12-01-2011, 12:44 PM
[attachment=8165]
Outline
Theory of Photoelasticity
The application of photoelasticity
Method
Limitations
Conclusion
Theory of Photoelasticity
Photoelasticity
applying a given stress state to model
utilising the induced birefringence of the
transparent material by polarized light
examining the stress distribution
Natural and polarized light
experimental method to determine stress distribution in a material.
fairly accurate picture of stress distribution even around abrupt discontinuities in a material.
critical stress points.
The method is based on the property of birefringence, which is exhibited by certain transparent materials.
Based on stress optic law.
Birefringence
A transparent material is birefringent if a ray of light passing through it experiences two refractive indices. The effect of this is to change the polarization state of the transmitted light to form interference fringes
When a ray of plane polarised light is passed through a photoelastic material, it gets resolved along the two principal stress directions and each of these components experiences different refractive indices.
The two waves are then brought together in a polariscope.
The phenomena of optical interference takes place and we get a fringe pattern, which depends on relative retardation.
Thus studying the fringe pattern one can determine the state of stress at various points in the material.
Method
In this method the test part, is first coated with a strain-sensitive plastic coating and then subjected to an external load.
The strains which exist throughout the part and over its surface are transferred to the coating and observed as optical interference fringes with a reflection polariscope.
Two different paterns are produced with the polariscope -- isochromatics via circular polarization and isoclinics via linear polarization.
Isochromatic fringes appear as a series of successive and continuous different-colored bands each representing a different degree of birefringence corresponding to the underlying strain.
The patterns can be read like a topographic map to visualize the stress distribution over the surface of the coated test part
