Lecture 17 - Applications and Case Studies (glass vehicle doors)

Question 1

_Case Study _

What are the 6 questions you ask when you begin conducting an experimental stress analysis?

Answer 1

1. What is the component ?
2. What are the operating conditions ?
3. What are the design criteria ?
4. What are the stress analysis requirements ?
5. What are the available methods ?
6. How are the appropriate techniques applied ?

Question 2

_Casestudy - Define The Problem _

what are the other 2 different ways that stress is categorised in the flat glass industry ?

  1-  Permanent and temporary (describe)

  2-

  3-

Answer 2

– variation with time (residual stress locked in) (temporary stress disappears whe the load is removed

2- Thickness, area and localised – variation with position

3- Surface and centre – values that actually cause fracture

Question 3

Stress variation with time (glass) - 1

Permanent stress - locked into the glass, typically as a result of …….-………….. processing around the ……… ………… ………. (typically ~560ºC).

Otherwise known as ……….. stress.

Must be in …………. ……………. – i.e. across any section of the glass, ………… forces and …………. forces must balance out.

Answer 3

thermal-mechanical

glass transition temperature

residual stress

mechanical equilibrium

compression , tension

Question 4

Stress variation with time (glass) - 2

………………. ………. - would disappear if the loading was removed.

The type of loading can be
……………. - bending, stretching, twisting, contact etc
…………… (non-uniform temperature gradients through the thickness or across the width of the glass)

The duration of loading can be
…………. – eg induced by forcing glass to fit in a frame
………….. – eg induced by temperature gradients (which soon disappear by thermal conduction)

Does not have to be in mechanical equilibrium just within glass.

Answer 4

Temporary stress

mechanical

thermal

prolonged

temporary

Question 5

Stress variation with (glass) - 3

………….. stress - varies through the thickness of the glass.

Arises from mechanical ……….. forces or from non-uniform ………….. ………… through the glass.

Also called toughening or ………… stress.

Answer 5

thickness

bending

temperature gradients

tempering

Question 6

Stress variation with (glass) - 4

draw the force distribution in tempered glass

Answer 6

Question 7

Stress variation with (glass) - 5

what is area stress?

Answer 7

Area stress is that stress that varies across the dimensions of a piece of glass, but is constant through the thickness.

Question 8

Surface and Centre Stress

The thickness stresses (permanent and temporary) can be further subdivided into surface and centre values. Obviously for the area stress, the surface and centre values are the same.

What is surface stress ?

Why is this important for glass ?

what are the two reasons why you need to make sure surface stress is correct ?

Answer 8

Surface stress is that stress in the near surface region of the glass – typically from the surface down to ~100m into the glass.

1. Need to make sure surface stress is correct so that glass breaks properly when we want it to – eg when scoring glass.
2. Want surface stress correct so that glass doesn’t break when you don’t want it to.

Question 9

Strength Improvement - Thermal Strengthening

………….. is the most common method used to improve the strength of glass.

Glass is heated to about 650oC, and then rapidly ………… with air jets.

This cools the surface ……….. and the core more …………

At ambient temperature, the core continues to ……. and ……………. stress develops in the ……….., balanced by ……….. in the centre.

Answer 9

Tempering

cool

quickly , slowly

compressive, surface

tension

Question 10

Strength Improvements - Thermal Strengthening

draw a sketch of the thermal strengthening process

Answer 10

Question 11

Strength Improvements - Thermal Strengthening 2

The tempering cycle gives rise to a …………… stress profile within the glass,

sketch a diagram of this and also show how it buffers crack growth

the magnitude at the surface is ~ …. that of the centre tensile region

what other stresses does tempering leads to ?

what does too high compression indicate?

Answer 11

parabolic

2x

Tempering also leads to area stress – because of temperature
gradients across the piece of glass

this indicates high tension somewhere else