Glasses- Stresses and Strength Flashcards
What are temporary stresses?
Stresses which only exist while an external constraints is applied, e.g mechanical load or a temperature change
Why are temperature gradients induced on heating?
Because there is a finite thermal conductivity (low for glasses) and non-zero heat capacity
Consequence of thermal gradients in glass during heating
Most solids expand on heating. Temperature gradient means only the outer layers try to expand as they heat up the most. But these are not free to expand independently and therefore stresses are induced. These are in compression for the outer layers and tension for inner layers. Finally thermal equilibrium is attained so there is a net expansion but with 0 stress as all parts have expanded equally
Stress calculation for 2D sheet
Stress=plate modulus x strain
This is for stretching in both dimensions. Plate modulus is
E/(1-ν)
When does glass fail?
Only under tension from the surface. Rapid cooling or non-uniform heating induced tension somewhere. It is strong in compression
When are stresses smaller?
When thermal expansion coefficient is small. When walls are thin so induced T gradients small
How do permanent stresses form in glass?
Consider hot glass above Tg. Cooling indices temperature gradients. No stresses because of liquid state. On achieving equilibrium at RT differential thermal contraction has occurred. The outer surface has cooled less so has contracted less. Results in permanent stresses where the surface is under compression and the centre under tension
What causes vacuum bubbles?
Contraction of hot molten core within a solid exterior
How else can permanent stresses arise?
If glasses of different TECs are joined together, e.g vacuum seals or chemical inhomogeneity
Formula for the surface compressive stresses
σmax=EαRL^2/3(1-ν)κ E is Young’s modulus α is TEC R is cooling rate L is half thickness of sheet ν is poisson’s ratio κ is thermal diffusivity
What is thermal diffusivity?
Thermal conductivity over (density x specific heat)
What is annealing?
Where the finished article is cooled slowly through the transformation range (Tg) to minimise stresses present.
Problem of remaining tensile stresses
Cause problems when cutting glass into sheets. Also bottles cool from the outside leaving the inside hot. Gives external compression and tension on inside. Unannelaed bottles are unstable so spontaneous failure can occur if the inner surface is weakened by damage
How to strengthen glass sheets
Once cut to shale they can be reheated and quenched to induce surface compression
Annealing process steps
Heat or cool to 5C above a Tg and hold for sufficient time to allow stress release. Cool slowly from this to Tg-50C after which no significant permanent stresses will be induced. Cool more quickly to RT but avoid thermal shock cracking. This process achieved using annealing lehr. Total time about 50mins
