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Structures and Controls Lab - AE 461 Chasiotis I.
Measurement of Stress by
Photoelasticity
References: M.M. Frocht, Photoelasticity, John Wiley and Sons, Boston, (1941)
A.J. Durrelli, Introduction to Photomechanics, Prentice-Hall, (1965)
Structures and Controls Lab - AE 461 Chasiotis I.
Photoelastic Experiment
Based on: When tractions are prescribed in a boundary value problem, the stressdistribution is independent of material (properties), i.e. E and
Photoelasticity helps us visualize stresses inside an object
Objectives:
1. Introduce the use of photoelastic principles in stress analysis
2. Use the stress distribution in a polymer beam to find the material fringe constant
3. Measure the stress concentration in a notched polymer beam
Photoelastic Effect: Optical property of transparent materials in which the speed oflight through the material is affected by the state of stress
How we use it: Pass light through a model of an engineering structure and measure
the changes in the wave speed for a given applied load.
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Structures and Controls Lab - AE 461 Chasiotis I.
Use Photoelasticity to Find the Stress in the Object
Structures and Controls Lab - AE 461 Chasiotis I.
Use Photoelasticity to Find the Stress in the Object
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Structures and Controls Lab - AE 461 Chasiotis I.
Definition of Light and Dark Fringes
Light and dark field isochromatics in a circular disk under diametric compression
Dark fringes fall between light fringes
High fringe density denotes high stress gradient (and perhaps stress concentration)
Structures and Controls Lab - AE 461 Chasiotis I.
We Need Polarized Light
PLANE POLARIZED LIGHT
Light going through a polarizer is transmitted in one plane only
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Structures and Controls Lab - AE 461 Chasiotis I.
The Method Applies to BirefringentMaterials
Gary Cloud, Experimental Techniques, 2008
Birefringent materials are those in which
the index of refraction varies with the
direction of polarization of the light passingthrough them.
The surface of the
birefringent slab acts as a
beam splitter, dividing the
entering wave into two waves
that are polarized inorthogonal directions called
the principal axes of the
refractive index.
Structures and Controls Lab - AE 461 Chasiotis I.
The Plane Polariscope
Polarizer and Analyzerare crossed Dark field
The incident light that enters the model is split into two components that are
orthogonally polarized and vibrate in the planes of1 and2
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Structures and Controls Lab - AE 461 Chasiotis I.
Isocromatics in the Object
Structures and Controls Lab - AE 461 Chasiotis I.
Isocromatics Shadowed by Isoclinics
Gary Cloud, Experimental Techniques,
2008
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Structures and Controls Lab - AE 461 Chasiotis I.
Circularly Polarized Light
The tip of the light vector traces a circular path with time forming a helix
as it propagates along the z-axis
How do we create circularly polarized light?
Use a plane polarizerand a quarter wave plate
Structures and Controls Lab - AE 461 Chasiotis I.
The Quarter Wave Plate
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Structures and Controls Lab - AE 461 Chasiotis I.
Crossing Quarter Wave Plates
Two parallel quarter plates shift the light plane by 90
Two crossing quarter plates shift the light plane by 0
Structures and Controls Lab - AE 461 Chasiotis I.
The Circular Polariscope Dark Field
If analyzer is crossed Dark field
If analyzer is parallel Light field
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Structures and Controls Lab - AE 461 Chasiotis I.
Dark field Dark field Light field
Configurations of the Circular Polariscope
Structures and Controls Lab - AE 461 Chasiotis I.
Configurations of the Circular Polariscope
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Structures and Controls Lab - AE 461 Chasiotis I.
Configurations of the Circular Polariscope
Structures and Controls Lab - AE 461 Chasiotis I.
The Circular Polariscope ? Field
WHAT HAPPENS IF I PLACE AN OBJECT IN THE MIDDLE?
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Structures and Controls Lab - AE 461 Chasiotis I.
The Circular Polariscope
Structures and Controls Lab - AE 461 Chasiotis I.
The Circular Polariscope Light Field Arrangement
When light passes through the analyzer we have Light Field Arrangement
2 cos cos2
A k t
AmplitudeFrequency Phase shift
2 2 2 2Intensity: 4 cos cos2
I A k t
Dark fields are generated when:
1. cos2(t)0 ; cannot be observed since is very high (~1012 Hz)
2. cos2(/2)0
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Structures and Controls Lab - AE 461 Chasiotis I.
Experimental Set-up for Bending Dark Field
fixed fixed
M M
Structures and Controls Lab - AE 461 Chasiotis I.
Pure Bending
1-2=0
1 2 fmt
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Structures and Controls Lab - AE 461 Chasiotis I.
Four Point Bending
1 2 mt
Structures and Controls Lab - AE 461 Chasiotis I.
Stress Concentration at a Circular Hole
Plate thickness = t
: 0 0r rr a r aBCs
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Structures and Controls Lab - AE 461 Chasiotis I.
1-2=0
Stress Concentration
Structures and Controls Lab - AE 461 Chasiotis I.
Stress Concentration
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Structures and Controls Lab - AE 461 Chasiotis I.
Pre-Lab Assignment
For the isotropic photoelastic beam shown below (in a light-field polariscope), what is the
highest fringe number? Locate the n = 0 fringe(s) and the highest fringe(s).
The fringes are denoted by the black lines. (Note: In a light-field polariscope, these are
actually half-fringes, thus the need for (n+1/2) rather than n)
Structures and Controls Lab - AE 461 Chasiotis I.
Pre-Lab Assignment
h = 1.5 in
h = 1.5 in
w = 0.25 in
Beam cross-section
0.125 in from highest fringe order to the edge of beam
Determine the fringe constant fs for the following beam with an applied
moment of 12.00 in-lb at the ends. Note that the fringe pattern is symmetric
about the neutral axis.
1 21
2
t
nf
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Structures and Controls Lab - AE 461 Chasiotis I.
Pre-Lab Assignment
Which would give a greater stress resolution in a photoelastic beam, a high
fs or a low fs? Which is more applicable for sensitive experimental stressanalysis? Explain.
1 21
2
t
nf
1 2
t
mf
or
What purpose do the quarter wave plates serve in the experimentalpolariscope setup?
Structures and Controls Lab - AE 461 Chasiotis I.
Pre-Lab Assignment
Around a circular hole the maximum stress is the nominal stress magnified by
a factor of 3.0. If nfar= 2 (number of fringes seen in the far field):
a) Derive the relationship between the stress concentration ratio to the
number of near and far field fringes.
b) How many fringe orders will be seen in the near field under this applied
stress?
nnear
nfar
nom max
PP
1
1 2
1
2
tn
f
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Structures and Controls Lab - AE 461 Chasiotis I.
Other Examples
Gary Cloud, Experimental Techniques, 2009
Structures and Controls Lab - AE 461 Chasiotis I.
Other Examples
http://www.flickr.com/photos/core-materials/3841043722/sizes/o/in/photostream/
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