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 Δn 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