4. Two-dimensional and Reflection Photoelasticity Flashcards
What are the 3 assumptions when using photoelastic stress analysis?
- Isotropic homogeneous materials
- A state of plane stress exists in the photoelastic material
- Stress-optic law applies in the elastic region only
Fringe Constant Evaluation
Need a test specimen of known stress distribution to measure fringe order against the load.
What are 3 common choices for these test specimens?
- Tension Test
- Disc in commpression
- Pure bending test
Sketch a tension test and write the equations relating the stresses to the stress optic law - hence find an equation for f,
this is independant of specimen ………… ?
Sketch a compression test and write the equations relating the stresses to the stress optic law - hence write an equation for f
This is independant of specimen …………?
Sketch a pure bending test and write the equations relating the stresses to the stress optic law hence write an equation for f
Materials 2d p.e
All ………….. materials exhibit temporary …………
What are 4 requirements for selecting materials for p.e tests
Transparent , birefringence
- High figure of merit
- Good machinability
- Low creep
- Low time edge effect , dont absorb moisture and show stresses on the edges due to this moisture
Materials for 2d p.e.
what is the figure of merit ?
Q = E/ f
The modulus divided by the fringe constant
A high number is desirable for p.e. models as it gives a high number of fringes for a given load. More fringes - more accurate results
Integer fringe oders
the basic p.e relationship is ……….
Hence the neighbouring fringe order is either
- ………
- ………
- ………
Location of the …….. order fringe allows identification of remainder
- increased by one
- decreased by one
- are the same order
zero
Zero order fringes
Zero order fringes are …….. in a circular polariscope using white light (e.g. isotropic points)
A loaded square corner in any model is an isotropic point & automatically has a zero fringe order
here. Draw a sketch of this
black
Fractional Fringe Orders
Interpolate on a graph of integer fringe order against ……………
Use Goniometric …………. ………….. and compensators
distance
compensation techniques
Procedure for Tardy Compensation
Use plane ……….. with white light
Rotate all elements until isoclininc covers ………. .. ………
Convert to ………… , circular polariscope
Rotate analyser until ………. covers the ……… .. ………
Fringe order at ………. .. …….. is N = ………
polariscope
point of interest
monochromatic
fringe, point of interest
point of interest
Boundary Stresses
………… stress usually occurs on a boundary
…….. free boundaries are ……….. planes
with no contact σnormal = …… => σtang = ………
Maximum
shear , principle
σnormal = 0
σtang = N.f/t
Sign of boundary stresses
- use a ………. of known ……….. or ……….. stress
- compress edge with finger nail
(N increases if σtang is ………) - boundary stress can only change sign when N=…….
compensator ,compressive , tensile
tensile
N = 0
Reflection Photoelasticity
sketch a diagram of reflection p.e
Strain Witness role
what are the 2 assumptions ?
Strain witness - surface strain in prototype transmitted to coating without distortion
How does this relate to Hooke’s law ?
- plane stress
- thin coating
Hookes law - see picture
Strain optic law in reflection
what is the equation
Why is 2t used ?
2t is used as light passes through the birefringent material twice
Once the strain has been meaured using reflection p.e
what equation is used to find the stress?
compine Hookes law and the strain optic law
Poisson’s ratio mismatch
Occurs when put a coating on a component . Paissons ratio are very different
Occurs when
- ………..
- ……….
Remedy
- ……….
- ………
Occurs when :
paisson ratio component > paissons ratio prototype
bordere 4t deep effected
Remedy:
apply corection factor (in picture)
Dont take data from the edge of the specimen
Selection of Coating
……. fringe constant
- …….. coating sensativity , K
- ……… thickness, t
…….. to avoid reinforcement
- particulary in bending
Compromise using .. ………… of coatings
low
high
large
thin
a range
Relative performance of reflection photoelasiticity
what are 3 advantages ?
- Applied to real components
- Full-field view of surface strain (can take pictures)
- Well- established ,simple eqipment (commonly used in industry)
Relative performance of reflection photoelasticiy
What are 3 disadvantages of this method
- Detailed analysis difficult (can only look at the analysis normal to the specimen)
- Limited temperature range (melting point of the polymer)
- Require surface preparation of the specimen
