Experimental Mechanics

, Volume 3, Issue 3, pp 67–72 | Cite as

New oblique-incidence method for direct photoelastic measurement of principal strains

Investigation indicates that method of separation of principal strains using a selected angle of oblique incidence has a potentially high accuracy
  • S. S. Redner


In most photoelastic-coating analyses, separation of principal stresses is accomplished by the oblique-incidence method, with rotation about one of the principal stress axes. Then, the relationship between relative retardation (\(\delta _\theta\)) and the principal strains (∈ x and ∈ y ) has the form
$$\delta _{\theta x }=A _{ \in x }+B _{ \in y} $$
whereA andB are functions of the angle of incidence θ and Poisson's ratio of the photoelastic material. It is shown here thatB=0 when cos2 θ x =v e . For this condition, individual principal strain ∈ x or ∈ y can be obtained directly by a single birefringence measurement.

The potential sources of error, including errors in Poisson's ratio, have been analyzed and evaluated. A 5 percent error in Poisson's ratio determination leads to a 2.1 percent error in the calculated principal strain.

In an experimental evaluation of the proposed method, the departure from strain-gage results did not exceed 5 percent of the measured strain.


Mechanical Engineer Fluid Dynamics Experimental Evaluation Principal Stress Percent Error 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



Normal strain


Normal stress


Poisson's ratio


Modulus of elasticity


Angle of incidence


Wave length of light




Relative retardation between two polarized light beams


Photoelastic fringe order

x, y, z

Mutually perpendicular directions of principal strains or stresses

F1, F2, F3

Coefficients, defined by eqs (3′) and (4′)

A, B

Coefficients, defined by eq (5)


Strain-optical coefficient


x, y, z

pertaining to values in thex, y, z directions


pertaining to structural part


pertaining to the coating


pertaining to the direction normal to the surface


pertaining to the direction forming an angle θ with the normal as defined in Fig. 1


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Copyright information

© Society for Experimental Mechanics, Inc. 1963

Authors and Affiliations

  • S. S. Redner
    • 1
  1. 1.Photoplastic, Inc.Malvern

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