5. Three-dimensional photoelasticity Flashcards
Why can you not just look at a birefringent 3D part through a polariscope to see strains
You would just see an integral of the stresses
What bonds do Diphase polymers have
Primary and secondary molecular bond
Primary bond = stronger, Secondary bond = weaker
How do we lock strains into a material using a diphase polymer?
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
What is the temperature called when the secondary bonds break down?
Glass trasition temperature
What happens to the material modulus at the glass transition temp?
E drops dramatically
What happens to the optical sensitivity K
K drops dramatically
Why do we have to be careful with our loading when passing through the glass transition temperature?
Because the strength of the material drops greatly and we only want slight deformation
Why do we need to put a calibration specimine in the oven with the diphase polymer?
Because K drops, we need the stress optic constant at the glass transition temperature
How do we see the stress inside a 3d object after stress freezing?
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
Why do we need to be careful with slicing thicknesses when cutting up a stress frozen specamine?
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)
What is the result if you slice specamine on a plane that is not along a principal strain?
You obtain secondary principal stresses
What is the result if you slice the stress frozen specamine along princiapl plane?
you obtain principal stresses
How do we process our resluts from 3D stress analysis?
By using graphs of quantitative data
How do we know what kind of loads to put on a scaled model?
Use equilibrium an mathematical compatability (i.e. dimensional analysis) to give a distribution of stress that is independant of load and scale
How do we rectify the stresses on a model with the stresses in a prototype or part?
We use the ratio of stresses between prototype and part
W is load, t is thickness and L is characteristic length
How do we rectify the strains on a model with the strains on the prototype or part?
We use the ratio of strains between the model and the prototype or part
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
What are 3 general rules for 3d stress analysis
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)
What is St Venant’s principal
the difference between the effects of two different but statically equivalent loads becomes very small at sufficiently large distances from load
