Glass compressive and tensile stress

[marellasunny]

http://www.glassalchemy.com/media/upload/image/stressfig1.gif

In the manufacturing of glass(flat glass),the outside cools first followed by the inside.So,this means that the outer surfaces compresses on the hot inner surface.What I don't understand is how the tensile stresses are developed on the inner surface?

1.Since the outer surfaces are already compressing on the interior,how can the contraction of the inside still create tensile stresses?http://www.ljstar.com/technical/images/sightglass_image_9.jpg

2.For example in a car windshield :By creating compressive and tensile stresses in such a manner as Fig.2,it would be suitable only for side crashes.The normal force shown would be a car/tree impacting from the side. In case of frontal normal force,my compressive stress aids the normal force.Am I right?

[For the windshield to be suitable for frontal crash,I would need to create compressive stresses at the top and bottom part of the windshield(looking from the front) and tensile stresses in the centre.Which is kind-of weird.]

 

[Baluncore]

I think you may have misinterpreted the terms inside and outside as referring to the section when they were intended to refer to the area of the pane.
When glass is being manufactured the central area of the pane is cooled first. The peripheral area then cools slowly and so shrinks onto that central zone. That helps prevent tension cracks when impacted in the central zone, because the central zone is under pre-compression. The outer band is cooled slower so it is stronger in tension than the central zone, it is also mounted in a rubber strip that reduces stress and vibration.

 

[marellasunny]

So,if I hit the windshield of my car(with a hammer) on the top and bottom areas,it would crack and shatter much easier?This when compared to the central portion where cracks are held together because of the compressive stress from the outer layers.

 

[Baluncore]

A windshield is made from laminated glass, not toughened glass. The side and rear windows will be toughened, not laminated.

Windshields were once made from toughened glass that shattered into small cubes. It took some time to develop a safe laminated glass for windshields that did not trap the thing that broke it.

 

[Baluncore]

In your original post you showed a section being Fig 1, with compression and tension forces perpendicular to the surface of the glass. I believe those forces should have been shown parallel with the surfaces. Whatever the diagram shows, it is quite misleading. Your second diagram showed a quite different situation, where a force applied to the centre of a constrained glass disk caused a fracture to begin. That constraint could be achieved by an external tensioned hoop, a fluid pressure or by cooling the outer annular zone last so it shrinks onto the inner disk. A critical difference between the diagrams is that the second shows no layer differentiation. There is a paradoxical reversal of logic in the toughening process. By chilling and shrinking the outer surface it actually ends up in compression.

Consider a flat sheet of hot glass, chill the flat surfaces rapidly, the surfaces shrink in area as they solidify. That surface area contraction effects the hotter still fluid interior which thickens very slightly to maintain it's volume as it's envelope shrinks and hardens. The interior then cools and contracts in area as it solidifies. This produces internal tension forces parallel with the surface that place the outer surface area in compression. That sequence results in outer surface compression due to the inner tension. At no time do you need to consider forces perpendicular to the surfaces.

Cracks can only start at points of tension, but the points of highest tension are inside the toughened sheet, not on the surface. When you hit the front surface of toughened glass you must exceed the high internal tension forces, that were frozen into the sheet, before the back surface can enter tension and cause the glass to shatter.