Usually, during the structural design of flat glass, one does not need to take into account the effects of changes from residual stresses.
Question:
How to understand and predict the residual stress distribution of hot bent glass?
I assume, that the surface of the concave and the convex side are the same.
Firstly, I assume that the residual stresses from heat-soaking are only a function of the thickness of a glass pane (and the holding time and temperature distribution over that time) - then it does not matter if the glass is bent or not. However, heat flux behind a concave surface tends to hinder convection while the opposite is true at a convex surface.
Does this geometric influence even matter or is it insensitive to the final residual stress?
Are there any other relevant parameters that could influence residual stresses?
Is there any research available of which I am not aware of?
Reminder:
Residual stresses of heat-soaked flat glass acc. to EN 14179.1:2016 (Heating up to +260°C +/-10°C during a 2-hour holding time) lead to about the same residual surface compression as non-heat soaked glass; therefore the structural design resistance values are not affected.
However, the inner core residual tension stresses are approximately 50% of that of a non-heat soaked glass. (Therefore, the overall residual energy stored is less).
Dear Franz Nikolaus Pölzl
By performing FE simulation you can predict the residual stress distribution of hot bent glass, but at first you should validate your model and result with an experimental data. Also residual stresses in addition to the thickness, holding time and temperature are highly dependent to the cooling rate and micro-structures (unfortunately FE simulations are not good in prediction of micro-structures and it need to check in your case).
Residual stresses on the surface of the concave and the convex side are not same because geometry and forming shape are important in the creation of residual stresses.
Best regards,
Dear Farshad Nazari,
if it is not possible to cover the micro-structure in a FEM calculation in order to predict residual stresses, then the question shall be how it can be simplified?
Further, you mention that "forming shape" is important - what do you exactly mean by that?
- At first you should simulate your process by FEM and compare residual stress results with an experimental data. Maybe in your case the effect of the micro-structure on the residual stresses is not significant. This work, if doesn't solve our problem, at least give you a point about your sample conditions and residual stresses.
- Residual stresses are highly dependent to the elastic and plastic bonds that they are created in the forming. For example in a simple bend (by a positive torque), the top surface is compressed and the bottom surface is stretched that after unloading it is caused to create tension residual stresses on the top surface and compression residual stresses on the bottom surface of the bent sheet.
Farshad Nazari; thank you for further explanations -
I attached a sketch because I have still open questions caused by your latest answer.
From hot bending (gravity bending), residual stresses remain in the glass pane due to the bent geometry, as I understand from your answer?
My thought is that the glass transition temperature is exceeded usually during hot bending (~620°C process temperature). Therefore, I just assumed that the stress relaxation expunges residual stresses caused by the bending process (viscosity is dominant and allows redistribution of "local bonds").
Now, I interpret from your answer that you disagree with this assumption as you state
".... is caused to create tension residual stresses on the top surface and compression residual stresses on the bottom surface of the bent sheet. "
Could you explain, I would be grateful for your further input!
Dear Franz Nikolaus Pölzl
My answer was a example about the effect of forming shape on the residual stress for a bent metal in room temperature, but as I explained before the residual stresses are dependence to the other parameters such as temperature, micro-structure (or material) and etc.
I'm appreciated to share this sketch for more clarity. According to presented studies on glass planes, because of wide elastic bond of glass and the effect of the product process and high temperature, the balance of residual stresses on the surface and bottom are compression and at the mid-plane is tension, that you truly presented in shared sketch.
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