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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.
Photoelasticity
Photoelasticity
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.
Photoelasticity
Circular Polariscope
This polariscope employs circularly polarized light. The photo elastic apparatus contains four optical elements and a light source.
Photoelasticity
Visualization of stresses in a straight beam subjected to four point bending.
Photoelasticity
· 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.
Experimental setup
Photoelasticity
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).
Experimental setup
Photoelasticity
Solution
When stressed, some transparent materials behave as bifringent material.
The speeds of the refracted lights are directly proportional to the
principal stresses σ1 and σ 2, major and minor principal stresses, respectively.
How to determine stress?
Photoelasticity
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.
Test Specimen
SupportSupport
Sample specimen Dimensions
Photoelasticity
StartThe Forces acting various sections are indicated as shown in the figure.
Click to start animation.
Photoelasticity
Due to application of load the beam is subjected to bending in the transverse plane. The red lines indicate the new position of
the beam.
Interpretation of 2-D photoelastic fringe patterns is based on the stress-optic law:
σ1- σ2 = (N *Fσ )/b
Where b is thickness of photo elastic model.
Photoelasticity
Calculation of material fringe order.
σ1- σ2 = (N *Fσ )/b
Hence fs can be determined by plotting the quantity Z versus the fringe order, N
Photoelasticity
2D Photo Elastic fringes under exposure of monochromatic light.
Photoelasticity
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
Photoelasticity
What does ∆n represent in the equation for the
Stress-Optic Law?
a Thickness of material
b Stress-Optical coeffcient
c Birefringence
d Stress
Photoelasticity
The color pattern indicate the ---------------------- of the stressed material?
oStress
obifrigence
opolarization of light
ostrain
Photoelasticity
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
Photoelasticity
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.
Photoelasticity
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/matproptest/ESA.html
http://en.wikipedia.org/wiki/Photoelasticity#References
www3.ntu.edu.sg/.../photoelasticity/index.html