5. Three-dimensional photoelasticity

Question 1

Why can you not just look at a birefringent 3D part through a polariscope to see strains

Answer 1

You would just see an integral of the stresses

Question 2

What bonds do Diphase polymers have

Answer 2

Primary and secondary molecular bond

Primary bond = stronger, Secondary bond = weaker

Question 3

How do we lock strains into a material using a diphase polymer?

Answer 3

1) Place part in oven
2) Load part
3) Heat up part (until secondary bonds break down)
4) Load is taken by primary bond
5) Cool part down v.slowly with load still applied
6) allow secondary bond to reform around loaded primary bond

Question 4

What is the temperature called when the secondary bonds break down?

Answer 4

Glass trasition temperature

Question 5

What happens to the material modulus at the glass transition temp?

Answer 5

E drops dramatically

Question 6

What happens to the optical sensitivity K

Answer 6

K drops dramatically

Question 7

Why do we have to be careful with our loading when passing through the glass transition temperature?

Answer 7

Because the strength of the material drops greatly and we only want slight deformation

Question 8

Why do we need to put a calibration specimine in the oven with the diphase polymer?

Answer 8

Because K drops, we need the stress optic constant at the glass transition temperature

Question 9

How do we see the stress inside a 3d object after stress freezing?

Answer 9

we slice the part up into effectively 2d parts along the principal strains and analyse it in the same way as we would a 2d part

Question 10

Why do we need to be careful with slicing thicknesses when cutting up a stress frozen specamine?

Answer 10

If the slice is too thin then we wont get any observable fringes as number of fringes is directly proportional to thickness of slice

If the slice is too thick then we wont get a 2d stress field (we dont want the stress profile to change through the thickness of the slice)

Question 11

What is the result if you slice specamine on a plane that is not along a principal strain?

Answer 11

You obtain secondary principal stresses

Question 12

What is the result if you slice the stress frozen specamine along princiapl plane?

Answer 12

you obtain principal stresses

Question 13

How do we process our resluts from 3D stress analysis?

Answer 13

By using graphs of quantitative data

Question 14

How do we know what kind of loads to put on a scaled model?

Answer 14

Use equilibrium an mathematical compatability (i.e. dimensional analysis) to give a distribution of stress that is independant of load and scale

Question 15

How do we rectify the stresses on a model with the stresses in a prototype or part?

Answer 15

We use the ratio of stresses between prototype and part

W is load, t is thickness and L is characteristic length

Question 16

How do we rectify the strains on a model with the strains on the prototype or part?

Answer 16

We use the ratio of strains between the model and the prototype or part

Question 17

Write an equation for the load on the model relating to the load on the prototype assuming poissons ratio is the same for both model and prototype

Answer 17

Question 18

What are 3 general rules for 3d stress analysis

Answer 18

1) Force depends on model scale and ratio of moduli
2) when body forces are not uniform the poissons ratio of the model and of the part must be roughly equal
3) data must be taken away from loading points due to St Venant’s principal (i.e. loads too close to point of loading cannot be scaled)

Question 19

What is St Venant’s principal

Answer 19

the difference between the effects of two different but statically equivalent loads becomes very small at sufficiently large distances from load