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Two new optical techniques to measure strain

A point-by-point technique and a field technique of strain determination based on optical filtering are presented

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Abstract

A grating engraved on the surface to be analyzed is utilized to measure strain. The grating is illuminated by a monochromatic, plane wavefront. A lens produces the diffraction spectrum of the grating, and an opaque screen is located in the plane of the spectrum. A window allows a single diffraction order to go through the screen. Two techniques are presented.

In the point-by-point technique, the illuminating wavefront is reduced to a very small area. Behind the window a light sensor detects the changes of light intensity that are produced by loading the specimen. In the field technique, an image of the specimen is produced, and the light sensor is located in the image plane. In both cases, changes of light intensity are related to strains. Using crossed gratings, a rosette strain gage is obtained.

Examples of application of both techniques are presented, showing a very good agreement between the strains measured by the proposed methods and by independent means.

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Abbreviations

A o :

amplitude of thenth order wavefront

a m :

coefficients of the Fourier series expansion of the frequency-modulated diffraction order

f :

focal distance of field lenses

F(x) :

amplitude function of a diffraction order

I(x) :

light-intensity distribution of the image plane

p :

pitch of the model grating

T(x) :

intensity distribution of a diffraction order in the frequency plane

W(ν):

energy going through the measuring window in the frequency plane

ε:

strain

λ:

wavelength of light

ν:

spatial frequency

ψ(x):

frequency-modulating function

w a :

angular frequency of the Fourier series expansion of the frequency-modulated single order

References

  1. Sciammarella, C. A., “Use of Gratings in Strain Analysis,” J. of Physics E: Scientific Instruments (Sept. 1972).

  2. Sciammarella, C. A. andSturgeon, D. L., “Digital-filtering Techniques Applied to the Interpolation of Moiré-fringe Data,”Experimental Mechanics,7(11),468–475 (1967).

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  3. Sciammarella, C. A., “Symposium, Applications of Holography in Mechanics,” ASME (1971).

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Authors and Affiliations

  1. Department of Mechanics and Aerospace Engineering, Illinois Institute of Technology, 60616, Chicago, IL

    C. A. Sciammarella (Professor)

  2. Tokushima-Prefecture, Japan

    H. Nyuko

Authors
  1. C. A. Sciammarella
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  2. H. Nyuko
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Sciammarella, C.A., Nyuko, H. Two new optical techniques to measure strain. Experimental Mechanics 14, 311–316 (1974). https://doi.org/10.1007/BF02324953

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  • DOI: https://doi.org/10.1007/BF02324953

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