Photo-elasticity is a visual technique for measuring stresses. When a photo elastic material is strained and viewed under polarized light, beautifully colored pattern can be observed. This colored pattern provides information on stress-state of the

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Photoelasticity
Photo-elasticity is a visual technique for
measuring stresses. When a photo elastic material
is strained and viewed under polarized light,
beautifully colored pattern can be observed. This
colored pattern provides information on
stress-state of the strained material.
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Plane Polariscope
It consists of two linear polarizer (which
transmit light only along their axis of
polarization) and a light source. The linear
polarizer nearest the light source is called the
polarizer, while the second linear polarizer is
known as the analyzer.
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Circular Polariscope
This polariscope employs circularly polarized
light. The photo elastic apparatus contains four
optical elements and a light source.
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Visualization of stresses in a straight beam
subjected to four point bending.
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Experimental setup
Light source emits light waves vibrating in
infinite number of planes. Polarization Filter
(Polarizer) Polarizer restricts the vibration of
light waves to a single plane.
Click START.
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Experimental setup
Analyzer
Analyzer Second polarizer used to analyze
polarized light passed through material in
testing. Doubly refracting lights which passed
through the birefringent are resolved by
analyzer. Speed difference in the refracting
lights causes that a certain color from the light
disappears (wave shift).
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How to determine stress?
Solution
When stressed, some transparent materials behave
as bifringent material. The speeds of the
refracted lights are directly proportional to the
principal stresses s1 and s 2, major and minor
principal stresses, respectively.
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Test Specimen
Support
Support
Sample specimen Dimensions
Four point Bending

• The straight beam is subjected to tranverse
  loading.The beam is supported at two ends and
  downward load is applied at the two points shown
  in figure.

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Start
The Forces acting various sections are indicated
as shown in the figure.
Click to start animation.
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Calculation of material fringe order.
s1- s2 (N Fs )/b
Hence fs can be determined by plotting the
quantity Z versus the fringe order, N
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2D Photo Elastic fringes under exposure of
monochromatic light.
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Photo elasticity is the phenomenon of
-------------------------------------

• Elastic behaviour of material in lighting
  conditions.
• inducing birefringence in a substance through
  the application of a stress system.
• Bending stresses and shear stress determination
  using light
• Normal stresses

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What does ?n represent in the equation for the
Stress-Optic Law? a Thickness of material b
Stress-Optical coeffcient c Birefringence d
Stress
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The color pattern indicate the
---------------------- of the stressed material?

• Stress
• bifrigence
• polarization of light
• strain

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What are the quantities on the axes of a
Michel-Levy chart? a Retardation against
thickness of specimen b Birefringence against
thickness of specimen c Birefringence against
time d Retardation against time
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What is the difference between isochromatic and
isoclinic fringes? a Isochromatic fringes are
obtained using monochromatic light, whereas
isoclinic fringes are obtained using white light.
b Isoclinic fringes are obtained when the
principal stress direction coincides with the
polarisation of the polariser isochromatic
fringes are lines of constant stress difference.
c Isochromatic and isoclinic fringes occur in
different types of plastic. d Isoclinic fringes
are lines of constant stress difference
isochromatic fringes are obtained when the
principal stress direction coincides with the
polarisation of the polariser.
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Resources
Books Experimental Stress Analysis. Second
Edition Dally, J W Riley, W FMcGraw-Hill Book
Co., 1978, pp. 571, 1978 Reference
Links http//dynamark-engineering.com/services/ma
tproptest/ESA.html http//en.wikipedia.org/wiki/Ph
otoelasticityReferences www3.ntu.edu.sg/.../photo
elasticity/index.html