Photoelasticity Experiment: Stress Analysis with a Polariscope

The photoelasticity experiment reveals the stress distribution inside a transparent model by viewing it under polarised light in a polariscope, where stress produces coloured fringe patterns. It is a powerful visual method for finding stress concentrations at holes, notches and fillets — and for validating finite-element results.

Aim of the experiment

To determine the stress distribution and stress concentration in a transparent model using a polariscope, and to apply the stress-optic law.

Theory

Photoelasticity relies on birefringence: certain transparent materials (such as epoxy or polycarbonate) become doubly refracting when stressed. Viewed between crossed polarisers in a polariscope, a stressed model shows coloured fringes. The isochromatic fringes are lines of constant principal-stress difference, given by the stress-optic law:

σ₁ − σ₂ = N · f_σ / t

where N is the fringe order, f_σ is the material fringe value, and t is the model thickness. The isoclinic fringes give the directions of the principal stresses.

Apparatus required

• Polariscope (plane or circular): light source, polariser, analyser and quarter-wave plates
• Loading frame to stress the model
• Transparent photoelastic models (with holes, notches or fillets)

Procedure

1. Place the photoelastic model in the loading frame between crossed polarisers.
2. Apply a known load and observe (or photograph) the fringe pattern.
3. Count the fringe order N at the points of interest.
4. Compute the principal-stress difference from the stress-optic law and locate the regions of highest stress.

Result

Fringe density is highest at stress concentrations — around holes, notches and sharp fillets — giving a direct visual map of where a component is most likely to fail.

Applications

Photoelasticity is used for stress analysis of complex components, validating finite-element models, and teaching stress concentration. It complements destructive tests such as the tensile test on a UTM, hardness testing, and the buckling of columns experiment. See related terms in the engineering lab glossary.

Frequently asked questions

What is photoelasticity?

It is an experimental stress-analysis technique that uses the birefringence of transparent materials to show stress as coloured fringe patterns under polarised light.

What is the stress-optic law?

It relates the fringe order to stress: σ₁ − σ₂ = N · f_σ / t, where N is the fringe order, f_σ the material fringe value and t the model thickness.

What is the difference between isochromatics and isoclinics?

Isochromatics are fringes of constant principal-stress difference (magnitude); isoclinics give the directions of the principal stresses.

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