Glass Analysis

[HungryDinosaur_imported]

Hi All, for glass there is no yield stress as it is a brittle material. So the failure criteria for glass materials under a certain amount of external pressure will be the compressive strength? Any comments on this please?

 

[3DDave]

Glass is very sensitive to stress concentration. As mentioned in the the book The New Science of Strong Materials, the author notes that a freshly created glass fiber has a tensile failure strength of around 1 million psi, but the touch of a fingertip and action of acidic oils left by it reduce it to around 5,000 psi.

In use, glass beads in syntactic foam can resist many thousands of psi in a liquid, cushioned against contact with epoxy, but a slight roughness in compression could result in failing at far less.

The main defense for glass is to avoid point loads, scratches, and any free edge/corner exposure, as well as corrosive materials.

Tempered glass can be an improvement as it maintains the external surface in compression, preventing scratches from becoming advancing cracks.

 

[SWComposites]

No, because it is unlikely the glass part is purely in compression.

 

[HungryDinosaur_imported]

Thanks Dave, SWComposites.

 

[MintJulep]

under a certain amount of external pressure

Do you think geometry might be important?

 

[BEMPE16524]

we design a subsea light many years ago.
we used clear borosilicate to make the glass dome. Carlton Glass can customise the shape you want.

the glass is very sensitive to stress concentration, so we have to carefully design the holding mechanism for it.
also, we will not allow long duration heat up on deck because it may crack when it is dump into the sea.

R.Efendy

 

[LiftDivergence]

So the failure criteria for glass materials under a certain amount of external pressure will be the compressive strength?

Well, it really depends on what you are doing. As others have mentioned, it's a sensitive material to work with and I imagine most of the time critical loads are going to be dictated by boundary conditions and contact mechanics.

But in general, like all materials, pure tension or pure compression are not the cause of failure. Shear distortion energy is the culprit. A passage from "Strength of Materials" by J.P. den Hartog has always been interesting to me... he is describing experiments related to maximum distortion energy theories of failure:

An experiment with three dimensional tension is much more difficult than with compression, but it has been performed once, by the Russian scientist Joffe. He took a solid glass sphere and cooled it off slowly to the temperature of liquid air...the marble was taken out of the liquid air bottle and exposed to room temperature...the outside layers of the marble would warm up while the center was still cold. The thermal expansion of the outer layers was prevented by the cold inside, this putting the center of the sphere in hydrostatic tension...the calculations showed hydrostatic tension at the center of the sphere far greater than ordinary, one-dimensional tensile strength of glass, but the center remained clear and did not crack or tear.

That being said, for brittle materials, experiments have generally shown that failure envelopes are better predicted by maximum normal stress theory, since many of these materials have shear strength in excess of their tensile strength. For static failure of brittle materials I suggest you read Section 5.2 of "Machine Design" by Robert Norton, and research the Modified-Mohr failure criterion.



Keep em' Flying
//Fight Corrosion!

 

[JStephen]

You don't say what your application is. But there have been a number of posts through the years in the Structural Engineering forum, dealing with glass railings, or glass-filler in railings, that might be of interest if your application resembles that.