spontaneous tempered glass breakage on greenhouse

Dear OOlitic,
It seems to me that the glass which is breaking up is due to the thermal shock.Cold in night and hot in morning.Glass is by nature insulator. It takes time to heat up and release the heat so as to cool down.A toughned glass will be more prone to thermal shock breakages. Toughned glasses are good for resisting mechanical shocks.Try a glass with lower alpha value. i.e,a glass having low dilatometric coefficient value. Since I do not know the detailed data. I would prefer not to comment on the value of alpha that you should go for.
Regards
Shudipto

A tempered glass is one where the surface is put into compression to give it additional strength. From a thermal shock standpoint, tempered glass has much greater thermal shock resistance than untempered. When a tempered glass fails, it is primarily due to a crack that has penetrated the compressive layer. If this is truly tempered glass, it is not failing from thermal shock it is being mechanically damaged.

A more likely cause is the difference in thermal expansion between the metal clips and the glass. This could easily be causing your problem.

Thank you for the responses. The SS clips were installed loosely and should have very little if any contact with the glass, except at the lap where the top lite overlaps the lite below it. The clip will hold the upper lite in position from sliding down the slope. The clip was designed to withstand uplift on the glass. The clip is very small ~1" square. The aluminum bar is ~24' long eave to ridge and the lites (4) are ~3' x 6'.
Thank you again
Oolitic

In my opinion the braking of the glass is caused by the constraints of the metal structural parts that support the glass. The effect is most seen with long greenhouses, with a length of the gutter more then 100 m. It may be that the complete structure is a bit inclinded. As the concrete base of the greenhouse will not move, while the metal parts expand, the topology of the structure can change when heated up (for instance rectangular shapes get diamond- shaped). In this way the glass pane can be loaded with high compression forces, while the sharp edges of the glass initiate a crack.

Take a good look at the greenhouse. It may help if the structure is straitened out.

Kees Douma
Caravell Engineering Design

Stresses generated by thermal expansion (usually non-uniform heat absorption patterns) might be causing your problem, as suggested by others.

Another possibility is breakage caused by nickel sulfide inclusions. NiS induced fractures often occur during cold weather when direct solar beam radiation strikes the glass.

Our consulting group does failure analysis on glass and glazing systems, but I am not sure that a greenhouse application would bear the costs.

NiS inclusions look like tiny spheres (barely visible with a magnifying glass) in 13mm glass. In 6mm material, they are egg-shaped. In 3mm glass, they are cigar-shaped. The shape varies with thickness because a float glass line is a constant-volume system. A thin raw glass ribbon is pulled through the aperture (drawn) more than a thick one.

If neither of these causes prove to be the cause breakage in the case you mentioned, I would have to know more about the specific glazing system to be of any help.