Moreover, the typical breakage pattern (Butterfly) is one indication for nickel sulphide inclusions.

• Nickel-Sulphide is a mineral with a high and low temperature phase.
• The volume increase leads to failure in thermally tempered glass if the inclusion is in or near the tension zone of the temper stress.

Heat-soak test is the most efficient measure to bring panes with inclusions to failure in advance.

Heat-Soak-Test 

• Heating
• Temperature 280-320 Celsius
• Holding time 2 or 4 hours
• Cooling

Determination

Phase change of nickel-sulphide is strongly accelerated

Pane brake already in the owen