Lecture 17 part 3

Question 1

What is a tempering pattern?

Answer 1

To make toughened glass it must be tempered. This is a process of controlled thermal or chemical treatments to increase the strength of the glass

Tempering puts outer parts into compression and inner parts into tension. This can be seen as a pattern when observed with polarised light

Question 2

When establishing the loads applied to a glass door why would a birefringant coating be used rather than the naural birefringence of the glass?

Answer 2

The glass has an inherent tempering strain pattern that would complicate the analysis, a birefringent coating bypasses this tempering pattern allowing only the applied loads to be looked at

Question 3

When looking at the loads on a large object, using a glass door as an example, which areas of the object would it be most advantageous to focus on?

Answer 3

It would be most advantageous to focus upon the stress raising elements

i.e. the edges of the glass and around the fittings

Question 4

How could a birefringent coating be applied to a glass door?

Answer 4

1) Mark on positions of all furniture and outer seal on glass to ensure the resin coating did not come into contact with direct clamping pressure upon re-fitting
- Fringe patterns would only be due to strain witness from glass
2) Apply coating of silver paint to top face of the glass
3) Apply epoxy resin to areas covered by silver paint
4) bake glass at 27 degrees to cure the resin

Question 5

Upon investagating the fracture of a glass door it was seen that the loading stresses alone were not enough to cause fracture, once this fact has been established what should be investigated?

Answer 5

The residual tempering stress combined with the loading stresses should be investigated

Question 6

When the fracture of a glass door was investigated the tempering stress patterns of a control sample of glass were compared to the patterns in the test glass.

The compressive edge stresses were high and in a concentrated 3mm deep zone around the glass edge

The average variation of compressive edge stresses in the test glass was around 70MPa compared to 35MPa for the control glass, these variations occur over a small distance, this is coupled with pockets of high tensile stress close to the compressive edge stresses

What problems are discovered and what is the root cause?

Answer 6

• The high edge stress gradients comprimise intended material properties
• The pockets of tensile stresses reduces the effect of the the compressive edge stresses
• The root cause is the toughening process, it is not sufficiently controlled and so will not give consistent products. The integrity of the glass under working conditions cannot be relied upon untill this is rectified