Experimental Mechanics

, Volume 27, Issue 3, pp 304–313 | Cite as

A summation strain-gage alternative to oblique incidence in photoelastic coatings

  • Wayne E. Nickola
Article

Abstract

A special strain gage (PhotoStress® Separator Gage, Measurements Group, Inc., Raleigh, NC) designed to measure the sum (ɛ1 +ɛ2) of the principal strains, is used in conjunction with photoelastic-coating measurements (ɛ1 -ɛ2) to establish the value of each principal strain (ɛ1 andɛ2). The summation strain signal is effectively independent of angular orientation (measurement direction), and by design, the gage negates soldering risks, self-heating, and localized-reinforcement considerations normally associated with strain-gage measurements on plastic parts.

Keywords

Mechanical Engineer Fluid Dynamics Measurement Direction Strain Gage Principal Strain 

List of Symbols

CB

pure-bending correction factor for photoelastic coatings

CPS

plane-stress correction factor for photoelastic coatings

Es

elastic modulus of test part

E*

E c /E s =elastic-modulus ratio, photoelastic coating to test part

KB

pure-bending correction factor for summation gage

KPS

plane-stress correction factor for summation gage

Nn

normal-incidence fringes

No

oblique-incidence fringes

\(\hat N_n \)

uncorrected normal-incidence-fringe measurement

R1

reading displayed on strain indicator

SG

output signal from summation gage

f

λ/2t c k=fringe value of photoelastic coating

k

strain-optical coefficient of photoelastic coating

tc,ts

thickness of photoelastic coating and specimen, respectively

t*

t c /t s =thickness ratio, photoelastic coating to test part

ɛ

sum of principal strains at summation gage

\(\widehat\sum _ \in \)

uncorrected sum of principal strains at summation gage

ɛ1,ɛ2

maximum and minimum principal strains, respectively

ɛx,ɛy

normal strains, at a point, in any two perpendicular directions

λ

wavelength of yellow light (22.7×10−6 in. or 577 nm)

ν

Poisson's ratio of test part

σ1, σ2

maximum and minimum principal stresses respectively

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References

  1. 1.
    “Introduction to Stress Analysis by the PhotoStress Method.” Measurements Group Tech Note TN-702, Measurements Group, Inc., Raleigh, NC (1981).Google Scholar
  2. 2.
    Zandman, F., Redner, S. and Dally, J.W., Photoelastic Coatings, Soc. for Exp. Mech., Bethel, CT.Google Scholar
  3. 3.
    “Strain Measurement with the 030-Series Reflection Polariscope,” Measurements Group, Inc., Raleigh, NC.Google Scholar
  4. 4.
    “Materials for Photoelastic Coatings/Models,” Photolastic Bulletin S-116, Measurements Group, Inc., Raleigh, NC.Google Scholar
  5. 5.
    Zandman, F., Redner, S. andRiegner, E., “Reinforcing Effect of Birefringent Coatings,”Proc. of the SESA,19 (1),55–64 (1962).Google Scholar
  6. 6.
    “Corrections to Photoelastic Fringe Order Measurements,” Measurements Group Tech Note TN-706, Measurements Group, Inc., Raleigh, NC (1982).Google Scholar
  7. 7.
    Post, D. andZandman, F., “Accuracy of Birefringent-coating Method for Coatings of Arbitrary Thickness,”Proc. of the SESA,18, (1),21–32 (1961).Google Scholar

Copyright information

© Society for Experimental Mechanics, Inc. 1987

Authors and Affiliations

  • Wayne E. Nickola
    • 1
  1. 1.Measurements Group, Inc.Raleigh

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